Gradients within gradients: The mesoscale distribution patterns of palms in a central Amazonian forest

Questions: What are the relative contributions of environmental factors and geographic distance to palm community structure at the mesoscale, and how do they depend on the length of the environmental gradient covered? How do soil and topography affect variation of the canopy and understory palm community structure at the mesoscale? How does fine‐scale variation within the broad edaphic/topographic classes affect palm community composition? Location: Reserva Ducke, terra‐firme forest, Manaus, Brazil. Methods: Palms were sampled in 72 plots 250 m × 4 m, systematically distributed over an area of 100 km². Soil, topography and distance to watercourses were measured for all plots. The relationship between community structure axes, summarized by NMDS ordinations, and environmental predictors, was analysed with multivariate regressions. Matrix regressions were used to determine the proportions of variance explained by environmental and geographic predictors. Results: Floristic variation at the mesoscale was mostly related to environmental variation, and the proportion of variance explained depended on the amplitude of the environmental gradient. Soil was the main predictor of floristic change, but its effects differed between life forms, with the understory palm community structured within one of the edaphic/topographic classes, in association with distance to watercourses. Conclusions: Dispersal limitation does not explain palm composition at the mesoscale, and the amplitude of environmental gradients covered by the analysis can be as important as the scale of analysis, in determining the relative contributions of environmental and geographical components to community structure. Soil and topography can predict a large proportion of palm composition, but gradients differ in scale, with some environmental gradients being nested within others. Therefore, although all environmental gradients are nested within distance, they do not necessarily coincide.     

山西灵空山天然松栎混交林群落特征与冠层结构

以2011年建设的山西灵空山4 hm²天然松栎混交林森林动态监测样地为研究平台,以400个10 m×10 m样方为测量单元,于2016年进行群落特征研究,采用半球面影像法(DHP)分析冠层结构和林下光照特征.结果表明: 样地内共有乔木5558株,共计25种,分属于10科15属.冠层开阔度(CO)集中在15.0%～25.0%,叶面积指数(LAI)集中在1.5～2.5,林下光环境参数集中在10.0%～30.0%.建群种在样地内的分布对冠层结构和林下光环境影响显著;冠层结构对林下光环境所有参数的影响方向一致,其中采用叶面积指数评价冠层结构动态的效果更佳;冠层开阔度和叶面积指数对林下光环境产生相反的影响,且均对散射光入射率影响程度最大.温性松栎混交林的林冠层整体较为均匀,林下光分布较为集中,林分树种组成与冠层结构对林下光照影响显著.     

How Does Restoration of Native Canopy Affect Understory Vegetation Composition? Evidence from Riparian Communities of the Hunter Valley Australia

Restoration of native vegetation often focuses on the canopy layer species, with the assumption that regeneration of the understory elements will occur as a consequence. The goal of this study was to assess the influence of canopy restoration on the composition and abundance of understory plant species assemblages along riparian margins in the Hunter Valley, NSW, Australia. We compared the floristic composition (richness, abundance, and diversity) of understory species between nonrevegetated (open) and canopy revegetated plots across five sites. A number of other factors that may also influence understory vegetation, including soil nutrients, proximity to main channel, and light availability, were also measured. We found that sites where the canopy had been restored had lower exotic species richness and abundance, as well as higher native species cover, but not native species richness, compared with open sites. Multivariate analysis of plots based on plant community composition showed that revegetated sites were associated with lower total species diversity, light availability, and exotic cover. This study has found that the restoration of the canopy layer does result in lower exotic species richness and cover, and higher native species cover and diversity in the understory, a desirable restoration outcome. Our results provide evidence that restoration of native canopy species may facilitate restoration of native understory species; however, other interventions to increase native species richness of the understory should also be considered as part of management practice.     

