Soil phosphorus heterogeneity promotes tree species diversity and phylogenetic clustering in a tropical seasonal rainforest

The niche theory predicts that environmental heterogeneity and species diversity are positively correlated in tropical forests, whereas the neutral theory suggests that stochastic processes are more important in determining species diversity. This study sought to investigate the effects of soil nutrient (nitrogen and phosphorus) heterogeneity on tree species diversity in the Xishuangbanna tropical seasonal rainforest in southwestern China. Thirty‐nine plots of 400 m² (20 × 20 m) were randomly located in the Xishuangbanna tropical seasonal rainforest. Within each plot, soil nutrient (nitrogen and phosphorus) availability and heterogeneity, tree species diversity, and community phylogenetic structure were measured. Soil phosphorus heterogeneity and tree species diversity in each plot were positively correlated, while phosphorus availability and tree species diversity were not. The trees in plots with low soil phosphorus heterogeneity were phylogenetically overdispersed, while the phylogenetic structure of trees within the plots became clustered as heterogeneity increased. Neither nitrogen availability nor its heterogeneity was correlated to tree species diversity or the phylogenetic structure of trees within the plots. The interspecific competition in the forest plots with low soil phosphorus heterogeneity could lead to an overdispersed community. However, as heterogeneity increase, more closely related species may be able to coexist together and lead to a clustered community. Our results indicate that soil phosphorus heterogeneity significantly affects tree diversity in the Xishuangbanna tropical seasonal rainforest, suggesting that deterministic processes are dominant in this tropical forest assembly.     

Density and species diversity of trees in four tropical forests of the Albertine rift, western Uganda

We assessed tree species density and diversity in 12 1‐ha plots in four forests of the Albertine rift, western Uganda. There were 5747 trees of diameter ≥ 10 cm in 53 families, 159 genera, and 212 species. Density ranged between 344 and 557 trees ha^?1 (average 479 trees ha^?1). Tree species diversity was highest in the Bwindi and Budongo forests. The Euphorbiaceae family was the most species rich (25 species) followed by Rubiaceae and Meliaceae with 16 species each. Canonical Correspondence Analysis (CCA) showed that major gradients in environmental variables influenced tree species distribution. Sample scores on ordination axes 1 and 2 were strongly correlated with pH and altitude, respectively. Correlated with rainfall and other soil factors, pH and altitude are presumed to be among the most important in influencing the distribution of tree species in the Albertine rift forests. Strategies that take account of variations in pH and elevation are required to conserve tree species in forests of the Albertine rift.     

Habitat Associations and Community Structure of Dipterocarps in Response to Environment and Soil Conditions in Brunei Darussalam,Northwest Borneo

Plant habitat associations are well documented in Bornean lowland tropical forests, but few studies contrast the prevalence of associations across sites. We examined habitat associations and community composition of Dipterocarpaceae trees in two contrasting Bornean lowland mixed dipterocarp forests separated by approximately 100 km: Andulau (uniform topography, lower altitudinal range, sandy soils) and Belalong (highly dissected topography, higher altitudinal range, clay‐rich soils). Dipterocarpaceae trees ≥ 1 cm diameter at breast height (dbh) were censused in 20‐m wide belt transects established along topographic gradients at each site. Dipterocarp density, evenness, species richness, and diversity were significantly higher at Andulau than Belalong. Significant site associations (with either Andulau or Belalong) were detected for 19 (52%) of the 37 dipterocarp species tested. Dipterocarpaceae community composition at Belalong correlated with soil nutrient concentrations as well as measures of vegetation and topographic structure, but community composition at Andulau correlated with fewer habitat variables. Within each site, dipterocarp density, species richness, and diversity were consistently higher on ridges than in slopes and valleys. Significant within‐site associations to topographic habitats were less common at Andulau (10% of species tested) than at Belalong (15%). We conclude that edaphic and other environmental factors influence dipterocarp community composition at a local scale, and are more important drivers of community structure in the more variable environment at Belalong. Species richness and diversity of dipterocarps on small plots, however, were higher at Andulau, suggesting that factors other than environmental heterogeneity contribute to contrasts in dipterocarp tree species richness at small scales.     

