Showing papers in "Journal of Vegetation Science in 2015"

Experimental warming interacts with soil moisture to discriminate plant responses in an ombrotrophic peatland

Abstract: Question A better understanding of the response of Sphagnum mosses and associated vascular plants to climate warming is relevant for predicting the carbon balance of peatlands in a warmer world. Open-top chambers (OTCs) have been used to investigate the effect on soil biogeochemical processes in peatlands, but little information is available on the effects of OTCs on microclimate conditions and the associated response of the plant community. We aimed to understand how simulated warming and differences in soil moisture affect plant species cover. Location A Sphagnum-dominated peatlands in French Jura. Methods We used OTCs to measure the effect of a near-ground temperature increase (+1.5 °C on average) on vegetation dynamics over five growing seasons (2008–2012) in a Sphagnum-dominated peatland, in two adjacent microhabitats with different hydrological conditions – wet and dry. Microclimatic conditions and plant species abundance were monitored at peak biomass in years 1, 2, 3 and 5 and monthly during the plant growing season in year 5. Results The response to warming differed between vascular plants and bryophytes, as well as among species within these groups, and also varied in relation to soil moisture. Andromeda polifolia abundance responded positively to warming, while Vaccinium oxycoccus responded negatively, and Eriophorum vaginatum showed a high resistance. Conclusion Depth of rooting of vascular plants appeared to control the response in plant abundance, while moss abundance depended on various other interacting factors, such as shading by the vascular plant community, precipitation and soil moisture.

Positive fire feedbacks contribute to shifts from Nothofagus pumilio forests to fire‐prone shrublands in Patagonia

Abstract: Question: Under climate change and increased ignitions by humans, burning of forests in which severe fires were naturally infrequent may result in environmental changes that increase the probability that they will burn again. On the eastern slopes of the northern Patagonian Andes, after fire-resistant Nothofagus pumilio forests burn they are typically replaced by fire-prone shrublands dominated by resprouting shrubs. We examine fuel properties and microclimatic conditions at the community level as potential fire feedback mechanisms mediating switches from fire-resistant N. pumilio forests to fire-prone shrublands. Location: Northwestern Chubut province, Patagonia, Argentina. Methods: We characterized the volume and vertical distribution of fine fuels, understorey woody and semi-woody plant composition, stand structure and microclimatic conditions in unburned and burned N. pumilio forest and shrublands 14–29 yr after severe fire. Results: Fuel amount and arrangement in unburned N. pumilio forests are unfavourable for fire activity compared with post-fire N. pumilio forests and shrublands. Unburned N. pumilio forests presented vertical discontinuities in fine fuel distribution and lesser amounts of fine fuels near the ground in comparison to fuels in shrublands. Floristic understorey composition of unburned and burned shrublands was very similar, while composition of unburned and burned N. pumilio forests showed clear differences. Additionally, microclimatic conditions following burning of N. pumilio forests and shrublands were significantly warmer and drier than in the unburned forest, and more frequently exceeded thresholds associated with fire activity in this region. Conclusions: Positive feedbacks from initial burning of otherwise fire-resistant N. pumilio forest will accelerate the rate of fire-induced conversion of forests to non-forest assemblages. Once transformed to the alternative state of shrublands, return to a forest cover is unlikely due to increased probability of burning in shrublands, as well as the unfavourable effects of warmer and drier conditions on tree establishment.

Pattern and process in the largest primeval beech forest of Europe (Ukrainian Carpathians)

Abstract: Questions Are the structural characteristics of natural beech forests predominant on small monitoring plots representative of the large-scale features of these forests? Do the findings of our large-scale investigation support the hypothesis that this primeval beech forest is shaped by fine-scale processes, or is there evidence that high-severity disturbance events have affected its structure? Location Ukrainian Carpathians, Uholka-Shyrokyi Luh, the largest primeval beech forest in Europe, covering 102.8 km2. Methods On 314 (500 m2) circular plots, systematically distributed across the forest, all living and dead trees with a DBH ≥6 cm were assessed. Lying deadwood, tree regeneration, the size of canopy gaps and the number of canopy layers were recorded. Spatial analyses were conducted using Moran's I. Dendrochronological analysis was used to reconstruct tree ages and growth patterns. Results The forest is characterized overall by a density of 435.0 ± 12.2 ha−1 (mean ± SE) living trees, a basal area of 36.6 ± 0.8 m2·ha−1, a volume of living trees of 582.1 ± 13.5 m3·ha−1 and a total deadwood volume of 162.5 ± 8.4 m3·ha−1. Beech is the dominant species (97.3 ± 0.7%, by basal area), interspersed with mostly deciduous species with moderate shade tolerance. The forest canopy is multi-layered, with a high abundance of old trees, and with canopy gaps rarely larger than the crown projection area of a few trees. Conclusions The results lend support to findings from small monitoring plots, but our landscape approach allows a reliable estimation of key forest characteristics such as basal area and standing volume, which tend to be overestimated in studies on subjectively placed small monitoring plots. We conclude that disturbance events of moderate or higher severity have left only few discernible traces in forest structure or species composition. All findings suggest that this forest is characterized by a mainly small-scale disturbance regime, leading to a homogeneity of forest characteristics of the living stand at larger spatial scales and causing only minor fluctuations around the average values of forest parameters observed.