Individual Canopy‐tree Species Effects on Their Immediate Understory Microsite and Sapling Community Dynamics

Canopy trees are largely responsible for the environmental heterogeneity in the understory of tropical and subtropical species‐rich forests, which in turn may influence sapling community dynamics. We tested the effect of the specific identity of four cloud forest canopy trees on total solar radiation, canopy openness, soil moisture, litter depth, and soil temperature, as well as on the structure and dynamics of the sapling community growing beneath their canopies. We observed significant effects of the specific identity of canopy trees on most understory microenvironmental variables. Soil moisture was higher and canopy openness lower beneath Cornus disciflora. In turn, canopy openness and total solar radiation were higher beneath Oreopanax xalapensis, while the lowest soil moisture occurred beneath Quercus laurina. Moreover, Chiranthodendron pentadactylon was the only species having a positive effect on litter depth under its canopy. In spite of these between‐species environmental differences, only C. pentadactylon had significant, negative effects on sapling density and species richness, which may be associated to low seed germination and seedling establishment due to an increased litter depth in its vicinity. The relevance of the specific identity of canopy trees for natural regeneration processes and species richness maintenance depends on its potential to differentially affect sapling dynamics through species‐specific modifications of microenvironmental conditions.     

The combined role of topography and overstorey tree composition in promoting edaphic and floristic variation in a Mediterranean forest

This study aimed to address which factors, other than topography, contribute to the floristic variation of forested slopes. The natural forest studied is located in the Sudoeste Alentejano e Costa Vicentina Nature Park (southwestern Portugal). We sampled topographic, edaphic, floristic and community structure variables along three bottom–top hillside transects. Multivariate analyses of soil variables (by PCA), and of woody species composition and floristic–environmental relationships (by CCA and pCCA) were performed. Environmental–floristic trends strongly associated with the elevation gradient were identified. At lower altitudes, the lowest species richness, the highest soil fertility, and the tallest and most dense (least available light) canopy occurred. The spatial variation in woody species composition and abundance was closely associated with Zn availability in the soil and litter groundcover, but these varibles had significant spatial structure in the studied forest. The non-spatially structured species variance was better predicted by soil NO₃^– and NH₄⁺. The spatial variation of species data not shared with environmental variables was also calculated. We suggest that the influence of topographic gradient on the variation of edaphic variables and on the distribution and abundance of woody species was mediated by overstorey tree composition. Locally dominant tree species, in particular Quercus faginea and Quercus suber, may function as ecosystem engineers promoting environmental changes (i.e., Zn availability in soil, litter accumulation and light availability) that influence overall floristic variation.     

The influence of Prosopis canopies on understorey vegetation: Effects of landscape position

Abstract. The influence of canopy trees and shrubs on under‐storey plants is complex and context‐dependent. Canopy plants can exert positive, negative or neutral effects on production, composition and diversity of understorey plant communities, depending on local environmental conditions and position in the landscape. We studied the influence of Prosopis velutina (mesquite) on soil moisture and nitrogen availability, and understorey vegetation along a topographic gradient in the Sonoran Desert. We found significant increases in both soil moisture and N along the gradient from desert to riparian zone. In addition, P. velutina canopies had positive effects, relative to open areas, on soil moisture in the desert, and soil N in both desert and intermediate terrace. Biomass of understorey vegetation was highest and species richness was lowest in the riparian zone. Canopies had a positive effect on biomass in both desert and terrace, and a negative effect on species richness in the terrace. The effect of the canopy depended on landscape position, with desert canopies more strongly influencing soil moisture and biomass and terrace canopies more strongly influencing soil N and species richness. Individual species distributions suggested interspecific variation in response to water‐ vs. N‐availability; they strongly influence species composition at both patch and landscape position levels.     

