Xugao Wang

Rate of tree carbon accumulation increases continuously with tree size

Forests are major components of the global carbon cycle, providing substantial feedback to atmospheric greenhouse gas concentrations. Our ability to understand and predict changes in the forest carbon cycle—particularly net primary productivity and carbon storage—increasingly relies on models that represent biological processes across several scales of biological organization, from tree leaves to forest stands. Yet, despite advances in our understanding of productivity at the scales of leaves and stands, no consensus exists about the nature of productivity at the scale of the individual tree, in part because we lack a broad empirical assessment of whether rates of absolute tree mass growth (and thus carbon accumulation) decrease, remain constant, or increase as trees increase in size and age. Here we present a global analysis of 403 tropical and temperate tree species, showing that for most species mass growth rate increases continuously with tree size. Thus, large, old trees do not act simply as senescent carbon reservoirs but actively fix large amounts of carbon compared to smaller trees; at the extreme, a single big tree can add the same amount of carbon to the forest within a year as is contained in an entire mid-sized tree. The apparent paradoxes of individual tree growth increasing with tree size despite declining leaf-level and stand-level productivity can be explained, respectively, by increases in a tree’s total leaf area that outpace declines in productivity per unit of leaf area and, among other factors, age-related reductions in population density. Our results resolve conflicting assumptions about the nature of tree growth, inform efforts to undertand and model forest carbon dynamics, and have additional implications for theories of resource allocation and plant senescence.

Testing the independent species’ arrangement assertion made by theories of stochastic geometry of biodiversity

The assertion that the spatial location of different species is independent of each other is fundamental in major ecological theories such as neutral theory that describes a stochastic geometry of biodiversity. However, this assertion has rarely been tested. Here we use techniques of spatial point pattern analysis to conduct a comprehensive test of the independence assertion by analysing data from three large forest plots with different species richness: a species-rich tropical forest at Barro Colorado Island (Panama), a tropical forest in Sinharaja (Sri Lanka), and a temperate forest in Changbaishan (China). We hypothesize that stochastic dilution effects owing to increasing species richness overpower signals of species associations, thereby yielding approximate species independence. Indeed, the proportion of species pairs showing: (i) no significant interspecific association increased with species richness, (ii) segregation decreased with species richness, and (iii) small-scale interspecific interaction decreased with species richness. This suggests that independence may indeed be a good approximation in the limit of very species-rich communities. Our findings are a step towards a better understanding of factors governing species-rich communities and we propose a hypothesis to explain why species placement in species-rich communities approximates independence.

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.

Distribution of Soil Organic Carbon Fractions Along the Altitudinal Gradient in Changbai Mountain, China

Understanding the responses of soil organic carbon (SOC) fractions to altitudinal gradient variation is important for understanding changes in the carbon balance of forest ecosystems. In our study the SOC and its fractions of readily oxidizable carbon (ROC), water-soluble carbon (WSC) and microbial biomass carbon (MBC) in the soil organic and mineral horizons were investigated for four typical forest types, including mixed coniferous broad-leaved forest (MCB), dark coniferous spruce-fir forest (DCSF), dark coniferous spruce forest (DCS), and Ermans birch forest (EB), along an altitudinal gradient in the Changbai Mountain Nature Reserve in Northeast China. The results showed that there was no obvious altitudinal pattern in the SOC. Similar variation trends of SOC with altitude were observed between the organic and mineral horizons. Significant differences in the contents of SOC, WSC, MBC and ROC were found among the four forest types and between horizons. The contents of ROC in the mineral horizon, WSC in the organic horizon and MBC in both horizons in the MCB and EB forests were significantly greater than those in either DCSF or DCS forest. The proportion of soil WSC to SOC was the lowest among the three main fractions. The contents of WSC, MBC and ROC were significantly correlated (P < 0.05) with SOC content. It can be concluded that vegetation types and climate were crucial factors in regulating the distribution of soil organic carbon fractions in Changbai Mountain.

Density dependence on tree survival in an old-growth temperate forest in northeastern China

Abstract• Density dependence is a major mechanism for shaping plant communities. However, its role in regulating diverse, mixed natural tree communities is less certain.• In this study we investigated density-dependent effects in a large-scale (25 ha) old-growth temperate forest in northeastern China. Spatial patterns of neighborhood distribution in the plot were analyzed using various methods for inferring competition, including (1) pair correlation function to determine spatial patterns of pre-mortality and post-mortality and (2) neighborhood analysis of individuals to examine the extent to which tree survival is correlated with other covariates.• Results showed that, for common species, 3 of 5 canopy species and 3 of 8 midstory and understory species were random in mortality. Negative density-dependent mortality was not found when trees reach 1 cm in DBH. There was no significant correlation for canopy species between tree survival and conspecific abundance, but largely positive correlations for midstory and understory species. In contrast, tree survival was found to negatively correlate with conspecific basal area for most species, indicating strong intraspecific competition. No strong interspecific density dependence was found in the forest.Résumé• La dépendance par rapport à la densité est un important mécanisme pour la formation des communautés végétales. Toutefois, son rôle dans la régulation de diverses communautés mélangées d’arbre est moins certain.• Dans cette étude, nous avons enquêté sur les effets densité-dépendance à une grande échelle (25 ha) dans une ancienne forêt tempérée, dans le nord de la Chine. Les modes de distribution spatiale de voisinage ont été analysés en utilisant diverses méthodes pour estimer la concurrence, y compris : (1) des paires de fonction de corrélation afin de déterminer les structures spatiales de pré et postmortalité et (2) l’analyse du voisinage des individus pour examiner dans quelle mesure la survie de l’arbre survivant est corrélée avec d’autres variables.• Les résultats ont montré que, pour les espèces communes, 3 des 5 espèces de la canopée et 3 des 8 de l’étage moyen et du sous-étage, la mortalité était aléatoire. Aucune mortalité densité-dépendante négative n’a été détectée lorsque les arbres atteignent 1 cm de diamètre à hauteur d’homme. Il n’y avait pas de corrélation significative pour les espèces de la canopée entre la survie des arbres et l’abondance conspécifique, mais il y avait des corrélations positives pour l’étage moyen et le sous-étage.• En revanche, la survie des arbres était corrélée négativement avec la surface terrière conspécifique pour la plupart des espèces, indiquant une forte concurrence intraspécifique. Aucune dépendance visible de la densité interspécifique n’a été détectée dans la forêt.

