Decoupled responses of tree and shrub leaf and litter trait values to ecosystem retrogression across an island area gradient

Aims

In the long term absence of catastrophic disturbance ecosystem retrogression occurs, and this is characterized by reduced soil fertility, and impairment of plant biomass and productivity. The response of plant traits to retrogression remains little explored. We investigated how changes plant traits and litter decomposability shift during retrogression for dominant trees and understory shrubs.

Methods

We characterized changes in intraspecific, interspecific and community-averaged values of plant traits and litter decomposability, for six abundant species across thirty lake islands in boreal forest that undergo retrogression with increasing time since fire.

Results

For understory shrubs, trait values and litter decomposability often changed as soil fertility declined in a manner reflective of greater conservation (versus acquisition) of nutrients, particularly at the interspecific and whole community levels. Such responses were seldom observed for trees, meaning that trees and shrubs show a decoupled response to declining soil fertility during retrogression.

Conclusions

Our results only partially agree with previous studies on temperate and subtropical retrogressive chronosequences. Because traits of only shrubs were responsive, they also highlight that impairment of belowground ecosystem processes during retrogression is primarily driven by changes in the trait spectra of understory vegetation rather than that of the trees.     

Patterns of tree community composition along a coastal dune chronosequence in lowland temperate rain forest in New Zealand

Soil chronosequences provide an opportunity to examine the influence of long-term pedogenesis on the biomass and composition of associated tree communities. We assessed variation in the species composition of trees, saplings, and seedlings, and the basal area of adult trees, in lowland temperate rain forest along the Haast chronosequence on the west coast of the South Island of New Zealand. The sequence consists of Holocene dune ridges formed following periodic earthquake disturbance and is characterized by rapid podzol development, including a marked decline in phosphorus concentrations, accumulation of a thick organic horizon, and formation of a cemented iron pan. Tree basal area increased for the first few hundred years and then declined in parallel with the decline in total soil phosphorus, consistent with the concept of forest retrogression. There were also marked changes in the composition of the tree community, from dominance by conifers on young soils to a mixed conifer?Cangiosperm forest on old soils. Although a variety of factors could account for these changes, partial Mantel tests revealed strong correlations between tree community composition and soil nutrients. The relationships differed among life history stages, however, because the adult tree community composition was correlated strongly with nutrients in the mineral soil, whereas the seedling community composition was correlated with nutrients in the organic horizon, presumably reflecting differences in rooting depth. The changes in the tree community at Haast are consistent with disturbance-related succession in conifer?Cangiosperm forests in the region, but the opposite of patterns along the nearby Franz Josef post-glacial chronosequence, where conifers are most abundant on old soils. The Haast chronosequence is therefore an important additional example of forest retrogression linked to long-term soil phosphorus depletion, and provides evidence for the role of soil nutrients in determining the distribution of tree species during long-term succession in lowland temperate rain forests in New Zealand.     

From bottom-up to top-down control of invertebrate herbivores in a retrogressive chronosequence

In the long-term absence of disturbance, ecosystems often enter a decline or retrogressive phase which leads to reductions in primary productivity, plant biomass, nutrient cycling and foliar quality. However, the consequences of ecosystem retrogression for higher trophic levels such as herbivores and predators, are less clear. Using a post-fire forested island-chronosequence across which retrogression occurs, we provide evidence that nutrient availability strongly controls invertebrate herbivore biomass when predators are few, but that there is a switch from bottom-up to top-down control when predators are common. This trophic flip in herbivore control probably arises because invertebrate predators respond to alternative energy channels from the adjacent aquatic matrix, which were independent of terrestrial plant biomass. Our results suggest that effects of nutrient limitation resulting from ecosystem retrogression on trophic cascades are modified by nutrient-independent variation in predator abundance, and this calls for a more holistic approach to trophic ecology to better understand herbivore effects on plant communities.     

