Showing papers in "Journal of Ecology in 2007"

Facilitation in plant communities: the past, the present, and the future

Abstract: Summary 1 Once neglected, the role of facilitative interactions in plant communities has received considerable attention in the last two decades, and is now widely recognized It is timely to consider the progress made by research in this field 2 We review the development of plant facilitation research, focusing on the history of the field, the relationship between plant‐plant interactions and environmental severity gradients, and attempts to integrate facilitation into mainstream ecological theory We then consider future directions for facilitation research 3 With respect to our fundamental understanding of plant facilitation, clarification of the relationship between interactions and environmental gradients is central for further progress, and necessitates the design and implementation of experiments that move beyond the clear limitations of previous studies 4 There is substantial scope for exploring indirect facilitative effects in plant communities, including their impacts on diversity and evolution, and future studies should connect the degree of non-transitivity in plant competitive networks to community diversity and facilitative promotion of species coexistence, and explore how the role of indirect facilitation varies with environmental severity 5 Certain ecological modelling approaches (eg individual-based modelling), although thus far largely neglected, provide highly useful tools for exploring these fundamental processes 6 Evolutionary responses might result from facilitative interactions, and consideration of facilitation might lead to re-assessment of the evolution of plant growth forms

Effects of size, competition and altitude on tree growth

Abstract: Summary 1 Understanding the factors influencing tree growth is central to forest ecology because of the significance of growth to forest structure and biomass. One of the simplest, yet most controversial growth models, proposed by Enquist and colleagues, predicts that stem-diameter growth scales as the one-third power of stem diameter. Recent analyses of large-scale data sets have challenged the generality of this theory and highlighted the influence of resource competition on the scaling of growth with size. 2Here we explore the factors regulating the diameter growth of 3334 trees of mountain beech (Nothofagus solandri var. cliffortioides) growing in natural single-species forests in New Zealand. Maximum-likelihood modelling was used to quantify the influences of tree size, altitude, the basal area of taller neighbours (BL) and the basal area of all neighbours (BT) on growth. Our interpretation of the models assumed that taller neighbours compete for light whereas all neighbours compete for nutrients. 3The regression analyses indicate that competition for light has a strong influence on the growth of small trees, whereas competition for nutrients affects trees of all sizes. These findings are consistent with experimental manipulation studies showing that competition for light and nutrients inhibits the growth of small mountain beech trees, and fertilizer application studies showing that nitrogen limits the growth of large trees. 4Tree growth declined with altitude. The regression analyses suggest that the intensity of light competition also declines with altitude, when trees with similar BT and BL values were compared along the gradient. These results are consistent with observations that trees become stunted and have more open canopies at high altitudes. 5Our study is the first to build the effects of competition and environment into Enquist's model of tree growth. We show that competitive interactions alter the scaling of mean growth rate with size, whereas altitude does not influence the scaling of potential growth rate with size.

Role of niche restrictions and dispersal in the composition of arbuscular mycorrhizal fungal communities

Abstract: Summary 1 Metacommunity and neutral theory have reinvigorated the study of ‘niches’ and have emphasized the importance of understanding the influences of competition, abiotic factors and regional spatial processes in shaping communities. 2 We conducted a field survey to examine the effects of soil characteristics and distance on arbuscular mycorrhizal (AM) fungal communities of maize (Zea mays) in sand and clay soils. To address whether the field distributions of AM fungal species represented their fundamental or realized niches, we grew representative species of the two dominant genera, Glomus and Gigaspora, alone or together on Sorghum bicolor plants in sand, clay or a sand/clay mixture in the glasshouse. 3 In the field, soil characteristics and spatial structure accounted for significant proportions of the variation in community composition among sites, suggesting that both environmental variables and dispersal were important factors shaping AM fungal communities. 4 AM fungi in the family Glomeraceae occurred predominately in clay soils, whereas AM fungi in the family Gigasporaceae dominated in sand soils. Niche space of Glomeraceae was further partitioned by levels of soil organic carbon and nitrogen. 5 In the glasshouse, root colonization by Glomus was high in all three soils when grown in the absence of Gigaspora, indicating a broad fundamental niche. Root colonization by Gigaspora was negatively correlated with percentage clay when grown in the absence of Glomus, consistent with the low abundance of this family in clay soils in the field. When grown together, spore production of both Glomus and Gigaspora was significantly reduced only in the sand soil, indicating that competition could limit niches of both families in certain soil environments. 6 Our results suggest that AM fungal distributions are the product of environment, interspecific competition and regional spatial dynamics, emphasizing the importance of using a metacommunity perspective in community ecology.

Variability, contingency and rapid change in recent subarctic alpine tree line dynamics

Abstract: Summary 1 Boundaries between forest and tundra ecosystems, tree lines, are expected to advance in altitude and latitude in response to climate warming. However, varied responses to 20th century warming suggest that in addition to temperature, tree line dynamics are mediated by species-specific traits and environmental conditions at landscape and local scales. 2 We examined recent tree line dynamics at six topographically different, but climatically similar, sites in south-west Yukon, Canada. Dendroecological techniques were used to reconstruct changes in density of the dominant tree species, white spruce (Picea glauca), and to construct static age distributions of willow (Salix spp.), one of two dominant shrub genera. Data were analysed to identify periods and rates of establishment and mortality and to relate these to past climate. 3 Tree line elevation and stand density increased significantly during the early to mid 20th century. However, this change was not uniform across sites. Spruce advanced rapidly on south-facing slopes and tree line rose 65–85 m in elevation. Tree line did not advance on north-facing slopes, but stand density increased 40–65%. Differences observed between aspects were due primarily to the differential presence of permafrost. Additional variability among sites was related to slope and vegetation type. Results were less conclusive for willow, but evidence for an advance was found at two sites. 4 Increases in stand density were strongly correlated with summer temperatures. The period of rapid change coincided with a 30-year period of above average temperatures, beginning in 1920. The highest correlations were obtained using a forward average of 30–50 years, supporting the hypothesis that tree line dynamics are controlled more by conditions influencing recruitment than by establishment alone. 5 The changes observed at several sites are suggestive of a threshold response and challenge the notion that tree lines respond gradually to climate warming. Overall, the results provide further evidence to support the idea that the pattern and timing of change is contingent on local, landscape, and regional-scale factors, as well as species’ biology.