Tree community structure and stem mortality along a water availability gradient in a Mexican tropical dry forest

We document spatial changes in species diversity, composition, community structure, and mortality of trees across a gradient of water availability in a tropical dry forest in western Mexico. This gradient occurs along the main stream of a small watershed of less than 1 km in length. Four 30 × 80 m plots were established systematically to include the driest (ridge top of the watershed) to the wettest sites (watershed bottom) within this watershed. All stems larger than 5 cm were identified, and measured for diameter and height. Dead stems larger than 5 cm were measured and classified as: a) found on live or dead trees, and b) standing (“snags”) or lying (“downlogs”) on the ground. The number of recorded species per plot declined from 73 to 44 species as water availability decreased. A decline in estimated total richness, and in Shannon-Wiener and Simpson diversity indices was also observed in the drier plots. Species composition strongly changed along the gradient, with the two ends of the gradient sharing only 11% of the species. Stem density and percentage of dead stems and trees increased in abundance and basal area from the wetter to the drier sites. Tree and stem size (basal area, height and stem diameter) showed the opposite trend. Nonetheless, total basal area of live trees was largest at the two end gradient locations and oscillated between 12.22 m² ha⁻¹ and 7.93 m² ha⁻¹. Proportion of snags increased towards the driest site (from 46 to 72%), while that of down logs decreased. Overall, our results suggest that small-scale gradients of water availability play a paramount role in the spatial organization of tree communities in seasonal tropical environments. This revised version was published online in June 2006 with corrections to the Cover Date.     

Within-community vegetation structure in the conifer woodlands of the Siskiyou Mountains,Oregon

The conifer woodlands found on serpentine-derived soils in southwest Oregon, USA, are characterized by a debse but discontinuous shrub stratum and a species-rich herbaceous stratum. Quantitative analysis of shrub and herb distributions within 10 sites showed that small-scale patterns within the conifer woodland sites are as important as landscape-scale patterns in community organization. Gradient analysis was used to describe the distribution of herbaceous species with respect to gradients of shrub influence (shading, decreased soil temperature, increased soil moisture, increased litter depth) within sites and topographic moisture among sites. Regression analysis of the resulting species distributions and calculations of alpha and beta diversity showed that (a) the shrub-influence microgradient significantly affected distributions for 15 out of 20 major herbaceous species, and the topographic-moisture gradient influenced 13 out of the 20 species, (b) species richness was higher under intermediate conditions along both gradients, and (c) beta diversity within communities was 2.5 to 3.8 times the beta diversity of the site-to-site topographic-moisture gradient.     

Abundance of trees and grasses in a woodland savanna in relation to environmental factors

The relationships of plant species associations and underlying environmental factors in a woodland savanna in South Africa were investigated. 40 plots were included with 25 tree and 17 grass species dominating the arboreal and ground cover. Correspondence Analysis described the relationships between soil moisture retention, soil nutrients and the abundance of trees and grasses. Dry matter indices represented the accumulated effects of rainfall, fire and grazing of the herbaceous layer. Variations in the abundance of plants corresponded to well-defined gradients of soil nutrients. The distribution of grass and tree species along the ordination axes indicated that soils with high water retention capacity and high nutrient contents provided a suitable substrate for many of the tree species sampled. However, grass species abundance was high in plots with porous soils and poor nutrient availability.     