Soil phosphorus and microbial response to a long-term wildfire chronosequence in northern Sweden

In the prolonged absence of major disturbances, ecosystems may enter a stage of retrogression, which is characterized by decreased ecosystem process rates both above and belowground, and often reduced availability of phosphorus (P). Disturbance through wildfire can increase soil P losses through leaching or erosion, but in the long-term absence of fire, soil P could potentially become increasingly bound in more stable forms that are less available to microbes. We studied forms of P and microbial respiration kinetics in the humus layer of a group of islands that vary considerably in wildfire frequency (40–5,300 years since last fire), and which are known to enter retrogression in the prolonged absence of fire. We found a decrease in labile P with decreasing fire frequency but no change in total P. Soil microorganisms responded more strongly to N than to P addition, and microbial biomass N:P ratios remained unchanged across the gradient. However, the concentration of labile P was the best predictor of microbial respiration responses across the islands, and this provides some evidence that declining access to P could contribute to the decline in soil microbial activity during retrogression. Our results show that even though N is arguably the main limiting nutrient during retrogression in this chronosequence, long term absence of fire also causes a decline in P availability which negatively affects microbial activity. This in turn could potentially impair microbially driven processes such as decomposition and mineralization and further contribute to the reduced availability of soil nutrients during retrogression.     

Response of photosynthetic carbon gain to ecosystem retrogression of vascular plants and mosses in the boreal forest

In the long-term absence of rejuvenating disturbances, forest succession frequently proceeds from a maximal biomass phase to a retrogressive phase characterized by reduced nutrient availability [notably nitrogen (N) and phosphorus (P)] and net primary productivity. Few studies have considered how retrogression induces changes in ecophysiological responses associated with photosynthetic carbon (C) gain, and only for trees. We tested the hypothesis that retrogression would negatively impact photosynthetic C gain of four contrasting species, and that this impact would be greater for vascular plants (i.e., trees and shrubs) than for non-vascular plants (i.e., mosses). We used a 5,000-year-old chronosequence of forested islands in Sweden, where retrogression occurs in the long-term absence of lightning-ignited wildfires. Despite fundamental differences in plant form and ecological niche among species, vascular plants and mosses showed similar ecophysiological responses to retrogression. The most common effects of retrogression were reductions in photosynthesis and respiration per unit foliar N, increases in foliar N, δ(13)C and δ(15)N, and decreases in specific leaf areas. In contrast, photosynthesis per unit mass or area generally did not change along the chronosequence, but did vary many-fold between vascular plants and mosses. The consistent increases in foliar N without corresponding increases in mass- or area-based photosynthesis suggest that other factor(s), such as P co-limitation, light conditions or water availability, may co-regulate C gain in retrogressive boreal forests. Against our predictions, traits of mosses associated with C and N were generally highly responsive to retrogression, which has implications for how mosses influence ecosystem processes in boreal forests.     

Are the eucalypt and non-eucalypt components of Australian tropical savannas independent?

Eucalypts (Eucalyptus and Corymbia spp.) dominate (>60%) the tree biomass of Australia's tropical savannas but account for only a fraction (28%) of the tree diversity. Because of their considerable biomass and adaptation to environmental stressors, such as fire, the eucalypts may drive tree dynamics in these savannas, possibly to the exclusion of non-eucalypts. We evaluated whether the eucalypt and non-eucalypt components in tropical savannas are dependent so that changes in one component are matched by opposite trends in the other. Using tree inventory data from 127 savanna sites across the rainfall and fire frequency gradients, we found that eucalypt and non-eucalypt basal area and species richness had a negative relationship. This relationship was maintained across the rainfall gradient, with rainfall having a positive effect on the basal area and species richness of both components, but with a greater effect in non-eucalypts. Fire frequency negatively affected basal area, but not species richness, although basal area and species richness of eucalypts and non-eucalypts did not differ in their response to fire. Rainfall appears to set the upper bounds to woody biomass in these mesic savannas, while fire maintains woody biomass below carrying capacity and facilitates coexistence of the components. The magnitude of the component responses, particularly for non-eucalypts, is determined by rainfall, but their dependence is likely due to their differential response to both rainfall and fire, but not to competition for resources. Thus, while eucalypts dominate biomass overall, at high rainfall sites non-eucalypt basal area and diversity are highest, especially where fire frequency is low.     