Patterns of tree diversity and composition in Amazonian floodplain paleo-várzea forest

Abstract: Question In a recent re-classification of Amazonian floodplain types, a new category was established for paleo-varzea. Although the geological basis for this classification is sound, a key question is whether it can be supported by botanical criteria, where floodplain paleo-varzea forests contain distinct tree assemblages compared to other major floodplain types. Location Central Amazonian floodplains, Brazil. Methods We used 41 tree inventories from floodplain paleo-varzea, varzea and igapo forests to perform a meta-analysis. Diversity and richness estimators were applied at the level of genus and the examination of floristic composition was performed at family and generic levels. Generic composition across the three floodplain types was investigated using GNMDS and cluster analysis. Similarity coefficients and ANOSIM were applied to quantify floristic resemblance at the generic level between forest types. Indicator species analysis (ISA) was used to identify any indicator genera for the forest types evaluated. Results Alpha diversity and generic richness in paleo-varzea forest was similar to varzea. Igapo forest was clearly the least diverse. Family composition was similar across the three forest types; Fabaceae was the dominant family on all three floodplains. Pouteria, Licania and Eschweilera presented the highest importance values in paleo-varzea, but these genera were also important in varzea and igapo. Nevertheless, ANOSIM analyses show that generic composition differed significantly between the three forest types, and a GNMDS ordination and cluster analysis portrays paleo-varzea tree composition as intermediate between varzea and igapo. ISA revealed that Euterpe, Dipteryx and Goupia were the most important indicator genera of paleo-varzea. Conclusions We conclude that paleo-varzea floodplain tree assemblages are dissimilar to those of varzea and igapo, but contain components typical of both these floodplain types, explaining their intermediate positioning in our analyses. This floral mix probably allows paleo-varzea alpha diversity to be similar to that of varzea.

Native and exotic plant species respond differently to wildfire and prescribed fire as revealed by meta- analysis

Abstract: Questions: Wildfire is a natural disturbance that shapes vegetation characteristics worldwide, while prescribed fire is increasingly used to modify vegetation composition and structure. Due to invasion of many ecosystems by exotic species, a concern of land managers is whether wildfire and prescribed fire alter plant communities in favour of exotics. We assessed the global literature describing community-level responses of native and exotic species groups to wildfire and prescribed fire and characterized the geographic and temporal scope of the data to inform research needs. Location: Predominantly the United States of America and Australia. Methods: We used meta-analysis to (1) test whether native and exotic species composition (richness or diversity) and performance (cover, density and biomass) differ in response to wildfire and prescribed fire, and (2) assess how the composition and performance of these species groups vary with time elapsed since fire and habitat types. Results: Most community-level research on how native and exotic species respond to fire has been conducted in the US and Australia, typically over short time scales, and with a focus on temperate forest and grassland ecosystems. Prescribed fire benefited native composition over short time scales (<1 yr) but, on average, had no effect on native performance, nor on exotic composition or performance. In contrast, wildfire consistently enhanced exotic composition and performance over all time scales, while having no effect on native species composition and significantly reducing native performance. Additionally, responses varied by habitat: native species groups responded positively to prescribed fire in heathlands and to a lesser extent in temperate grasslands, while responding negatively to wildfire in arid shrublands and heathlands, and to a more modest degree in tropical savannas. Exotic species groups responded positively to wildfire in arid shrublands, temperate forests and heathlands. Conclusions: This quantitative assessment of the literature revealed strong evidence for a positive response of exotics to wildfire, coupled with a striking near-absence of negative responses. The assessment additionally suggests that while prescribed fire does benefit native composition, on average ,i t does not appear to greatly facilitate native performance; however site-specific variation in how communities responded to fire was pronounced, underscoring the importance of local assessments in determining mechanistic drivers and management policy.

Explaining within‐community variation in plant biomass allocation: a balance between organ biomass and morphology above vs below ground?

Abstract: Questions It remains unresolved why, despite the obvious functional importance of leaves and roots, co-existing plant species can display highly contrasting biomass distributions of these organs. Building on the ‘functional equilibrium’ hypothesis, we hypothesize that co-existing species can each achieve balanced resource acquisition above vs below ground by trading off the biomass vs morphology of structures responsible for resource acquisition, i.e. leaves and fine roots. Methods We tested this hypothesis in a natural field setting by measuring plant above- and below-ground biomass and morphological traits associated with resource uptake – specific leaf area (SLA) and specific root length (SRL) – of 18 dominant angiosperm species from a sub-alpine plant community. Location New Zealand South Island. Results We found a significant negative relationship between the species leaf mass to fine root mass ratio and the SLA to SRL ratio when we considered eudicot species only. The SLA to SRL ratio and plant size explained 31% and 34% of the total variation in the species leaf to fine root mass ratio respectively, and 45% when used in combination (P < 0.05 in all cases). Within a given plant size, 90% of the variation among species in total leaf area was due to differences in SLA, whereas variation in the fine root mass fraction was responsible for 71% of the variation among species in fine root length. Conclusions In support of our hypothesis, part of the difference between co-occurring species in leaf and fine root biomass distribution could be explained by the variable morphologies of these organs as well as variation in plant size, independent of the plant economic strategy. We expect that this outcome may result from environmental and evolutionary constraints on plant species-average traits, as well as plastic responses to local environmental conditions. These findings help explain why a diversity of strategies for achieving balanced resource acquisition can co-exist within a single plant community.

Communities: are they groups of hidden interactions?