Floristic diversity of an eastern Mediterranean dwarf shrubland: the importance of soil pH

Questions: Does plant species richness and composition of eastern Mediterranean dwarf shrubland (phrygana) correlate with soil pH? How important is the effect of pH on species diversity in relation to other environmental factors in this ecosystem? What is the evolutionary background of the diversity–pH relationship? Location: Western Crete, Greece. Methods: Species composition of vascular plants, soil and other environmental variables were sampled in 100‐m² plots on acidic and basic bedrock in phrygana vegetation. The relationships between species composition and environmental variables (including climate) were tested using canonical correspondence analysis, and relationships between species richness and environment using correlation and regression analyses. Data were analysed separately for different plant functional types based on life form and life span. Results: Although soil pH varied across a narrow range (5.9‐8.1), species composition changed significantly along the pH gradient within all plant functional types. For most functional types, the effect of soil pH on species composition was stronger than that of other environmental variables. Species richness of annuals, geophytes and suffruticose chamaephytes increased with soil pH, while richness of hemicryptophytes and shrubs was not correlated with pH. Conclusions: The results are consistent with the evolutionary species pool hypothesis. High numbers of calcicole annuals, geophytes and suffruticose chamaephytes may be a result of the evolution of these groups on base‐rich dry soils in the Mediterranean climate. In contrast, hemicryptophytes, a life form typical of the temperate zone, evolved on both acidic and basic soils and therefore their species numbers do not respond to soil pH across the narrow range studied. The lack of a relationship between shrub species richness and pH is difficult to explain: it may reflect the more diverse or older origin of Mediterranean woody species and their conservative niches.     

Seedling regeneration,environment and management in a semi‐deciduous African tropical rain forest

Questions: How is seedling regeneration of woody species of semi‐deciduous rain forests affected by (a) historical management for combinations of logging, arboricide treatment or no treatment, (b) forest community type and (c) environmental gradients of topography, light and soil nutrients? Location: Budongo Forest Reserve, Uganda. Methods: Seedling regeneration patterns of trees and shrubs in relation to environmental factors and historical management types were studied using 32 0.5‐ha plots laid out in transects along a topographic gradient. We compared seedling species diversity, composition and distribution patterns along topographic gradients and within types of historical management regimes and forest communities to test whether environmental factors contributed to differences in species composition of seedlings. Results: A total of 85 624 woody seedlings representing 237 species and 46 families were recorded in this rain forest. Cynometra alexandri C.H. Wright and Lasiodiscus mildbraedii Engl. had high seedling densities and were widely distributed throughout the plots. The most species‐rich families were Euphorbiaceae, Fabaceae, Rubiaceae, Meliaceae, Moraceae and Rutaceae. Only total seedling density was significantly different between sites with different historical management, with densities highest in logged, intermediate in logged/arboricided and lowest in the nature reserve. Forest communities differed significantly in terms of seedling diversity and density. Seedling composition differed significantly between transects and forest communities, but not between topographic positions or historical management types. Both Chao‐Jaccard and Chao‐Sørensen abundance‐based similarity estimators were relatively high in the plot, forest community and in terms of historical management levels, corroborating the lack of significant differences in species richness within these groups. The measured environmental variables explained 59.4% of variance in seedling species distributions, with the three most important being soil organic matter, total soil titanium and leaf area index (LAI). Total seedling density was positively correlated with LAI. Differences in diversity of >2.0 cm dbh plants (juveniles and adults) also explained variations in seedling species diversity. Conclusions: The seedling bank is the major route for regeneration in this semi‐deciduous tropical rain forest, with the wide distribution of many species suggesting that these species regenerate continuously. Seedling diversity, density and distribution are largely a function of adult diversity, historical management type and environmental gradients in factors such as soil nutrient content and LAI. The species richness of seedlings was higher in soils both rich in titanium and with low exchangeable cations, as well as in logged areas that were more open and had a low LAI.     

