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Background and aims: Insight for vegetation restoration in karst mountains could come from understanding the methods by which communities are built along a successional chronosequence. We integrated species diversity, phylogenetic structure, and functional trait diversity to investigate the mechanisms governing community assembly along a successional chronosequence in the northern tropical karst mountains, South China. Methods: Five successional sequences, including grassland, shrubland, young forest, middle-aged forest, and primary forest, were selected in the northern tropical karst mountains using the space-for-time substitution method. The relationships between species diversity and environmental variables (elevation, slope degree and rock exposed rate) were investigated using redundancy analysis and variance partitioning methods. Results and discussion: Our results showed that as succession progressed, species and phylogenetic diversity showed an increasing trend, then a decreasing trend, with the maximum value occurring in young forests, which displayed the optimum curves over the successional gradient. In terms of functional diversity, shrubland, and young forest had significantly higher functional richness than grassland and middle-aged forest, and grassland and young forest had significantly higher quadratic entropy coefficients than primary forest. The vegetative community changed in phylogenetic structure from clustering in the early stages of succession to diverging in the late stages.In contrast, the leaf thickness density decreased as succession progressed, suggesting that the young forest vegetation had the highest resource acquisition ability and utilization rate as the successional progressed. Additionally, the chlorophyll content, leaf thickness, and leaf area showed a first increased then decreased trend as successional progressed. Conclusions: Our results suggest that due to harsher soil conditions, the vegetation in early successional stage was dominated by environmental filtering (i.e., limitation of environmental resources), whereas the vegetation in late successional stage was governed by competitive exclusion (i.e., species competition). Our findings can guide plants conservation and ecological restoration in the tropical karst regions. [ABSTRACT FROM AUTHOR]

Nebkhas play a tremendous role in maintaining the ecological functions of deserts and in slowing land desertification processes. Numerous nebkhas are distributed in the northwestern edge of the Ulan Buh desert, Nitraria tangutorum nebkhas are the most typical dunes in this area. However, our knowledge of the effect of those factors on the succession of nebkhas, including their succession in different sandy land types, is limited. The goal of this study was to reveal their factors influencing the morphological characteristics of N. tangutorum nebkhas in different sandy land types (shifting sandy land, semi-fixed sandy land, and fixed sandy land) in the Ulan Buh desert. Our results showed the densities of N. tangutorum nebkhas were 34, 51, and 77 dunes per ha in the shifting, semi-fixed, and fixed sandy lands, respectively. The density of N. tangutorum nebkhas gradually increased and N. tangutorum nebkhas started to degenerate with the increasing fixation of sandy land. The successional stage had significant effects on all the morphological characteristics, and sandy land types had significant effects on the length (L), height (H), volume (V), horizontal scale (D) and ratio of width to height (H/W). Almost all of the morphological parameters of N. tangutorum nebkhas were correlated at different successional stages, indicating a strong interrelationship among the morphological parameters of nebkhas. The successional stages of N. tangutorum nebkhas had significant effects on the density of branch (M), height (h), coverage (C), deadwood rate (DR), and length of new branches (LB) of the N. tangutorum shrub. The M, h, C, and LB first increased and then decreased, and DR increased with the succession of N. tangutorum nebkhas. These results indicated that we should focus on nebkhas succession to ensure the ecological stability of oases in a dessert. [ABSTRACT FROM AUTHOR]