Abstract: QuestionsEcologists are increasingly interested in community-level consequences of biotic interactions. However, community-level studies have not considered that biotic interactions might have contrasting directions within communities, and indirect interactions are rarely quantified although they may influence community-level outcomes. We tested the hypotheses that in species-rich plant communities from intermediate severe environmental conditions: (1) direct facilitation by dominant functional groups is balanced by negative indirect interactions among beneficiary species with no net effect at the community level on diversity and biomass, and (2) both direct and indirect interactions contribute to community composition. LocationA species-rich subalpine community of the eastern Tibet Plateau (China). MethodsWe removed dominant shrubs and graminoids and quantified, at the community and species levels, their direct and indirect effects on 43 forb species. We used multivariate analyses to assess the contribution of direct and indirect effects on community composition. ResultsThere were no community-level effects of either dominant life form on forb diversity and biomass. There were multiple species-level interactions that we grouped into six types based on the direction and intensity of indirect effects. We found significant relationships between species-level interactions and community composition. ConclusionsOur study highlights that communities are sets of hidden interactions that contribute to community composition, although no interaction might be detected at the community level because hidden interactions balance each other. Future studies should assess the ecological and functional drivers of these hidden interactions.

Geographic patterns of lake macrophyte communities and species richness at regional scale

Abstract: Questions Are community composition and species richness of aquatic macrophytes determined primarily by local (habitat heterogeneity and water quality) or regional (climate) patterns at regional scale? Do two macrophyte functional groups (i.e. emergent and submerged macrophytes) respond similarly to local and regional patterns? Are lake macrophytes and explanatory variables geographically structured? Location The US state of Minnesota. Methods The community composition and species richness of aquatic flora was studied using presence–absence data in 454 lakes, covering the entire US state of Minnesota. In addition, community composition and species richness of emergent and submerged macrophytes was investigated separately. Variation partitioning based on partial redundancy analysis and partial linear regression was used to study the relative roles of water quality, habitat heterogeneity, climate and sampling effort in explaining community composition and species richness of lake macrophytes, respectively. Results Macrophyte community composition and species richness (all taxa and two functional groups) were explained by water quality and climate. Alkalinity and total phosphorus were water quality variables that most affected community composition of aquatic flora, and macrophyte species richness decreased with increasing concentrations of these two variables. Maximum temperature of the warmest month and mean annual temperature most affected plant community composition, whereas species richness had a negative relationship with minimum temperature of the coldest month. Most significant explanatory variables (e.g. alkalinity, total phosphorus and temperature) were geographically structured, showing a latitudinal change. Conclusions Community composition and species richness of macrophytes were congruently influenced by regional (climate) and local patterns (water quality) at regional scale. Community composition and species richness of helophytes and submerged macrophytes were equally explained by environmental gradients. The latitudinal change in these most significant environmental variables was related to calcareous soils and intensive agriculture, which were situated in the southern part of the state. Macrophyte species richness showed a reverse latitudinal gradient, which was likely due to high nutrient concentrations in southern latitude lakes. Water quality primarily filters species from the regional species pool, allowing only species tolerating high nutrient concentrations, e.g. invasive plants, to survive at southern latitudes.

Between- and within-species trait variability and the assembly of sapling communities in forest patches

Abstract: Questions What is the magnitude of between-species trait variability (BSV) and within-species trait variability (WSV) of specific leaf area (SLA) in a sapling meta-community? To what extent do species turnover and WSV influence community-level mean trait responses to an environmental gradient and trait spread patterns across this gradient? What is the role of WSV for mean plant responses to environmental variation and niche partitioning in structuring sapling communities? Location Forest patches within a native grassland matrix in southern Brazil. Methods We recorded saplings in community plots across a canopy openness gradient in forest patches and described each of the 1129 individuals using SLA. First, we partitioned trait variation into BSV and WSV irrespective of co-occurrence in plots. Then, using the community data, we partitioned the total variation of community-weighted trait means (CWM) and Rao's functional diversity (FD) into components explained by canopy openness, species turnover and WSV. We also partitioned the effects of WSV between and within plots on FD. Finally, we explored the responses of CWM and FD to the gradient using the whole trait variability, only BSV or only WSV. Results Specific leaf area presented a substantial proportion of variation within species (37%), although it varied more between species (63%). Species turnover and WSV explained 48% and 19% of the variation in CWM across the gradient, respectively. Species turnover and WSV explained 51% and 45% of the variation in FD across the gradient, respectively. SLA varied within species more along the gradient than within communities. Within-species variability enhanced shifts in CWM and FD across the gradient. Canopy openness significantly predicted CWM at all levels, and FD at all but the within-species level. Conclusions Plastic responses of species mirrored the average response of communities to the environmental gradient. Within-species trait variability enhanced the mean plant responses to environmental variation as well as niche partitioning, and was especially important in enabling species to establish in a wider portion of the environmental gradient. Our study provides new evidence that population-level phenomena matter for community assembly.