Linking woody species diversity with plant available water at a landscape scale in a Brazilian savanna

Question: How is the diversity of woody species in a seasonally dry savanna related to plant available water (PAW)? Location: Savannas in central Brazil. Methods: Two‐dimensional soil resistivity profiles to 10‐m depth previously measured along three 10 m × 275 m replicate transects revealed differences in belowground water resources among and within transects: (1) driest/most heterogeneous; (2) wettest/least heterogeneous; and (3) PAW‐intermediate. All woody plants along these transects were identified to species, and height and basal circumference measured. Species diversity was evaluated for the whole transect (total diversity), 100‐m² plots (alpha‐diversity) and dissimilarity among 100‐m² plots within transects (beta‐diversity). Correlation analyses were conducted between PAW and vegetation variables at the 100‐m² scale. Results: The driest/most heterogeneous transect had the lowest total species diversity, while the wettest/least heterogeneous transect showed the lowest beta‐diversity. Floristic variation was correlated with PAW in all transects. In the most heterogeneous transect, species density was positively correlated with PAW in the 0‐400 cm soil layer. Evenness and Simpson's diversity were negatively correlated with PAW in the 700‐1000 cm soil layer. Conclusion: Woody species diversity was related to PAW at a fine spatial scale. Abundant PAW in the top 4 m of soil may favour many species and increase species total diversity. Conversely, abundant PAW at depth may result in lower evenness and total diversity, probably because the few species adapted to obtaining deep soil water can become dominant. Environmental changes altering soil water availability and partitioning in soil layers could affect the diversity of woody plants in this savanna.     

Vegetation patterns and environmental gradients in tropical dry forests of the northern Yucatan Peninsula

Patterns of plant species composition and their relationships to soil and topographic variables were investigated in tropical dry forests across the north central Yucatan, Mexico. Seven sites were studied in the oldest accessible forests along a 200–km transect oriented northwest to southeast; an eighth site was located in a little‐disturbed area located 75 km northeast of the transect. Two of the sites were on Mayan ruins. All sites were sampled using 9–24, 10m × 20m plots (<n= 132) for woody stems ≥ 3.0 cm diameter breast height. The important natural forest species were Bursera simaruba, Caesalpinia gaumeri, Gymnopodium floribundum, Piscidia piscipula, and Thouinia paucidentata. The two most important woody species in ruin woodlands were Brosimum alicastrum and Croton lundellii. Forest plots (n=108) had 17 species on average, ruin plots (n= 24) nine species. Mean basal area of stems at the forest plots (20.7 m².ha^‐1) was lower than in ruin plots (28.4 m².ha^‐1). Detrended Correspondence Analysis generally placed plots by site along the geographic transect. Natural forest plots and sites were separated from the plots on ruin sites. The five soil and topographic variables (slope, soil depth, percent surface rock, soil pH, total soil organic matter) differed significantly among sites. Plot values were correlated with DCA axe scores. Intersite floristic variation reflects an overall west to east environmental gradient affected by climate.     

Tropical tree species diversity in a mountain system in southern Mexico: local and regional patterns and determinant factors

Mechanisms explaining patterns of biodiversity along elevation gradients in tropical mountain systems remain controversial. We use a set of climatic, topographic, and soil variables encompassing regional, landscape, and local‐level spatial scales to explain the spatial variation of tree species diversity in the Sierra Madre of Chiapas, Mexico. We sampled 128 circular plots (0.1‐ha each) in four elevational bands along four elevation gradients or transects encompassing 100–2200 m. A total of 12,533 trees belonging to 444 species were recorded. Diversity patterns along the elevation gradient and the explanatory power of independent variables were dependent on spatial scale (regional vs transect) and functional group (total vs late‐successional or pioneer species). Diversity of all species and late‐successional species (1 – proportion of pioneer species) showed a constant pattern at the regional and transect scales, with low predictive power of climatic variables and/or elevation. A linear decrease in either number or proportion of pioneer species diversity was observed with increasing elevation, which was correlated with temperature, rainfall, and human disturbance trends. Total species diversity showed an increase with rainfall of the warmest quarter, indicating a regional‐level limiting effect of seasonality (drought duration). Yet the explanatory power of climatic and topographic variables was higher at the individual transect level than at the regional scale, suggesting the parallel but differential influence of evolutionary and geological history factors on diversification not so far studied to explain elevation patterns of species diversity in tropical mountain systems.     