Soil bacterial communities of different natural forest types in Northeast China

Background and aimsThe types of natural forests have long been suggested to shape below-ground microbial communities in forest ecosystem. However, detailed information on the impressionable bacterial groups and the potential mechanisms of these influences are still missing. The present study aims to deepen the current understanding on the soil microbial communities under four typical forest types in Northeast Asia, and to reveal the environmental factors driving the abundance, diversity and composition of soil bacterial communities.MethodsFour forest types from Changbai Nature Reserve, representing mixed conifer-broadleaf forest and its natural secondary forest, evergreen coniferous forest, and deciduous coniferous forest were selected for this study. Namely, Broadleaf-Korean pine mixed forest (BLKP), secondary Poplar-Birch forest (PB), Spruce-Fir forest (SF), and Larch forest (LA), respectively. Soil bacterial community was analyzed using bar-coded pyrosequencing. Nonmetric multidimensional scaling (NMDS) was used to illustrate the clustering of different samples based on both Bray-Curtis distances and UniFrac distances. The relationship between environmental variables and the overall community structure was analyzed using the Mantel test.ResultsThe two mixed conifer-broadleaf forests (BLKP and PB) displayed higher total soil nutrients (organic carbon, nitrogen, and phosphorus) and soil pH, but a lower C/N ratio as compared to the two coniferous forests (SF and LA). The mixed conifer-broadleaf forests had higher alpha-diversity and had distinct bacterial communities from the coniferous forests. Soil texture and pH were found as the principle factors for shaping soil bacterial diversity and community composition. The two mixed conifer-broadleaf forests were associated with higher proportion of Acidobacteria, Verrucomicrobia, Bacteroidetes, and Chloroflexi. While the SF and LA forests were dominated by Proteobacteria and Gemmatimonadetes.ConclusionsDifferent natural forest type each selects for distinct microbial communities beneath them, with mixed conifer-broadleaf forests being associated with the low-activity bacterial groups, and the coniferous forests being dominated by the so-called high-activity members. The differentiation of soil bacterial communities in natural forests are presumably mediated by the differentiation in terms of soil properties, and could be partially explained by the copiotroph/oligotroph ecological classification model and non-random co-occurrence patterns.

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.

Spatial variation of species diversity across scales in an old-growth temperate forest of China

Species richness and abundance are the two most important diversity variables. Species abundance is additive when aggregated across spatial scale, whereas species richness is non-additive. This study analyzes the effect of spatial scale and site on species abundance and richness in a 25-ha temperate forest plot in the Changbai Mountains, northeastern China. The result shows that species abundance and richness are not only dependent on spatial scales, but also dependent on site. Species abundance responds linearly to changes of spatial scale with no intersection in different sites of the study area. However, although species richness also increases with the increase of spatial scale, there are some intersections for the different sites, suggesting that a species-rich site does not always have a high value if the spatial scale is changed. In all, with respect to additive variables, it is relatively easy to extrapolate them from one spatial scale to another spatial scale, as they and the spatial scale usually form a linear relationship. In contrast, non-additive variables are difficult to extrapolate across spatial scales, because they often respond nonlinearly to spatial scale changes. In order to extrapolate these non-additive variables across spatial scales, it is necessary to estimate the relationships between them and spatial scales. As a result, extrapolation of information among spatial scales may be possible, but very difficult, especially for non-additive variables. Because the 25-ha Changbai plot is very small compared to the extent of the world temperate forests, and the vegetation is a relatively uniform type, more such studies in other ecosystems are needed before theories and generalization about scaling effects can be formulated.

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.

Aboveground-belowground biodiversity linkages differ in early and late successional temperate forests

Understanding ecological linkages between above- and below-ground biota is critical for deepening our knowledge on the maintenance and stability of ecosystem processes. Nevertheless, direct comparisons of plant-microbe diversity at the community level remain scarce due to the knowledge gap between microbial ecology and plant ecology. We compared the α- and β- diversities of plant and soil bacterial communities in two temperate forests that represented early and late successional stages. We documented different patterns of aboveground-belowground diversity relationships in these forests. We observed no linkage between plant and bacterial α-diversity in the early successional forest, and even a negative correlation in the late successional forest, indicating that high bacterial α-diversity is not always linked to high plant α-diversity. Beta-diversity coupling was only found at the late successional stage, while in the early successional forest, the bacterial β-diversity was closely correlated with soil property distances. Additionally, we showed that the dominant competitive tree species in the late successional forest may play key roles in driving forest succession by shaping the soil bacterial community in the early successional stage. This study sheds new light on the potential aboveground-belowground linkage in natural ecosystems, which may help us understand the mechanisms that drive ecosystem succession.