A prominent tree species coexistence mechanism suggests host-specific natural enemies inhibit seedling recruitment at high conspecific density (negative conspecific density dependence). ...Natural-enemy-mediated conspecific density dependence affects numerous tree populations, but its strength varies substantially among species. Understanding how conspecific density dependence varies with species' traits and influences the dynamics of whole communities remains a challenge. Using a three-year manipulative community-scale experiment in a temperate forest, we show that plant-associated fungi, and to a lesser extent insect herbivores, reduce seedling recruitment and survival at high adult conspecific density. Plant-associated fungi are primarily responsible for reducing seedling recruitment near conspecific adults in ectomycorrhizal and shade-tolerant species. Insects, in contrast, primarily inhibit seedling recruitment of shade-intolerant species near conspecific adults. Our results suggest that natural enemies drive conspecific density dependence in this temperate forest and that which natural enemies are responsible depends on the mycorrhizal association and shade tolerance of tree species.
The theory of “top-down” ecological regulation predicts that herbivory suppresses plant abundance, biomass, and survival but increases diversity through the disproportionate consumption of dominant ...species, which inhibits competitive exclusion. To date, these outcomes have been clear in aquatic ecosystems but not on land. We explicate this discrepancy using a meta-analysis of experimental results from 123 native animal exclusions in natural terrestrial ecosystems (623 pairwise comparisons). Consistent with topdown predictions, we found that herbivores significantly reduced plant abundance, biomass, survival, and reproduction (all P < 0.01) and increased species evenness but not richness (P = 0.06 and P = 0.59, respectively). However, when examining patterns in the strength of top-down effects, with few exceptions, we were unable to detect significantly different effect sizes among biomes, based on local site characteristics (climate or productivity) or study characteristics (study duration or exclosure size). The positive effects on diversity were only significant in studies excluding large animals or located in temperate grasslands. The results demonstrate that top-down regulation by herbivores is a pervasive process shaping terrestrial plant communities at the global scale, but its strength is highly site specific and not predicted by basic site conditions. We suggest that including herbivore densities as a covariate in future exclosure studies will facilitate the discovery of unresolved macroecology trends in the strength of herbivore–plant interactions.
The elevational pattern of soil microbial diversity along mountain slopes has received considerable interest over the last decade. An increasing amount of taxonomic data on soil microbial community ...composition along elevation gradients have been collected, however the trophic patterns and environmental drivers of elevational changes remain largely unclear. Here, we examined the distribution patterns of major soil bacterial and fungal taxa along the northern slope of Changbai Mountain, Northeast China, at five typical vegetation types located between 740 and 2,691 m above sea level. Elevational patterns of the relative abundance of specific microbial taxa could be partially explained by the oligotrophic-copiotrophic theory. Specifically, two dark-coniferous forests, located at mid-elevation sites, were considered to be oligotrophic habitats, with relatively higher soil C/N ratio and Formula: see text-N concentrations. As expected, oligotrophic microbial taxa, belonging to the bacterial phyla Acidobacteria and Gemmatimonadetes, and fungal phylum Basidiomycota, were predominant in the two dark-coniferous forests, exhibiting a mid-elevation maximum pattern. In contrast, the broad leaf-Korean pine mixed forest located at the foot of the mountain,
-dominated forest located below the tree line, and alpine tundra at the highest elevation were considered more copiotrophic habitats, characterized by higher substrate-induced-respiration rates and Formula: see text-N concentrations. Microbial taxa considered to be so called copiotrophic members, such as bacterial phyla Proteobacteria and Actinobacteria, and fungal phylum Ascomycota, were relatively abundant in these locations, resulting in a mid-elevation minimum pattern. At finer taxonomic levels, the two most abundant proteobacterial classes, alpha- and beta-Proteobacteria, along with Acidobacteria Gp1, 2, 3, 15, and the Basidiomycotal class of Tremellomycetes were classified with the copiotrophic group. Gamma- and delta-Proteobacteria, Acidobacteria Gp4, 6, 7, 16, and Basidiomycotal class of Agaricomycetes were classified as oligotrophic taxa. This work uses the oligotrophic-copiotrophic theory to explain the elevational distribution pattern of the relative abundance of specific microbial taxa, confirming some of the existing trophic classifications of microbial taxa and expanding on the theory to include a broader range of taxonomic levels.