Plant diversity in a calcareous wooded meadow – The significance of management continuity

Questions: What is the contribution of management continuity during the last 30–40 years to variation in species diversity and composition of a calcareous wooded meadow plant community? Is tree cover related to species diversity and composition of the herbaceous layer? What are the effects of local soil gradients on species diversity? Location: Laelatu calcareous wooded meadow, Western Estonian coastal zone. Methods: Plant community composition was assessed in 150 1 m × 1 m plots, located at 30 sites with known management history within Laelatu meadow (7 ha). Light and soil conditions and relative altitude were measured at each plot. DCA was used to analyse variation in species composition and general linear mixed models to analyse the effects of management and environmental parameters on diversity. Results: Management continuity was the primary determinant of plant community composition, followed by light conditions and soil parameters. Species richness, diversity and evenness are positively dependent on management continuity. Spatial autocorrelation is important as well. Diversity started to decline under the tree canopy where 50% or less irradiation reached the level of the herbaceous layer. We did not find significant effects of soil conditions on small‐scale diversity. Conclusions: Management continuity, together with the cover of the tree layer, are the most important determinants of diversity. Despite grassland stands with different management history are located side by side, the regeneration of diversity and composition of plant communities after restoring regular management practices is a slow process.     

Structure of herbaceous plant assemblages in a forested riparian landscape

We assessed patterns of herbaceous and woody species richness, plant-environment interactions, and correspondence between the herb and tree layer in a riparian landscape (the Ozark National Scenic Riverways, Missouri, USA). A total of 269 herb and 70 tree species were identified on 94 sample plots. Gradient analysis revealed that environmental variables and vegetation were influenced by a strong elevation gradient. However, high variability in environmental variables (pH, elevation, slope, sand, clay, organic matter) indicated a high level of substrate heterogeneity across the riparian landscape. We were unable to predict the composition of the herb understory from the canopy trees with any detailed accuracy and no clear characterization of herb species assemblages was found using cluster analysis or ecological land type (ELT) classifications. Canonical correspondence analysis (CCA) results for both tree and herb plots showed that elevation (height above river) and pH were the dominant environmental gradients influencing vegetation patterns on the first CCA axis while soil particle size exhibited the strongest correlation with the second CCA axis. Secondary gradients of importance included slope, soil container capacity, and organic matter. No significant linear or quadratic correlation was found between elevation and herb or woody species richness. Environmental variables alone or in combination, were weak predictors of herb and woody species richness, despite the patterns observed in the gradient analysis and the correlations observed in the CCA results. Ecotonal analysis showed that the herb layer exhibited a high species replacement rate at the lower elevations most susceptible to flooding (0–3 m). Above the flooding zone, there was more or less continuous species replacement, suggesting the presence of a gradual ecotone/ecocline. The tree layer exhibited much stronger discontinuities than the herb layer in the lower elevations along the height gradient (0–10 m). Recognizing the limitations of classification techniques for riparian herb assemblages and the importance of scale and heterogeneity in vegetation layers is especially important in light of mandates to preserve, protect, and manage for plant diversity.     

Soil heterogeneity in tree mixtures depends on spatial clustering of tree species

Heterogeneity in soil characteristics promotes and maintains coexistence between a diverse set of species. In forests, trees have species-specific impacts on soil abiotic characteristics and mixing of tree species is being promoted as a tool to ensure high levels of diversity and functioning. Yet, limited knowledge is available on the effect of tree species composition and spatial clustering on heterogeneity in soil characteristics. In this paper we derived heterogeneity of key characteristics of the leaf litterfall, the forest floor and the mineral topsoil (C, N and base cation concentration, C:N ratio and mass) in 53 plots of 7 different tree species compositions. We found that heterogeneity increased from the leaf litterfall, through the forest floor down to the mineral topsoil. Mixing tree species did not lead to an increased heterogeneity in the forest floor and topsoil compared to monocultures. However, we did find that mixed plots where conspecific trees stand in groups are more heterogeneous than plots where species are intimately mixed. Our results imply that heterogeneity in soil characteristics does not necessarily increase with tree diversity, but that within mixed stands the spatial organization of tree species should be considered in relation to the scale at which heterogeneity is desired.     

