Showing papers in "Journal of Vegetation Science in 1997"

Plant functional types and ecosystem function in relation to global change

Abstract: Plant functional types (PFTs) bridge the gap between plant physiology and community and ecosystem processes, thus providing a powerful tool in climate change research We aimed at identifying PFTs within the flora of central-western Argentina, and to explore their possible consequences for ecosystem function We analyzed 24 vegetative and regenerative traits of the 100 most abundant species along a steep climatic gradient Based on plant traits and standard multivariate techniques, we identified eight PFTs Our results confirmed, over a wide range of climatic conditions, the occurrence of broad recurrent patterns of association among plant traits reported for other floras; namely trade-offs between high investment in photosynthesis and growth on the one hand, and preferential allocation to storage and defence on the other Regenerative traits were only partially coupled with vegetative traits Using easily-measured plant traits and individual species cover in 63 sites, we predicted main community-ecosystem processes along the regional gradient We hypothesized likely impacts of global climatic change on PFTs and ecosystems in situ, and analysed their probabilities of migrating in response to changing climatic conditions Finally, we discuss the advantages and limitations of this kind of approach in predicting changes in plant distribution and in ecosystem processes over the next century

Distribution patterns of tree species in a Malaysian tropical rain forest

Abstract: Spatial patterns of tree species were studied in a 50-ha tropical rain forest plot in the Pasoh forest, Malaysia. This forest is characterized by a high diversity and very high number of rare species. Out of the 745 species occurring with > five individuals, 80.4 % had an aggregated distribution, 19.5% were randomly distributed and one species had a regu- lar distribution. The spatial patterns of rare vs. common spe- cies, juvenile vs. adult trees, and coarse vs. fine scales were compared. Rare species are generally less aggregated than common ones and most of the randomly distributed species are rare. Spatial patterns shift from high clumping to looser intensity or random distribution when moving from juveniles to adults for the same species. No adult tree species display a regular pattern, however. Regular distributions were rarely found; this is probably due to intraspecific competition at a local scale. There is a negative correlation between per capita death rate and population density. This study suggests that the Pasoh forest and its high diversity are subjected to multiple controlling factors, e.g., topography, spacing effect, density-dependent processes and species rarity. The importance of any factor changes across spatial and temporal scales.

Instability of ordination results under changes in input data order: explanations and remedies

Abstract: Correspondence analysis (CA) and its Detrended form (DCA) produced by the program CANOCO are unstable under reordering of the species and sites in the input data matrix In CA, the main cause of the instability is the use of insufficiently stringent convergence criteria in the power algorithm used to estimate the eigenvalues The use of stricter criteria gives results that are acceptably stable The divisive classification program TWINSPAN uses CA based on a similar algorithm, but with extremely lax convergence criteria, and is thus susceptible to extreme instability We detected an order-dependent programming error in the non-linear rescaling procedure that forms part of DCA When this bug is corrected, much of the instability in DCA disappears The stability of DCA solutions is further enhanced by the use of strict convergence criteria In our trials, much of the instability occurred on axes 3 and 4, but one should not assume that published two-dimensional ordinations are sufficiently accurate Data sets which have pairs of almost equal eigenvalues among the first three axes could suffer from marked instability in the first two dimensions We recommend that a debugged, strict version of CANOCO be released Meanwhile, users can check the stability of their CA and DCA ordinations using the software that we have made available on the World Wide Web (http://wwwhelsinkifi/jhoksane/) An accurate program for CA, a debugged, strict version of DECORANA (for DCA) and a strict version of TWINSPAN are also available at our site

Prediction of yield in the Rothamsted Park Grass Experiment by Ellenberg indicator values

Abstract: . The Rothamsted Park Grass Experiment was established in 1856, with experimental plots subjected to annual applications of fertilizer and twice-yearly cutting of hay. There were two major responses to fertilizer, one reflecting high ammonium-nitrogen and increased acidity and the other reflecting high herbage yield without increased acidity. We calculated mean Ellenberg indicator values for N (nitrogen) and R (soil reaction) for the hay harvested between 1948 and 1975, using both unweighted and abundance-weighted means. Plot Ellenberg values were compared with herbage yield and with fertilizer application rates and published soil data. Annual yield of hay varied from 1.5 to 7.4 t/ha and was well predicted by the unweighted mean Ellenberg N-values (r = 0.91). Relatively large negative residuals from the relationship were found in plots whose soil combined low K and low pH. Soil pH was poorly predicted by the unweighted mean R-value, but showed a moderately good relation with weighted mean R (r = 0.73). The fact that Ellenberg N-values correlated better with yield than with applied nitrogen suggests that they might rather be called productivity values.

