Subtropical Forests In Southern China Research Articles

Role of the Dominant Species on the Distributions of Neighbor Species in a Subtropical Forest

Understanding the role of dominant species in structuring the distribution of neighbor species is an important part of understanding community assembly, a central goal of ecology. Phylogenetic information helps resolve the multitude of processes driving community assembly and the importance of evolution in the assembly process. In this study, we classified species in a 20-ha subtropical forest in southern China into groups with different degrees of phylogenetic relatedness to the dominant species Castanopsis chinensis. Species surrounding individuals of C. chinensis were sampled in an equal area annulus at six spatial scales, counting the percent of relatives and comparing this to permutation tests of a null model and variance among species groups. The results demonstrated that dominant species affected their relatives depending on community successional stage. Theory would predict that competitive exclusion and density-dependence mechanisms should lead to neighbors that are more distant in phylogeny from C. chinensis. However, in mature forests distant relatives were subjected to competitive repulsion by C. chinensis, while environment filtering led to fewer distant species, regardless of scale. A variety of biological and non-biological factors appear to result in a U-shaped quantitative distribution determined by the dominant species C. chinensis. Scale effects also influenced the dominant species. As a dominant species, C. chinensis played an important role in structuring the species distributions and coexistence of neighbor species in a subtropical forest.

Native Bamboo Invasions into Subtropical Forests Alter Microbial Communities in Litter and Soil

Both exotic and native plant invasions can have profound impacts on ecosystems. While many studies have examined the effects of exotic plant invasions on soil properties, relatively few have tested the effects of native plant invasions on soil microbial communities. Furthermore, we know little about the effects of native plant invasions on microbial communities in litter. In subtropical forests in southern China, we sampled litter at three decomposition stages and top soil in three forest sands representing three stages of the invasion (not invaded, moderately and heavily invaded) by the Moso bamboo (Phyllostachys edulis (Carriere) J. Houzeau), a native species in China. We measured chemical properties (concentrations of C, N, P, Mg, Al, K, Ca, Mn, Cu, and Zn, and concentrations of cellulose and lignin) and microbial communities in litter and/or soil. The bamboo invasion, in general, decreased the element concentrations in litter and soil and also decreased total microbial abundance and diversity. Considering bacteria and fungi separately, the bamboo invasion decreased fungal diversity in litter and soil, but had little impact on bacterial diversity, suggesting that fungi are more sensitive and vulnerable to the bamboo invasion than bacteria. We conclude that native Moso bamboo invasions into subtropical forests may lead to a complex biogeochemical process in the litter–soil system, which may threaten local forest ecosystems by affecting microbial communities and, thus, litter decomposition and nutrient cycling.

Subtropical Forests Act as Mercury Sinks but as Net Sources of Gaseous Elemental Mercury in South China.

Comprehensive mercury (Hg) budgets were constructed in two typical subtropical forests in southern China in 2014 to quantify Hg (gaseous elemental Hg, Hg0, and reactive Hg, HgII) input and output fluxes and Hg retention in forests, consequently exploring the roles of subtropical forests in the global Hg cycle. At site Qianyanzhou, representing a background region with an enhanced atmospheric Hg0 concentration, the total HgII deposition (67.7 μg·m-2·year-1, 73% as dry HgII deposition) was found to be slightly higher than the Hg0 emission above the canopy (58.5 μg·m-2·year-1), indicating that the forest is a minor Hg sink but a significant net Hg0 source on a yearly basis. In contrast, the forest in the moderately polluted region (site Huitong) acted as a significant Hg sink but a minor net Hg0 source with a higher HgII deposition (73.7 μg·m-2·year-1) and relatively negligible Hg0 emission (2.65 μg·m-2·year-1). The decreasing atmospheric Hg0 concentrations declined the total Hg sink based on the Hg budgets synthesized of this and previous studies and may promote forest Hg0 emissions. Consequently, it was expected that the re-emission of historically deposited Hg may be enhanced from subtropical forests by recent decreases in atmospheric Hg0 concentrations throughout China.