Changes in woody vegetation abundance and diversity after natural disturbances causing different levels of mortality

Questions: How does woody vegetation abundance and diversity differ after natural disturbances causing different levels of mortality? Location: Abies balsamea–Betula papyrifera boreal mixed‐wood stands of southeast Quebec, Canada. Methods: Woody vegetation abundance and diversity were quantified and compared among three disturbance‐caused mortality classes, canopy gap, moderate‐severity disturbances, and catastrophic fire, using redundancy analysis, a constrained linear ordination technique, and diversity indices. Results: Substantial changes in canopy tree species abundance and diversity only occurred after catastrophic fire. Shade‐tolerant, late‐successional conifer species remained dominant after canopy gap and moderate‐severity disturbances, whereas shade‐intolerant, early‐successional colonizers dominated canopy tree regeneration after catastrophic fire. Density and diversity of mid‐tolerant and shade‐intolerant understory tree and shrub species increased as the impact of disturbance increased. Highest species richness estimates were observed after catastrophic fire, with several species establishing exclusively under these conditions. Relative abundance of canopy tree regeneration was most similar after canopy gap and moderate‐severity disturbances. For the sub‐canopy tree and shrub community, relative species abundances were most similar after moderate‐severity disturbances and catastrophic fire. Vegetation responses to moderate‐severity disturbances thus had commonalities with both extremes of the disturbance‐caused mortality gradient, but for different regeneration layers. Conclusions: Current spatio‐temporal parameters of natural disturbances causing varying degrees of mortality promote the development of a complex, multi‐cohort forest condition throughout the landscape. The projected increase in time intervals between catastrophic fires may lead to reduced diversity within the system.     

Lichen Specific Thallus Mass and Secondary Compounds Change across a Retrogressive Fire-Driven Chronosequence

In the long-term absence of major disturbances ecosystems enter a state of retrogression, which involves declining soil fertility and consequently a reduction in decomposition rates. Recent studies have looked at how plant traits such as specific leaf mass and amounts of secondary compounds respond to declining soil fertility during retrogression, but there are no comparable studies for lichen traits despite increasing recognition of the role that lichens can play in ecosystem processes. We studied a group of 30 forested islands in northern Sweden differing greatly in fire history, and collectively representing a retrogressive chronosequence, spanning 5000 years. We used this system to explore how specific thallus mass (STM) and carbon based secondary compounds (CBSCs) change in three common epiphytic lichen species (Hypogymnia phsyodes, Melanohalea olivacea and Parmelia sulcata) as soil fertility declines during this retrogression. We found that STMs of lichens increased sharply during retrogression, and for all species soil N to P ratio (which increased during retrogression) was a strong predictor of STM. When expressed per unit area, medullary CBSCs in all species and cortical CBSCs in P. sulcata increased during retrogression. Meanwhile, when expressed per unit mass, only cortical CBSCs in H. physodes responded to retrogression, and in the opposite direction. Given that lichen functional traits are likely to be important in driving ecological processes that drive nutrient and carbon cycling in the way that plant functional traits are, the changes that they undergo during retrogression could potentially be significant for the functioning of the ecosystem.     