Does diversity beget diversity? A case study of crops and weeds

Abstract: The ecological literature is ambiguous as to whether the initial diversity of a plant community facilitates or deters the diversity of colonizing species. We experimentally planted annual crop species in monoculture and polyculture, and ex- amined the resulting weed communities. The species compo- sition of weeds was similar among treatments, but the species richness of weeds was significantly higher in the polycultures than in the monocultures. This supports the 'diversity begets diversity' hypothesis. Environmental microheterogeneity, di- versity promoters, and ecological equivalency do not seem able to explain the observed patterns.

Resprouting of Quercus suber in NE Spain after fire.

Abstract: . Many Mediterranean species have evolved strategies that allow them to survive periodic wildfires. Quercus suber trees resprout after fire, some from stem buds and others from basal buds only. In the former case the canopy recovers quickly. In the latter case the stem dies but the tree survives and regrows from basal sprouts. The probability of stem death and the degree of height recovery were studied after a fire in a Q. suber forest in NE Spain using logistic regression analysis. The results suggest that most trees survive after fire; the probability of stem death is negatively related to tree diameter; and recovery is positively related to tree diameter and to bark thickness. Implication for management and conservation of cork-oak forests are discussed.

Forest species in an agricultural landscape in The Netherlands: effects of habitat fragmentation

Abstract: For 312 forest patches on sandy soils in the Netherlands, effects of fragmentation are studied of forest habitat in the past on the present occurrence of forest plato species. Using regression techniques, the numbers of forest edge, interior, zoochorous and anemochorous species, as well as occurrence of 24 individual species were related to patch area and connectivity measures. Connectivity was defined as the amount of forest habitat around patches within three zones up to 1000 m. Plant categories were distinguished by habitat type anti dispersal mechanism. The results showed that number of total species and number of species of all habitat and dispersal categories increased with area. The occurrence of ten individually studied species were also positively related to area. Most of them were interior species. The number of zoochorous species increased with increasing connectivity. Also occurrence of ten individually studied species were affected by connectivity. Interior zoochorous species showed the highest percentage of affected species. The relationship of interior, animal-dispersed plants to connectivity can be explained by the limited distances covered by their dispersal agents (forest birds and ants) in a non-forest habitat. Also, some anemochorous plants appeared to be affected by connectivity, especially those with heavy seeds and potentially short distance dispersal. As not all species within a certain dispersal or habitat category react similar to area or isolation, it is suggested that differences in underlying processes of fragmentation such as local extinction and colonization need more focus.

Long-term vegetation dynamics mediated by herbivores, weather and fire in a Juniperus-Quercus savanna

Abstract: Long-term (45-yr) basal area dynamics of dominant graminoid species were analyzed across three grazing inten- sity treatments (heavily grazed, moderately grazed and ungrazed) at the Texas A&M University Agricultural Re- search Station on the Edwards Plateau, Texas. Grazing inten- sity was identified as the primary influence on long-term variations in species composition. Periodic weather events, including a severe drought (1951-1956), had little direct influ- ence on composition dynamics. However, the drought inter- acted with grazing intensity in the heavily grazed treatment to exacerbate directional changes caused by grazing intensity. Species response to grazing was individualistic and noisy. Three response groups were identified. Taller, more produc- tive mid-grasses were most abundant under moderate or no grazing. Short grasses were most abundant under heavy graz- ing. Intermediate species were most abundant under moderate grazing and opportunistic to weather patterns. Graminoid diver- sity increased with the removal or reduction of grazing inten- sity. The moderately and ungrazed treatments appeared most resistant to short-term weather fluctuations, while the heavily grazed treatment demonstrated significant resilience when graz- ing intensity was reduced after over 110 yr of overgrazing. Identification of a 'climax' state is difficult. Significant directional change, which took nearly 20 yr, appears to con- tinue in the ungrazed treatment after 45 yr of succession. The observed, relatively linear patterns of perennial grass compo- sition within the herbaceous patches of this savanna were generally explained by traditional Clementsian succession. However, when dynamics of the herbaceous community are combined with the woody component of this savanna, the frequency and intensity of fire becomes more important. Across the landscape, successional changes follow several pathways. When vegetation change is influenced by several factors, a multi-scale model is necessary to demonstrate interactions and feedbacks and accurately describe successional patterns. Ab- sence of fires, with or without grazing, leads ultimately to a Juniperus/Quercus woodland with grazing intensity primarily influencing the fuel load and hence fire intensity.