Homogenization of forest plant communities and weakening of species–environment relationships via agricultural land use

Abstract: Summary 1 Disturbance may cause community composition across sites to become more or less homogenous, depending on the importance of different processes involved in community assembly. In north-eastern North America and Europe local (alpha) diversity of forest plants is lower in forests growing on former agricultural fields (recent forests) than in older (ancient) forests, but little is known about the influence of land-use history on the degree of compositional differentiation among sites (beta diversity). 2 Here we analyse data from 1446 sites in ancient and recent forests across 11 different landscapes in north-eastern North America and Europe to demonstrate decreases in beta diversity and in the strength of species‐environment relationships in recent vs. ancient forests. 3 The magnitude of environmental variability among sites did not differ between the two forest types. This suggests the difference in beta diversity between ancient and recent forests was not due to different degrees of environmental heterogeneity, but rather to dispersal filters that constrain the pool of species initially colonizing recent forests. 4 The observed effects of community homogenization and weakened relationships between species distributions and environmental gradients appear to persist for decades or longer. The legacy of human land-use history in spatial patterns of biodiversity may endure, both within individual sites and across sites, for decades if not centuries.

Carbohydrate storage enhances seedling shade and stress tolerance in a neotropical forest

Abstract: Summary 1 To survive in forest understoreys, seedlings must depend on carbohydrate reserves when they experience negative carbon balance imposed by occasional light reduction and tissue loss to herbivores and diseases. We present the first experimental evidence in support of this hypothesis, using seven woody neotropical species. 2 We transplanted seedlings that had recently expanded their first photosynthetic cotyledon or leaf to the forest understorey (1% of full sun) and quantified initial biomass and total non-structural carbohydrate (TNC) in stems, roots and storage cotyledons. We then randomly assigned seedlings to control and two stress treatments: light reduction (0.08% of full sun for 8 weeks) and complete defoliation. 3 First-year survival of control seedlings, a comparative measure of shade tolerance, differed widely among species. The two stress treatments reduced survival and relative growth rates (RGR) of all species. Shade-tolerant species were little impacted by the stress treatments, whereas the two least shade-tolerant species experienced 100% mortality. 4 In all treatments, 8-week and first-year survival was positively correlated with initial TNC pool size in stems and roots. By contrast, survival was generally not correlated with initial TNC concentration in any organ, TNC pools in cotyledons, seed mass or seedling biomass. 5 TNC in stems and roots, but not in cotyledons, decreased in response to light reduction and defoliation over 8 weeks. Leaf area recovery of defoliated seedlings was positively correlated with initial TNC pools in stems and roots. 6 First-year survival in each treatment was negatively correlated with 0–8 week RGR of control seedlings, suggesting higher stress tolerance of species with inherently slow growth rates in shade. RGR of control seedlings from 0 to 8 weeks was negatively correlated with initial TNC pools, but not concentrations, in stems and roots. After 8 weeks, RGR was positive for all species, without clear relationships with survival or TNC. 7 We conclude that carbohydrate storage in stems and roots enhances long-term survival in shade by enabling seedlings to cope with periods of biotic and abiotic stress. Carbohydrate storage is a key functional trait that can explain species differences in growth and survival that lead to species coexistence through niche assembly processes and life-history trade-offs.

Exotic invasive plant accumulates native soil pathogens which inhibit native plants

Abstract: Summary 1We investigated the role of a native generalist soil pathogen through which a non-native invasive plant species may suppress naturalized/native plant species. 2We found that rhizosphere soils of Chromolaena odorata, one of the world's most destructive tropical invasive weeds, accumulate high concentrations of the generalist soil borne fungi, Fusarium (tentatively identified as F. semitectum), thus creating a negative feedback for native plant species. 3Soils collected beneath Chromolaena in the Western Ghats of India inhibited naturalized/native species and contained over 25 times more spores of the pathogenic fungi Fusarium semitectum than soils collected at the same locations beneath neighbouring native species that were at least 20 m from any Chromolaena plant. Sterilization of these soils eliminated their inhibitory effect. Chromolaena root leachate experimentally added to uninvaded soils increased Fusarium spore density by over an order of magnitude, and increased the inhibitory effect of the soils. 4The positive effect of Chromolaena root leachates on Fusarium spores was attenuated by activated carbon, suggesting a biochemical basis for how the invader stimulated the pathogen. 5Synthesis. Invasive plants have been shown to escape inhibitory soil biota in their native range and to inhibit soil biota in their invaded range, but our results indicate that the impacts of Chromolaena are due to the exacerbation of biotic interactions among native plants and native soil biota, which is to our knowledge a new invasive pathway.

Evenness drives consistent diversity effects in intensive grassland systems across 28 European sites

Abstract: 1 Ecological and agronomic research suggests that increased crop diversity in species-poor intensive systems may improve their provision of ecosystem services. Such general predictions can have critical importance for worldwide food production and agricultural practice but are largely untested at higher levels of diversity. 2 We propose new methodology for the design and analysis of experiments to quantify diversity-function relationships. Our methodology can quantify the relative strength of inter-specific interactions that contribute to a functional response, and can disentangle the separate contributions of species richness and relative abundance. 3 Applying our methodology to data from a common experiment at 28 European sites, we show that the above-ground biomass of four-species mixtures (two legumes and two grasses) in intensive grassland systems was consistently greater than that expected from monoculture performance, even at high productivity levels. The magnitude of this effect generally resulted in transgressive overyielding. 4 A combined analysis of first-year results across sites showed that the additional performance of mixtures was driven by the number and strength of pairwise inter-specific interactions and the evenness of the community. In general, all pairwise interactions contributed equally to the additional performance of mixtures; the grass-grass and legume-legume interactions were as strong as those between grasses and legumes. 5 The combined analysis across geographical and temporal scales in our study provides a generality of interpretation of our results that would not have been possible from individual site analyses or experimentation at a single site. 6 Our four-species agricultural grassland communities have proved a simple yet relevant model system for experimentation and development of methodology in diversity-function research. Our study establishes that principles derived from biodiversity research in extensive, semi-natural grassland systems are applicable in intensively managed grasslands with agricultural plant species.

