Structural Diversity And Thermal Energy Habitat Association In A Subtropical Mountain Forest Community

Under global warming scenario,investigating habitat association of forest community structural diversity and thermal energy gradient by analyzing mechanism and adaptations of forest communities will facilitate the revelation of mechanisms for species coexistence and biodiversity maintenance.In the meantime,understanding warming effects on forest ecosystems and forest plant feedback will help policy-makers design scientific strategies for biodiversity conservation and forest management.In this study,a 10-hm² permanent forest census plot was established to collect field data in a subtropical evergreen broad-leaved forest in the Guangdong Kanghe Provincial Nature Reserve on Dongjiang River Basin,with 20 m×20 m square cell as the primary sampling unit for the tally of trees?1 cm,and understory plant census in five 2 m×2 m quarats within each primary sampling unit.The coordinates and topographic data of each sample unit were also measured and recorded.Nonparametric multiplicative regression?NPMR?was used to build habitat models and calculate heat load index associated with the slope aspect.On the basis of this,a variety of nonparametric statistical techniques were used to study the habitat association of plant community and spatial variation in energy,and the differences in species composition,structure and diversity along the topography-associated thermal gradient were analyzed to reveal the response of plant community attributes.By analyzing the response of the indicator species and common species across various aspects to the heat load index,we understand the effect of environmental filtering based on the heat distribution.A“space-for-time”method was applied to predict the responses and adaptive strategies of canopy and understory plants to global warming,in which the spatial thermal gradient from low to high was used as a temporal warming trend.The main conclusions of this thesis are as follows:?1?Heat load index and potential direct incidence radiation?PDIR?were calculated through habitat modeling using nonparametric multiplicative regression?NPMR?.The NPMR technique quantified the heterogeneity in thermal distribution in various aspects of the mountain forest stand,which facilitated quantitative analyses of habitat association between forest community structural diversity and the thermal gradient.PDIR and heat load index significantly increase from the cold shady aspect to the warmer sunny aspect.Since PDIR is modeled base don the symmetry of the south-north axis,it fails to reflect the variation in the length and intensity of radiations from the morning sun and afternoon;however,heat load index is calculated with a correction for this.Consequently,heat load index and its related aspect gradient are the better predictor variables to predict and analyze changes in plant diversity.?2?Warming increased community productivity by enhancing the growth of large trees,but decrease species diversity and inhibit the regeneration of tree seedlings and saplings.Structural diversity attributes of seedlings?1 cm?DBH?2.4 cm?and saplings?2.5cm?DBH?12.4 cm?significantly decreased with the increase in heat load and across a warming gradient,while adult trees?DBH?12.5 cm?exhibited an opposite trend.With the increase in heat load,number of stems,number of species,and the number of stems of the most abundant species(N_max)for the three size classes,i.e.,all sizes combined,seedlings and saplings,significantly decreased,while number of stems,basal area,and N_max for the adult trees significantly increased.Therefore,competition for thermal energy might be the major driver for trees of different sizes or different growth stages to respond to climate warming.?3?Both species composition and functional group composition showed remarkable associations with heat load index and elevation,with significant variations across a thermal gradient.Canonical correspondence analysis?CCA?indicated that both species composition and functional group composition were markedly associated with heat load index and elevation;while multi-response permutation procedures?MRPP?identify significant difference in species composition and functional group composition across a thermal gradient.However,pairwise comparison using MRPP detected no significant difference in species composition and functional group composition between the sunny and semi-sunny aspects,showing that understory plants had different response in composition and distribution to warming in different thermal gradients.?4?As compared with canopy trees,the structural diversity of understory plants was different in their response to thermal gradient.Functional diversity of the understory plants did not significantly change across a topography-association thermal gradient,indicating that functional group composition might have greater effect and complementarity than species composition in their response to habitat shift.?5?Different species had different strategies in their response to thermal gradient as shown by their response curves to heat load index.The indicator species to a particular thermal environment reflected the adaptation and exclusion of species by environmental filtering,while the common species shared by various thermal environments were the result of species adaptation to heterogeneous habitats,representing their competitiveness and vitality.The response in abundance to heat load index had species-specific variation.The relative abundance?RA?of the indicator species with a significant indicator value to the shaded and semi-shaded aspects decreased with greater heat load index,while RA of such species on the sunny aspect increased with greater heat load index.Differences existed in RA response of the common species shared by various thermal environments.This suggested that common species also had different strategies for resource utilization in response to thermal gradient,but they had wider ecological magnitude for their adaptation to thermal gradient.This study revealed different responses and different adaptation strategies of forest community to thermal gradient,which justified the role of heat factor in the control of community assembly and species diversity maintenance.By using a“space-for-time”approach,in which the spatial thermal gradient from low to high was used as a temporal warming trend,this study have provided a novel method for predicting the responses and adaptive strategies of canopy and understory plants to global warming.The findings from this study will have significant scientific implications for biodiversity conservation and adaptation to climate change of the forest sector.