Formalized classification of species-poor vegetation: a proposal of a consistent protocol for aquatic vegetation

Abstract: Aims: Most vegetation classification systems developed for large areas include various inconsistencies. Therefore, we (1) propose a new consistent Cocktail-based approach to redefine the traditional phytosociological classification of species-poor vegetation; (2) apply it to create a classification protocol for aquatic vegetation; (3) implement this protocol in a computer expert system; and (4) test it with a data set previously classified using an older version of the Cocktail method. Methods: The new approach uses formal logic to provide formal definitions of vegetation units. In the classification protocol for aquatic vegetation we defined consistent criteria for delimitation of associations according to the concepts that are predominantly used in phytosociology, based on species cover, dominance patterns and functional species groups. We applied these criteria in a computer expert system running in the JUICE 7.0 program, and applied them to a test data set of 12171 vegetation plots from the Czech Republic containing at least one aquatic species. The new classification was compared with (1) the previous national Cocktail classification based on species cover values and in few cases on sociological species groups, and (2) a non-formalized expert-based classification. Results: Thirteen functional species groups were created to build logical formulas of 64 aquatic associations and 5297 (44% of the total data set) vegetation plots were assigned to these associations, i.e. by 4% and 12% more than in the previous Cocktail and expert-based classifications, respectively. There was 94% and 83% classification agreement with the previous Cocktail and expert-based classification. Conclusions: The new approach produces a formal, consistent and unequivocal classification of species-poor vegetation with several advantages over similar approaches. It provides not only a set of formal definitions of vegetation units, but also a set of rules for building such definitions. All associations with common characteristics are defined by structurally identical formulas, ensuring consistency of the classification. While similar approaches for species-rich vegetation use sociological species groups, which are not applicable to species-poor vegetation, the new approach introduces the use of functional species groups, which reflect vegetation physiognomy and spatial structure and, in combination with species dominance, enable the classification of species-poor vegetation in a similar manner as in traditional phytosociology.

Epiphyte community development throughout tree ontogeny: an island ontogeny framework

Abstract: Aim Epiphytes are a conspicuous feature of numerous forests, yet they are poorly understood compared with terrestrial plants. Theoretical frameworks have helped bridge this gap; however, important questions relating to epiphyte community development have yet to be answered. For example, at what point in tree ontogeny do epiphytes first establish? Do epiphyte communities develop uniformly, or is there variation among host trees? Methods In this study, we build upon Darwin's geological theory of island ontogeny to explore changes in epiphyte species richness throughout the life span of their respective host trees. Based on the general features of island ontogeny, we predict that there are three stages of epiphyte community development: (i) an initial stage where host trees are devoid of epiphytes because they lack sufficient architectural and physiological characteristics suitable for epiphyte establishment, (ii) a second stage where trees acquire epiphytes as adults and continue to do so into maturity, and (iii) a final stage where epiphyte communities progress through a period of species decline following host tree mortality. To test our model predictions, we censused epiphyte communities on 371 host trees from six New Zealand tree species. We first assessed the relationship between epiphyte species richness and host tree diameter. We then tested whether relationships between epiphyte species richness and host tree diameter varied between host species. Results and conclusion Results were consistent with model predictions. Our investigation found variation in the ontogenetic stage at which host trees become favourable for epiphyte establishment. Moreover, the rate at which epiphyte species richness increased with host tree diameter varied among host species. Our findings indicate that an island ontogeny framework is useful for guiding investigations on epiphyte community development.

Inter‐annual fluctuations in rainfall shift the functional structure of Mediterranean grasslands across gradients of productivity and disturbance

Abstract: This is the peer reviewed version of the following article: Journal of Vegetation Science 26.3 (2015): 538-551, which has been published in final form at http://dx.doi.org/10.1111/jvs.12260. This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Self-Archiving.

Clonal and bud bank traits: patterns across temperate plant communities

Abstract: Questions We examine patterns of clonal traits and below-ground bud bank traits in plant communities over a large set of temperate vegetation types. We asked (i) how clonal traits are distributed in different community types and (ii) what are within-community patterns of these traits as an indication of their role in species coexistence and community assembly. Location Czech Republic. Methods We use a stratified set of more than 20,000 vegetation records from the Czech National Phytosociological Database as a source of species co-occurrence and habitat type data, and combined it with data on clonal and bud bank traits from the CLO-PLA database. We calculated both community trait means and dispersions for all records in the database and used a series of randomization tests to assess effects of environmental filtering and within-community assembly. Results Habitat filtering was stronger in bud bank traits than in clonal traits. The key factors driving structure of clonal traits in Central Europe seem to be differences in productivity and in wetness. These gradients are responsible for a major change in clonal growth parameters, from clonal plants with short and persistent connections in dry and unproductive habitats to plants with long- and short-lived connections in wet and productive sites. Size of the bud bank is rather uncorrelated with this gradient: species with small below-ground bud bank size prevail in highly disturbed and in water habitats. None of the traits showed significant overdispersion, either at the level of the whole data set, or at the level of individual habitat types. Conclusions Capacity for clonal growth occurs under many different environmental regimes and can give rise to very different growth forms. Therefore, using capacity for clonal growth as a proxy for clonal reproduction, capture of horizontal space or resprouting capacity may hinder understanding of its role. Absence of overdispersion after accounting for habitat filtering means that we found no evidence that clonal traits play a role in niche-based coexistence processes.