Effects of anthropogenic disturbance on tree population structure and diversity of a rain forest biosphere reserve in Ghana,West Africa

We evaluated the impacts of anthropogenic disturbance on community structure and diversity along three management zones of the Bia biosphere reserve in Ghana. Sixty sample plots were distributed among the core, buffer and transition zones. We estimated the degree of disturbances from discernible indicators on the field and satellite images. All tree species ≥10 cm dbh (diameter at breast height) were identified and enumerated. Inventory data were compared across the zones and related to intensity of disturbances. A total of 1176 individual trees from 108 species and 33 families were encountered. Number of species varied from 27 in the highly disturbed (HD) to 61 in the least disturbed (LD) zone. Mean basal area (BA) varied from 11.71 in the HD to 28.26 in the LD. Both Margalef's species richness and Shannon‐Weiner's α‐diversity were highest in the moderately disturbed (MD) than either the least and most disturbed zones. Our study revealed significant differences in tree abundance, stem density, BA and species diversity, attributable to differences in degree of anthropogenic disturbances among zones. Given the different levels of anthropogenic disturbance and corresponding impacts across the reserve, we recommend an integrated management strategy for the conservation of biodiversity in the Bia biosphere reserve.     

AOB community structure and richness under European beech, sessile oak, Norway spruce and Douglas-fir at three temperate forest sites

Background and aims

The relations between tree species, microbial diversity and activity can alter ecosystem functioning. We investigated ammonia oxidizing bacteria (AOB) community structure and richness, microbial/environmental factors related to AOB diversity and the relationship between AOB diversity and the nitrification process under several tree species.

Methods

Forest floor (Of, Oh) was sampled under European beech, sessile oak, Norway spruce and Douglas-fir at three sites. AOB community structure was assessed by PCR-DGGE and sequencing. Samples were analyzed for net N mineralization, potential nitrification, basal respiration, microbial biomass, microbial or metabolic quotient, pH, total nitrogen, extractable ammonium, organic matter content and exchangeable cations.

Results

AOB community structure and tree species effect on AOB diversity were site-specific. AOB richness was not related to nitrification. Factors regulating ammonium availability, i.e. net N mineralization or microbial biomass, were related to AOB community structure.

Conclusion

Our research shows that, at larger spatial scales, site specific characteristics may be more important than the nature of tree species in determining AOB diversity (richness and community structure). Within sites, tree species influence AOB diversity. The absence of a relation between AOB richness and nitrification points to a possibly role of AOB abundance, phenotypic plasticity or the implication of ammonia oxidizing archaea.     

Effects of a natural flood disturbance on species richness and beta diversity of stream benthic diatom communities

Natural hydrological disturbances in streams may reduce biomass and species richness and change community composition within streams. Disturbances can also affect beta diversity among streams if their effects are species specific or vary across sites. We investigated the effect of a natural flood on species richness, community composition and among-streams beta diversity of benthic diatoms (total community and three functional groups: low profile, high profile and motile) of seven streams in New Zealand. Sampling occurred shortly before, 10 days after and 40 days after the flood. Species richness of the total diatom community did not change after the flood. The high-profile group was the only one whose species richness declined after the flood, whereas species richness of the low-profile group increased. Community composition changed after the flood, mostly as a result of species replacement rather than richness differences over time. Finally, among-streams beta diversity did not change after the flood, suggesting that variation in species composition of benthic diatoms among streams may be maintained in the face of flood disturbances.     