The impact of nitrogen deposition on carbon sequestration in European forests and forest soils

An estimate of net carbon (C) pool changes and long‐term C sequestration in trees and soils was made at more than 100 intensively monitored forest plots (level II plots) and scaled up to Europe based on data for more than 6000 forested plots in a systematic 16 km × 16 km grid (level I plots). C pool changes in trees at the level II plots were based on repeated forest growth surveys At the level I plots, an estimate of the mean annual C pool changes was derived from stand age and available site quality characteristics. C sequestration, being equal to the long‐term C pool changes accounting for CO₂ emissions because of harvest and forest fires, was assumed 33% of the overall C pool changes by growth. C sequestration in the soil were based on calculated nitrogen (N) retention (N deposition minus net N uptake minus N leaching) rates in soils, multiplied by the C/N ratio of the forest soils, using measured data only (level II plots) or a combination of measurements and model calculations (level I plots). Net C sequestration by forests in Europe (both trees and soil) was estimated at 0.117 Gton yr^?1, with the C sequestration in stem wood being approximately four times as high (0.094 Gton yr^?1) as the C sequestration in the soil (0.023 Gton yr^?1). The European average impact of an additional N input on the net C sequestration was estimated at approximately 25 kg C kg^?1 N for both tree wood and soil. The contribution of an average additional N deposition on European forests of 2.8 kg ha^?1 yr^?1 in the period 1960–2000 was estimated at 0.0118 Gton yr^?1, being equal to 10% of the net C sequestration in both trees and soil in that period (0.117 Gton yr^?1). The C sequestration in trees increased from Northern to Central Europe, whereas the C sequestration in soil was high in Central Europe and low in Northern and Southern Europe. The result of this study implies that the impact of forest management on tree growth is most important in explaining the C pool changes in European forests.     

Tree growth and soil acidification in response to 30 years of experimental nitrogen loading on boreal forest

Relations among nitrogen load, soil acidification and forest growth have been evaluated based on short‐term (<15 years) experiments, or on surveys across gradients of N deposition that may also include variations in edaphic conditions and other pollutants, which confound the interpretation of effects of N per se. We report effects on trees and soils in a uniquely long‐term (30 years) experiment with annual N loading on an un‐polluted boreal forest. Ammonium nitrate was added to replicated (N=3) 0.09 ha plots at two doses, N1 and N2, 34 and 68 kg N ha^?1 yr^?1, respectively. A third treatment, N3, 108 kg N ha^?1 yr^?1, was terminated after 20 years, allowing assessment of recovery during 10 years. Tree growth initially responded positively to all N treatments, but the longer term response was highly rate dependent with no gain in N3, a gain of 50 m³ ha^?1 stemwood in N2 and a gain of 100 m³ ha^?1 stemwood in excess of the control (N0) in N1. High N treatments caused losses of up to 70% of exchangeable base cations (Ca²⁺, Mg²⁺, K⁺) in the mineral soil, along with decreases in pH and increases in exchangeable Al³⁺. In contrast, the organic mor‐layer (forest floor) in the N‐treated plots had similar amounts per hectare of exchangeable base cations as in the N0 treatment. Magnesium was even higher in the mor of N‐treated plots, providing evidence of up‐lift by the trees from the mineral soil. Tree growth did not correlate with the soil Ca/Al ratio (a suggested predictor of effects of soil acidity on tree growth). A boron deficiency occurred on N‐treated plots, but was corrected at an early stage. Extractable NH₄⁺ and NO₃^?were high in mor and mineral soils of on‐going N treatments, while NH₄+ was elevated in the mor only in N3 plots. Ten years after termination of N addition in the N3 treatment, the pH had increased significantly in the mineral soil; there were also tendencies of higher soil base status and concentrations of base cations in the foliage. Our data suggest the recovery of soil chemical properties, notably pH, may be quicker after removal of the N‐load than predicted. Our long‐term experiment demonstrated the fundamental importance of the rate of N application relative to the total amount of N applied, in particular with regard to tree growth and C sequestration. Hence, experiments adding high doses of N over short periods do not mimic the long‐term effects of N deposition at lower rates.     