Fine roots have been hypothesized to be 'leaves underground' in terms of vascular network, but this hypothesis has rarely been tested within the framework of metabolic scaling theory (MST). We measured average fine‐root (diameter < 1 mm) mass (M), surface area (A), volume (V), diameter (D) and length (L) for 216 soil cores from 24 plots across four successional stages in tropical forests of Xishuangbanna (southwest China), and examined eight scaling relationships between these variables at the individual root scale. We tested whether fine‐root allometries conformed to MST's model for leaf (MSTl) or model (MSTw) for woody organs (e.g. trunk). We also assessed the relative effects of environmental factors, tree size, species composition and diversity, and stand structural factors on allometric relationships using structural equation models (SEMs). Our results showed that: 1) fine‐root scaling exponents rarely conformed to MSTl's predictions. 2) The scaling exponents between fine‐root M, A, V and D all conformed to MSTw's predictions in later successional forests, but showed greater deviation towards early successional stage. 3) The scaling exponents associated with fine‐root length differed markedly from MSTw's predictions. 4) Changes of some fine‐root scaling exponents across successional stage were mainly affected by tree size or soil fertility, and species composition affected allometry only indirectly via tree size. Our results suggested that the allometries of individual fine roots largely conform to the scaling rules governing woody organs instead of leaves, probably because leaves are nearly two‐dimensional objects while the other two are three‐dimensional. We showed that MSTw can well predict some fine‐root allometries in later successional forests, suggesting great potential of utilizing MSTw to better estimate fine‐root biomass and productivity. However, the present MSTw still needs to be improved for predicting the scaling relationships concerning fine‐root length, and also for better quantifying allometric exponents in earlier successional forests. [ABSTRACT FROM AUTHOR]

Desertification is a worldwide problem caused by climate change and human activity in dryland regions. In China, approximately 27% of the land surface is desertified. In these desertified ecosystems, aboveground interactions are known to greatly affect ecological succession, but belowground soil microbial communities are relatively unstudied. It is not yet clear how these communities change with different vegetation patch types (e.g., shrub vs. grass) across successional stages in dryland ecosystems. In this study, we assessed the effects of shrub patches (dominated by Artemisia ordosica) and grass patches on soil microbial community diversity, soil environment, and soil substrates across three successional stages (mobile, semi‐mobile, and fixed dunes) in a sand dune ecosystem in the Kubuqi Desert, China. Microbial alpha diversity, as indicated by the Shannon index or operational taxonomic unit (OTU) richness, was not affected by patch type but increased with successional stages. Patch type‐ or successional stage‐induced changes in soil bacterial and fungal alpha diversity were mainly associated with soil organic carbon. Unlike alpha diversity, soil microbial beta diversity was greatly affected by successional stage and patch type. The Mantel test revealed that changes in soil bacterial beta diversity were mainly associated with soil organic carbon, and changes in soil fungal beta diversity were mainly associated with soil organic carbon and soil pH. Microbial OTU richness responded more strongly to the successional stage than the microbial Shannon index, indicating that microbial OTU richness can be used as an indicator of successional status in desert ecosystems. The consistent differences in microbial beta diversity between shrub and grass patches across the three successional stages indicate that shrubs significantly influence soil microbial communities in desert ecosystems. [ABSTRACT FROM AUTHOR]

Soil phosphorus (P) availability often limits plant productivity. Classical theories suggest that total P content declines at the temporal scale of pedogenesis, and ecosystems develop toward the efficient use of scarce P during succession. However, the trajectory of ecosystem P within shorter time scales of succession remains unclear. We analyzed changes to P pools at the early (I), middle (II), and late (III) stages of growth of plantation forests (PFs) and the successional stages of natural forests (NFs) at 1969 sites in China. We found significantly lower P contents at later growth stages compared to earlier ones in the PF (p <.05), but higher contents at late successional stages than in earlier stages in the NF (p <.05). Our results indicate that increasing P demand of natural vegetation during succession, may raise, retain, and accumulate P from deeper soil layers. In contrast, ecosystem P in PF was depleted by the more rapidly increasing demand outpacing the development of a P‐efficient system. We advocate for more studies to illuminate the mechanisms for determining the divergent changes, which would improve forest management and avoid the vast degradation of PF ecosystems suffering from the ongoing depletion of P. [ABSTRACT FROM AUTHOR]