Spatiotemporal changes of beetle communities across a tree diversity gradient

Aim Plant and arthropod diversity are often related, but data on the role of mature tree diversity on canopy insect communities are fragmentary. We compare species richness of canopy beetles across a tree diversity gradient ranging from mono‐dominant beech to mixed stands within a deciduous forest, and analyse community composition changes across space and time. Location Germany’s largest exclusively deciduous forest, the Hainich National Park (Thuringia). Methods We used flight interception traps to assess the beetle fauna of various tree species, and applied additive partitioning to examine spatiotemporal patterns of diversity. Results Species richness of beetle communities increased across the tree diversity gradient from 99 to 181 species per forest stand. Intra‐ and interspecific spatial turnover among trees contributed more than temporal turnover among months to the total γ‐beetle diversity of the sampled stands. However, due to parallel increases in the number of habitat generalists and the number of species in each feeding guild (herbivores, predators and fungivores), no proportional changes in community composition could be observed. If only beech trees were analysed across the gradient, patterns were similar but temporal (monthly) species turnover was higher compared to spatial turnover among trees and not related to tree diversity. Main conclusions The changes in species richness and community composition across the gradient can be explained by habitat heterogeneity, which increased with the mix of tree species. We conclude that understanding temporal and spatial species turnover is the key to understanding biodiversity patterns. Mono‐dominant beech stands are insufficient to conserve fully the regional species richness of the remaining semi‐natural deciduous forest habitats in Central Europe, and analysing beech alone would have resulted in the misleading conclusion that temporal (monthly) turnover contributes more to beetle diversity than spatial turnover among different tree species or tree individuals.     

Is shadier better? The effect of agroforestry management on small mammal diversity

Agroforestry systems play fundamental roles for wildlife conservation, but are prone to disturbances from management practices aiming at increasing local productivity. This work investigates the small mammal assemblages present in cacao agroforests, which differ in shade tree density. We tested the prediction that higher tree density increases shade level, analyzed how some environmental variables important for small mammals (vegetation complexity, tree basal area, and invertebrate biomass) vary across the shade level gradient, and how the assemblages respond to these variations. We also tested the effect of the environmental variables on the abundance of the three most common species: Rhipidomys mastacalis, Hylaeamys seuanezi, and Marmosa murina. We captured 651 individuals belonging to 18 species. A positive relationship was observed between an abundance of non-forest specialists and tree basal area, while species diversity within this group showed positive association with vegetation complexity. Assemblage structure (described by a matrix of species abundance per site) was not affected by our environmental variables, but R. mastacalis was more abundant in sites with lower vegetation complexity. Higher shade levels in cacao agroforest tended to occur in sites with greater tree basal area, which was not a good predictor of small mammal diversity. This suggests that environmental management to reduce shade with the purpose of increasing cacao productivity is not necessarily negative for small mammal conservation. Species diversity was favored by structurally complex systems, a possible response to greater niche diversity.     

Species richness peaks for intermediate levels of biomass in a fractal succession with quasi-neutral interactions

The mechanisms that promote species richness, including net community interactions, are considered central to the investigation of the consequences of biodiversity loss for ecosystem functioning. Recently, some empirical studies at large spatiotemporal scales suggest that increasing species richness within natural communities results in a finer division of biomass among species rather than an increase in total biomass. In parallel, the most common pattern observed in nature is the peaked relationship between diversity and productivity estimated as total biomass. Thus, the aim of our study is to provide model predictions for the diversity–biomass relationship with various levels of net species interactions within communities: negative, neutral, quasi-neutral and positive. Using a scaling relationship between the number of species and total community biomass, we propose a new self-similar process of biomass partitioning during a community assembly process. At each step of the succession, K more species appear that are A times less abundant on average giving K=A^d; the parameter d being a fractal dimension related to the nature of interactions among coexisting species. Our results, compared to those from meta-analyses about empirical diversity–productivity relationships, illustrate that quasi-neutral interactions among coexisting species lead to the most commonly observed pattern: an 'envelope' where diversity peaks at intermediate values of total biomass, i.e. that the area below the hump-backed line (considered as the upper boundary) is filled with data points.     