Observation bias and its causes in botanical surveys on high‐alpine summits

Abstract: Question To determine long-term vegetation changes in revisitation studies, it is crucial to know how much of measured species turnover over time can be attributed to pseudo-turnover (i.e. turnover caused by imperfect data acquisition), and which factors contribute to observation bias and pseudo-turnover. Independent simultaneous surveys provide a powerful tool to quantify pseudo-turnover and to indentify factors causing it, which may vary strongly between lowland and mountain areas. Location Alpine mountain summits (2616 m to 3418 m a.s.l.) in the southeastern Swiss Alps. Methods Plant inventories of 48 summits were collected by two independent observers simultaneously. Pseudo-turnover between observers was compared to species turnover over one century based on historical species lists of the same summits. Variables linked to observer characteristics and external (observer-independent) factors were tested for their influence on pseudo-turnover and number of species missed by one of the observers, and plant characteristics were tested for their effect on species detection probability. Results Mean pseudo-turnover between observers (13.6%) was almost three times smaller than species turnover over one century (41.4%). Pseudo-turnover and the number of species missed increased with difference in botanizing time between observers and with a longer ascent to the summit, especially in combination with a high species richness on the summit. Species had a higher probability to be missed if occurring on many summits but with a low abundance, if small in stature and if belonging to certain taxonomic plant groups (e.g. Asteraceae). Conclusions Our critical evaluation of turnover over time vs pseudo-turnover confirms that floristic changes on alpine summits over time represent an ecological pattern. In mountainous terrain, factors related to observer characteristics play a major role, as we found the best correspondence between simultaneous records when the difference in botanizing time was small and the ascent was short. Our results help to improve data quality in mountainous terrain by pointing out possible causes for observation bias. Long-term vegetation studies in alpine ecosystems should make a strong effort to identify and minimize such causes in advance, for instance by reducing between-observer differences in botanical skills, fitness and time management through appropriate training.

Alternative fire-driven vegetation states

Abstract: There is increasing evidence that alternative stable vegetation types exist for a given climate that are maintained by distinct fire regimes. Paritsis et al. (2014, this issue) provide an example in a temperate ecosystem. Here I briefly review cases of bi-stability in various climates, and present a simple model for the transition between states in their system.

From soaking wet to bone dry: predicting plant community composition along a steep hydrological gradient

Abstract: Question Why do plants grow where they grow? Prediction of species' occurrence and abundance in relation to the environment is a core aim of ecology, as is understanding the link between environmental stressors and adaptive traits. Community assembly may be viewed as a sequence of filters, sorting species according to their functional traits. We ask if the strength of filtering changes along a strong hydrological gradient? Can we predict species′ relative abundance using few, but biologically relevant traits? And are strongly filtered traits better predictors of community assembly? Location A hydrological gradient from pond to dry limestone pavements on the Island of Oland, South Sweden. Methods Plant community composition and six morpho-physiogical plant traits were measured along a pronounced gradient in water supply and soil depth. The strength of filtering was quantified using a trait dispersion index, while the prediction of species′ relative abundance and importance of individual traits was assessed with the community assembly by trait selection (CATS) model. Results We show that species are filtered by the hydrological environment through the traits root porosity, specific leaf area and resistance to water loss on drying. For individual traits, the strength of filtering waxes and wanes along the gradient. This strongly suggests that the mechanism, through which species are filtered into communities, acts through different traits as environmental conditions change along the gradient. The CATS model predicted 66% of the variation in species' relative abundances using six traits. In general, the traits subject to filtering also were most important in predicting species abundance. Conclusions Few plant traits are exposed to environmental filtering across the entire hydrological gradient, and most traits are strongly filtered only in parts of the gradient (e.g. root porosity in wet soils and water loss on drying on thin dry soils). Evidence for congruence between trait dispersion indices and the CATS model was established, underpinning the importance to plant community assembly of environmental filtering of species through their traits. New functional traits relevant to a specific environmental gradient – and not just some standard traits from a public database – can contribute significantly to resolve how plant communities are assembled.

Estimating themissing species bias in plant trait measurements

Abstract: Aim Do plant trait databases represent a biased sample of species, and if so, can that bias be corrected? Ecologists are increasingly collecting and analysing data on plant functional traits, and contributing them to large plant trait databases. Many applications of such databases involve merging trait measurements with other data such as species distributions in vegetation plots; a process that invariably produces matrices with incomplete trait and species data. Typically, missing data are simply ignored and it is assumed that the missing species are missing at random. Methods Here, we argue that this assumption is unlikely to be valid and propose an approach for estimating the strength of the bias regarding which species are represented in trait databases. The method leverages the fact that, within a given database, some species have many measurements of a trait and others have few (high vs low measurement intensity). In the absence of bias, there should be no relationship between measurement intensity and trait values. We demonstrate the method using five traits that are part of the TRY database, a global archive of plant traits. Our method also leads naturally to a correction for this bias, which we validate and apply to two examples. Results Specific leaf area and seed mass were strongly positively biased (frequently measured species had higher trait values than rarely measured species), leaf nitrogen per unit mass and maximum height were moderately negatively biased, and maximum photosynthetic capacity per unit leaf area was weakly negatively biased. The bias-correction method yielded greatly improved estimates in the validation tests for the two most biased traits. Further, in our two applications, ecological interpretations were shown to be sensitive to uncorrected bias in the data. Conclusions Species inclusion in trait databases appears to be strongly biased in some cases, and failure to correct this can lead to incorrect conclusions.

