Gaoming Xiong

Effects of size, neighbors, and site condition on tree growth in a subtropical evergreen and deciduous broad‐leaved mixed forest, China

Abstract Successful growth of a tree is the result of combined effects of biotic and abiotic factors. It is important to understand how biotic and abiotic factors affect changes in forest structure and dynamics under environmental fluctuations. In this study, we explored the effects of initial size [diameter at breast height (DBH)], neighborhood competition, and site condition on tree growth, based on a 3‐year monitoring of tree growth rate in a permanent plot (120 × 80 m) of montane Fagus engleriana–Cyclobalanopsis multiervis mixed forest on Mt. Shennongjia, China. We measured DBH increments every 6 months from October 2011 to October 2014 by field‐made dendrometers and calculated the mean annual growth rate over the 3 years for each individual tree. We also measured and calculated twelve soil properties and five topographic variables for 384 grids of 5 × 5 m. We defined two distance‐dependent neighborhood competition indices with and without considerations of phylogenetic relatedness between trees and tested for significant differences in growth rates among functional groups. On average, trees in this mixed montane forest grew 0.07 cm year−1 in DBH. Deciduous, canopy, and early‐successional species grew faster than evergreen, small‐statured, and late‐successional species, respectively. Growth rates increased with initial DBH, but were not significantly related to neighborhood competition and site condition for overall trees. Phylogenetic relatedness between trees did not influence the neighborhood competition. Different factors were found to influence tree growth rates of different functional groups: Initial DBH was the dominant factor for all tree groups; neighborhood competition within 5 m radius decreased growth rates of evergreen trees; and site condition tended to be more related to growth rates of fast‐growing trees (deciduous, canopy, pioneer, and early‐successional species) than the slow‐growing trees (evergreen, understory, and late‐successional species).

Patterns of plant carbon, nitrogen, and phosphorus concentration in relation to productivity in China’s terrestrial ecosystems

Significance Estimates of nutrient allocation in different plant tissues and the relationships between the nutrient contents and photosynthetic capacity are critical to predicting ecosystem carbon sequestration under global change. Here, we provide an assessment of large-scale patterns of community-level nitrogen and phosphorus concentrations in different plant tissues and then examine how nutrient allocations are coupled with plant productivity. The results show that nutrient concentrations in leaves are less responsive to abiotic environments than those in woody stems and roots (stable leaf nutrient concentration hypothesis); the relationships between vegetation primary productivity and leaf nutrient contents are stronger when less nutrients are allocated to the woody tissues (productivity–nutrient allocation hypothesis) and are stronger in deciduous than in evergreen vegetation (productivity–leaf lifespan hypothesis). Plant nitrogen (N) and phosphorus (P) content regulate productivity and carbon (C) sequestration in terrestrial ecosystems. Estimates of the allocation of N and P content in plant tissues and the relationship between nutrient content and photosynthetic capacity are critical to predicting future ecosystem C sequestration under global change. In this study, by investigating the nutrient concentrations of plant leaves, stems, and roots across China’s terrestrial biomes, we document large-scale patterns of community-level concentrations of C, N, and P. We also examine the possible correlation between nutrient content and plant production as indicated by vegetation gross primary productivity (GPP). The nationally averaged community concentrations of C, N, and P were 436.8, 14.14, and 1.11 mg·g−1 for leaves; 448.3, 3.04 and 0.31 mg·g−1 for stems; and 418.2, 4.85, and 0.47 mg·g−1 for roots, respectively. The nationally averaged leaf N and P productivity was 249.5 g C GPP·g-1 N·y−1 and 3,157.9 g C GPP·g–1 P·y−1, respectively. The N and P concentrations in stems and roots were generally more sensitive to the abiotic environment than those in leaves. There were strong power-law relationships between N (or P) content in different tissues for all biomes, which were closely coupled with vegetation GPP. These findings not only provide key parameters to develop empirical models to scale the responses of plants to global change from a single tissue to the whole community but also offer large-scale evidence of biome-dependent regulation of C sequestration by nutrients.