Forests play a key role in regulating the global carbon cycle, and yet the abiotic and biotic conditions that drive the demographic processes that underpin forest carbon dynamics remain poorly ...understood in natural ecosystems. To address this knowledge gap, we used repeat forest inventory data from 92,285 trees across four large permanent plots (4–25 ha in size) in temperate mixed forests in northeast China to ask the following questions: (1) How do soil conditions and stand age drive biomass demographic processes? (2) How do vegetation quality (i.e., functional trait diversity and composition) and quantity (i.e., initial biomass stocks) influence biomass demographic processes independently from soil conditions and stand age? (3) What is the relative contribution of growth, recruitment, and mortality to net biomass change? Using structural equation modeling, we showed that all three demographic processes were jointly constrained by multiple abiotic and biotic factors and that mortality was the strongest determinant on net biomass change over time. Growth and mortality, as well as functional trait diversity and the community-weighted mean of specific leaf area (CWMSLA), declined with stand age. By contrast, high soil phosphorous concentrations were associated with greater functional diversity and faster dynamics (i.e., high growth and mortality rates), but associated with lower CWMSLA and initial biomass stock. More functionally diverse communities also had higher recruitment rates, but did not exhibit faster growth and mortality. Instead, initial biomass stocks and CWMSLA were stronger predictors of biomass growth and mortality, respectively. By integrating the full spectrum of abiotic and biotic drivers of forest biomass dynamics, our study provides critical system-level insights needed to predict the possible consequences of regional changes in forest diversity, composition, structure and function in the context of global change.
Tree–fungal symbioses are increasingly recognized to affect soil nitrogen (N) transformations, yet the role of free-living soil microbes in the process is largely unclear. Soil microbes directly ...interact with trees and are a primary driver of many N transformation processes. Here, we explored the linkage among tree mycorrhizal associations, free-living soil microbes and N transformation rates in a temperate forest of Northeast China. Across a gradient of increasing ectomycorrhizal (ECM) tree dominance, we measured soil acid–base chemistry, bacterial and fungal abundances, N-hydrolyzing enzyme activities, abundances and community composition of ammonia-oxidizing archaea (AOA) and bacteria, and net N mineralization and net nitrification rates. Results showed that soil pH, exchangeable base cations, inorganic N concentrations and N transformation rates decreased with increasing ECM tree dominance. The ECM tree dominance was negatively related to soil bacterial and AOA
amoA
gene abundances, and positively to soil fungal abundances and β-
N
-acetylglucosaminidase activities. These shifts in soil microbial abundances and enzyme activities along the mycorrhizal gradient were linked with the increase in soil acidity with increasing ECM tree dominance. Piecewise structural equation models revealed that ECM tree dominance was not directly related to N transformation rates, but indirectly to net N mineralization rates by affecting bacterial and fungal abundances, and indirectly to net nitrification rates by influencing AOA
amoA
gene abundances. Collectively, our results indicate that soil microbes provide a mechanistic link between mycorrhizal associations and soil N transformations, and suggest that shifts in forest mycorrhizal associations under global change could have profound consequences for biogeochemical cycling of temperate forests.
Soil fertility influences plant community structure, yet few studies have focused on how this influence is affected by the type of mycorrhizal association formed by tree species within local ...communities.
We examined the relationship of aboveground biomass (AGB) and diversity of adult trees with soil fertility (nitrogen, phosphorus, organic matter, etc.) in the context of different spatial distributions of arbuscular mycorrhizal (AM) and ectomycorrhizal (EM) trees in a temperate forest in Northeast China.