Effects of population density on forest structure and species richness and diversity of trees in Kampong Thom Province, Cambodia

This study examined differences in stand structure, tree species richness, and tree species diversity in relation to population density in Kampong Thom Province, Cambodia. Tree data were obtained from a 1997 forest inventory involving 60 clusters (540 plots) systematically distributed over 30% of the provincial forest area. Spatially referenced population data were obtained from the 1998 national population census. The average number of trees per cluster was 356/ha, the average basal area, 23 m²/ha, the average stand volume, 217 m³/ha, and the average aboveground biomass, 273 Mg/ha for all trees of DBH 10 cm and larger. The average species richness per cluster was 37 species, while average species diversity was measured as 0.916 using Simpson’s index and 2.98 by Shannon’s index. Significant negative correlations were generally found between population density surrounding clusters and tree density, basal area, stand volume, aboveground biomass, and species richness and diversity for three examined diameter classes (DBH of 10–30, ≥30, and ≥10 cm). As the distance from clusters for calculating population density increased, the correlation levels increased up to 5 or 7 km, depending on the variables and diameter class, and then stayed relatively constant for stand structure variables and decreased for species richness and diversity. The results indicate that evidence of disturbance was more pronounced at higher population density up to around 5 to 7 km. We suggest that introduction of greater controls on human disturbance should be a high priority for resource management and conservation in Kampong Thom Province and, presumably, Cambodia as a whole.     

Biodiversity Promotes Tree Growth during Succession in Subtropical Forest

Losses of plant species diversity can affect ecosystem functioning, with decreased primary productivity being the most frequently reported effect in experimental plant assemblages, including tree plantations. Less is known about the role of biodiversity in natural ecosystems, including forests, despite their importance for global biogeochemical cycling and climate. In general, experimental manipulations of tree diversity will take decades to yield final results. To date, biodiversity effects in natural forests therefore have only been reported from sample surveys or meta-analyses with plots not initially selected for diversity. We studied biomass and growth of subtropical forests stands in southeastern China. Taking advantage of variation in species recruitment during secondary succession, we adopted a comparative study design selecting forest plots to span a gradient in species richness. We repeatedly censored the stem diameter of two tree size cohorts, comprising 93 species belonging to 57 genera and 33 families. Tree size and growth were analyzed in dependence of species richness, the functional diversity of growth-related traits, and phylogenetic diversity, using both general linear and structural equation modeling. Successional age covaried with diversity, but differently so in the two size cohorts. Plot-level stem basal area and growth were positively related with species richness, while growth was negatively related to successional age. The productivity increase in species-rich, functionally and phylogenetically diverse plots was driven by both larger mean sizes and larger numbers of trees. The biodiversity effects we report exceed those from experimental studies, sample surveys and meta-analyses, suggesting that subtropical tree diversity is an important driver of forest productivity and re-growth after disturbance that supports the provision of ecological services by these ecosystems.     

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.     

Plant diversity,structure and regeneration potential in tropical forests of Western Ghats,India

Diversity, stand structure and regeneration potential are the key elements of any forest ecosystem. For the present study, seven sites were selected with the aims of assessing plant diversity, structure and regeneration potential in tropical forests across Kanyakumari Wildlife Sanctuary (KWLS), Western Ghats, India. The sites were classified based on the similarity: tropical dry deciduous sites (TDDs I and II), tropical semi-evergreen sites (TSEs I and II) and tropical evergreen sites (TEFs I-III). The phytosociological survey was done by laying a total of 70 plots (10 plots in each study site). Standard methods were followed for the assessment of diversity, structure and regeneration patterns. A total of 267 species (205 genera, 70 families) were recorded. The tree species richness ranged 24 (TDD II) – 76 (TEF III). Of the vegetation spectrum, trees, vines and understorey accounted 56.5, 15.3 and 28.2% respectively to the total flora documented. A total of 66 species were endemic. The total tree density and tree basal area (seedlings, saplings, juveniles and adults) were 18,790 individuals (mean 2684) and 137.6 m² (mean 19.7 m²) in 70 plots respectively. The mean tree adult density and basal area ranged 370 (TDD I) – 900 (TEF I) individuals/ha and 24.2 (TDD I) – 75.3 (TEF III) m²/ha respectively. The overall species richness was highest in TDD I, but TEF III had the highest tree species richness. The diameter class-wise distribution showed the characteristic ‘reverse J-shaped’ curve. Most tree species were ‘newly recruited’. The dominant species had ‘fair’ to ‘good’ regeneration potential. However, 12 tree species showed ‘no’ regeneration. The overall regeneration pattern of trees was ‘good’, but ‘no’ or ‘poor’ regeneration patterns in some tree species, especially endemics is a point of concern. Since a majority of tree species were ‘new recruits’, species composition may likely change in the future. The results obtained would help in understanding diversity patterns, structural attributes and regeneration potential in tropical forests of protected areas for better forest management and conservation.     