Mortality and tree‐size distributions in natural mixed‐age forests

Abstract: Summary 1Tree-size distributions are changing in many natural forests around the world, and it is important to understand the underlying processes that are causing these changes. Here we use a classic conceptual framework – the shifting mosaic of patches model – to explore the ways in which competitive thinning and disturbance influence tree-size distributions, and to consider the effects of temporal variability in disturbance frequency on the size structure of forests. 2We monitored 250 stands of Nothofagus solandri var. cliffortiodes (mountain beech), randomly distributed over 9000 hectares, for 19 years. Mountain beech is a light-demanding species that forms monospecific forests in New Zealand mountains. For the purposes of our model, we assumed that each stand functions as an even-aged population: it is initiated by a pulse of recruitment, undergoes competitive thinning as it matures, and is eventually destroyed by a disturbance event. The tree-size distribution of the whole forest is driven partly by the frequency and temporal patchiness of disturbance events and partly by competitive processes within the constituent stands. 3Temporal changes in stem density and mean tree size were observed to be remarkably similar in all young stands, indicating that a consistent packing rule operates during this phase of stand development. A popular idea in the self-thinning literature is that the maintenance of constant leaf area index (LAI) provides the mechanism for this packing rule, but our analyses suggest that LAI increased by about 30% during the thinning phase. We use leaf economic theory to develop a new packing rule based on light interception, and argue that LAI increases with stand age because of changes in canopy organisation. 4Smaller trees were significantly more likely to die than larger trees within the young stands. Tree-diameter distributions within young stands were left skewed but those of older populations were normally distributed. These observations are consistent with asymmetric competition winnowing out small, suppressed trees from young stands but having less effect in older stands. 5Large-scale disturbances created gaps of sufficient size to allow mass recruitment of seedlings in about 0.8% of stands each year. Older stands were most susceptible to such large-scale disturbance, but the trend was weak. 6The diameter-distribution of the whole Nothofagus forest was found to be approximately exponential in form. Simulation models only produced realistic diameter distributions when competitive packing rules and disturbance were included. Therefore, the shifting mosaic model provides a general framework for understand the ways in which these mortality processes determine forest size structure. 7The diameter distribution of the forest was not in equilibrium over the 19-year study. Using simulation models, we show that temporal variability in disturbance frequency can generate enormous deviations in tree-diameter distributions away from the long-term mean, leading us to conclude that modern-day disequilibrium in natural forests may be the legacy of past disturbance events.

Characterizing the phylogenetic structure of communities by an additive partitioning of phylogenetic diversity

Abstract: 1 Analysing the phylogenetic structure of natural communities may illuminate the processes governing the assembly and coexistence of species in ecological communities. 2 Unifying previous works, we present a statistical framework to quantify the phylogenetic structure of communities in terms of average divergence time between pairs of individuals or species, sampled from different sites. This framework allows an additive partitioning of the phylogenetic signal into alpha (within-site) and beta (among-site) components, and is closely linked to Simpson diversity. It unifies the treatment of intraspecific (genetic) and interspecific diversity, leading to the definition of differentiation coefficients among community samples (e.g. I-ST, P-ST) analogous to classical population genetics coefficients expressing differentiation among populations (e.g. F-ST, N-ST). 3 Two coefficients which express community differentiation among sites from species identity (I-ST) or species phylogeny (P-ST) require abundance data (number of individuals per species per site), and estimators that are unbiased with respect to sample size are given. Another coefficient (Pi(ST)) expresses the gain of the mean phylogenetic distance between species found in different sites compared with species found within sites, and requires only incidence data (presence/absence of each species in each site). 4 We present tests based on phylogenetic tree randomizations to detect community phylogenetic clustering (P-ST > I-ST or Pi(ST) > 0) or phylogenetic overdispersion (P-ST

Tree line population monitoring of Pinus sylvestris in the Swedish Scandes, 1973-2005: implications for tree line theory and climate change ecology

Abstract: Tree line population monitoring of Pinus sylvestris in the Swedish Scandes, 1973-2005: implications for tree line theory and climate change ecology

Presence and identity of arbuscular mycorrhizal fungi influence competitive interactions between plant species

Abstract: 1 Competition for nutrients is an important factor structuring plant communities. Plant symbionts such as arbuscular mycorrhizal fungi (AMF) can have considerable influence on nutrient uptake and are therefore likely to influence plant competition. In this study we investigated the influence of different AMF isolates on the competitive relationships between a legume (Lotus corniculatus L.) and a grass (Festuca ovina L.), and between L. corniculatus and a forb (Plantago lanceolata L.). 2 AMF altered the competitive interactions between the investigated plant species. The legume, which was the most AMF-dependent plant species, was favoured in the presence of AMF relative to the non-AMF treatment in seven out of eight cases. Competition between the grass and the legume was the most strongly affected by AMF. The grass and the legume both benefited from AMF when grown alone, but in competition AMF favoured the legume and grass biomass was reduced by 38%. 3 The presence and also the identity of AMF influenced the competitive relationships. The extent to which AMF influenced plant competition and changed the relative abundances of the competitors depended on the specific AMF isolate. 4 This study shows that both presence and identity of AMF can influence plant competition. These results indicate that AMF and the composition of AMF communities regulate plant interactions and determine the structure of plant communities.

Ecological networks, nestedness and sampling effort

Abstract: Summary 1 Ecological networks have been shown to display a nested structure. To be nested, a network must consist of a core group of generalists all interacting with each other, and with extreme specialists interacting only with generalist species. 2 Studies on ecological networks are especially prone to sampling effects, as they involve entire species assemblages. However, we know of no study addressing to what extent nestedness depends on sampling effort, despite the numerous studies discussing the ecological and evolutionary implications of nested networks. 3 Here we manipulate sampling effort in time and space and show that nestedness is less sensitive to sampling effort than number of species and links within the network. 4 That a structural property of an ecological network appears less prone to sampling bias is encouraging for other studies of ecological networks. This is because it indicates that the sensitivity of ecological networks properties to effects of sampling effort might be smaller than previously expected.

A new framework for predicting invasive plant species

Abstract: 1. Many studies have searched for traits that characterize successful invaders. Unfortunately, very few generalizations have emerged from this work. It seems that the traits of successful invaders are idiosyncratic and context-dependent. Unless we are to study each potential invader in each possible target community individually, we will need a new approach. 2. We introduce a framework for predicting traits that are likely to confer success in a given ecosystem. Our approach considers the prevailing environmental conditions, the traits of resident species, and the traits of potentially invading species. 3. Our approach can be applied to ecosystems where the environmental conditions and/or disturbance regime have recently changed, to predict the range of trait space occupied by (i) native species at risk of local extinction, (ii) native species that can persist under the present conditions, and (iii) successful invaders. Our approach can also be used to identify unoccupied viable trait space (i.e. vacant niches) that might be at risk of invasion. 4. Synthesis. Understanding invasions resulting from rapid changes in environmental conditions and invasions resulting from the colonization of vacant niches would be a major step forward for invasion biology. The conceptual framework described here is not limited to plant invasions: the same approach can be used for any taxa (e.g. insects, fish, mammals and marine invertebrates) and could also be used to predict species responses to environmental change.