Long‐term changes in composition, diversity and distribution patterns in four herbaceous plant communities along an elevational gradient

Abstract: Questions Have species expanded their ranges upward in elevation? Have α- and β-diversity changed over time? Have species richness changes along the gradient shown distributional patterns? Have community compositions changed over time, and if so, which plant families are responsible for variation? Do herbaceous changes correspond to increased warming, overstorey shifts and disturbance events at each site? Location Long-term Ecological Research (LTER) sites near Niwot Ridge, CO, US. Methods We surveyed four herbaceous plant communities along an elevational gradient for species composition, diversity and frequency. Our results were compared to those found in 1981 and 1996 in the same plots to determine long-term community change at each site and along the elevational gradient. We evaluated changes in elevational species range expansion, community diversity, and composition. We related herbaceous change to tree data and site-specific literature on climate and disturbance to analyze change patterns and drivers. Results Our investigation showed the upward range expansion of eight species. Alpha-diversity changes were site-specific, while species richness changes along the gradient did not exhibit distinctive patterns. Each community changed significantly (P < 0.05) in composition and in β-diversity. Shade-tolerant families increased in dominance at the three lower sites, which reflected succession-driven canopy closure. Changes correlated to increased warming, overstorey shifts and disturbance. Conclusions Herb aceous communities have changed in dynamic ways, as evidenced by shifts in composition, diversity and distribution. We confirm the need for more long-term studies that can offer robust insights into the interplay between change drivers, such as climate, biotic interactions and disturbance, which work in concert to determine community assemblage.

Phylogenetic structure of plant species pools reflects habitat age on the geological time scale

Abstract: Patterns of phylogenetic relatedness of species within community types (phylogenetic structure; PS) are often used to infer processes of community assembly, yet the causes of these patterns remain poorly understood. Here we ask whether PS of extant plant species pools is related to availability of correspoding habitats in the geological history.

Quantifying multimodal trait distributions improves trait-based predictions of species abundances and functional diversity

Abstract: Question Niche differentiation results in functionally diverse communities that are often composed of dominant species with contrasting trait values. However, many predictive trait-based models that emphasize environmental filtering have implicitly assumed that traits exhibit unimodal distributions among individuals within communities centred on an optimal trait value. Does accounting for more complex, multimodal trait distributions among individuals in a community improve predictions of species abundances and functional diversity along environmental gradients? Location Franz Josef soil chronosequence, central Westland, New Zealand. Methods Leaf nitrogen (N) and phosphorus (P) concentrations from 23 woody plant species were modelled as functions of soil total N and P from eight sites of declining soil P. We compared predictions to observations of species abundances and functional diversity along the soil chronosequence using two modelling approaches: (i) the standard application of the hierarchical Bayesian Traitspace model that assumes unimodally distributed traits at each point along the gradient, and (ii) a modified application of the model that accounts for multimodal trait distributions within each community. Results Soil P was the strongest predictor of traits and species abundances. The strength of the environmental filter of leaf traits changed along this gradient, as evidenced by highly constrained variances and low modality of the trait distribution at low soil P, and high variance and multimodality at high soil P. Both modelling approaches predicted species abundances that were significantly correlated with observations, but the multimodal approach significantly improved predictions of species abundances and functional diversity. Conclusions Our results indicate that predictive models that emphasize environmental filtering over niche differentiation by assuming unimodal trait distributions can be more parsimonious than more complex approaches, especially when predicting species abundances along strong environmental gradients. However, models need to account for trait multimodality if they are to accurately replicate spatial patterns in functional diversity. This is important since functional diversity may be a key predictor of ecosystem function and resilience to global change.

Novel insights into post‐glacial vegetation change: functional and phylogenetic diversity in pollen records

Abstract: QuestionHow do pollen-based functional and phylogenetic diversity help to explain post-glacial vegetation change in relation to climate and human influence? LocationEstonia and Latvia, NE Europe. MethodsWe used a data set of 1062 pollen samples from 20 sites covering the last 14500yrs to estimate plant richness, evenness, functional and phylogenetic diversity (community-weighted mean and mean pair-wise distance). We adjusted existing functional and phylogenetic diversity measures for the pollen data and tested the methods with a simulation study. The simulations showed that species-based and pollen-based diversity estimates were all significantly positively correlated. ResultsThe Late Glacial (14500-11650cal. yr BP) and the mid-Holocene (8000-4000cal. yr BP) periods showed contrasting values for most of the diversity components, and several diversity estimates were strongly associated with climate. The cold climate during the Late Glacial led to high phylogenetic diversity, and relatively low functional diversity. Climate warming during the transition from the Late Glacial to the Holocene was followed by a decrease in phylogenetic diversity but an increase in functional diversity based on plant height and seed weight. Increasing human impact in the late Holocene was associated with an increase in plant richness and decreases in functional diversity based on plant height and seed weight and in phylogenetic diversity of herbs. ConclusionsPollen-based functional and phylogenetic diversity provide novel insights into post-glacial vegetation change and its drivers. Both functional and phylogenetic diversity were closely related to climatic conditions, suggesting that trait differences play an important role in long-term community response to climate change. Our results indicate that human impact during the last two millennia has influenced functional and phylogenetic diversity negatively by suppressing plants with certain traits (functional convergence) and giving advantage to plants from certain phylogenetic lineages. We see great potential in the further development of functional and phylogenetic diversity methods for pollen data.