Functional Composition Changes of a Subtropical Monsoon Evergreen Broad-Leaved Forest Under Environmental Change

Long-term studies have revealed that forest species composition was shifting under environment change and disturbance induced by loss of large trees. Yet, few studies explicitly analyzed their impacts on composition concurrently. To learn more about impacts of environment change and disturbance on driving forest community, we investigated shifts in functional composition over past 24 years in an old-growth subtropical forest in southern China. We analyzed nine traits that are mainly related to leaf nutrients, photosynthetic capacity, hydraulic conductivity, and drought tolerance of plants and examined hypotheses: (1) The functional composition change over time was directional instead of random fluctuation, (2) drought-tolerant species increased their abundance under soil dryness, (3) both environmental change and disturbance related to changes of functional composition significantly, and (4) initial trait values of quadrats strongly influenced their subsequent change rates in quadrat level (10 × 10 m). We found that species composition had shifted to favor species with high leaf nutrient content, high photosynthesis rate, high hydraulic conductivity, low water-use efficiency, and high drought tolerance traits, which was due to soil dryness and disturbance. These two factors explained 47–58% of quadrats’ trait value changes together. Considering rapidly increasing stem density, this pattern may indicate ecological processes of which disturbance provided numerous recruits of resource-acquisition strategy species and soil dryness conducted a selecting effect on shaping composition in the forest. Additionally, quadrats with initial trait values at the far end of change direction shifted faster in three traits, which also indicated that functional composition changes in quadrats were directional and homogenized. Our results implied that environment change and accompanied disturbance events possibly drove species composition change along a different trajectory in the subtropical forest that experienced high climatic variability.

Disentangling the Contributions of Plant Taxonomic and Functional Diversities in Shaping Aboveground Biomass of a Restored Forest Landscape in Southern China.

Restoration is essential for supporting key ecosystem functions such as aboveground biomass production. However, the relative importance of functional versus taxonomic diversity in predicting aboveground biomass during restoration is poorly studied. Here, we used a trait-based approach to test for the importance of multiple plant diversity attributes in regulating aboveground biomass in a 30-years-old restored subtropical forest in southern China. We show that both taxonomic and functional diversities are significant and positive regulators of aboveground biomass; however, functional diversity (FD) was more important than taxonomic diversity (species richness) in controlling aboveground biomass. FD had the strongest direct effect on aboveground biomass compared with species richness, soil nutrients, and community weighted mean (CWM) traits. Our results further indicate that leaf and root morphological traits and traits related to the nutrient content in plant tissues represent the existence of a leaf and root economic spectrum, and the acquisitive resource use strategy influenced aboveground biomass. Our results suggest that both taxonomic and FD play a role in shaping aboveground biomass, but FD is more important in supporting aboveground biomass in this type of environments. These results imply that enhancing FD is important to restoring and managing degraded forest landscapes.

Species Differences in Nitrogen Acquisition in Humid Subtropical Forest Inferred From 15N Natural Abundance and Its Response to Tracer Addition

Differences in nitrogen (N) acquisition patterns between plant species are often reflected in the natural 15N isotope ratios (δ15N) of the plant tissues, however, such differences are poorly understood for co-occurring plants in tropical and subtropical forests. To evaluate species variation in N acquisition traits, we measured leaf N concentration (%N) and δ15N in tree and understory plant species under ambient N deposition (control) and after a decade of N addition at 50 kg N ha−1 yr−1 (N-plots) in an old-growth subtropical forest in southern China. We also measured changes in leaf δ15N after one-year of 15N addition in both the control and N-plots. The results show consistent significant species variation in leaf %N in both control and N-plots, but decadal N addition did not significantly affect leaf %N. Leaf δ15N values were also significantly different among the plant species both in tree and understory layers, and both in control and N-plots, suggesting differences in N acquisition strategies such as variation in N sources and dominant forms of N uptake and dependence on mycorrhizal associations among the co-occurring plant species. Significant differences between the plant species (in both control and N-plots) in changes in leaf δ15N after 15N addition were observed only in the understory plants, indicating difference in access (or use) of deposited N among the plants. Decadal N addition had species-dependent effects on leaf δ15N, suggesting the N acquisition patterns of these plant species are differently affected by N deposition. These results suggest that co-occurring plants in N-rich and subtropical forests vary in their N acquisition traits; these differences need to be accounted for when evaluating the impact of N deposition on N cycling in these ecosystems.