Developmental changes in habitat associations of tropical trees

Abstract: Summary 1 Recent studies have documented local-scale associations between tree species and topographic and edaphic habitat types in forests worldwide. To determine whether such associations form at early life stages, we compared species’ positive associations with five habitat types (high plateau, low plateau, slope, streamside, and swamp) at two life stages for 80 tree and shrub species in a Panamanian lowland forest. 2 Nineteen significant, positive habitat associations were detected at the small tree stage (seedlings and saplings ≥ 20 cm tall and < 1 cm d.b.h.), and 18 at the large tree stage (individuals ≥ 1 cm d.b.h.), according to results of torus-translation randomization tests. The majority of species did not show consistent associations at the two stages. Of the 30 species significantly associated with a habitat, only five were associated with the same habitat at both stages. Overall, more species were associated with the wetter slope habitat at the large tree stage compared with the small tree stage. 3 For a subset of species, we examined the relationship between observed habitat associations and seed dispersal and seedling establishment patterns by using species-specific seed dispersal kernels to predict seed rain into each habitat. 4 Two-thirds of species associated with a habitat at the large tree stage had higher predicted seed densities in the associated habitat relative to other habitat types, indicating that limited seed dispersal acts to reinforce habitat associations for most species. In contrast, only one-third of the species associated with a habitat at the large tree stage showed evidence of higher seedling establishment rates in the associated habitat compared with other habitats, and an equal number of species appeared to have lower rates of establishment in the habitat that large trees of the species were associated with. 5 Overall, our results indicate that habitat associations of large trees typically do not form at early life stages. Rather, many species appear to exhibit different ecological habitat preferences across life stages. Future studies of species’ habitat associations should therefore include multiple life stages in order to detect developmental shifts in ecological preferences.

What is the relationship between changes in canopy leaf area and changes in photosynthetic CO2 flux in arctic ecosystems

Abstract: 1 The arctic environment is highly heterogeneous in terms of plant distribution and productivity. If we are to make regional scale predictions of carbon exchange it is necessary to find robust relationships that can simplify this variability. One such potential relationship is that of leaf area to photosynthetic CO2 flux at the canopy scale. 2 In this paper we assess the effectiveness of canopy leaf area in explaining variation in gross primary productivity (GPP): (i) across different vegetation types; (ii) at various stages of leaf development; and (iii) under enhanced nutrient availability. To do this we measure net CO2 flux light response curves with a 1 × 1 m chamber, and calculate GPP at a photosynthetic photon flux density (PPFD) of 600 µmol m2 s1. 3 At a subarctic site in Sweden, we report 10-fold variation in GPP among natural vegetation types with leaf area index (LAI) values of 0.05?2.31 m2 m2. At a site of similar latitude in Alaska we document substantially elevated rates of GPP in fertilized vegetation. 4 We can explain 80% of the observed variation in GPP in natural vegetation (including vegetation measured before deciduous leaf bud burst) by leaf area alone, when leaf area is predicted from measurements of normalized difference vegetation index (NDVI). 5 In fertilized vegetation the relative increase in leaf area between control and fertilized treatments exceeds the relative increase in GPP. This suggests that higher leaf area causes increased self-shading, or that lower leaf nitrogen per unit leaf area causes a reduction in the rate of photosynthesis. 6 The results of this study indicate that canopy leaf area is an excellent predictor of GPP in diverse low arctic tundra, across a wide range of plant functional types.

Fire drives phylogenetic clustering in Mediterranean Basin woody plant communities

Abstract: Summary 1 Many Mediterranean plant species persist after fire because their seeds are protected from the heat of the fire (e.g. hard-coated seeds, serotinous cones), thus permitting rapid post-fire recruitment. For simplicity, this trait will hereafter be called P and its two possible phenotypes P+ (seeder) and P– (non-seeder). 2 Because P+ appears in a narrow taxonomic spectrum and confers persistence under high fire frequencies, we test the extent to which communities with different fire histories have different phenotypic and phylogenetic structures. Specifically, we compare coastal vegetation growing in a warm and dry Mediterranean climate subject to high fire frequency (HiFi vegetation) with montane vegetation subject to a subhumid climate where fires are rare (LowFi) under the hypothesis that P+ species will be over-represented in HiFi communities, thus producing phenotypic and phylogenetic clustering. 3 Trait conservatism on P is evaluated by testing the presence of a phylogenetic signal, phenotypic clustering is tested by correlating co-occurrence and phenotypic distance matrices, and the phylogenetic structure is evaluated by testing whether the phylogenetic distances between species in each community are different from those expected by chance. 4 The results suggest that: (a) P is a strongly conserved trait; (b) co-occurring species have similar P phenotypes (phenotypic clustering); and (c) the phylogenetic structure in HiFi vegetation is significantly clustered while LowFi vegetation tends to be overdispersed. 5 Synthesis: Fire is a strong driving force in assembling HiFi communities while other forces, such as competitive interactions, are the main assembly mechanisms in LowFi communities. This result supports the role of recurrent disturbances as filters driving phylogenetic community structure.

