Zuoqiang Yuan

Scale‐dependent relationships between tree species richness and ecosystem function in forests

1. The relationship between species richness and ecosystem function, as measured by productivity or biomass, is of long-standing theoretical and practical interest in ecology. This is especially true for forests, which represent a majority of global biomass, productivity and biodiversity.

Local-Scale Drivers of Tree Survival in a Temperate Forest

Tree survival plays a central role in forest ecosystems. Although many factors such as tree size, abiotic and biotic neighborhoods have been proposed as being important in explaining patterns of tree survival, their contributions are still subject to debate. We used generalized linear mixed models to examine the relative importance of tree size, local abiotic conditions and the density and identity of neighbors on tree survival in an old-growth temperate forest in northeastern China at three levels (community, guild and species). Tree size and both abiotic and biotic neighborhood variables influenced tree survival under current forest conditions, but their relative importance varied dramatically within and among the community, guild and species levels. Of the variables tested, tree size was typically the most important predictor of tree survival, followed by biotic and then abiotic variables. The effect of tree size on survival varied from strongly positive for small trees (1–20 cm dbh) and medium trees (20–40 cm dbh), to slightly negative for large trees (>40 cm dbh). Among the biotic factors, we found strong evidence for negative density and frequency dependence in this temperate forest, as indicated by negative effects of both total basal area of neighbors and the frequency of conspecific neighbors. Among the abiotic factors tested, soil nutrients tended to be more important in affecting tree survival than topographic variables. Abiotic factors generally influenced survival for species with relatively high abundance, for individuals in smaller size classes and for shade-tolerant species. Our study demonstrates that the relative importance of variables driving patterns of tree survival differs greatly among size classes, species guilds and abundance classes in temperate forest, which can further understanding of forest dynamics and offer important insights into forest management.

Mechanisms underlying local functional and phylogenetic beta diversity in two temperate forests

Although trait information has been widely used to explore underlying mechanisms of forest community structure, most studies have focused on local patterns of phylogenetic or functional alpha diversity. Investigations of functional beta diversity, on the other hand, have not been conducted at local scales in a spatially explicit way. In this study, we provide a powerful methodology based on recent advances in spatial point pattern analysis using fully mapped data of large and small trees in two large temperate forest plots. This approach allowed us to assess the relative importance of different ecological processes and mechanisms for explaining patterns of local phylogenetic and functional beta diversity. For both forests and size classes, we found a clear hierarchy of scales: habitat filtering accounted for patterns of phylogenetic and functional beta diversity at larger distances (150-250 m), dispersal limitation accounted for the observed decline in beta diversity at distances below 150 m, and species interactions explained small departures from functional and phylogenetic beta diversity at the immediate plant-neighborhood scale (below 20 m). Thus, both habitat filtering and dispersal limitation influenced the observed patterns in phylogenetic and functional beta diversity at local scales. This result contrasts with a previous study from the same forests, where dispersal limitation alone approximated the observed species beta diversity for distances up to 250 m. In addition, species interactions were relatively unimportant for predicting phylogenetic and functional beta diversity. Our analysis suggests that phylogenetic and functional beta diversity can provide insights into the mechanisms of local community assembly that are missed by studies focusing exclusively on species beta diversity.

The contribution of understory light availability and biotic neighborhood to seedling survival in secondary versus old-growth temperate forest

Seedling survival plays an important role in the maintenance of species diversity and forest dynamics. Although substantial gains have been made in understanding the factors driving patterns of seedling survival in forests, few studies have considered the simultaneous contribution of understory light availability and the local biotic neighborhood to seedling survival in temperate forests at different successional stages. Here, we used generalized linear mixed models to assess the relative importance of understory light availability and biotic neighborhood variables on seedling survival in secondary and old-growth temperate forest in north eastern China at two levels (community and guild). At the community level, biotic neighborhood effects on seedling survival were more important than understory light availability in both forests. In both the old-growth and secondary forests, conspecific basal area had a negative effect on seedling survival, consistent with negative conspecific density dependence. At guild levels, the relative importance of light and biotic neighborhood on seedling survival showed considerable variation among guilds in both forests. Available understory light tended to have positive effects on seedling survival for shrub and light-demanding species in the old-growth forest, but negative effects on survival of shade-tolerant seedlings in the secondary forest. For tree species and shade-tolerant species, the best fit models included neighborhood variables, but that was not the case for shrubs, light-demanding, or mid shade-tolerant species. Overall, our results demonstrate that the relative importance of understory light availability and biotic factors on seedling survival vary with species life-history strategy and forest successional stage.

Abiotic and biotic determinants of coarse woody productivity in temperate mixed 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.