Seasonal Variations in N2O Emissions in a Subtropical Forest With Exogenous Nitrogen Enrichment are Predominately Influenced by the Abundances of Soil Nitrifiers and Denitrifiers

AbstractElevated nitrogen (N) deposition has induced substantial impacts on the emissions of nitrous oxide (N2O) from forest ecosystems, but how soil microbes regulate the production/consumption of N2O under elevated N deposition remains poorly understood, particularly in high N deposition subtropical forests that are characterized by distinct wet‐dry seasonality. We established a field N addition experiment in a subtropical forest in southern China to explore the influences of low, medium and high (35, 70, and 105 kg N ha‐1 yr‐1, respectively) N addition on N2O efflux and its associated microbial functional genes [amoA for nitrifiers (ammonia‐oxidizing bacteria (AOB) and ammonia‐oxidizing archaea (AOA)) and nirK and nosZ for denitrifiers]. The results showed the following: (1) The N2O emissions were stimulated by N addition in the dry season but were depressed in the wet season. (2) The nirK and nosZ abundances were generally stimulated by N addition, whereas the AOB‐amoA and AOA‐amoA abundances showed divergent responses to N addition. (3) Based on the results of principal component and Pearson correlation analyses, N2O effluxes were associated with microbial biomass in the wet season but with nirK and nosZ abundances in the dry season. Structural equation modeling analyses further indicated that both nitrifiers and denitrifiers under N addition contributed to the generation of N2O in the dry season, whereas the decreased production of N2O in the wet season was primarily caused by denitrifiers. Therefore, seasonally specific strategies should be developed to mitigate the emissions of N2O from subtropical forests with distinct seasonal precipitation patterns.

Fine root dynamics responses to nitrogen addition depend on root order, soil layer, and experimental duration in a subtropical forest

Elevated atmospheric N deposition has been well documented to enhance fine root production in N-limited temperate forests, but how fine roots respond to N deposition in N-rich tropical and subtropical forests remains poorly understood. The sequential coring and minirhizotron methods were applied to quantify fine root biomass, production, and turnover of a N-rich but P-limited subtropical forest in southern China and to assess the responses of these root variables to a gradient of N additions (control (0), low-N (35), medium-N (70), and high-N (105 kg N ha−1 year−1)) during the first 3 years of experimentation. The high- and medium-N additions significantly reduced fine root diameter by about 30% but increased the specific root length by 20–105%, i.e., fine roots became thinner and longer under the experimental N addition. Both low- and medium-N additions generally stimulated fine root production (10–88%) and turnover (3–40%), whereas high-N suppressed them by 32–70% and 8–54%, respectively, varying with sampling season and estimation method. The stimulatory effects were presumably ascribed to the increased fine root growth for P acquisition, the suppressive effect, to the deleterious damage to the root health and micronutrient availability. Overall, the N effects were more pronounced in the surface (0–10 cm) than in the deeper (10–40 cm) soil layers and for the first-order than the higher-order fine roots. Our results indicate that lower-order absorptive fine roots are responsive to elevated N deposition, and complex responses could emerge due to the interactive influences of the N deposition rate, seasonality, and soil depth.

Coordination of leaf, stem and root traits in determining seedling mortality in a subtropical forest

Plant traits from different organs are thought to be coordinated to achieve main vital functions. However, evidence on how the coordination of traits affect plant vital rates (e.g. mortality rates) is rare due to the poor representation of root traits, which play important roles in water and soil nutrients uptake. In this study, we collected plant traits from 13,733 seedlings of 57 species across 10-year monitoring in a subtropical forest in Southern China, asking whether traits from root and aboveground organs are coordinated, and whether they have consistent effects on seedling mortality (e.g. all fast resource-acquisitive traits reduce mortality). We performed phylogenetic principal component analysis (PPCA) to test trait coordination and used generalized linear mixed models (GLMMs) to examine trait-mortality relationships. We found that some of the root and aboveground traits were highly correlated. PPCA of traits separated species to the strategy of resource acquisition or conservation, supporting the plant economics spectrum. Traits from root, stem and leaf showed coordinated effects on seeding mortality, in which species with conservative traits tended to have lower mortality rates than species with acquisitive traits. Root traits, such as root nitrogen content, tissue density and specific root length significantly related to seedling mortality. We concluded that traits from different organs were coordinated describing an acquisitive-conservative continuum of strategies and have consistent effects on seedling mortality, providing the first evidence for the plant economics spectrum and for the root trait-mortality relationships in subtropical seedling communities. Our results emphasized that besides aboveground traits, key root traits significantly impact seedling mortality. Integrating root traits is necessary to gain further understanding in the relationships between plant performance and traits.