Forest plant community as a driver of soil biodiversity: experimental evidence from collembolan assemblages through large‐scale and long‐term removal of oak canopy trees Quercus petraea

Plant–soil interactions are increasingly recognized to play a major role in terrestrial ecosystems functioning. However, few studies to date have focused on slow dynamic ecosystems such as forests. As they are vertically stratified by multiple vegetation strata, canopy tree removal by thinning operations could alter forest plant community through tree canopy opening. Very little is known about cascading effects on soil biodiversity. We conducted a large‐scale, multi‐site assessment of collembolan assemblage response to long‐term canopy tree removal in sessile oak Quercus petraea temperate forests. A total of 33 experimental plots were studied covering a large gradient of canopy tree basal area, stand age and local abiotic contexts. Collembolan abundance strongly declined with canopy tree removal in early forest successional stage and this was mediated by negative effect of understory plant community composition changes, i.e. shift from moss and forb to tree seedling, fern, shrub and grass species. Negative effect of this composition shift on collembolan species richness was largely offset by positive effect of the increase in understory plant species richness. This gives support to both the plant mass‐ratio and functional diversity hypotheses. Collembolan functional groups had contrasting response patterns, which were mediated by different ecological factors. Epedaphic (r‐strategist) abundance and species richness increased with canopy tree removal in relation with the increase in understory plant species richness. In contrast, euedaphic (K‐strategist) abundance and species richness declined with canopy tree removal in early forest successional stage in relation with changes in understory plant community composition and species richness, as well as microclimatic conditions. Overall, our study provides experimental evidence that forest plant community can be a strong driver of collembolan assemblages. It also emphasizes the role of trees as foundation species of forest ecosystems that can shape soil biodiversity through their regulation of understory plant community and ecosystem abiotic conditions.     

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.     

Variation in stand structure,light and seedling abundance across a tropical moist forest chronosequence,Panama

Abstract. We asked whether forest structure and understory light environments across a tropical moist forest chronosequence followed predictions of a 4‐phase model of secondary succession (establishment, thinning, transition and steady‐state) and whether seedling density and diversity were functions of light availability as predicted by this model. Using aerial photographs, we identified eight second‐growth stands (two each aged ca. 20, 40, 70, and 100 yr) and two old‐growth stands within Barro Colorado Nature Monument, Panama. Trees and seedlings were sampled in nested, contiguous quadrats in 2 160‐m transects in each stand. Light was measured as percent transmittance of diffuse photosynthetically active radiation (TPAR) at each seedling quadrat and by estimation of percent total incident radiation during the growing season from hemispherical canopy photographs. Basal area, tree density, and canopy height followed predictions of the 4‐phase model. Percent total radiation, but not TPAR, declined with stand age as did seedling density. While seedlings were more likely to occur in quadrats at higher light levels, much variation in seedling density was not related to light availability. Seedling patch sizes were small irrespective of light patches, estimated as semivariance ranges. Seedling species richness was a function of seedling density; estimates of species diversity unbiased by density did not vary systematically as a function of stand age. Proximate seed sources, efficient dispersal mechanisms, and appropriate establishment conditions can promote establishment of species‐rich communities early in successions of heterogeneous tropical moist forest.     

Spatial and environmental determinants of plant species diversity in a temperate desert

Aims Deserts are one of the ecosystems most sensitive to global climate change. However, there are few studies examining how changing abiotic and biotic factors under climate change will affect plant species diversity in the temperate deserts of Asia. This study aimed to: (i) characterize species distributions and diversity patterns in an Asian temperate desert; and (ii) to quantify the effects of spatial and environment variables on plant species diversity.Methods We surveyed 61 sites to examine the relationship between plant species diversity and several spatial/environmental variables in the Gurbantunggut Desert. Spatial and environmental variables were used to predict plant species diversity in separate multiple regression and ordination models. Variation in species responses to spatial and environmental conditions was partitioned by combining these variables in a redundancy analysis (RDA) and by creating multivariate regression trees (MRT).Important findings We found 92 plant species across the 61 sites. Elevation and geographic location were the dominant environmental factors underlying variation in site species richness. A RDA indicated that 93% of the variance in the species–environment relationships was explained by altitude, latitude, longitude, precipitation and slope position. Precipitation and topographic heterogeneity, through their effects on water availability, were more important than soil chemistry in determining the distribution of species. MRT analyses categorized communities into four groups based on latitude, soil pH and elevation, explaining 42.3% of the standardized species variance. Soil pH strongly influenced community composition within homogeneous geographic areas. Our findings suggest that precipitation and topographic heterogeneity, rather than edaphic heterogeneity, are more closely correlated to the number of species and their distributions in the temperate desert.     