Plant assemblages do not respond homogenously to local variation in environmental conditions: functional responses differ with species identity and abundance

Abstract: Questions We investigated some commonly held assumptions of community assembly theory needed to provide accurate predictions of changes in plant species assemblages across environmental gradients or following environmental change. Do (1) dominant and subordinate species respond in the same way to changes in environmental variables; (2) plant species assemblages show higher interspecific than intraspecific trait responses; and (3) co-existing dominant species differ in their responses to the same environmental variables? Location Islands in Lakes Uddjaure and Hornavan, northern Sweden. Methods We explored the responses of forest understorey vegetation assemblages to variation in environmental resources across a chronosequence of 30 lake islands that differ in fire history, above-ground and below-ground resource availability and species diversity. For one plot on each island, we measured specific leaf area, leaf dry matter content and foliar N and P of all dominant and subordinate understorey plant species to assess species-specific and weighted and non-weighted community-level trait responses to variation across islands in all major local environmental drivers. Results Consistent with our expectations, we found that species responses to environmental conditions were not homogenous within assemblages, and that responses of dominant and subordinate species differed. Further, intraspecific variation was often an important component of local-scale plant community-level responses. Responses were often relatively consistent across species, but dominant species sometimes showed contrasting responses of the same trait to the same environmental factor. Finally, environmental factors that influenced community average trait values also affected functional diversity. Conclusions This study has shown that several common assumptions that underpin community assembly theory do not necessarily hold, and this can cause inaccuracies in predicting plant functional composition responses to changes in environmental variables. Because these assumptions are central to current models that predict vegetation responses to environmental change, it is crucial to further test in which particular environmental context and to what extent these assumptions are critical for model accuracy.

No evidence of facilitation collapse in the Tibetan plateau

Abstract: Questions The relative importance of facilitation under different environmental conditions has raised some recent controversy centred on predictions of the stress gradient hypothesis (SGH) for its apparent lack of predictive power under very stressful conditions. This criticism, however, is not widely shared nor fully supported by empirical data. We addressed whether changes in interaction intensity in plots under severe environmental stress determine an increase in facilitation and whether facilitation would expand the realized niche of some plant species. Location Four sites differing in elevation and environmental conditions along a N–S transect in the Central Tibetan Plateau, Qinghai Province, China. Methods We assessed plant interaction intensity between the cushion plant, Androsace tapete, and species growing inside its canopy. We recorded species and number of individuals in cushions and in gaps, and mass of the two most frequent species in each site. Results Interactions ranged from competition to facilitation, depending on environmental severity. Net interaction balance followed predictions of the SGH, from competition on the relatively more mesic sites to facilitation in the relatively more stressful sites. There were no hints of facilitation collapse even though our harsher sites posed extreme conditions for plant survival. The main limiting factor seems to be temperature, as interaction intensity responded most to this factor, rather than to elevation or rainfall. Conclusions Facilitation expanded the realized niche of some species, countering adverse environmental conditions. We found no collapse of facilitation under the most stressful conditions, even though some sites were at the edge of the distribution range for the cushion species.

Non‐parametric methods reveal non‐linear functional trait variation of lichens along environmental and fire age gradients

Abstract: Questions Popular methods to analyse community–trait–environment relationships constrain community patterns by trait and environment relationships. What if some traits are strongly associated with community composition but unrelated to environmental variables and vice versa? We take a different approach, unconstrained by this assumption using non-parametric methods. We applied this technique to lichen (fungal/algal and/or cyanobacterial symbioses) communities across environmental and fire age gradients by measuring richness and cover of four important functional traits: energy generation (type of photosynthetic symbiont), water relations (inferred from growth form), dispersal capability (from vegetative propagules) and microsite specificity (measured by substrate affinity). Location Denali National Park and Preserve, Alaska, USA. Methods We ordinated plots in species space and regressed trait and environmental variables against ordination axes, resulting in one- or two-dimensional trait and environment surfaces. We then superimposed these surfaces on the ordination to create a new visual display, the ‘hilltop plot’, which enabled simultaneous measurement and display of one- and two-dimensional, non-linear community–trait–environment associations. Results Most traits examined show non-linear relationships with community structure. Fire favoured simple cladoniiform lichens, species with higher vegetative dispersal capacity and specificity to grow on wood, but excluded the ‘reindeer’ lichens, which had lower cover even more than 20 yrs after fire. Forests had more sorediate lichens than non-forested habitats, whereas high elevation, rocky areas had more green algal and fruticose lichens. Cyanobacterial lichen richness was positively related to shrub cover, while tripartite (cyanobacteria and green algae in a single lichen) and foliose lichen richness was highest in areas with higher moss cover. Conclusions Different combinations of lichen functional traits peaked along environmental and disturbance gradients, which we interpreted as balancing energy generation, water relations, vegetative dispersal and habitat specificity. Our method of trait–environment–community analysis revealed numerous one- and two-dimensional, non-linear relationships between community composition and functional traits, environmental variables and fire age gradients, which informed mechanisms behind community assembly. Our results indicate non-parametric and non-linear methods of trait–environment–community analysis have the potential to detect patterns that would have been missed using current popular techniques.

Cushion plants can have a positive effect on diversity at high elevations in the Himalayan Hengduan Mountains

Abstract: QuestionsWhat is the community-level consequence of biotic processes, in particular the importance of facilitation, in determining patterns of diversity in alpine plant communities of the Himalayan Hengduan Mountains? Does facilitation intensity change with environmental severity, and are these changes due to environmental severity or nurse trait effects?