Variation and synchrony of tree species mast seeding in an old-growth temperate forest

Questions Mast seeding, synchronous highly variable seed production among years, occurs in many perennial plant species and across diverse plant communities. Two predominant ultimate hypotheses for mast seeding are pollination efficiency and predator satiation, with weather conditions as a proximate cause. Little consensus has been achieved regarding the relative impacts of ultimate selection and proximate weather on variation in seed production. Moreover, mast-seeding studies often focus on a single species or phylogenetically closely related species, while studies on plant communities consisting of tree species with a diversity of reproductive strategies are less common. Location The 25-ha Changbaishan temperate forest dynamic plot, Northeast China. Methods We used eight-years of seed rain data to characterize the patterns and evaluate underlying selective drivers of mast seeding. We employed generalized linear mixed-effects models to analyze the relationships between interannual variability of mast seeding and weather conditions. Results All 20 species in this forest community exhibited high variability in annual seed production, but the magnitude of seed production among species was generally asynchronous across years. Wind-pollinated species had higher interannual variation of seed production than animal-pollinated species, while species dispersed by seed predators and abiotic modes (e.g., wind and gravity) showed little variation. Species responded individually to weather conditions for both temperature and precipitation, and spring phenology at the same year as seed production had a larger effect among species than both the weather conditions of the same summer and season-long lags. Conclusions Our findings suggest that pollination efficiency hypothesis hada much stronger effect than predation satiation hypothesis on mast seeding, and weather conditions showed the proximate role of weather drivers in producing the community-wide mast seeding pattern. We emphasize the necessity to simultaneously assess drivers of mast seeding for multiple species within a plant community. This article is protected by copyright. All rights reserved.

The effect of tree size, neighborhood competition and environment on tree growth in an old-growth temperate forest

Aims Understanding the controls influencing tree growth is central to forest ecology. Although many factors such as tree size, neighborhood competition and environmental variables, have been proposed as being important in explaining patterns of tree growth, but their relative contributions are still subject to debate. We aimed to examine the relative importance of tree size, local abiotic conditions and the density and identity of neighbors on tree growth in an old-growth temperate forest in northeast china. Methods We used linear mixed models with data from a 25 ha (500 × 500 m) broad-leaved Korean pine (Pinus koraiensis) mixed forest permanent plot to examine the relative importance of these local drivers on tree growth at three organizational levels (community, guild and species). Subplot was included as a random effect to account for spatial autocorrelation in growth of trees located within the same subplot, and species was included as a random effect to account for variation among species in growth. Important Findings Tree size was typically the most important predictor of growth, followed by neighborhood competition and then soil nutrients. The correlation of tree size to growth varied from strongly positive for large trees and medium trees to slightly positive for small trees. The effect of neighborhood competition on growth showed weak negative density dependence as indicated by slightly negative effect of total basal area of neighbors and the proportion of conspecific neighbors. Environmental factors influenced growth of very common species, smaller tree-size classes and shade-tolerant species. We concluded that the relative importance of variables driving patterns of tree growth varied greatly among tree size classes, shade tolerance and abundance classes in this temperate forest. These results provide critical information for future studies of forest dynamics and offer insight into forest management in this region.

Dynamics of Two Multi-Stemmed Understory Shrubs in Two Temperate Forests

A multi-stemmed growth form may be an important trait enabling the persistence of individual shrubs in the forest understory. With the aim of evaluating the role of multiple stems, neighbor competition and soil nutrients in shrub performance, we study the dynamics of two temperate multi-stemmed shrub species. We modeled stem growth and survival of Corylus mandshurica and Acer barbinerve in two temperate forests with differing structure in northeastern China. One forest was an old growth broad-leaved Korean pine (Pinus koraiensis) mixed forest; the other was a secondary poplar-birch forest. Growth of the two species and survival of C. mandshurica increased with stem number in the old growth forest, but not the secondary forest, suggesting the benefits of a multi-stemmed growth form are facultative. C. mandshurica also suffered more from overstory neighbor competition in the old growth forest, which may suggest that this species is less shade-tolerant than A. barbinerve. Moreover, the performance of the two species were clearly influenced by understory neighbors and soil variables in the old growth forest relative to the secondary forest, which may be due to different forest structure. We conclude that multiple stems are not always important for the persistence of shrub species. Even within the same species, the multi-stemmed benefits might be facultative, differing among forests and neighborhood compositions.

Reproductive traits and their correlation among woody plants in a broadleaf-Korean pine (Pinus koraiensis) mixed forest in Northeast China

We examined the sexual reproduction of woody plants in a 25-hm2 plot of broadleaf–Korean pine(Pinus koraiensis) mixed forest in the Changbai Mountains, Northeast China. Reproductive traits differed among species, and sexual systems were associated with pollination syndromes, fruit types, seed mass and life forms. Among the species present in the plot, 49% were hermaphroditic, 27.5% were monoecious and 23.5% were dioecious. The percentage of monoecious species in the forest studied was higher than that of tropical forests. The flowering phenology among species was almost synchronous, whereas fruiting phenology varied greatly among species. Seed weight differed dramatically among different sexual systems and between life forms. Monoecious species produced the heaviest seeds, whereas hermaphroditic species produced the lightest seeds. Seeds of shrubs were heavier those of trees. In general, complex correlations were observed among reproductive traits and ecological characteristics in the broadleaf–Korean pine mixed forest, which was characterized by a generally entomophilous pollination syndrome among the common species and a high proportion of monoecious species.

Global signal of top-down control of terrestrial plant communities by herbivores

Significance A focus in ecology is understanding the processes that govern ecosystem productivity and biodiversity. A multitude of co-occurring biological mechanisms shape these properties in plant communities, but the relative importance of specific processes remains ambiguous, such as competition among individuals and species for resources (bottom-up regulation) and the role of herbivory in controlling plant populations (top-down regulation). In this global synthesis of herbivore impacts on terrestrial plants, we find strong evidence that herbivores regulate most plant communities, but their positive effects on diversity may be contingent on a subset of animals and specific habitats. We conclude that the strength of top-down regulation in terrestrial ecosystems appears more variable and context-dependent than in aquatic systems. 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 top-down 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.