Intra-seasonal precipitation patterns and above-ground productivity in three perennial grasslands

Abstract: Summary 1 Relationships between above-ground net primary productivity (ANPP) of grasslands and annual precipitation are often weak at the site level, with much of the inter-annual variation in ANPP left unexplained. A potential reason for this is that the distribution of precipitation within a growing season affects productivity in addition to the total amount. 2 We analysed long-term ANPP data for three southern African temperate grasslands (mean annual precipitation ranging from 538 mm to 798 mm) to determine the effects of precipitation event size, number and spacing relative to seasonal totals. 3 Ungrazed, non-manipulated treatments at each site showed contrasting results despite sharing a common, dominant species. At the driest site, a model combining average event size and number of events per growing season provided a substantially better fit to the ANPP data than precipitation amount (seasonal total). At the wettest site, the interval between events was the most important precipitation variable. Precipitation distribution was not important at the intermediate site where amount was the best predictor of ANPP. A limit to the size of precipitation events efficiently utilized for ANPP was evident for the driest site only. 4 At each site, experimental treatments that altered species composition and soil fertility had little effect on precipitation–ANPP relationships. The lack of consistency in the relative importance of the precipitation variables among sites suggests that local, edaphic factors modify precipitation–ANPP relationships. 5 This analysis demonstrates that the distribution and size of precipitation events can affect ANPP independent of precipitation amount. As altered precipitation regimes are forecast by global climate models, the sensitivity of ecosystems to precipitation distribution should be considered when predicting responses to climate change. 6 While mean values of precipitation, and other ecosystem drivers, are typically used to predict function at the level of whole ecosystems, our results show that more complex measures of environmental variability may be required to understand ecosystem function, and to increase the accuracy of predictions of ecosystem responses to global change.

Threshold changes in vegetation along a grazing gradient in Mongolian rangelands

Abstract: Summary 1 The concept of threshold has become important in ecology, but the nature of potential threshold responses of vegetation to grazing in rangeland ecosystems remains poorly understood. We aimed to identify ecological thresholds in vegetation changes along a grazing gradient and to examine whether threshold changes were expressed similarly at a variety of ecological sites. 2 To accomplish this, we surveyed the vegetation along grazing gradients at 10 ecological sites, each located at different landscape positions in Mongolia's central and southern rangelands. Evidence for a threshold in changes in floristic composition along the grazing gradient was examined by comparing linear models of the data with nonlinear models fitted using an exponential curve, an inverse curve, a piecewise regression and a sigmoid logistic curve. 3 Three nonlinear models (piecewise, exponential and sigmoid) provided a much better fit to the data than the linear models, highlighting the presence of a discontinuity in vegetation changes along the grazing gradient. The shapes of the best-fit models and their fit to the data were generally similar across sites, indicating that the changes in floristic composition were relatively constant below a threshold level of grazing, after which the curve changed sharply. 4 Except for two sites, the best-fit models had relatively narrow bootstrap confidence intervals (95% CI), especially around threshold points or zones where the rate of change accelerated, emphasizing that our results were robust and conclusive. 5 Synthesis. Our study provided strong evidence for the existence of ecological thresholds in vegetation change along a grazing gradient across all ecological sites. This suggests that vegetation responses to grazing in the study areas are essentially nonlinear. The recognition that real threshold changes exist in real grazing gradients will help land managers to prevent the occurrence of undesirable states and promote the occurrence of desirable states, and will therefore permit a major step forward in the sustainable management of rangeland ecosystems.

Atmosphere, ecology and evolution: what drove the Miocene expansion of C4 grasslands?

Abstract: Grasses using the C4 photosynthetic pathway dominate today's savanna ecosystems and account for ∼20% of terrestrial carbon fixation. However, this dominant status was reached only recently, during a period of C4 grassland expansion in the Late Miocene and Early Pliocene (4–8 Myr ago). Declining atmospheric CO2 has long been considered the key driver of this event, but new geological evidence casts doubt on the idea, forcing a reconsideration of the environmental cues for C4 plant success. Here, I evaluate the current hypotheses and debate in this field, beginning with a discussion of the role of CO2 in the evolutionary origins, rather than expansion, of C4 grasses. Atmospheric CO2 starvation is a plausible selection agent for the C4 pathway, but a time gap of around 10 Myr remains between major decreases in CO2 during the Oligocene, and the earliest current evidence of C4 plants. An emerging ecological perspective explains the Miocene expansion of C4 grasslands via changes in climatic seasonality and the occurrence of fire. However, the climatic drivers of this event are debated and may vary among geographical regions. Uncertainty in these areas could be reduced significantly by new directions in ecological research, especially the discovery that grass species richness along rainfall gradients shows contrasting patterns in different C4 clades. By re-evaluating a published data set, I show that increasing seasonality of rainfall is linked to changes in the relative abundance of the major C4 grass clades Paniceae and Andropogoneae. I propose that the explicit inclusion of these ecological patterns would significantly strengthen climate change hypotheses of Miocene C4 grassland expansion. Critically, they allow a new series of testable predictions to be made about the fossil record. Synthesis. This paper offers a novel framework for integrating modern ecological patterns into theories about the geological history of C4 plants. Keywords: atmospheric CO2, C4 plants, climate change, fire, grassland, grazing, Poaceae, rainfall, savanna, seasonality Carbon dioxide and the expansion of C4 grasslands Major contrasts in the climatic preferences of grass subfamilies have been noted for more than half a century (Hartley 1950), but their significance was only recognized following the discovery of C4 photosynthesis. Tropical and subtropical grasslands are dominated by the predominantly C4 Panicoideae and Chloridoideae (Hartley 1958a, b; Hartley & Slater 1960), which together account for more than half of the world's grass species (Fig. 1; Linder & Rudall 2005) and ∼20% of gross terrestrial carbon fixation (Lloyd & Farquhar 1994). Their carbon-concentrating mechanism suppresses the energetically wasteful process of photorespiration that plagues C3 grasses at high temperatures, and significantly raises the efficiency of photosynthesis in warm climate regions. In contrast, C3 grass subfamilies such as the Pooideae are largely confined to temperate climates, where photorespiration is naturally limited by lower temperatures (Hartley 1961, 1973). Fig. 1 Cladogram displaying the hypothesized relationships among subfamilies of the Poaceae based on multiple markers, with the number of species shown for each, and filled circles (•) indicating the unequivocal origins of C4 photosynthesis in independent ... The evolutionary history of these patterns was elucidated only in the past two decades, following the realization that C4 photosynthesis imparts a distinctive carbon isotope signature to plant materials and trophic pathways. By analysing the isotopic composition of fossilized soils (palaeosols) and the teeth of herbivores, geochemists uncovered a surprise; the domination of low-latitude ecosystems by C4 grasses is a recent phenomenon in geological terms, occurring only 4–8 Myr ago (Ma) at the Miocene–Pliocene boundary, when C4 grasslands expanded across at least four continents (Fig. 2; Quade et al. 1989; Cerling et al. 1997). The factors behind this evolutionary phenomenon have remained controversial since its discovery. Fig. 2 Examples of the shifts in stable carbon isotope ratio (δ13C) characterizing the Miocene rise of C4 plants in (a) Pakistan (Quade & Cerling 1995), (b) the Great Plains (Fox & Koch 2003) and (c) East Africa (Cerling et al. 1997). ... At first, debate focused on the relative merits of CO2 as a driver of C4 grassland expansion (e.g. Cerling et al. 1994; Morgan et al. 1994a, b). The case for CO2 was persuasive, and based on the premise that selective and competitive advantages of C4 photosynthesis result from the energetic benefits of eliminating photorespiration at high temperatures and low atmospheric CO2 (Ehleringer et al. 1991, 1997; Cerling et al. 1997). These advantages over the ancestral C3 condition are reversed when photorespiration is naturally suppressed by low temperatures or high CO2, because the C4 carbon-concentrating mechanism requires energy. This leads to critical thresholds of temperature (around 20–25 °C at today's atmospheric CO2 concentration) and CO2 (around 500 p.p.m. in tropical environments) where C3 and C4 photosynthesis have equal energy requirements and, by extension, equal competitive and selective advantages (Cerling et al. 1997). The proponents of this elegant hypothesis noted the close correspondence between the theoretical temperature threshold and mean growing season value in modern regions of equal C3 and C4 grass species richness, and postulated that declining CO2 crossed an equivalent threshold at the Miocene–Pliocene boundary (Ehleringer et al. 1991, 1997; Cerling et al. 1997). Their ideas were supported by a geochemical model of atmospheric CO2, which indicated the necessary decline during the Cenozoic (Berner 1998). By the late 1990s, the ideas underpinning the CO2 starvation hypothesis were widely accepted, but still awaited the crucial test provided by palaeo-CO2 reconstructions. This soon came with the publication of three data sets (Pagani et al. 1999; Pearson & Palmer 2000; Royer et al. 2001), each using an independent proxy for CO2, and each showing a long period of stasis in the level of atmospheric CO2 during the expansion of C4 grasslands (Fig. 3b; reviewed by Royer 2006). Unless these palaeo-CO2 proxy records are challenged on technical or theoretical grounds, the geological evidence therefore stands firmly against the CO2 starvation hypothesis for C4 grassland expansion, and new mechanisms must be sought. Instead, the latest evidence suggests that atmospheric CO2 dropped sharply through the C3–C4‘crossover threshold’ at 25–30 Ma during the Oligocene, when it initiated our modern ‘icehouse’ era of advancing and retreating polar ice sheets (Fig. 3; Pagani et al. 2005; Royer 2006). The emerging picture of palaeoenvironmental change therefore lends credence to an alternative hypothesis, proposing declining CO2 concentration as a key selection pressure for the evolutionary origins of C4 photosynthesis in the grasses, rather than C4 grassland expansion (Ehleringer et al. 1991; Pagani et al. 2005). Fig. 3 CO2 and climate change from the Oligocene to the present day. (a) Oxygen isotope values of deep-sea foraminifera (δ18O) displayed as a locally weighted running mean calculated by Zachos et al. (2001). This serves as a combined proxy for the global ... Here, I review two major issues currently facing geologists, physiological ecologists and ecosystem scientists with interests in this field. I first evaluate the proposed role of CO2 in the evolutionary origins of C4 photosynthesis, focusing on the question of when the pathway first arose in the grasses. However, my principal focus is on the current debate surrounding the causes of C4 grassland expansion, particularly the hypothesized effects of climate change and fire. I argue that a deeper understanding of these proposed abiotic drivers could be achieved by explicitly considering their contrasting interactions within independent groups of C4 grasses.