Plant species as indicators of ancient woodland in northwestern Germany

Abstract: . 315 isolated semi-natural and natural stands in NW Germany were investigated floristically, 285 stands of ancient woodlands and 30 recent ones. In the study area nearly all semi-natural and natural ancient woodlands are found on mesotrophic or eutrophic sites and can be assigned to the Fagetalia. 54 selected herbaceous and five woody Fagetalia-species have been tested in their association to ancient woodlands. 21 of the 59 selected plant species show a highly significant association to ancient woodlands and seven species show a weak significance. 31 plant species are not significantly correlated with ancient woodland sites, but 25 of them have a low frequency. The result shows that investigations of the historical ecology can help to understand floristic composition of present-day woodlands. Restriction of many woodland species to ancient woodlands, especially rare species, emphasizes the importance of woodlands with a long continuous history for the preservation of endangered species. Irrespective of ecological conditions, the restriction of plant species to ancient woodlands seems to be mainly caused by their low ability to colonize recent woodlands, especially isolated stands. Differences in the association to ancient woodlands between European countries are assumed to be a function of both time and degree of isolation of woodlands, of the dispersal mode of the plant species and of the availability of suitable habitats.

A long‐term study of Pinus banksiana population dynamics

Abstract: . The long-term population dynamics of a pure, naturally-established stand of Pinus banksiana (jack pine) in southeastern Manitoba, Canada is described. The study was initiated at stand age 15, when all 468 trees in a plot were mapped and their trunk diameter at breast height (DBH) measured. The plot was remeasured eight times — every five years (six years in one case) — and mortality and DBH changes recorded. Total mortality over the 41-yr study period was ca. 84 %. Mortality was initially very low, increased once the stand entered the self-thinning stage from ages 25–46, and declined at ages 46–56. Mortality was restricted to the smallest size classes throughout. The stand reached the self-thinning line at ca. age 30. The self-thinning slope was significantly less steep than the theoretically expected value of −0.5. The distribution of DBH values was initially symmetric, showed increasing positive skewness during the period of highest mortality, and became symmetric again at later stages. Size (DBH) inequality was highest just prior to the onset of density-dependent mortality, and subsequently declined. Tree DBH values were positively autocorrelated both initially and at later stages of stand development, but were spatially independent during the period of highest density-dependent mortality. The stand initially had a strongly clumped pattern at all spatial scales. Patterns of mortality were non-random during stand development, however, resulting in increased spatial regularity over time. Mortality was initially restricted to high density patches of the stand, but occurred throughout the plot once the self-thinning line was reached. Mortality during the self-thinning stage deviated from random expectation at local spatial scales (1–2m radius), suggesting that individuals were competing with their immediate neighbours. It is argued that an integrated approach, incorporating both population size and spatial structures, is essential in improving our understanding of long-term plant population dynamics.

Classification of vegetation: past, present and future

Abstract: This paper is a report on the past, status-quo and perspectives of vegetation classification, still a major occupa- tion of many vegetation scientists. The history of vegetation classification is discussed against a background of several controversial issues such as the problem of continuum vs. discontinuum, naturalness vs. arbitrariness of the nature of plant communities, universality vs. ad hoc character of syn- taxonomic schemes, as well as classical versus numerical approaches to data analysis for classification purposes. The development of the methodology of vegetation sci- ence and the present image of vegetation classification is documented by a bibliometric analysis of the publication record of four major journals: Journal of Vegetation Science, Vegetatio, Phytocoenologia and Tuexenia. This analysis revealed a per- sisting controversy between traditional and numerical ap- proaches to vegetation classification. A series of important changes in vegetation science (foundation of new journals, change of editorial policy by the established, important meet- ings) punctuate a period called the 'Innovation period'. Several trends in the development of methods of vegeta- tion systematics are summarized under the headings formal- ism, pluralism, functionalism, pragmatism and indetermin- ism. Some new features, such as the development and im- provement of numerical tools, use of large data banks and attempts to summarize the theory of vegetation classification are discussed. The new growth-form system of Barkman initiated a revival of physiognomy-based vegetation classifi- cation. Within this framework the use of the character-type concept and the development of new numerical methods for studying the hierarchical structure of character-set types seems to be a promising approach. The achievements of population biology and ecophysiology have affected vegetation science by emphasizing the functionality of species within plant communi- ties. The use of guilds and other functional groups has experi- enced an increasing interest from vegetation scientists. Applied in vegetation science, fuzzy-set theory has bridged the tech- niques of classification and ordination of plant communities.