Drought‐related tree death of savanna eucalypts: Species susceptibility,soil conditions and root architecture

Questions: For eucalypt savanna in northeast Australia subject to multi‐year rainfall deficits this paper asks whether (1) dominant tree species (Ironbarks, Boxes) are more drought susceptible than the sub‐dominant Bloodwoods; (2) whether soil moisture is beyond wilting point in surface soil layers but available at depth; (3) soil conditions (moisture availability and texture) are related to tree death during drought; (4) the root systems of the Boxes and Ironbarks are shallower than the Bloodwoods; and the survivors of drought within species have deeper root systems than those that died. Location: Central Queensland, Australia. Methods: Patterns of tree death between eucalypt species were compared from field data collected after drought. Soil conditions during drought were described and compared with patterns of tree death for the Ironbark Eucalyptus melanophloia. The basal area and orientation of coarse roots were measured on upturned trees after broad‐scale tree clearing, and compared between species, and between live and dead trees with tree size as a covariate. Results: Drought‐induced tree death was higher for dominant Ironbark‐Box than for sub‐dominant Bloodwoods. During a moderate to severe drought in 2004, 41% of 100 cm deep subsoils had soil matric potential less than‐5600 kPa. The drought hardy Bloodwoods had a greater root basal area and particularly so for vertical roots compared to the drought sensitive Ironbark‐Box. Within species there was no significant difference in root basal area characteristics between trees that were recently killed by drought and those that remained relatively healthy. Surface soil moisture availability was lower where tree densities were high, and tree death increased as surface soil moisture became less available. Tree death was also greater as the clay content of sub‐soils increased. Discussion: The study suggests species with roots confined to upper soil layers will suffer severe water stress. The results strongly indicate that root architecture, and the way it facilitates water use during drought, is important for the relative dominance of the tree species. Patchiness in drought‐induced tree death seems to be at least partially a product of heterogeneity in sub‐soil conditions and competition for soil moisture.     

Change within and among forest communities: the influence of historic disturbance,environmental gradients,and community attributes

Understanding how ecological communities change over time is critical for biodiversity conservation, but few long‐term studies directly address decadal‐scale changes in both the within‐ and among‐community components of diversity. In this study, we use a network of permanent forest vegetation plots, established in Great Smoky Mountains National Park (USA) in 1978, to examine the factors that influence change in community composition within and among communities. In 2007, we resampled 15 plots that were logged in the late 1920s and 15 plots that had no documented history of intensive human disturbance. We found that understory species richness decreased by an average of 4.3 species over the 30‐yr study period in the logged plots, but remained relatively unchanged in the unlogged plots. In addition, tree density decreased by an average of 145 stems ha^?1 in the logged plots, but was relatively stable in the unlogged plots. However, we found that historic logging had no effect on within‐community understory or tree compositional turnover during this time period. Instead, sites at lower elevations and sites with lower understory biomass in 1978 had higher understory compositional turnover than did sites at higher elevations and sites with higher understory biomass. In addition, sites with lower soil cation exchange capacity (CEC) and with lower tree basal area in 1978 had higher tree compositional turnover than did sites with higher soil CEC and higher tree basal area. Among‐community similarity was unchanged from 1978 to 2007 for both the logged and unlogged plots. Overall, our results indicate that human disturbance can affect plant communities for decades, but the extent of temporal change in community composition may nevertheless depend more on environmental gradients and community attributes.     