The Janzen-Connell effect on the population dynamics of a Fagus engleriana- Cyclobalanopsis oxyodon community in a subtropical zone of China

The Janzen-Connell (J-C) hypothesis provides a mechanism explaining the high species diversity in tropical rainforests. It postulates that predation could cause greater mortality on seeds and seedlings near their parental trees. In this study, we tested the hypothesis in a subtropical zone, a mixed evergreen-deciduous broad-leaved forest dominated by the Fagus engleriana and Cyclobalanopsis oxyodon. The study area was in the Shennongjia region, a key area of biodiversity conservation in both China and the world. The recruitment probability index was used to detect the J-C effect on nine species of the community, which were more than 50 individuals. Six large adults of each species were selected, and the numbers of saplings and adults were counted at the distance intervals of 0–5, 5–10, 10–15, 15–20, and 20–25 m from each focal tree. Two species in saplings stage and six in adult stage supported the J-C hypothesis, but their χ2 was not significant. Three species, the F. engleriana, Rhododendron hypoglaucum, and Toona sinensis, showed a strong Hubbell pattern in the adult stage. Because of these results, we reject the J-C hypothesis and conclude that species could recruit near the conspecific trees in subtropical forest. The reasons why the J-C hypothesis fails to explain the species diversity in this community are the shortage of seed-consuming agents of subtropical forest and the influence of microsite topographic variation.

Different composition and distribution patterns of mineral‐protected versus hydrolyzable lipids in shrubland soils

Mineral protection is known as an important mechanism stabilizing soil organic carbon (SOC). However, the composition, sources and variations of mineral-protected SOC remain poorly constrained. To fill this knowledge gap, we used hydrofluoric acid to demineralize soil matrix and compared the sources and distribution of mineral-protected lipids (ML) versus hydrolysable lipids (HL) of four typical Chinese shrubland soils. ML was found to represent a sizable fraction (9–32%) of total aliphatic lipids (including n-alkanols, n-alkanoic acids, α,ω-alkanedioic acids, hydroxyalkanoic acids and mid-chain-substituted acids) in all soils. Based on carbon chain length and branch positions, microbial- and plant-derived lipids were distinguished. No significant difference was found in the ratio of microbial- to plant-derived lipids in ML versus HL, implying that plant and microbial inputs are equally important for the mineral-associated soil lipids. However, ML contained a higher proportion of non-specific lipids, especially at depths. Furthermore, to evaluate key environmental variable(s) controlling the distribution of different lipid components, a multiple stepwise regression analysis was conducted. Notably, ML was mainly affected by SOC-to-nitrogen ratio instead of mineralogical properties, implying that the accrual of mineral-associated soil lipids relies strongly on organic matter properties. Collectively, our findings provide novel insights on sources and accumulation mechanisms of mineral-protected soil lipids. SOC decomposition and subsequent accretion of degradation products appear to be vital for the sequestration of mineral-associated soil lipids and warrant better recognition in the investigations of stable soil carbon accumulation mechanisms.

Nitrogen and phosphorus concentrations and allocation strategies among shrub organs: the effects of plant growth forms and nitrogen-fixation types

AimsWe aimed to explore the influences of plant functional groups on nutrient concentrations and allocation strategies among shrub organs, as well as to examine the effects of climate, soil and species on nutrient concentrations in shrubs of different plant functional groups.MethodsWe investigated the nitrogen (N) and phosphorus (P) concentrations in roots, stems and leaves and their influencing factors of 187 shrub species in the shrublands across southern China, and we also examined the relationships between N and P among various organs using scaling analysis.ResultsThe scaling relationships of N and P tended to be allometric between leaf and non-leaf organs, while they tended to be isometric among non-leaf organs. Plant functional groups affected nutrient allocation among shrub organs, where a higher proportion of nutrients were present in the stems and roots of evergreen shrubs and non-legume shrubs when compared to deciduous shrubs and legume shrubs as nutrients within a plant increased. Among organs, N and P concentrations were higher in leaves than in stems and roots. Among functional groups, evergreen shrubs and legume shrubs were more P-limited than deciduous shrubs and non-legume shrubs, respectively. The N and P concentrations in evergreen shrubs were lower and more sensitive to environmental change than in deciduous shrubs. Both N and P contents in legume shrubs were higher and more homeostatic than those of non-legume shrubs.ConclusionsPlant growth forms and N-fixation types exerted strong effects on nutrient concentrations and allocations among shrub organs. The influences of climate and soil on shrub N and P concentrations differed by plant functional groups.