Small-scale and multi-species approaches for assessing litter decomposition and soil dynamics in high-diversity forests.

Premise of the StudyThe relationship between tree species abundance and diversity and soil chemistry has been studied in several ecosystems and at different spatial scales. However, species‐specific assessments have mainly been conducted in temperate ecosystems and in monospecific settings, calling for studies from diverse, mixed forests from different ecosystems.MethodsIn a subtropical forest in southern China, under four dominant tree canopy species (Lithocarpus chintungensis, Castanopsis wattii, Schima noronhae, and Manglietia insignis), we assessed species’ effect on inter‐ and intraspecific percentages of litter mass loss, and the effect of species on soil nutrients and soil microbial biomass.ResultsOur results show significant differences in litter decomposition for all four species; however, the percentage of litter mass loss was stable under different species. Microbial biomass and soil nutrients presented strong differences under different tree species. Species‐specific differences in soil characteristics were seen for carbon‐nitrogen‐phosphorus relationships. Surprisingly, the correlations between carbon and phosphorus and between nitrogen and phosphorus showed opposite slopes in soils collected under different tree species.DiscussionOur results provide insights into the importance of tree species identity in providing variety to ecosystem processes occurring on the forest floor. We recommend this methodological approach—combining analysis of litter decomposition, soil nutrient concentrations, and microbial biomass—when dealing with species‐rich forests.

Future forest dynamics under climate change, land use change, and harvest in subtropical forests in Southern China

Subtropical forests have and will continue to face tremendous pressure from various disturbances, which have the potential to alter forest composition, structure, and function. Forest dynamics relate to spatial patterns, ecological processes, and their interactions. However, integrating forest ecosystems and land systems has seldom been attempted in southern China. We explore the spatiotemporal response and trajectories of forest dynamics at different scales under climate change, harvesting, and land-use disturbances in the near future. We simulated forest landscape dynamics by integrating a forest landscape model (LANDIS-II), an ecosystem model (PnET-II), and a land change model (CA-Markov) for 2010 to 2050. We identified changes in forest composition, aboveground biomass, and landscape patterns under individual and integrated scenarios, including a control scenario, climate change, harvesting, and land-use change for tree species, ecoregions, and forest types. For forest composition, the forest area continued to increase, and coniferous forests increased approximately 3.7 times that of broad-leaved forests. Harvesting reduced aboveground biomass, with a reduction of 30.3% in comparison to the control scenario. The integrated disturbances showed a greater impact on the forest landscape. Landscape fragmentation increased, showing that the patch density increased by 52.3% (control scenario), 46.2% (climate change), 118.4% (harvest), 55.0% (land use change) and 139.5% (integrated scenarios), respectively. Our results suggest that climate change will contribute to forest growth, especially for coniferous forests. Harvesting will reduce forest area and aboveground biomass. The interaction between human activities and climate change contributes to diminished forest expansion and increased landscape fragmentation.

Will heterotrophic soil respiration be more sensitive to warming than autotrophic respiration in subtropical forests?