长江口及其邻近海域春季无脊椎动物群落时空变化

为了研究长江口及其邻近海域无脊椎动物群落结构和多样性的时空变化特征, 我们于1999-2012年春季在长江口及其邻近海域采用定点底层双拖网调查方式进行无脊椎动物调查。结果表明: (1)1999-2012年长江口水域共记录无脊椎动物41种, 隶属6纲10目23科, 其中甲壳动物种类最多(26种), 其次为软体动物(13种)。不同年份种类数量呈先下降后上升的趋势。(2)优势种主要包括日本枪乌贼(Loligo japonica)、三疣梭子蟹(Portunus trituberculatus)、葛氏长臂虾(Palaemon gravieri)和鹰爪虾(Trachypenaeus curvirostris)等, 其在年间存在剧烈变动, 但日本枪乌贼几乎每年都是优势种。(3)长江口无脊椎动物丰度、种类丰富度和多样性在年间均存在显著差异, 1999年和2001年最高, 2004年后呈先下降后恢复上升的趋势。(4)长江口水域的无脊椎动物在每个航次调查中都存在2-3个群聚类型, 并有不同的指示种类。(5)1999-2012年长江口无脊椎动物群落的时间变化可划分为3个阶段: 1999-2001年多样性程度最高, 2004-2007年下降至最低水平, 2009-2012年多样性显著回升, 但尚未恢复到1999-2001年的水平。与20世纪80年代相比, 蟹类减少导致长江口无脊椎动物生物量整体水平下降, 高营养级生物资源衰退带来了无脊椎动物中低营养级生物种群的迅速发展。     

Positive tree diversity effect on fine root biomass: via density dependence rather than spatial root partitioning

The importance of species richness to ecosystem functioning and services is a central tenet of biological conservation. However, most of our theory and mechanistic understanding is based on diversity found aboveground. Our study sought to better understand the relationship between diversity and belowground function by studying root biomass across a plant diversity gradient. We collected soil cores from 91 plots with between 1 and 12 aboveground tree species in three natural secondary forests to measure fine root (≤ 2 mm in diameter) biomass. Molecular methods were used to identify the tree species of fine roots and to estimate fine root biomass for each species. This study tested whether the spatial root partitioning (species differ by belowground territory) and symmetric growth (the capacity to colonize nutrient-rich hotspots) underpin the relationship between aboveground species richness and fine root biomass. All species preferred to grow in nutrient-rich areas and symmetric growth could explain the positive relationship between aboveground species richness and fine root biomass. However, symmetric growth only appeared in the nutrient-rich upper soil layer (0–10 cm). Structural equation modelling indicated that aboveground species richness and stand density significantly affected fine root biomass. Specifically, fine root biomass depended on the interaction between aboveground species richness and stand density, with fine root biomass increasing with species richness at lower stand density, but not at higher stand density. Overall, evidence for spatial (i.e. vertical) root partitioning was inconsistent; assumingly any roots growing into deeper unexplored soil layers were not sufficient contributors to the positive diversity–function relationship. Alternatively, density-dependent biotic interactions affecting tree recruitment are an important driver affecting productivity in diverse subtropical forests but the usual root distribution patterns in line with the spatial root partitioning hypothesis are unrealistic in contexts where soil nutrients are heterogeneously distributed.