Seed floating ability and distribution of alpine plants along a northern Swedish river

Abstract: . In this study we tested the hypothesis that diasporebuoyancy is an important factor in determining the distribu-tion range of riparian plants, by measuring the floating capac-ity of fruits and seeds from 17 alpine vascular plant speciesduring 1 yr. Minimum, median, mean, and maximum ( ≤ 1 yr)floating times of fruits and seeds were then related to thedownstream distribution of plants along the free-flowing VindelRiver in northern Sweden. The distances from the alpineheadwater region to the most downstream occurrence of plants,and to the cessation of a more continuous distribution, wereevaluated. The variation in downstream limits of plants couldnot be statistically explained by variation in seed buoyancybetween species, indicating that floating ability of seeds is notimportant for plant distribution patterns. There may be severalreasons for this, and they may differ between species.Keywords: Downstream dispersal; Hydrochory; Riparianplant; Seed buoyancy; Vindel River.Nomenclature: Lid (1985).

Simulated pattern formation around artificial waterholes in the semi‐arid Kalahari

Abstract: . Sinking boreholes to tap groundwater supplies facilitated expansion of all-year round livestock production into the semi-arid Kalahari. Increased grazing and trampling pressure around the boreholes often caused vegetation changes and range degradation. The long-term influences of cattle grazing on vegetation pattern around watering points in the southern part of the semi-arid Kalahari are investigated using a grid-based simulation model. Shrub-grass dynamics are modelled for two regimes with high and low rainfall and under various stocking rates. Results indicate the formation of distinct vegetation zones (‘piosphere’ zones) at the high rainfall site. Under all tested stocking rates distinct zones of bare soil, woody shrubs and a mixed grass-shrub savanna develop. The piosphere zones expand outwards at a rate correlated with the grazing pressure. At the lower-rainfall site zone development is limited and influenced by rainfall. Under abnormally high stocking rates an increase in shrub cover occurs within 50 yr under the low rainfall regime, leading to less distinct zones than under the high rainfall scenario. Modelling results suggest that the recovery potential of shrub-encroached piosphere zones after withdrawal of cattle is negligible in a time span of 100 yr.

Chemical and mechanical defense against herbivory in two sympatric species of desert Acacia

Abstract: . Most African Acacia trees occur in semi-arid savannas, but some species grow in deserts at the periphery of their range in northern Africa and the Middle East. We studied Acacia raddiana and A. tortilis in the southern Negev desert of Israel. According to the resource availability hypothesis, plants in poor habitats invest heavily into anti-herbivore defense because losses to browsers are costly when growth rates are low. Few plants invest simultaneously in different categories of defense. This suggests that trade-offs exist, although little is known about how individuals allocate resources to defence in natural populations. In a field experiment, we studied chemical and mechanical defences as a response to differential browsing for more than 10 yr. Both Acacia species increased spinescence but there was no clear response in the production of total polyphenols, condensed tannins, and protein-precipitating tannins (only A. raddiana may increase secondary compounds under very high browsing levels). There was an age effect in spinescence but not in secondary compounds for both species, with younger trees investing more into anti-herbivore defense than older individuals. We were unable to find negative correlations between traits of chemical and mechanical defense. We conclude that inducibility and age effects suggest a cost for mechanical but not necessarily for chemical defense. Contrary to the predictions of the resource availability hypothesis, concentrations of secondary compounds were not higher in this desert environment than in savannas.

The alien flora of Europe: a taxonomic and biogeographic review

Abstract: . A geographic and taxonomic overview of the non-indigenous plant species of Europe, based on the ‘Flora Europaea’ is given. The flora of Europe includes 1568 species which have either expanded their ranges within Europe under human influence (naturalized European species) or are of non-European origin (exotic species). The latter group consists of 580 species (37%) which form a diverse group in terms of their taxonomic composition and geographic origin. The exotics are represented by 113 families, of which the Compositae, Rosaceae and Gramineae are most important. The ratio of species to families is low. Most exotic species in Europe originate from the Americas and Asia. Countries of southern Europe have a higher relative number of exotics in their non-native flora than northern ones. The species-range size distribution differs between naturalized European and exotic species; the latter are on average more widespread than the naturalized.