Tree Diversity,Forest Structure and Productivity along Altitudinal and Topographical Gradients in a Species-Rich Ecuadorian Montane Rain Forest

We studied the spatial heterogeneity of tree diversity, and of forest structure and productivity in a highly diverse tropical mountain area in southern Ecuador with the aim of understanding the causes of the large variation in these parameters. Two major environmental gradients, elevation and topography, representing a broad range of climatic and edaphic site conditions, were analyzed. We found the highest species richness of trees in valleys <2100 m. Valleys showed highest values of basal area, leaf area index and tree basal area increment as well. Tree diversity also increased from ridges to valleys, while canopy openness decreased. Significant relationships existed between tree diversity and soil parameters (pH, total contents of Mg, K, Ca, N and P), and between diversity and the spatial variability of pH and Ca and Mg contents suggesting a dependence of tree diversity on both absolute levels and on the small-scale heterogeneity of soil nutrient availability. Tree diversity and basal area increment were positively correlated, partly because both are similarly affected by soil conditions. We conclude that the extraordinarily high tree species richness in the area is primarily caused by three factors: (1) the existence of steep altitudinal and topographic gradients in a rather limited area creating a small-scale mosaic of edaphically different habitats; (2) the intermingling of Amazonian lowland plant species, that reach their upper distribution limits, and of montane forest species; and (3) the geographical position of the study area between the humid eastern Andean slope and the dry interandean forests of South Ecuador.     

Habitat niche specialization in an understory species in a warm temperate forest

Relationships between microhabitat variables; understory light conditions in summer and winter, altitude, slope inclination and topographic categories (valley, ridge, and slope) and the distribution of Aucuba japonica Thunb. (Cornaceae), a common understory shrub species in Japan were examined using non-contagious 66, 20 × 20 m² quadrats. The Morishita’s I _δ suggested that A. japonica distributions were strongly heterogeneous among the quadrats. Therefore positive spatial autocorrelation of A. japonica at a within-quadrat level (≤20 m) was obvious. Moran’s I statistics showed a significant positive spatial autocorrelation in A. japonica abundance within the distance shorter than 60 m. But the partial Mantel tests suggested that the mass effect from neighboring quadrats would little explain A. japonica abundance in a quadrat. The partial Mantel tests also clearly showed that A. japonica distributions were strongly structured by topography and understory light conditions. Using Monte Carlo randomization tests, we found that A. japonica was aggregately distributed in quadrats in valley which were covered by deciduous canopies. Better understory light conditions in winter together with valley edaphic conditions may increase the abundance of A. japonica there. It is concluded that habitat niche specialization is important in structuring distribution of A. japonica in this forest community.     

Canopy Openness Enhances Diversity of Ant–Plant Interactions in the Brazilian Amazon Rain Forest

In closed‐canopy tropical forest understory, light availability is a significant determinant of habitat diversity because canopy structure is highly variable in most tropical forests. Consequently, variation in canopy cover affects the composition and distribution of plant species via creating variable light environments. Nevertheless, little is known about how variation in canopy openness structures patterns of plant–animal interactions. Because of the great diversity and dominance of ants in tropical environments, we used ant–plant interactions as a focal network to evaluate how variation in canopy cover influences patterns of plant–insect interactions in the Brazilian Amazon rain forest. We observed that small increases in canopy openness are associated with increased diversity of ant–plant interactions in our study area, and this change is independent of plant or ant species richness. Additionally, we found smaller niche overlap for both ants and plants associated with greater canopy openness. We hypothesize that enhanced light availability increases the breadth of ant foraging sources because variation in light availability gives rise to plant resources of different quality and amounts. Moreover, greater light availability promotes vegetative growth in plants, creating ant foraging ‘bridges’ between plants. In sum, our results highlight the importance of environmental heterogeneity as a determinant of ant–plant interaction diversity in tropical environments.     