Understanding the responses of heterotrophic (Rh) and autotrophic (Ra) components of soil respiration (Rs) to warming is important in evaluating and modelling the effects of changes in climate on soil carbon (C) cycling in terrestrial ecosystems. We used a mesocosm system with buried heating cables (5°C warming) to investigate the responses of Rs, Rh and Ra to warming in a subtropical forest in southern China. Soil CO2 effluxes were measured with a portable automatic soil CO2 flux system from March 2014 to July 2015. We found that warming increased mean Rs and Rh from 788 to 1036 g C m−2 year−1 (+31%) and from 512 to 707 g C m−2 year−1 (+38%), respectively. There was no difference in Ra between the warming treatment and the control. The lack of response of Ra to warming was probably because the fine root biomass did not change with warming treatment. Soil warming also increased available dissolved organic carbon, microbial biomass carbon, actinomycetal biomass and arbuscular mycorrhizal biomass. Our results suggest that Rh might be more sensitive to climate warming than Ra, and future climate warming could increase soil C loss from increased Rh in subtropical forest ecosystems.Highlights A field warming experiment with partitioning of soil respiration in a humid subtropical forest. Warming increased Rs and Rh without significantly altering soil microbial substrate availability. Heterotrophic respiration appeared more sensitive to warming than autotrophic respiration. Warming increased Actinomycetes bacteria and Arbuscular mycorrhizal fungi.

Testing potassium limitation on soil microbial activity in a sub-tropical forest

Because potassium (K) is a rock-derived essential element that can be depleted in highly-weathered tropical soils, K availability may limit some portion of soil microbial activity in tropical forest ecosystems. In this paper we tested if K limits microbial activity in the condition of sufficient labile C supply. An incubation experiment was performed using surface soil samples (0–10 cm depth) obtained from four permanent ecological research plots in a natural sub-tropical forest in southern China. Soil samples were taken in September 2016. Heterotrophic soil respiration rates and microbial biomass were measured after the addition of glucose (both D and L) with and without K (potassium chloride). We did not observe any effects of K addition on soil microbial respiration, suggesting that K does not limit the microbial activity in the condition of sufficient labile C supply. The lack of microbial response to added K can be attributed to the high mobility of K in forest ecosystems, which may have provided sufficient K to microbes in our soil samples (already provided at the beginning of the incubation). However, at the present stage, we cannot conclude that K is not a limiting factor of soil microbial activity in other tropical forest ecosystems because of the heterogeneity of tropical forest ecosystems and few observations. The hypothesis needs to be tested in larger numbers of tropical forests.

Nitrogen and phosphorus stoichiometry of Schima superba under nitrogen deposition

In this study, leaf nitrogen (N) and phosphorus (P) stoichiometry were used as indicators of nitrogen saturation and to assess ecosystem nutrient limitations. Schima superba, a representative and widely distributed dominant evergreen broadleaf tree species of the subtropical forests in southern China, was used for this purpose. A nutrient-addition experiment and a field survey were conducted to test the responses of trees from different provenances to N deposition. The relationships between leaf N and P stoichiometry and biomass, nutrient limitation, and soil N:P were analyzed. There was a relationship between leaf N, P, N:P, soil N:P and plant dry biomass. A threshold leaf N:P ratio (16.3) divided the five provenances into different nutrient-limitation classes that were related to the soil N:P ratio or N deposition. The leaf N:P ratio provided an indication of P limitation. A higher soil P level reduced the N deposition effect on plant growth. The leaf N:P ratio of individuals from different provenances can be used as a predictor of nutrient limitation, and this was related to the soil N:P ratio.

Impacts of a High Nitrogen Load on Foliar Nutrient Status, N Metabolism, and Photosynthetic Capacity in a Cupressus lusitanica Mill. Plantation