Acidification and eutrophication of deciduous forests in northwestern Germany demonstrated by indicator species analysis

Abstract: A new method of indicator species analysis is suggested that can be used to demonstrate changes in the species composition of forests resulting from atmospheric deposition of acidifying substances and nitrogen. The method is not dependent on an exact re-localization of sites. 2162 published releves of Fagus sylvatica and Quercus robur- Carpinus betulus forests from two study areas in Germany (S Niedersachsen and Nordrhein-Westfalen) were compiled and divided into releves from before 1975 and more recent releves. Weighted averages of Ellenberg indicator values for light (L), moisture (M), reaction (R), and nitrogen (N) were calculated and for each of the four data sets - mesic and dry sites in two areas - N was regressed on R to give a predicted value (Npred) for any given R. The differences between observed values Nobs and Npred were calculated separately for old and new releves (Ndev-old and Ndev-new). The same analysis was applied to model data including 16 different scenarios of acidification and/or eutrophication. The model runs resulted in six different types of regression plots for Ndev and R. All models had in common that the regression lines for Ndev-new lay, completely or largely, above the zero line of no change and above the lines for Ndev-old. Also, in the four data sets, Ndev-new was generally larger than Ndev-old, and particularly so in Nordrhein-Westfalen. The values of Ldev-old and Ldev-new indicate that the differences in Ndev are unlikely to be caused by altered forest management, but must result from atmospheric pollution.

Nutrient limitation in species‐rich lowland fens

Abstract: . Nitrogen, phosphorus and potassium were supplied to some Belgian fens of varying nutrient status and productivity. Plant growth in the lowest productive fen with a species-rich Caricion davallianae vegetation was strongly P-limited. N was ineffective when applied alone, but increased the effect of P-addition when applied together. Summer biomass and plant nutrient concentrations were monitored for four years, and showed partial recovery of nutrient limitation. In a more productive fen dominated by Carex lasiocarpa and in a fen meadow, nutrient limitation was less strong. N limited growth in the productive fen, and N and K were co-limiting in the fen meadow. The P-concentration in the productive fen vegetation showed a marked increase after P-fertilization, but it did not result in higher standing crop. The significance of P-limitation for the conservation of species rich low productive fens is discussed. P-limitation may be an essential feature in the conservation of low productive rich fens: because it is less mobile in the landscape than N and/or because it is an intrinsic property of this vegetation type. Plant nutrient concentrations and N:P-ratios may be used as an indication for the presence and type of nutrient limitation in the vegetation. We found N:P-ratios of 23 to 31 for a P-limited site and 8 to 15 in N-limited sites. This was in agreement with critical values from the literature: N:P > ca. 20 for P-limitation and N:P < 14 for N-limitation. Thus, this technique appears valid in the vegetation types that were studied here.

Scales of above-ground and below-ground competition in a semi-arid woodland detected from spatial pattern

Abstract: . Semi-arid woodlands are two-phase mosaics of canopy and inter-canopy patches. We hypothesized that both aboveground competition (within canopy patches), and below-ground competition (between canopy patches), would be important structuring processes in these communities. We investigated the spatial pattern of trees in a Pinus edulis-Juniperus monosperma woodland in New Mexico using Ripley's K-function. We found strong aggregation of trees at scales of 2 to 4 m, which indicates the scale of canopy patches. Canopy patches were composed of individuals of both species. Crown centers of both species were always less aggregated than stem centers at scales less than canopy patch size, indicating morphological plasticity of competing crowns. In the smallest size classes of both species, aggregation was most intense, and occurred over a larger range of scales; aggregation decreased with increasing size as is consistent with density-dependent mortality from intraspecific competition. Within canopy patches, younger trees were associated with older trees of the other species. At scales larger than canopy patches, younger trees showed repulsion from older conspecifics, indicating below-ground competition. Hence, intraspecific competition was stronger than interspecific competition, probably because the species differ in rooting depth. Woodland dynamics depend on the scale and composition of canopy patches, aggregated seed deposition and facilitation, above- and below-ground competition, and temporal changes in the spatial scale of interactions. This woodland is intermediate in a grassland-forest continuum (a gradient of increasing woody canopy cover) and hence we expected, and were able to detect, the effects of both above- and below-ground competition.

Tree and grass rooting patterns in an African humid savanna

Abstract: . Spatial and temporal soil partitioning between roots of the two savanna plant components, i.e. trees and grasses, were investigated in a West African humid savanna. Vertical root phytomass distribution was described for grass roots, large (> 2 mm) and fine (< 2 mm) tree roots, in open sites and beneath tree canopies. These profiles were established monthly over one year of vegetation growth. Natural 13C abundance measurement was used to determine the woody/herbaceous phytomass ratio in root samples. Tree and grass root distributions widely overlapped and both were mostly located in the top 20 cm of the soil. Grass root phytomass decreased with depth whereas woody root phytomass peaked at about 10 cm depth. No time partitioning was detected. These structural results do not support the hypothesis of soil resource partitioning between trees and grasses and are thus consistent with functional results previously reported.