The effect of fertilization on the below-ground diversity and community composition of ectomycorrhizal fungi associated with western hemlock (Tsuga heterophylla)

Fertilization typically reduces ectomycorrhizal diversity shortly after its application but less is known about its longer-term influence on fungal species. Long-term effects are important in forests where fertilizer is rarely applied. We compared fungal species composition in western hemlock control plots with plots last fertilized 7 years ago with nitrogen (N) or nitrogen plus phosphorus (N + P). The N + P fertilization had a significant lingering effect, increasing the tree size and foliar P content of the western hemlocks. From ectomycorrhizal roots of 24-year-old trees from northern Vancouver Island, Canada, we identified fungi from 12 samples per treatment, by amplifying, cloning, and sequencing fungal ribosomal DNA fragments, placing sequences with 97% or more identity in the same operational taxonomic unit (OTU). Diversity was high across treatments; we detected 77 fungal OTUs, 52 from ectomycorrhizal genera, among 922 clone sequences. The five most frequent OTUs were similar in abundance across treatments. Only 19 OTUs matched any of the 197 previously reported ectomycorrhizal species of western hemlock. Species composition but not diversity in nitrogen plus phosphorus plots differed significantly from control or nitrogen plots. Two Cortinarius OTUs were indicator species for nitrogen plus phosphorus plots and presence of Cortinarius cinnamomeus was correlated with control or nitrogen plots. After 7 years, fertilization history had made no detectable difference in ectomycorrhizal fungal diversity, but long-lasting changes in environment resulting from fertilization had a lingering effect on fungal ectomycorrhizal species composition. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Woody vegetation structure of xeric forest stands under different edaphic site conditions and disturbance histories in the Biosphere Reserve 'Parque Costero del Sur', Argentina

We studied stand structures and soil properties in an old-growth forest and two 30-yr-old second-growth stands. In the old-growth forest, the total density and basal area average 1566 trees > 1.25 m height ha^-1 and 46.73 m² ha^-1. The dominant trees are Scutia buxifolia and Celtis tala. Using multivariate techniques we distinguished three stands: PS1, dominated by S. buxifolia, is 1000 m far from the river. Its soil is shallow and loamy. PS2 and PS3, co-dominated by S. buxifolia and C. tala, are over 1200 m distant from the river. There the soil is deeper and has thicker texture and higher phosphorus and calcium concentrations than the near-the-river forest soil. Scutia buxifolia shows reverse J-shaped size-distributions and has morphological features of stress-tolerant species. Celtis tala shows oscillating decay size-curves that suggest recruitment pulses related to small gaps and it has morphological features of competitive species. Celtis tala was selectively cut in the past in the second-growth stands SNRD and SRD. The stand SNRD, 1000 m far from the river, is dominated by S. buxifolia. The second species is Schinus polygamus which presents the bell-shaped size-structure of the pioneer species. SNRD does not differ from its old-growth counterpart PS1 in total tree density, basal area and tree branching. The stand SRD, over 1200 m distant from the river, is co-dominated by S. buxifolia and by C. tala trees regenerated from stumps. SRD does not differ from its old-growth counterparts PS2 and PS3 in total tree density and basal area. As to tree branching, it does not differ from PS3, but differs from PS2. All the stands are being invaded by the exotic tree Ligustrum lucidum.The differences between the old-growth stands seem to be related to the gradients of soil texture and nutrient concentrations raising edaphic stress towards the river. In SNRD, the stress, the stress-tolerance of S. buxifolia, and the aptitude of S. polygamus to recruit in disturbed habitats seem to have prevented the post-logging recruitment of C. tala. In SRD, C. tala regenerated possibly due to a better competitive performance in a more favorable site. Under protection the second-growth stands recovered the old-growth quantitative features. We recommend the restoration of the qualitative features and the control of L. lucidum.