Xylem density, biomechanics and anatomical traits correlate with water stress in 17 evergreen shrub species of the Mediterranean-type climate region of South Africa

Abstract: 1 Climate change in South Africa may threaten the sclerophyllous evergreen shrubs of this region. Available data suggest that they are not as tolerant of water stress as chaparral shrubs occurring in climatically similar California, USA. 2 Seventeen species from nine angiosperm families, including both fynbos and succulent karoo species, were studied at a field site in Western Cape Province, South Africa. Minimum seasonal pressure potential (P min ), xylem specific conductivity (K s ), stem strength against breakage (modulus of rupture, MOR), xylem density, theoretical vessel implosion resistance ((t/b) 2 h ) and several fibre and vessel anatomical traits were measured. 3 Species displayed great variability in P min , similar to the range reported for chaparral and karoo shrub species, but in contrast to previous reports for fynbos shrubs. 4 More negative P min was associated with having greater xylem density, MOR and (t/b) 2 h . There was no relationship between P min and traits associated with increased water transport efficiency. 5 Xylem density integrates many xylem traits related to water stress tolerance, including P min , MOR and (t/b) 2 h [ , as well as percentage fibre wall, parenchyma, vessel area and fibre lumen diameter. 6 Xylem density may be an integral trait for predicting the impact of climate change on evergreen shrubs.

Wind-throw mortality in the southern boreal forest: effects of species, diameter and stand age

Abstract: Summary 1 Patterns of tree mortality as influenced by species, diameter and stand age were assessed across a gradient in wind disturbance intensity in a southern boreal forest in Minnesota, USA. Few previous studies have addressed how wind impacts boreal forests where fire was historically the dominant type of disturbance. 2 We surveyed 29 334 trees of nine species within a 236 000 ha blowdown in the Boundary Waters Canoe Area Wilderness (BWCAW), in forests that have never been logged and were not salvaged after the windstorm. Within the disturbed area, a range of disturbance severity from zero to complete canopy mortality was present, overlaying an existing mosaic of fire origin stands. For this study, we derived an index of wind disturbance intensity by standardizing the observed disturbance severity using common species with similar diameter at breast height (d.b.h.) distributions. We then used multiple logistic regression to assess patterns of tree mortality across gradients in tree size and wind intensity index, and for three stand ages. 3 Probability of mortality was higher with increasing ln d.b.h. for all nine species, with two species (Abies balsamea and Picea mariana) showing much more dramatic shifts in mortality with d.b.h. than the others. As hypothesized, the species most susceptible to windthrow at all d.b.h. classes were early successional and shade intolerant (Pinus banksiana, Pinus resinosa, Populus tremuloides) and those least susceptible were generally shade tolerant (e.g. Thuja occidentalis, Acer rubrum), although the intolerant species Betula papyrifera also had low mortality. 4 Mortality rates were higher in mature (c. 90 years old) stands than for old and very old (c. 126–200 years old) stands, probably because old stands had already gone through transition to a multi-aged stage of development. 5 Synthesis. Quantification of canopy mortality patterns generally supports disturbance-mediated accelerated succession following wind disturbance in the southern boreal forest. This wind-induced weeding of the forest favoured Thuja occidentalis, Betula papyrifera and Acer rubrum trees of all sizes, along with small Abies balsamea and Picea mariana trees. Overall, the net impact of wind disturbance must concurrently consider species mortality probability, abundance and diameter distributions.