Regeneration and coexistence of two subalpine conifer species in relation to dwarf bamboo in the understorey

Abstract: . Microhabitats for seedling establishment and gap regeneration in subalpine forests of northern Japan were studied for two conifers, Abies sachalinensis and Picea glehnii. The abundance of understorey dwarf bamboo (Sasa spp.) was different for the four plots examined. Two types of micro-habitats were recognized for the two conifers: ground and elevated woody substrates (fallen logs and buttresses). Picea regenerated mostly on elevated sites, while Abies regenerated on both ground and elevated sites. The densities of Picea were independent of those of Sasa, but Abies densities decreased with increasing abundance of Sasa because Sasa reduced regeneration on the ground. Density of Abies on elevated sites was higher than that of Picea, irrespective of Sasa and of the density of adult trees. There was no significant difference in growth in sapling trunk height between the two conifers, but Picea grew more slowly under the canopy than Abies and was aggregated into gaps. Thus, in forests with less Sasa, the recruitment capacity of Abies was greater than that of Picea. The long life span of Picea compensated for its low density on elevated sites. Examination of a dynamic system model showed that Picea was excluded by Abies in forests without Sasa because regeneration on the ground is more advantageous than on elevated sites, but the two conifers could coexist in forests with Sasa because of the increased relative success of regeneration on elevated sites by Picea saplings.

Tornado damage and initial recovery in three adjacent, lowland temperate forests in Missouri

Abstract: We surveyed tornado damage 14 months after disturbance in three adjacent forest sites in southeastern Missouri, USA: upland, an occasionally-flooded lowland and a frequently flooded swamp We analysed three 009-ha plots in the swamp, and three 004-ha plots in the other sites and recorded the herbaceous-layer in five 1-m2 quadrats per plot Overall, 30 % of the individuals and 64 % of the basal area of these sites was blown down However, mortality was less than structural damage: 20 % of the trees were dead 14 months post-disturbance, an intermediate level of mortality relative to other wind disturbances Damage varied widely among species, sites and tree sizes, but large trees of all species were most likely to be damaged and least likely to resprout While damage was greatest in the wettest site, we attribute the increase to larger tree sizes and lower density there, rather than to inherent rooting instability This study suggests that different forest composition and structure may be sufficient to cause differential damage, without topographic effects Herbaceous-layer response was more rapid than in a previously reported tornado windthrow in Pennsylvania, and was dominated by fast-growing, shade-intolerant forbs

The effects of treefall gaps on understory vegetation in New York State

Abstract: Treefalls are a common form of disturbance in northeastern United States forests. The resultant gaps contrib- ute to a high degree of environmental heterogeneity in the understory of these forests. Plant density, plant cover, and species richness in understory plant communities were moni- tored for three years during the growing season, May - Sep- tember. Differences between treefall gap and closed canopy vegetation were less pronounced early in the growing season for plant density and leaf cover. Species richness was signifi- cantly greater within treefall gaps during the entire growing season. Eight species were found in greater abundance within treefall gaps (i.e., gap-phase species), while one species was found more commonly under closed canopy. Ordination re- sults suggest that time since gap creation and treefall gap size marginally affect the species composition of vegetation found within treefall gaps.

Effects of trampling and vegetation removal on species diversity and micro‐environment under different shade conditions

Abstract: . The effects of disturbance by trampling and vegetation removal on herbaceous communities and their micro-environments were examined at two sites with different levels of shade. The dominant species of the original vegetation were the perennial herb Artemisia princeps at the sunny site and the dwarf-bamboo Pleioblastus chino at the shady site. With no disturbance and marked dominance of these species, diversity was low. After vegetation removal there was a rapid recovery, leading to a more diverse vegetation with many more species, each with a lower dominance. Continuous trampling induced a short vegetation while the light intensity under the foliage was increased. At the sunny site, species richness was depressed by trampling because the soil water potential decreased markedly during summer and only the stress-tolerant annual Digitaria adscendens grew vigorously. Thus, the lowest species diversity was observed in the heavily trampled vegetation in late summer. At the shady site, soil water availability was not affected by trampling. This allowed the survival of many species and prevented a strong decline in diversity. The results suggest that the pattern of change in diversity in communities subjected to various disturbances, was always determined by the original environments.