Diversity showed a positive trend along the soil fertility gradient driven mostly by a positive relationship between AM tree abundance and soil fertility. By contrast, the AGB showed a negative trend along the soil fertility gradient driven mostly by a negative relationship between EM tree AGB and soil fertility. Furthermore, the opposite trend in the AGB and tree species diversity along the soil fertility gradient led to an overall negative diversity–biomass relationship at the 50-m scale but not the 20-m scale.
These results suggest that tree mycorrhizal associations play a critical role in driving forest community structure along soil fertility gradients and highlight the importance of tree mycorrhizal associations in influencing how the diversity–ecosystem function (e.g. biomass) relationships change with soil fertility.
Biodiversity plays a fundamental role in provisioning and regulating forest ecosystem functions and services. Above‐ground (plants) and below‐ground (soil microbes) biodiversity could have ...asynchronous change paces to human‐driven land‐use impacts. Yet, we know very little how they affect the provision of multiple forest functions related to carbon accumulation, water retention capacity and nutrient cycling simultaneously (i.e. ecosystem multifunctionality; EMF). We used a dataset of 22,000 temperate forest trees from 260 plots within 11 permanent forest sites in Northeastern China, which are recovering from three post‐logging disturbances. We assessed the direct and mediating effects of multiple attributes of plant biodiversity (taxonomic, phylogenetic, functional and stand structure) and soil biodiversity (bacteria and fungi) on EMF under the three disturbance levels. We found the highest EMF in highly disturbed rather than undisturbed mature forests. Plant taxonomic, phylogenetic, functional and stand structural diversity had both positive and negative effects on EMF, depending on how the EMF index was quantified, whereas soil microbial diversity exhibited a consistent positive impact. Biodiversity indices explained on average 45% (26%–58%) of the variation in EMF, whereas climate and disturbance together explained on average 7% (0.4%–15%). Our result highlighted that the tremendous effect of biodiversity on EMF, largely overpassing those of both climate and disturbance. While above‐ (β = 0.02–0.19) and below‐ground (β = 0.16–0.26) biodiversity had direct positive effects on EMF, their opposite mediating effects (β = −0.22 vs. β = 0.35 respectively) played as divergent pathways to human disturbance impacts on EMF. Our study sheds light on the need for integrative frameworks simultaneously considering above‐ and below‐ground attributes to grasp the global picture of biodiversity effects on ecosystem functioning and services. Suitable management interventions could maintain both plant and soil microbial biodiversity, and thus guarantee a long‐term functioning and provisioning of ecosystem services in an increasing disturbance frequency world.
Higher EMF was found in disturbed forests rather than relatively undisturbed mature forests. Above‐and below‐ground biodiversity had direct positive effects on EMF, their opposite mediating effects played as divergent pathways to human disturbance impacts on EMF.
Biodiversity can be measured by taxonomic, phylogenetic, and functional diversity. How ecosystem functioning depends on these measures of diversity can vary from site to site and depends on ...successional stage. Here, we measured taxonomic, phylogenetic, and functional diversity, and examined their relationship with biomass in two successional stages of the broad-leaved Korean pine forest in northeastern China. Functional diversity was calculated from six plant traits, and aboveground biomass (AGB) and coarse woody productivity (CWP) were estimated using data from three forest censuses (10 years) in two large fully mapped forest plots (25 and 5 ha). 11 of the 12 regressions between biomass variables (AGB and CWP) and indices of diversity showed significant positive relationships, especially those with phylogenetic diversity. The mean tree diversity-biomass regressions increased from 0.11 in secondary forest to 0.31 in old-growth forest, implying a stronger biodiversity effect in more mature forest. Multi-model selection results showed that models including species richness, phylogenetic diversity, and single functional traits explained more variation in forest biomass than other candidate models. The models with a single functional trait, i. e., leaf area in secondary forest and wood density in mature forest, provided better explanations for forest biomass than models that combined all six functional traits. This finding may reflect different strategies in growth and resource acquisition in secondary and oldgrowth forests.