Functional convergence in regulation of net CO2 flux in heterogeneous tundra landscapes in Alaska and Sweden

Abstract: 1. Arctic landscapes are characterized by extreme vegetation patchiness, often with sharply defined borders between very different vegetation types. This patchiness makes it difficult to predict landscape-level C balance and its change in response to environment. 2. Here we develop a model of net CO2 flux by arctic landscapes that is independent of vegetation composition, using instead a measure of leaf area derived from NDVI (normalized-difference vegetation index). 3. Using the light response of CO2 flux (net ecosystem exchange, NEE) measured in a wide range of vegetation in arctic Alaska and Sweden, we exercise the model using various data subsets for parameter estimation and tests of predictions. 4. Overall, the model consistently explains similar to 80% of the variance in NEE knowing only the estimated leaf area index (LAI), photosynthetically active photon flux density (PPFD) and air temperature. 5. Model parameters derived from measurements made in one site or vegetation type can be used to predict NEE in other sites or vegetation types with acceptable accuracy and precision. Further improvements in model prediction may come from incorporating an estimate of moss area in addition to LAI, and from using vegetation-specific estimates of LAI. 6. The success of this model at predicting NEE independent of any information on species composition indicates a high level of convergence in canopy structure and function in the arctic landscape.

Reduced plant–soil feedback of plant species expanding their range as compared to natives

Abstract: 1. As a result of global warming, species may spread into previously cool regions. Species that disperse faster than their natural enemies may become released from top-down control. We investigated whether plants originating from southern Europe and recently established in north-western Europe experience less soil pathogen effects than native species. 2. We selected three plant species originating from southern Europe that have immigrated into the Netherlands and three similar native Dutch species. All six plant species were grown in sterilized soils with a soil inoculum collected from the rhizospheres of field populations. As a control we grew a series of all six plant species with a sterilized rhizosphere inoculum. 3. We harvested the plants, added the conditioned soil to sterilized soil and grew a second generation of all six plant species in order to test for each plant pair feedback effects from the conditioned soil communities to conspecifics and heterospecifics. 4. The effect of the soil community is dependent on plant species, and is dependent on soil fertility in only one of the three pairs. 5. Soil conditioning caused less biomass reduction to exotic plant species than to native species, suggesting that exotic immigrants are less exposed to soil pathogens than similar native plant species. 6. Our results suggest that plant species that expand their range as a result of climate change may become released from soil pathogenic activity. Whether the exotics are released from soil pathogens, or whether they experience enhanced benefit from mutualistic symbionts remains to be studied. We conclude that range expansion may result in enemy release patterns that are similar to artificially introduced invasive exotic plant species. 7. The escape from enemies through range shifts changes key biotic interactions and complicates predictions of future distribution and dominance.

Niche pre-emption increases with species richness in experimental plant communities

Abstract: In plant communities, invasion resistance may increase with diversity because empty niche space decreases simultaneously. However, it is not clear if this only applies to exotic species or also to native species arriving at a site with few other native species during community assembly. We tested the latter by transplanting four native species into experimental grassland communities varying in species richness form 1–16 (−60) species. In addition, we tested the hypothesis that invasion is less successful if the invading species belongs to a functional group that is already present in the community. The test invaders included a grass species (Festuca pratensis, FP), a short (Plantago lanceolata, PL) and a tall herb species (Knautia arvensis, KA), and a legume species (Trifolium pratense, TP). The same four functional groups also occurred alone or in all possible combinations in the different experimental communities. The overall performance of the transplants was negatively related to the logarithm of the species richness of host communities. Plant biomass declined by 58%, 90%, 84% and 62% in FP, PL, KA and TP, respectively, from monocultures to 16-species mixtures, indicating lower invasiveness of the two herbs than of the grass and the legume. Resident grasses showed a strong negative effect on the performance of all test invaders, whereas resident small and tall herbs had neutral, and resident legumes had positive effects. The case of the legumes indicates that contributions to invasion resistance need not parallel invasiveness. Communities containing resident species of only one functional group were most inhibitive to transplants of the same functional group. These results indicate that invasion resistance of experimental plant communities is related to the degree of niche overlap between resident species and invaders. This niche overlap can be high due to generally low amounts of empty niche space in species-rich resident communities or due to the occurrence of the same functional group as the one of the invader in the resident community. Stronger within- than between-functional-group invasion resistance may be the key mechanism underlying diversity effects on invasion resistance in grassland and other ecosystems at large.

Effects of rooting volume and nutrient availability as an alternative explanation for root self/non‐self discrimination

Abstract: 1 An increasing number of studies have shown that plants produce more root mass when sharing rooting space with an intraspecific neighbour as compared with plants growing alone. This so-called self/non-self discrimination has been suggested as a mechanism by which plants may prevent wasteful competition with their own roots and enhance their competitive ability for nutrients with roots of neighbouring plants. The overproduction of root biomass is said to result in a ‘tragedy of the commons’, because it appears to occur at the expense of reproductive biomass. 2 Studies on self/non-self root discrimination have commonly used a split-root design to distinguish self from non-self competition, while keeping the total amount of nutrients available per plant the same. This design has recently been criticized because the rooting volume differs between treatments. 3 Here, we use three general hypotheses to explain the published results without invoking the mechanism of self/non-self discrimination. The hypotheses propose that differences in root mass are due to differences in rooting volume, and differences in nutrient availability determine whole plant growth. More root mass without more growth results in less reproductive biomass. 4 A reanalysis of the results of root self/non-self discrimination confirms these hypotheses. Root overproduction in the presence of another plant, as found in nearly all studies, is consistent with effects of a larger soil volume available to these plants as compared with plants growing alone. Under the same total nutrient availability, total plant weight was the same or higher when more roots were produced. Inevitably, a larger root production with the same total biomass implies that less reproductive biomass is produced. 5 Although our analysis can explain most of the results of the split-root experiments, we cannot rule out the possibility that self/non-self root discrimination did take place. We discuss a limited set of experiments for which volume effects cannot explain the results, suggesting, in fact, that direct self/non-self root interactions have operated. We suggest experimental designs that can demonstrate their ecological significance in the future. 6 We conclude that there is ample evidence that plants can sense the volume of available rooting space, and a limited number of studies on individual roots show that plant roots may sense the identity of neighbouring roots and respond accordingly. The significance of these responses for whole plant growth and reproduction in relation to well-known resource competition effects is yet largely unknown and in urgent need of further study.