Diversity of the understory vascular vegetation in 40 year‐old and old‐growth forest stands on Vancouver Island, British Columbia

Abstract: We studied plant diversity of the understory vascu- lar vegetation in 40 yr-old plantations (immature stands) and old-growth forest stands on southwestern Vancouver Island, British Columbia, Canada. Site-specific comparisons using several indices of species diversity were made between: (1) immature stands segregated according to the canopy cover and dominant canopy tree species; and (2) immature and old- growth stands. There were no significant differences ( P < 0.05) among immature stands in species richness (S) and the Shan- non-Wiener index (H'), in relation to the canopy cover or in S, H' and evenness (E) in relation to the dominant canopy tree species. Using the same indices, the plant diversity varied with edaphic conditions (represented by five site associations) and time (represented by two developmental stages). At both stand- and site levels, plant diversity increased with increasing soil moisture, from slightly dry to moist sites, and with in- creasing plant-available soil nitrogen in both immature and old-growth stands; and the plant diversity of immature stands across the sites studied was considerably lower than in old- growth stands, regardless of site association. The indices of plant diversity, floristic similarity indices, and species turno- ver rates indicated that the immature stands had their plant diversity at a minimum, but a drastic loss of diversity expected in the stem exclusion stage had not materialized. We attributed decline in plant diversity to the absence of old-growth struc- tural features in immature stands. Several measures to foster the stand-level diversity were proposed.

Effects of small‐scale animal disturbances on plant assemblages of set‐aside land in Central Germany

Abstract: . The distribution, density and cover of small-scale natural disturbances was surveyed in semi-natural and set-aside vegetation in a hilly landscape in Central Germany in the spring of 1995. More of the landscape was disturbed on set-aside agricultural land in valley s (1.02 %) and on warm south-facing slopes(1.33 %) than on rocky hilltops (0.3 %) and cool north-facing slopes (0.56 %). The major agents of disturbance on set-aside fields were moles, rodents (and their predators) and wild boars. In surrounding semi-natural grasslands, rabbit warrens were common on south-facing slopes and mound-building ants on north-facing slopes. Disturbances were significantly clustered and frequently superimposed. In order to investigate the effects of disturbance quality on plant assemblages in set-aside fields, two common types of disturbances were compared. These were grazing lawns (0.4–1.0 m2) maintained throughout the summer by common voles (Microtus arvalis), and excavations, 0.7–1.6 m2 in area, where wild boars (Sus scrofd) grubbing for food had removed vegetation and top soil in early spring. Both types of disturbances increased plant species richness on the local scale. Wind-dispersed annual and pauciennial forbs of ruderal habitats (e.g. Carduus acanthoides, Cirsium vulgare, Matricaria maritima, Senecio vernalis) were most abundant on the superficial disturbances made by voles, whereas small, ephemeral field weeds (e.g. Polygonum aviculare, Anagallis arvensis, Chenopodium album, Fallopia convolvulus) predominated on patches grubbed by boars. This study confirms that small-scale disturbances by animals provide a variety of regeneration niches for field weeds and ruderals. The rate of decline in plant species richness on set-aside land is likely to be reduced where land is utilized by a variety of herbivorous and soil-moving mammals.

Spatial pattern in Anthyllis cytisoides shrubland on abandoned land in southeastern Spain

Abstract: . Univariate and bivariate distribution patterns of small shrubs on abandoned land in semi-arid southeastern Spain were investigated by second-order spatial analysis (Ripley's K-function). All shrubs (Anthyllis cytisoides and subdominant Artemisia barrelieri) were either randomly distributed or clumped at scales of 0.25 - 1.0 m. The pattern shown by A. cytisoides shrubs alone changed from clumped to random with decreasing density. Pattern analysis and demographic data suggest a successive replacement of A. barrelieri, which had high proportions (44 - 86 %) of dead shrubs, by the dominant A. cytisoides. In two of three plots there was a positive association between A. cytisoides and A. barrelieri at a scale of 0.25 - 0.5 m. In the third plot, believed to represent a more advanced stage of colonization, there was a negative association (repulsion) between the two species, presumably as a result of interspecific competition from A. cytisoides. Dead shrubs present in spring 1994 were randomly distributed in all plots. Living and dead A. cytisoides shrubs were positively associated at a scale of 0.5 m, suggesting that the shrubs died as a result of intraspecific competition in small clumps. The shrubland previously dominated by A. barrelieri is changing to A. cytisoides shrubland with increasing biomass and ground cover.