Vertical stratification of ichneumonid wasp communities: the effects of forest structure and life‐history traits

Parasitoid wasp communities of the canopy of temperate forests are still largely unexplored. Very little is known about the community composition of parasitoids between canopy and understory and how much of this difference is related to forest structure or parasitoid biological strategies. In this study we investigated upon the difference in the community composition of the parasitic wasps Ichneumonidae between canopy and understory in a lowland temperate forest in northern Italy. We used general linear models to test whether parasitic strategy modifies species vertical stratification and the effect of forest structure. We also tested differences in β‐diversity between canopy and understory traps and over time within single forest layers. We found that stand basal area was positively related to species richness, suggesting that the presence of mature trees can influence local wasp diversity, providing a higher number of microhabitats and hosts. The ichneumonid community of the canopy was different from that of the understory, and the β‐diversity analysis showed higher values for the canopy, due to a higher degree of species turnover between traps. In our analyses, the vertical stratification was different between groups of ichneumonids sharing different parasitic strategies. Idiobiont parasitoids of weakly or deeply concealed hosts were more diverse in the understory than in the canopy while parasitoids of spiders were equally distributed between the two layers. Even though the ichneumonid community was not particularly species‐rich in the canopy of the temperate forests, the extension of sampling to that habitat significantly increased the number of species recorded.     

Influence of soils and topography on Amazonian tree diversity: a landscape‐scale study

Question: How do soils and topography influence Amazonian tree diversity, a region with generally nutrient‐starved soils but some of the biologically richest tree communities on Earth? Location: Central Amazonia, near Manaus, Brazil. Methods: We evaluated the influence of 14 soil and topographic features on species diversity of rain forest trees (≥10 cm diameter at breast height), using data from 63 1‐ha plots scattered over an area of ～400 km². Results: An ordination analysis identified three major edaphic gradients: (1) flatter areas had generally higher nutrient soils (higher clay content, carbon, nitrogen, phosphorus, pH and exchangeable bases, and lower aluminium saturation) than did slopes and gullies; (2) sandier soils had lower water storage (plant available water capacity), phosphorus and nitrogen; and (3) soil pH varied among sites. Gradient 2 was the strongest predictor of tree diversity (species richness and Fisher's α values), with diversity increasing with higher soil fertility and water availability. Gradient 2 was also the best predictor of the number of rare (singleton) species, which accounted on average for over half (56%) of all species in each plot. Conclusions: Although our plots invariably supported diverse tree communities (≥225 species ha^?1), the most species‐rich sites (up to 310 species ha^?1) were least constrained by soil water and phosphorus availability. Intriguingly, the numbers of rare and common species were not significantly correlated in our plots, and they responded differently to major soil and topographic gradients. For unknown reasons rare species were significantly more frequent in plots with many large trees.     

A model of geographical, environmental and regional variation in vegetation composition of pyrogenic grasslands of Florida