Temperature and sheltering determine patterns of seedling establishment in an advancing subtropical treeline

Abstract: Questions An advance of high-altitude treelines has been reported in response to warming climate throughout the globe. Understanding local controls on the establishment of seedlings above the treeline is essential for predicting wider patterns of treeline response from a process-based perspective. Here we investigate patterns of seedling establishment in relation to microsite conditions in an advancing treeline ecotone in the subtropics, using climate data recorded at the plant-relevant scale. We sought to determine which temperature factors were of importance, if sheltering plays an important role in seedling establishment and if the response varied with seedling age. Location The Abies kawakamii treelines of the Central Mountain Range, Taiwan, 2800 to 3275 m a.s.l. Methods Seedlings were monitored in plots covering a range of treeline structural forms over a period of 2 yrs. Temperatures were recorded at plant-relevant height (5 cm) above ground and depth below ground with data loggers. Microtopographic sheltering (at a 10-m scale) and surrounding vegetation were measured. The influence of the above variables on seedling number and growth was investigated using generalized linear models and linear mixed effect models, respectively. Results Soil temperatures had more influence on seedling number than air temperature, whilst air temperature was positively associated with subsequent seedling growth. Establishment patterns were found to have a strong relationship with microtopographic sheltering, with more sheltered areas having elevated seedling numbers. Early growth may have significant implications for subsequent plant performance since smaller seedlings were more sensitive to both temperature and microtopography than larger seedlings. Conclusions Air and soil temperatures and microtopography determine spatial patterns of seedling establishment. Our results suggest that establishment above the treeline is likely to continue as the climate warms, although advance will not be spatially uniform due to the modifying influence of topography. This variability has important implications for the persistence and extinction of alpine plant communities occurring above treeline in topographically complex systems.

Intraspecific trait variability matters

Abstract: Intraspecific trait variability (ITV) plays a central part in various ecological processes, though using mean trait values may be sufficient in some instances. Ecologists need thus to find under which circumstances. Carlucci etal. (2014, this issue) bring new evidence on the importance of ITV for community assembly across a strong gradient. Sampling design may affect ITV quantification across gradients.

Impact of fine-scale edaphic heterogeneity on tree species assembly in a central African rainforest

Abstract: Soil properties have been shown to partially explain tree species distribution in tropical forests. Locally, species turnover across space can result not only from edaphic heterogeneities but also from limited seed dispersal. To characterize the contribution of each process, contact areas between contrasted soil types offer ideal settings. In the present study, we aimed to test species and species assemblage responses to a sharp edaphic discontinuity in a tropical forest tree community.We set up four 500–600-m long parallel transects crossing two contrasted edaphic habitats, one lying on clayey soil and the other on sandy soil. The canopy and subcanopy trees were identified and geo-referenced along the transects over a width of 50 m and 5 m, respectively, and soil samples were collected every 50 m to characterize each habitat.Correspondence analyses indicated a clear differentiation of tree communities between sandy and clayey soils. Using a torus-translation method combined with Chi-squared non-parametric tests, we observed that ca. 40% and 18% of the species represented by at least 12 individuals displayed significant density differences according to habitat in the canopy and subcanopy, respectively, although very few species displayed significant differences in their relative abundance. Nevertheless, whole community tests of differentiation (in species relative abundances) between soil types were significant in both strata, even after removing individual species or families displaying a significant habitat preference.While only a minority of species displayed a clear habitat preference, we still observed a community-wide impact of the edaphic discontinuity on species assemblages at a local scale. Our results provide further evidence for the major contribution of environmental heterogeneity in maintaining biodiversity in tropical forests.

Using imaging spectroscopy to predict above-ground plant biomass in alpine grasslands grazed by large ungulates

Abstract: Aims Imaging spectroscopy enables measurement of vegetation optical properties to predict vegetation characteristics that are important for a wide range of ecological applications Our aim was to predict fresh above-ground biomass of heterogeneous alpine grasslands in two areas and at two ecological scales We assessed model plausibility for an intensively studied alpine grassland site (plant community scale) having distinct biomass and ungulate grazing patterns Location Alpine grasslands in the Swiss National Park Methods Biomass data were collected in 51 plots and combined with imaging spectroscopy data to establish simple ratio models We analysed the predictive power and transferability of models developed in two areas (Val Trupchun, Il Fuorn) and at two ecological scales (regional, local) In a next step, we compared our results to the broadband normalized difference vegetation index (NDVI) Finally, we assessed the correlations between model predictions and plant biomass distribution at the plant community scale Results The best local simple ratio models yielded a model fit of R2 = 060 and R2 = 030, respectively, the best regional model a fit of R2 = 044 NDVI model performance was weaker for the regional and one local area, but slightly better for the other local area However, at the plant community scale only the local model showed a significant positive correlation (RS = 039) with the known biomass distribution Further, predictive power decreased when models were transferred from one local area to another or from one ecological scale to another Conclusions Our study demonstrated that imaging spectroscopy is generally useful to predict above-ground plant biomass in alpine grasslands with distinct grazing patterns Site-specific local models based on simple ratio indices performed better than the NDVI or regional models, suggesting that standardized approaches might not be adequate, particularly in heterogeneous grasslands inhabited by large ungulates We emphasize the importance of collecting ground reference data covering the expected range of productivity and plant species composition Moreover, plant community-scale data from a previous study proved to be extremely valuable to test model plausibility

Bole bryophyte diversity and distribution patterns along three altitudinal gradients in Yunnan, China

Abstract: QuestionWe investigated the bryophyte communities on tree boles along three altitudinal gradients located within different types of forested area to answer the following questions: (1) how many bole epiphytic bryophyte species are there; (2) what are the dominant species; (3) what are the richness patterns of epiphytic bryophyte communities along the altitudinal gradients; and (4) what are the major drivers and conservation implications of these patterns?