Fire and late‐Holocene expansion of Quercus ilex and Pinus pinaster on Corsica

Abstract: . The natural origin of old Quercus ilex (holm oak) forests on the west coast of Corsica is a matter of dispute. This paper discusses the use of pedoanthracology, especially adapted to approach topics in palaeoecology requiring a high spatial precision. It also shows the importance of fire in vegetation change during the Holocene. Pedoanthracology relies on botanical identification and AMS 14C dating of micro-charcoal found in soil. Three test pits were dug in a forest currently protected within the ‘Man and Biosphere’ program. These pits yielded large quantities of charcoal. There is evidence that Pinus nigra ssp. laricio (laricio pine) and understorey heather species (Ericaceae) played an important role in the vegetation at the beginning of the Subatlantic (ca. 2500 BP). The importance of Pinus pinaster (maritime pine), Quercus ilex and a few mesophilic species increased between 2000 and 1000 BP. Charcoal fragments found in the soil show the role of fire in the vegetation changes recorded. The late expansion of P. pinaster results from a high fire frequency. The hypothesis proposed in the paper suggests that fires destroyed the remaining toxic humus that had accumulated during the millennia which preceded the arrival of Q. ilex to our study sites. The late development of holm oak forest is a combination of the migration of the species and the increase of fire frequency since 2500 BP, most probably resulting from slash-and-burn agriculture.

Seedling establishment patterns on the Pumice Plain, Mount St. Helens, Washington

Abstract: We examined the factors that control seedling es- tablishment on barren substrates on the pyroclastic flows from Mount St. Helens. From June to September in 1993, we monitored seedling and microhabitat changes in 240 20 cm x 20 cm quadrats on the Pumice Plain. Seedlings emerged in 104 quadrats (43.3 %). The most abundant species were Anaphalis margaritacea, Hypochaeris radicata, Lupinus lepidus and Epilobium angustifolium. Measured site charac- teristics included topography, particle size distribution, ground surface movements, soil water content, organic matter, pH, and presence or absence of dead lupines. Quadrats with seed- lings had higher cover of dead lupines, higher amount of rock and gravel substrate, and a greater cover of rills. More seed- lings emerged where eroded material accumulated. Compared to coarse-textured surfaces, silt surfaces had higher organic matter, held more water, and showed higher pH. However, seedlings became established more frequently on coarse-tex- tured surfaces. In greenhouse experiments, a higher percent- age of Hypochaeris seeds germinated on silt than on sand or gravel. The germination of Anaphalis and Epilobium did not differ with soil texture, but was higher at higher moisture levels. Seedling colonization is more dependent on ground surface microtopography, particle size, and ground movement than on the chemical status of these volcanic deposits.

Local colonization and extinction of field layer plants in a deciduous forest and their dependence upon life history features

Abstract: . In this study we used species inventory data collected in 1970 and 1993 from 132 plots in a 14.5ha deciduous forest to examine local extinction and colonization processes among 45 field layer species. Local colonization rate was positively related to both seed size and seed dispersal features. Local extinction rate was negatively related to seed size. Growth form (clonal vs. non-clonal) and presence of a seed bank were not found to be associated with local dynamics. Despite an overall constancy in species composition during this period, plants exhibited a considerable mobility among the 132 plots. This pattern conformed to a suggested ‘carousel model’ of species mobility in grasslands. A tentative suggestion is that this mobility acts on a comparatively broader spatio-temporal scale in deciduous forests as compared with grasslands. Additional data are presented indicating that species abundance (frequency) among field layer plants in deciduous forest communities is consistent among forest fragments, and when comparing local and regional scales. The main conclusion is that life history features of the seed dispersal/recruitment phase, particularly seed size, are causally associated with abundance patterns at least at a local scale, but possibly also on a broader regional scale.

Predicting competitive interactions between pioneer plant species by using plant traits

Abstract: . A competitive effect hierarchy for 15 Namaqualand pioneer plant species was established by using the mean mass of the phytometer (Dimorphotheca sinuata) when grown in combination with itself and 14 other species. There were no clear groupings of species in the hierarchy. This competitive hierarchy (gradient) indicated which species are strong competitors (resulting in a low phytometer mass) with D. sinuata and which species are weak competitors (resulting in a high phytometer mass). Each plant species has a certain combination of plant traits which determines its life history strategy and competitive ability. Regressions of various plant traits (measured on plants grown singly) against phytometer biomass indicated which traits were significantly correlated. The traits, most being size-related, were: maximum shoot mass, total mass, stem mass, reproductive mass, leaf area, stem allocation, specific leaf area (SLA), vegetative height × diameter, leaf area ratio (LAR); and mean number of days to flower initiation. A forward stepwise multiple regression of the significant traits was used to determine an equation to predict competitive effect.