Aim To develop a landscape‐level model that partitions variance in plant community composition among local environmental, regional environmental, and purely spatial predictive variables for pyrogenic grasslands (prairies, savannas and woodlands) throughout northern and central Florida. Location North and central Florida, USA. Methods We measured plant species composition and cover in 271 plots throughout the study region. A variation‐partitioning model was used to quantify components of variation in species composition associated with the main and interaction effects of soil and topographic variables, climate variables and spatial coordinates. Partial correlations of environmental variables with community variation were identified using direct gradient analysis (redundancy analysis and partial redundancy analysis) and Monte Carlo tests of significance. Results Community composition was most strongly related to edaphic variables at local scales in association with topographic gradients, although geographically structured edaphic, climatic and pure spatial effects were also evident. Edaphic variables explained the largest portion of total variation explained (TVE) as a main effect (48%) compared with the main effects of climate (9%) and pure spatial factors (9%). The remaining TVE was explained by the interaction effect of climate and spatial factors (13%) and the three‐way interaction (22%). Correlation analyses revealed that the primary compositional gradient was related to soil fertility and topographic position corresponding to soil moisture. A second gradient represented distinct geographical separation between the Florida panhandle and peninsular regions, concurrent with differences in soil characteristics. Gradients in composition corresponded to species richness, which was lower in the Florida peninsula. Main conclusions Environmental variables have the strongest influence on the species composition of Florida pyrogenic grasslands at both local and regional scales. However, the limited distributions of many plant taxa suggest historical constraints on species distributions from one physiographical region to the other (Florida panhandle and peninsula), although this pattern is partially confounded by regionally spatially structured environmental variables. Our model provides insight into the relative importance of local‐ and regional‐scale environmental effects as well as possible historical constraints on floristic variation in pine‐dominated pyrogenic grasslands of the south‐eastern USA.     

Impacts of soil fertility on species and phylogenetic turnover in the high - rainfall zone of the Southwest Australian global biodiversity hotspot

The ancient landscape of the South - West Australian Floristic Region (SWAFR) is characterized by exceptional floristic diversity, attributed to a complex mosaic of nutrient - impoverished soils. Between - soil type differences in nutrient availability are expected to affect floristic assemblage patterns in the SWAFR. We compared patterns of floristic diversity between open - forest samples from three soil types in the high - rainfall zone of the SWAFR. The importance of environmental and spatial factors for species compositional turnover within soil types were evaluated within canonical correspondence analyses using variation partitioning. Patterns of phylogenetic diversity and dispersion were contrasted between soil types and related to differences in soil nutrient availability. Between - quadrat shared phylogenetic branch length for individual life form categories was correlated with explanatory variables using Mantel tests. Species and phylogenetic diversity increased with a decline in soil nutrients and basal area. Nutrient - poorer soils were differentiated by higher species density and phylogenetic diversity, and larger phylogenetic distances between species. Species turnover was best explained by environmental factors when soil nutrient concentrations and basal area were low. Coastal and inland quadrats from the most fertile soil type were distinguished by significantly differing patterns of phylogenetic diversity. Inland quadrats were characterized by strong relationships between phylogenetic diversity and environment, while phylogenetic patterns remained largely unaccounted for by explanatory variables within coastal quadrats. Phylogenetic diversity was more strongly related with environment within upland landform types for nutrient-poor soils. We highlight the complex relationships between climatic and edaphic factors within the SWAFR, and propose that the occurrence of refugial habitat for plant phylogenetic diversity is dynamically linked with these interactions. Climate change susceptibility was estimated to be especially high for inland locations within the high - rainfall zone. Despite the strong relationship between floristic diversity and soil fertility, holistic conservation approaches are required to conserve the mosaic of soil types regardless of soil nutrient status.     