At present, anthropogenic nitrogen deposition has dramatically increased worldwide and has shown negative impacts on temperate/boreal forest ecosystems. However, it remains unclear how an elevated N load affects plant growth in the relatively N-rich subtropical forests of Southern China. To address this question, a study was conducted in a six-year-old Cupressus lusitanica Mill. plantation at the Scientific Research and Teaching Base of Nanjing Forestry University, with N addition levels of N0 (0 kg ha−1 year−1), N1 (24 kg ha−1 year−1), N2 (48 kg ha−1 year−1), N3 (72 kg ha−1 year−1), N4 (96 kg ha−1 year−1), and N5 (120 kg ha−1 year−1). Leaf physiological traits associated with foliar nutrient status, photosynthetic capacity, pigment, and N metabolites were measured. The results showed that (1) N addition led to significant effects on foliar N, but had no marked effects on K concentration. Furthermore, remarkable increases of leaf physiological traits including foliar P, Ca, Mg, and Mn concentration; photosynthetic capacity; pigment; and N metabolites were always observed under low and middle-N supply. (2) High N supply notably decreased foliar P, Ca, and Mg concentration, but increased foliar Mn content. Regarding the chlorophyll, photosynthetic capacity, and N metabolites, marked declines were also observed under high N inputs. (3) Redundancy analysis showed that the net photosynthesis rate was positively correlated with foliar N, P, Ca, Mg, and Mn concentration; the Mn/Mg ratio; and concentrations of chlorophyll and N metabolites, while the net photosynthesis rate was negatively correlated with foliar K concentration and N/P ratios. These findings suggest that excess N inputs can promote nutrient imbalances and inhibit the photosynthetic capacity of Cupressus lusitanica Mill., indicating that high N deposition could threaten plant growth in tropical forests in the future. Meanwhile, further study is merited to track the effects of high N deposition on the relationship between foliar Mn accumulation and photosynthesis in Cupressus lusitanica Mill.

Composition and seasonal dynamics of seed rain in Chinese fir (Cunninghamia lanceolata) plantation.

Chinese fir plantation is an important part of the subtropical forests in southern China. It has a sustainable natural regeneration ability, which is the foundation of determining community succession direction and maintaining their large area. The main objective of this study was to investigate whether the seed pool was the main restricting factor for the natural regeneration of Chinese fir plantation. Mixed broad leaf-conifer forest and pure plantation of Chinese fir were selected to study the species composition, quantity and seasonal dynamics of all species and dominant species. The results showed that seeds from 21 species belonged to 13 families and 18 genera were collected in the mixed forest, while seeds from 19 species belonged to 12 families and 16 genera were collected from pure forest. Seed rain intensities of all species were 3797 and 3300 seeds·m-2 in mixed forest and pure plantation, respectively. The number of seeds from tree species was absolutely dominant in seed rain (mixed forest 89.1%, pure plantation 86.2%). The number of Chinese fir seeds was the largest, the intact seeds intensities were 825 and 345 seeds·m-2, respectively. The proportion of all types of seeds in both stands followed the order: the intact seeds > empty or rotten seeds > feeding seeds. The seed rain of both stands had significant seasonal dynamics, both reaching the peak in autumn. The seed rain mainly was intact seeds at the peak of seed-falling. Both mixed forest and pure plantation of Chinese fir had plenty of seeds. The results indicated that the seed rain is not the main factor that restricts natural regeneration in Chinese fir plantations.

Effects of nitrogen and phosphorus addition on leaf nutrient characteristics in a subtropical forest

P deposition can alleviate P limitation in subtropical forests and P deposited in conjunction with high quantities of N does not enhance P limitation. Increasing nitrogen (N) and phosphorus (P) deposition could influence plant growth and survival to varying extents. N and P deposition is essential for subtropical forests where plant growth is limited by lower soil P availability in highly weathered soils. However, whether N and P deposition can increase leaf P concentration and alleviate P limitation is unclear. We investigated changes in N and P concentrations, N:P ratios, and resorption for six dominant species (two tree species and four understory species), following 2 years of N and P additions in a subtropical forest in southern China. P addition either alone or together with the addition of N increased green leaf P concentrations (except in Schima superba) and senesced leaf P and decreased N:P ratios (except in Pinus massoniana), but had no influence on P resorption. N addition had no apparent influence on leaf N concentrations, N:P ratios, or N resorption in all species. The considerable influence of P addition can be explained by rising soil P availability. Our results suggest that subtropical forests are P limited and that increasing P deposition can moderately alleviate the limitation of P. Furthermore, P deposited in conjunction with high quantities of N does not enhance P limitation, as it is insensitive to elevated N input.