C: N: P stoichiometry of Ericaceae species in shrubland biomes across Southern China: influences of climate, soil and species identity

Aims Carbon (C), nitrogen (N) and phosphorus (P) stoichiometry strongly affect functions and nutrient cycling within ecosystems. However, the related researches in shrubs were very limited. In this study, we aimed to investigate leaf stoichiometry and its driving factors in shrubs, and whether stoichiometry significantly differs among closely related species. Methods We analyzed leaf C, N and P concentrations and their ratios in 32 species of Ericaceae from 161 sites across southern China. We examined the relationships of leaf stoichiometry with environmental variables using linear regressions, and quantified the interactive and independent effects of climate, soil and species on foliar stoichiometry using general linear models (GLM). Important Findings The foliar C, N and P contents of Ericaceae were 484.66, 14.44 and 1.06 mg g(-1), respectively. Leaf C, N and P concentrations and their ratios in Ericaceae were significantly related with latitude and altitude, except the N:P insignificantly correlated with latitude. Climate (mean annual temperature and precipitation) and soil properties (soil C, N and P and bulk density) were significantly influenced element stoichiometry. The GLM analysis showed that soil exerted a greater direct effect on leaf stoichiometry than climate did, and climate affected leaf traits mainly via indirect ways. Further, soil properties had stronger influences on leaf P than on leaf C and N. Among all independent factors examined, we found species accounted for the largest proportion of the variation in foliar stoichiometry. These results suggest that species can largely influence foliar stoichiometry, even at a lower taxonomic level.

Structures and topographical pattern of the tree layer of Fagus engleriana-Cyclobalanopsis oxyodon community in Shennongjia area, Hubei Province, China

Shennongjia represents an area of considerable plant biodiversity, not only for China but for the whole world. The numerous species are distributed along an altitude gradient. The genus Fagus, commonly known as Beeches, constitutes one of the dominant woody species of the humid temperate forests in China. This paper deals with the community structure of a mixed, broadleaved deciduous-evergreen beech forest dominated by the Fagus engleriana and Cyclobalanopsis oxyodon, which occurs along an altitude gradient range in this area. The community consists of 46 woody species, belonging to 22 families and 27 genera. The tree layer can be divided into three sub-strata. The upper layer is composed of deciduous trees, 80.7% of which is F. engleriana. The second and third layers are dominated by the evergreen species, such as C. oxyodon and the Rhododendron hypoglaucum. These species increase from 55.9% in the second layer to 80.5% in the third, and regeneration at that rate allows us to assume that the stability of the community may be guaranteed. The micro-site heterogeneity plays an important role in maintaining species diversity in plant communities. In this research, a terrain surface of 0.96 hm2 was simulated using the digital elevation model (DEM) in order to analyze the influence of the topography on plant community structures on a small scale. The slope relief aspect obtained with the DEM showed an accentuated heterogeneity. Semivariance analysis was used to measure the heterogeneity scale. The range and nugget variance of the semivariograms for slope were 285.8 and 280.5, respectively; for aspect, were 21.8 and 498, respectively. The elevated value of the nugget variance suggested that even on a small scale, the topographic variations influenced the species distribution. Twenty-four species were selected to estimate the correlation coefficient between the slope, aspect, and orientation. Then based on the correlation results using the Square Euclidean Distance cluster analysis, the 24 species were divided into four groups.