Linking ecological and built components of urban mosaics: an open cycle of ecological design

Abstract: Summary 1. By the end of this decade, the majority of people will live in cities and suburban areas. Urban areas, including suburbs and exurbs, are expanding rapidly worldwide. 2. Plant ecology has largely ignored cities, or has primarily focused on the discrete urban green spaces within cities. 3. Plant ecology is increasingly engaging urban ecosystems as integrated natural-human systems, in which human agency is part of the complex of feedbacks. 4. Linking plant ecology with urban design (architecture, landscape architecture, civil engineering and urban planning) can help to integrate research and understanding of plants into the structure of cities, and to make use of urban design projects as ecological research tools. 5. Synthesis . A cycle of ecological design illustrates the linkage of plant ecological research with the ongoing transformation of urban systems by urban designers and civil society. Quality of life, human health, public appreciation of ecological processes in cities, and scientific understanding can all be enhanced by participating in a cycle of ecological urban design.

Specificity and resilience in the arbuscular mycorrhizal fungi of a natural woodland community

Abstract: 1. While the composition of communities of arbuscular mycorrhizal (AM) fungi can have a large effect on the performance of their plant hosts, but the role of individual fungal species in shaping this response is as yet unresolved. 2. We have used the fungicide benomyl to alter the community of AM fungi in undisturbed monoliths of soil in a natural community. Changes in the community were characterised by root colonisation (%RLC), cloning, sequencing and tRFLP of a partial SSUrDNA fragment. Eleven plant species were sufficiently abundant in the monoliths to be examined. 3. In the highly mycorrhiza-dependent perennial herb Ajuga reptans, phosphate concentration was significantly reduced after benomyl treatment over a full growing season. The other plant species showed low colonisation and no significant difference in phosphate concentration after benomyl treatment. 4. Although colonisation in A. reptans was reduced, many mycorrhizal fungi survived in the roots. Some became more abundant following fungicide treatment, suggesting competitive release. Fungi that increased were generalists that have been identified in field samples from published studies colonising a wide range of plant species. Those that declined were specialists with a narrow host range; five types had not been recorded previously in field samples. 5. AM fungi in this study differed greatly in their response to perturbation, independent of the identity of the host plant. If such functional diversity is widespread, then elucidating the part played by AM fungal diversity in regulating plant community structure will be key to our understanding and management of ecosystems.

Reproductive phenology over a 10‐year period in a lowland evergreen rain forest of central Borneo

Abstract: Summary 1 The aim of this study was to document patterns in tree reproductive phenology in a rain forest of central Borneo and examine relationships between phenology and climatic patterns. 2 A 10-year data set (1990–2000) of monthly observations of flowering and fruit production of 171 trees (including 39 members of the Dipterocarpaceae) at Barito Ulu, Central Kalimantan, Indonesia, showed that most trees (73%) underwent reproductive activity on a supra-annual timescale. 3 There were three general flowering (GF) events, in 1991, 1994 and 1997, which were preceded by major drought periods (30-day sliding total rainfall of less than 100 mm for more than 10 days) in which at least 40% of dipterocarps and at least 18% of all other trees underwent synchronized reproductive activity; there was also a minor event in 1990. Around 1.3% of trees flowered and 3.8% produced fruit in months outside of these four events. 4 At the community level, the strongest negative correlation was found between the percentage of flowering individuals and total rainfall in the preceding 150 days. 5 Within three genera of dipterocarps examined in more detail (Dipterocarpus, Shorea and Vatica) there were clear and consistent patterns of sequential flowering with certain species flowering early in the GF events and others towards the end of these events. 6 Our results confirm the importance of large-scale climatic fluctuations (El Nino-Southern Oscillation) on plant reproductive phenology in South-east Asian tropical forests and indicate that drought may be a more important cue than low night-time temperatures.

Co-occurrence based assessment of habitat generalists and specialists: a new approach for the measurement of niche width

Abstract: Summary 1 Much ecological theory is based on the characterization of ecological habits of species as ‘generalist’ or ‘specialist’, but standard measures for placing species along a generalist-specialist gradient do not exist. 2We introduce a method for quantifying habitat specialization (i.e. relative niche widths) using species co-occurrence data. Generalists should co-occur with many species, whereas specialists should co-occur with relatively few species, given equal plot occurrences. We quantify this concept using a generalist-specialist metric (θ) derived from a beta diversity statistic. 3We evaluate the ability of our generalist-specialist metric to correctly rank species according to simulated (known) niche widths. Our technique is generally robust to a wide variety of niche distribution structures and sampling designs, but surveys strongly biased toward certain habitats can undermine the ability of θ to accurately describe niche widths for underrepresented species. 4We apply our technique to three spatially nested surveys of the large woody flora (> 1 cm d.b.h.) of the south-eastern USA. For each dataset we rank the generalist-specialist tendencies of all species of non-trivial occurrences, including 113 species across the Southeast, 71 species of southern Appalachian forests, and 44 species of the 6800-ha Joyce Kilmer-Slickrock Wilderness Area (NC and TN, USA). 5Rankings of species’θ-values were generally consistent among datasets of different spatial extent. Generalist species (e.g. Ilex opaca, Ulmus rubra, Morus rubra, Prunus serotina, Acer rubrum) were often those with large geographical ranges, particularly for θ-values from the largest dataset, and overall were more likely to be bird-dispersed, deciduous, and shade tolerant. South-eastern specialist species (e.g. Taxodium spp., Abies fraseri, Quercus laevis, Pinus pungens, Pinus palustris) were those associated with stressful or unusual conditions, such as a long duration of flooding, high fire frequency, or extreme cold or dry climates. 6Our study demonstrates that increasingly available, large-survey datasets can contribute niche-related species information in the absence of detailed environmental or habitat measurements. Applications include new assessments of relationships between species traits, ecological and environmental tolerances, and species packing in different assemblages.