Forests play an important role in regulating the global carbon cycle. Yet, how abiotic (i.e. soil nutrients) and biotic (i.e. tree diversity, stand structure and initial biomass) factors ...simultaneously contribute to aboveground biomass (coarse woody) productivity, and how the relative importance of these factors changes over succession remain poorly studied. Coarse woody productivity (CWP) was estimated as the annual aboveground biomass gain of stems using 10-year census data in old growth and secondary forests (25-ha and 4.8-ha, respectively) in northeast China. Boosted regression tree (BRT) model was used to evaluate the relative contribution of multiple metrics of tree diversity (taxonomic, functional and phylogenetic diversity and trait composition as well as stand structure attributes), stand initial biomass and soil nutrients on productivity in the studied forests. Our results showed that community-weighted mean of leaf phosphorus content, initial stand biomass and soil nutrients were the three most important individual predictors for CWP in secondary forest. Instead, initial stand biomass, rather than diversity and functional trait composition (vegetation quality) was the most parsimonious predictor of CWP in old growth forest. By comparing the results from secondary and old growth forest, the summed relative contribution of trait composition and soil nutrients on productivity decreased as those of diversity indices and initial biomass increased, suggesting the stronger effect of diversity and vegetation quantity over time. Vegetation quantity, rather than diversity and soil nutrients, is the main driver of forest productivity in temperate mixed forest. Our results imply that diversity effect for productivity in natural forests may not be so important as often suggested, at least not during the later stage of forest succession. This finding suggests that as a change of the importance of different divers of productivity, the environmentally driven filtering decreases and competitively driven niche differentiation increases with forest succession.
Relative importance of different predictors on coarse woody productivity (CWP) in secondary (PBF plot) and old growth forest (BKF plot) using the boosted regression tree analysis Pie charts show the summed relative influences of biodiversity indices, trait composition, soil nutrients, stand structure, initial stand biomass and first two principal component axes (PC) of soil nutrients variables. Display omitted
•Vegetation quantity outperforms vegetation quality among biotic factors for driving productivity.•Productivity markedly increased with functional composition in secondary forest.•In old growth forest, productivity greatly increased with initial stand biomass.•Soil nutrients enhanced productivity in secondary forest but not in old growth forest.•The effects of vegetation quantity and quality on productivity increased over succession.
Interactions between plants and soil microbial communities underpin soil processes and forest ecosystem function, but the links between tree diversity and soil microbial diversity are poorly ...characterized. Differences in both the taxonomic and functional diversity of trees and microbes can shape soil nutrient status and carbon storage, but the stoichiometry of carbon and nutrients in the soil also influences resource availability to plant and microbial communities. Given the key role of resource availability in plant–soil interactions, we hypothesized that relationships between tree diversity metrics and soil bacterial or fungal diversity are mediated by soil stoichiometry. To test our hypothesis, we measured tree diversity metrics (tree species richness, functional trait diversity and functional trait composition) and soil stoichiometry in a temperate forest in China, and we determined soil microbial diversity by Illumina sequencing. We used structural equation models to assess the relationships between tree diversity metrics and soil bacterial or fungal diversity and to evaluate the influence of soil stoichiometry. Overall, microbial diversity was strongly related to soil stoichiometry, whereby fungal diversity was associated with high soil N/P ratios, whereas bacterial diversity was related to high soil C/P ratios. Soil bacterial and fungal diversity were more closely related to tree functional trait diversity and composition than to tree species richness, and the links between tree and soil microbial diversity were mediated by soil stoichiometry. The strong links between tree functional traits, soil stoichiometry and soil bacteria or fungi suggest that resource quality plays a key role in plant–microbial interactions. Our results highlight the importance of nutrient stoichiometry in linkages between tree functional diversity and soil microbial diversity.
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