This study investigated seasonal patterns in stoichiometric ratios, nutrient resorption characteristics, and nutrient use strategies of dominant tree species at three successional stages in subtropical China, which have not been fully understood. Fresh leaf and leaf litterfall samples were collected in growing and nongrowing seasons for determining the concentrations of carbon (C), nitrogen (N), and phosphorus (P). Then, stoichiometric ratios (i.e., C:N, C:P, N:P, and C:N:P) and resorption parameters were calculated. Our results found that there was no consistent variation in leaf C:N and C:P ratios among different species. However, leaf N:P ratios in late-successional species became significantly higher, indicating that P limitation increases during successional development. Due to the P limitation in this study area, P resorption efficiency and proficiency were higher than corresponding N resorption parameters. Dominant tree species at early-successional stage adopted 'conservative consumption' nutrient use strategy, whereas the species at late-successional stage inclined to adopt 'resource spending' strategy. [ABSTRACT FROM AUTHOR]

Biological soil crusts (biocrusts) cover up to 60-70% of the soil surface in grasslands rehabilitated since the Grain for Green project was implemented in the hilly Loess Plateau region in 1999, which exerted significant impacts on runoff and soil loss from revegetated grasslands. In the study, field plots were used to investigate runoff and soil loss in sites of a 4- and a 13-year revegetated grassland, with each exhibiting an early and a later successional biocrust, respectively. The objectives of the study were to (1) examine the role of biocrusts on runoff and soil loss during their early and later successional stages in a semiarid region under water erosion, (2) determine the influence of biocrusts on soil antiscourability with different runoff intensities, and (3) isolate the effects of biocrust patches and vascular plant canopies on runoff and soil loss from revegetated grasslands. Treatments used in both sites included (1) retaining biocrusts and plant canopies intact (CP), (2) retaining biocrusts without plant canopies (CNP), (3) retaining plant canopies without biocrusts (PNC), and (4) removing both biocrusts and plant canopies (NCP). The simulated scouring water flux was designed as 7.8, 12.0, and to reflect local rainfall conditions. The results indicated that the runoff yield was increased by biocrust patches in their well-development stage. Runoff was increased by 15.1% when plant canopies were retained and 16.0% when plant canopies were removed in the 13-year revegetated grassland with the scouring water flux. Compared with biocrust patches, plant canopies reduced runoff by 11.3% (with biocrusts) and 8.4% (biocrusts was removed) with the same scouring water flux. No significant difference was found in runoff yield with respect to the four treatments in the 4-year revegetated grassland. In contrast, 92% of the sediments were reduced for the formation of biocrusts in their early successional stage (cyanobacteria-dominated biocrusts) in the 4-year revegetated grassland with respect to CNP compared with NCP at the scouring intensity. No sediment was generated on either CP or CNP treatments in grassland revegetated for 13 years (moss-dominated biocrusts) with the same intensity of simulated runoff. Compared with biocrusts, plant canopies had a limited influence on soil loss. This amounted to reductions of 45 and 10% in soil loss for grasslands that revegetated for 4 and 13 years, respectively. The results of the study suggest that biocrusts play an important role in soil loss control from water erosion in semiarid regions, although there was a potential increase in runoff yield. [ABSTRACT FROM AUTHOR]

Background: Metabolic scaling theory (MST) is still in debate because observed allometric exponents often deviate from MST predictions, and can change significantly depending on environment, phylogeny, and disturbance. We assembled published scaling exponents from literatures for three allometric relationships linked to biomass allocation: leaf biomass-diameter (L-D), stem biomass-diameter (S-D), and root biomass-diameter (R-D). We used data from natural forests and plantations across China to test the following hypotheses: 1) the allometric relationships of trees support the predictions of MST on a broad scale; 2) the observed deviations from MST predictions are caused by climate, biotic factors, and/or phylogeny; 3) abiotic and biotic factors influence allometric relationships in plantations and natural forests differently, and different allometric relationships (i.e. L-D, S-D, and R-D) are affected differently. We related these scaling exponents to geographic climate gradient, successional stage, stand density, leaf form and phenology, and phylogeny. We used mixed-effect models to examine the major factors affecting tree allometries. Results: In natural forests, S-D and R-D scaling exponents were consistent with MST predictions in primary forests, but were significantly lower in secondary forests. Both S-D and R-D scaling exponents in plantations had a medium value that fell between those of the secondary and primary forests, despite plantations being similar in species characteristics and age to secondary forests. The S-D and R-D exponents were significantly affected by factors that are not yet considered in MST, including winter coldness which explained 2.76% – 3.24% of variations, successional stage (7.91% – 8.20% of variations), density (a surrogate for competition, 5.86% – 8.54% of variations), and especially phylogeny (45.86% – 56.64% of variations explained). However, the L-D scaling exponents conformed to MST predictions in primary, secondary, and plantation forests, and was not strongly explained by most factors. Conclusion: MST is only applicable to primary (steady-state) forests, and climate, biotic factors and phylogeny are causes of the observed deviations of allometric relationships from MST predictions. Forest management practices in plantations have a strong influence on tree allometries. L-D allometry is more strongly controlled by biophysical constraints than S-D and R-D allometries, however, the mechanisms behind this difference still need further examinations. [ABSTRACT FROM AUTHOR]