Different responses of asymbiotic nitrogen fixation to nitrogen addition between disturbed and rehabilitated subtropical forests

Asymbiotic nitrogen (N) fixation is an important source of new N in ecosystems, and is sensitive to atmospheric N deposition. However, there is limited understanding of asymbiotic N fixation and its response to N deposition in the context of forest rehabilitation. In this study, we measured N fixation rates (acetylene reduction) in different ecosystem compartments (i.e. soil, forest floor, moss Syrrhopodon armatus, and canopy leaves) in a disturbed and a rehabilitated subtropical forest in southern China, under 12years of N treatments: control, low N addition (50kgNha−1yr−1), and medium N addition (100kgNha−1yr−1). The rehabilitated forest had higher nutrient (e.g. N) availability than the disturbed forest. In control plots, N fixation rates in forest floor were higher in the rehabilitated forest than in the disturbed forest, but N fixation rates in other compartments (soil, S. armatus, and canopy leaves) were comparable between the forests. Nitrogen addition significantly suppressed N fixation in soil, forest floor, S. armatus, and canopy leaves in the disturbed forest, but had no significant effect on those compartments in the rehabilitated forest. The main reasons for the negative effects of N addition on N fixation in the disturbed forest were NH4+ inhibition (soil), the P and C limitation (forest floor), and the reduced N dependence on canopy N-fixers (S. armatus and canopy leaves). We conclude that asymbiotic N fixation does not decline with increasing N availability after rehabilitation in the study forests. The inhibitory effects of N addition on asymbiotic N fixation occurred in the disturbed forest but not in the rehabilitated forest, indicating that forest rehabilitation may change the response of ecosystem function (i.e. N fixation) to N deposition, which merits further study in other tropical and subtropical regions.

Dominant Trees in a Subtropical Forest Respond to Drought Mainly via Adjusting Tissue Soluble Sugar and Proline Content.

It is well-known that drought has considerable effects on plant traits from leaf to ecosystem scales; however, little is known about the relative contributions of various traits within or between tree species in determining the plant’s sensitivity or the tolerance to drought under field conditions. We conducted a field throughfall exclusion experiment to simulate short-term drought (∼67% throughfall exclusion during the dry season from October to March) and prolonged drought (∼67% throughfall exclusion prolonging the dry season from October to May) and to understand the effects of drought on two dominant tree species (Michelia macclurei and Schima superba) in subtropical forests of southern China. The morphological, physiological, and nutritional responses of the two species to the two types of drought were determined. There were significantly different morphological (leaf max length, max width, leaf mass per area), physiological (leaf proline) and nutritional (P, S, N, K, Ca, Mg) responses by M. macclurei and S. superba to prolonged drought. Comparison between the drought treatments for each species indicated that the trees responded species–specifically to the short-term and prolonged drought, with S. superba exhibiting larger plasticity and higher adaption than M. macclurei. M. macclurei responded more sensitively to prolonged drought in terms of morphology, proline content, and nutritional traits and to short-term drought with regard to soluble sugars content. The differential species-specific responses to drought will allow us to estimate the changes in dominant trees in subtropical forests of China that have experienced a decade’s worth of annual seasonal drought.

Species-specific and general allometric equations for estimating tree biomass components of subtropical forests in southern China

Chinese subtropical forests contain a diversity of tree species and exhibit a high carbon (C) sequestration capacity, but biomass and C stock assessments in subtropical secondary forests remain uncertain because of a limited availability of allometric equations and an uncertain applicability of existing allometric equations that have not been tested for these forests. We developed allometric equations for important coniferous (Pinus massoniana), deciduous broadleaved (Alniphyllum fortunei, Choerospondias axillaris, Liquidambar formosana) and evergreen broadleaved (Cyclobalanopsis glauca, Litsea rotundifolia, Schima superba) species. A total of 70 trees (10 trees for each species) with diameters at breast height (D) ranging from 2.6 to 50.9 cm were destructively harvested and dissected into tree components (stem, branch, leaf and coarse root). Species-specific equations using D alone as the predictor variable fitted the data well (p 0.72) for each tree component. Including height (H) in the form of D2H only improved the regression fit for A. fortunei and L. rotundifolia. The relationships of branch, leaf and root biomass against D varied among tree species. General equations for functional groups and all species combined showed comparable bias for stem, aboveground and total tree biomass to species-specific equations. We recommend the general equations of multiple species to estimate forest biomass at regional scales and also to estimate stem, aboveground and total tree biomass for each species when species-specific allometric equations are not available at a given site. For branch, leaf and root biomass, species-specific equations are preferred, even though this requires biomass data for additional tree species.