西双版纳热带雨林林窗空间分布格局及其特征数与林窗下植物多样性的相关性

林窗作为森林群落中一种重要的干扰方式, 对林下物种构成有着重要的影响。开展林窗空间格局及其特征指数与林下植物多样性关系研究对于探讨林窗对林下生物多样性的影响有重要意义, 有助于进一步了解群落动态, 在物种多样性保护方面也具有指导作用。本研究在西双版纳热带雨林地区随机选取3块大小为1 ha的热带雨林为研究样地, 采用轻小型六旋翼无人机搭载Sony ILCE-A7r可见光传感器, 分别获取各个样地的高清数字影像, 结合数字表面高程模型以及各个样地的地形数据用以确定各样区的林窗分布格局, 并进一步提取出各林窗的景观格局指数。结合地面样方基础调查数据, 对各样地各林窗下植物多样性情况进行统计, 旨在分析热带雨林林窗空间分布格局以及林窗下植物多样性对各林窗空间格局特征的响应情况。研究表明, 西双版纳州热带雨林林窗呈大而分散的空间分布, 林窗空间格局特征指数如林窗形状复杂性指数、林窗面积都与林下植物多样性呈显著正相关关系。在面积小的林窗下, 较之林窗形状复杂性因子, 林窗面积大小对林下植物多样性影响更显著; 在面积达到一定程度后, 相对于面积因子, 林窗形状复杂性指数对林下植物多样性影响更显著, 各样地林窗皆趋于向各自所处样地顶极群落发展。     

Environmental factors influencing spatial patterns of shrub diversity in chaparral,Santa Ynez Mountains,California

We examined patterns of shrub species diversity relative to landscape‐scale variability in environmental factors within two watersheds on the coastal flank of the Santa Ynez Mountains, California. Shrub species richness and dominance was sampled at a hierarchy of spatial units using a high‐powered telescope from remote vantage points. Explanatory variables included field estimates of total canopy cover and percentage rock cover, and modeled distributions of slope, elevation, photosynthetically active radiation, topographic moisture index, and local topographic variability. Correlation, multiple regression, and regression tree analyses showed consistent relationships between field‐based measurements of species richness and dominance, and topographically‐mediated environmental variables. In general, higher richness and lower dominance occurred where environmental conditions indicated greater levels of resource limitation with respect to soil moisture and substrate availability. Maximum richness in shrub species occurred on high elevation sites with low topographic moisture index, rocky substrate, and steep slopes. Maximum dominance occurred at low elevation sites with low topographic variability, high potential solar insolation, and high total shrub canopy cover. The observed patterns are evaluated with respect to studies on species‐environment relations, resource use, and regeneration of shrubs in chaparral and coastal sage scrub. The results are discussed in the context of existing species‐diversity hypotheses that hinge on reduced competitive dominance and increased resource heterogeneity under conditions of resource limitation.     

Understory Plant Community Composition Is Associated with Fine-Scale Above- and Below-Ground Resource Heterogeneity in Mature Lodgepole Pine (Pinus contorta) Forests

Understory plant communities play critical ecological roles in forest ecosystems. Both above- and below-ground ecosystem properties and processes influence these communities but relatively little is known about such effects at fine (i.e., one to several meters within-stand) scales, particularly for forests in which the canopy is dominated by a single species. An improved understanding of these effects is critical for understanding how understory biodiversity is regulated in such forests and for anticipating impacts of changing disturbance regimes. Our primary objective was to examine the patterns of fine-scale variation in understory plant communities and their relationships to above- and below-ground resource and environmental heterogeneity within mature lodgepole pine forests. We assessed composition and diversity of understory vegetation in relation to heterogeneity of both the above-ground (canopy tree density, canopy and tall shrub basal area and cover, downed wood biomass, litter cover) and below-ground (soil nutrient availability, decomposition, forest floor thickness, pH, and phospholipid fatty acids (PLFAs) and multiple carbon-source substrate-induced respiration (MSIR) of the forest floor microbial community) environment. There was notable variation in fine-scale plant community composition; cluster and indicator species analyses of the 24 most commonly occurring understory species distinguished four assemblages, one for which a pioneer forb species had the highest cover levels, and three others that were characterized by different bryophyte species having the highest cover. Constrained ordination (distance-based redundancy analysis) showed that two above-ground (mean tree diameter, litter cover) and eight below-ground (forest floor pH, plant available boron, microbial community composition and function as indicated by MSIR and PLFAs) properties were associated with variation in understory plant community composition. These results provide novel insights into the important ecological associations between understory plant community composition and heterogeneity in ecosystem properties and processes within forests dominated by a single canopy species.