| Under the ongoing warming and alteration of precipitation patterns,the species composition,structure,and functions of forest ecosystem are affected and changing.It is a challenge to ecologists and forest managers to understand the influences of various biotic and abiotic factors on forest plant diversity and ecosystem functions as well as the underlying mechanisms,to protect forest plant diversity,and to maintain and promote the performance of ecosystem functions and the availability of ecosystem services under climate change.We established 129 1200 m2 permanent plots in 43 randomly selected sites in a subtropical mountain forest in eastern Guangzhou,China and then measure the geographic and topographic variables,determined soil and plant carbon and nitrogen concentrations,and conducted a tree census,to investigate the influences of driving factors on forest plant diversity and ecosystem functions as well as the underlying mechanisms.Nonparametric statistical methods were used to test the effects of various factors on woody plant alpha and beta diversity.Community assembly and diversity maintenance mechanisms were tested based on the integration of functional traits with null models,on species rank-abundance distribution models,and on variation partitioning of community composition/beta diversity.The simple linear or quadratic regression models were used to investigate the independent and joint effects of climate warming and decrease in precipitation on carbon and nitrogen allocations and cycling in the plant-soil system by means of“space for time”.Kruskal-Wallis test were used to test the differences in woody plant biomass among various organs and in the biomass proportions of various organs to the whole plant along the elevational gradient.Standard major axis models were used to test the effects of elevation on the biomass allocation strategies between various plant organs and consequently investigate the underlying mechanisms of biomass allocations between plant organs.Geographically weighted regression models were used to investigate the patterns and underlying mechanisms of the relationship between woody plant diversity and the spatial stability of forest biomass.The main findings were as follows:(1)As mean annual temperature increased and mean annual precipitation decreased with decreasing elevation,soil carbon concentration,soil nitrogen concentration,soil carbon to nitrogen ratio,number of individuals of woody plants,community weighted mean values(CWMs)of height to diameter ratio(HDR),species richness,evenness index,and functional richness decreased highly and significantly,while the CWMs of the maximum tree height and maximum diameter at breast height(DBH),Berger-Parker dominance index,and functional divergence increased highly and significantly.Mean annual temperature and CWMs of functional traits were the most important drivers for plant diversity and dominance.These findings indicated that warming and decrease in precipitation decreased woody plant diversity but increased plant dominance.(2)Beta diversity and the turnover components increased while the nestedness components decreased with increasing geographic distance.Beta diversity and the components decreased with increasing plot size except that the species nestedness component increased.On the whole,geographical scale and plot size exerted opposite effects on species and functional beta diversity and their components.(3)The standard effect sizes of intraspecific,interspecific,and individual variations in tree height,DBH,and HDR were significantly less than 0.The differences between the standard effect sizes and 0 were more significant for intraspecific and interspecific variations than for individual variations.Species rank-abundance distribution of woody plants was best fitted by Zipf-Mandelbrot model.According to variation partitioning of beta diversity,untested and spatial variables explained more variances than did environmental variables in species and functional beta diversity and their components.The relative importance of environmental variables for explaining variances in beta diversity and its components increased while those of spatial variables decreased with increasing plot size.These findings suggested that different results were observed while using different methods to test the diversity maintenance mechanisms;that habitat filtering was the major ecological process influencing community assembly and diversity maintenance,while other processes also played an important role;that the habitat filtering became stronger as climate warming and decrease in precipitation;and that intraspecific and interspecific trait variations were better than individual trait variations for explaining diversity maintenance mechanisms.(4)With climate warming and decrease in precipitation,soil carbon concentration([C]Soil),soil nitrogen concentration([N]Soil),soil carbon to nitrogen ratio([C:N]Soil),below-ground plant nitrogen concentration([N]BG),and above-ground plant carbon to nitrogen ratio([C:N]AG)decreased;above-and below-ground and total plant carbon concentration([C]AG,[C]BG,and[C]TP),total plant nitrogen concentration([N]TP),total plant carbon to nitrogen ratio([C:N]TP),and carbon ratio between above-and below-ground plant organs([AG:BG]C)remained stable;below-ground plant carbon to nitrogen ratio([C:N]BG),and nitrogen ratio between above-and below-ground plant organs([AG:BG]N)increased on the whole.The joint effects of warming and decrease in precipitation on carbon and nitrogen allocations in different nutrient pools in the plant-soil system were larger or less than the independent effects.These findings suggested that climate warming and decreasing precipitation affected carbon and nitrogen allocations in the plant-soil system and that carbon and nitrogen storages in soil decreased but those in plant remained stable;plant nitrogen was transported from below-ground to above-ground organs.(5)Various tree organ biomass,total plant biomass,and stem biomass proportions and above-ground biomass proportion highly and significantly increased,while leaf biomass proportion and root biomass proportion highly and significantly decreased with increasing elevation.Woody plant diversity and CWMs of maximum tree height,maximum DBH,and HDR were the most important drivers for tree biomass and biomass proportions of various organs.Higher diversity increased tree biomass through both selection effect and complementarity effect.The observed slopes of log linear regressions for stem biomass against root biomass and for above-ground biomass against root biomass did not significantly differ from the theoretical slope,i.e.,1,and the observed slopes did not significantly vary across elevational classes.The observed slope of log linear regression for leaf biomass against stem biomass did not significantly differ from the theoretical slope,i.e.,3/4,and the observed slopes did not significantly vary across elevational classes.The observed slope of log linear regression for leaf biomass against root biomass was highly and significantly larger than the theoretical slope,i.e.,3/4,and the observed slopes significantly varied across elevational classes.These findings suggested that tree biomass allocations between above-ground organs and roots,between stems and roots,and between leaves and stems were in accordance with the allometric partitioning hypothesis,while biomass allocation between leaves and roots was in line with neither allometric partitioning hypothesis nor optimal partitioning hypothesis.(6)Higher diversity promoted asynchronous responses of various plant species to environmental heterogeneity across space(i.e.,species asynchrony),but decreased the species dominance(Berger-Parker dominance index)and spatial stability of dominant species biomass.The negative effects of higher species dominance and higher spatial stability of dominant species biomass overwrote the positive effect of higher species asynchrony on spatial stability of forest biomass.Thus,higher woody plant diversity reduced spatial stability of forest biomass.The correlations of diversity with spatial stability of forest biomass locally varied among plots and increased with higher elevation,suggesting that the spatial non-stationarity existed in the diversity-stability relationships and was affected by elevation. |
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