Despite the accumulating evidence of the beneficial effects of diverse mixed species forests on ecosystem functioning and services, foresters in subtropical forest cultivation in China still prefer easily managed monocultures, which is also due to the complexity of mixed forests and the unknown underlying mechanisms related to relationships between biodiversity and forest growth. In a designed pot experiment, we selected two early-successional tree species (Pinus massoniana Lamb., Liquidambar formosana Hance.) and two late-successional tree species (Schima superba Champ., Elaeocarpus decipiens Hemsl.) and planted four saplings in one pot with regard to tree species diversity (monoculture, two species and four species mixtures), each combination replicated four times. In this three-year duration experiment, the effect of tree species diversity, tree identity, and functional traits on sapling growth (tree height, ground diameter, crown projection area), were analyzed. The results showed that the increments of ground diameter and crown projection area increased with tree species richness, whereas the mean tree height increment showed the opposite effect. This growth variation was species specific and related to functional traits (early or late succession), as the increments of the early successional tree species (P. massoniana Lamb. and L. formosana Hance.) had a positive correlation with tree species richness, while the late successional tree species (E. decipiens Hemsl. and S. superba Champ.) showed negative effects. In addition, our study provided evidence for the allometric differences between mixtures and monocultures, which have an important reference value on mixed-species forests. [ABSTRACT FROM AUTHOR]

Background and Aims: Empirical studies and allometric partitioning (AP) theory indicate that plant above-ground biomass (MA) scales, on average, one-to-one (isometrically) with below-ground biomass (MR) at the level of individual trees and at the level of entire forest communities. However, the ability of the AP theory to predict the biomass allocation patterns of understorey plants has not been established because most previous empirical tests have focused on canopy tree species or very large shrubs., Methods: In order to test the AP theory further, 1586 understorey sub-tropical forest plants from 30 sites in south-east China were harvested and examined. The numerical values of the scaling exponents and normalization constants (i.e. slopes and y-intercepts, respectively) of log-log linear MA vs. MR relationships were determined for all individual plants, for each site, across the entire data set, and for data sorted into a total of 19 sub-sets of forest types and successional stages. Similar comparisons of MA/MR were also made., Key Results: The data revealed that the mean MA/MR of understorey plants was 2·44 and 1·57 across all 1586 plants and for all communities, respectively, and MA scaled nearly isometrically with respect to MR, with scaling exponents of 1·01 for all individual plants and 0·99 for all communities. The scaling exponents did not differ significantly among different forest types or successional stages, but the normalization constants did, and were positively correlated with MA/MR and negatively correlated with scaling exponents across all 1586 plants., Conclusions: The results support the AP theory's prediction that MA scales nearly one-to-one with MR (i.e. MA ∝ MR (≈1·0)) and that plant biomass partitioning for individual plants and at the community level share a strikingly similar pattern, at least for the understorey plants examined in this study. Furthermore, variation in environmental conditions appears to affect the numerical values of normalization constants, but not the scaling exponents of the MA vs. MR relationship. This feature of the results suggests that plant size is the primary driver of the MA vs. MR biomass allocation pattern for understorey plants in sub-tropical forests., (© The Author 2015. Published by Oxford University Press on behalf of the Annals of Botany Company. All rights reserved. For Permissions, please email: journals.permissions@oup.com.)