Niche And Structural Diversity In Plant Communities Of Endangered Parrotia Subaequalis Together With Its Distribution Prediction

Parrotia subaequalis is an endangered Tertiary relict tree species endemic to east China,which is classified as a‘GradeⅠKey Protected Wild Plant’in China.It is now scattered only in the part of the subtropical mountains.Due to its own biological characteristics and human interference and destruction,the survival of the wild populations of P.subaequalis has been seriously threatened.In order to provide theoretical basis for scientific protection and management,in this paper we chose 3sites from Anhui,Jiangsu,and Zhejiang Province as study area where P.subaequalis was a dominant or co-dominant species to explore the species composition,growth conditions,and their relationship with surrounding trees by ecological niches and structural diversity.Meanwhile,we predicted the potential distribution of P.subaequalis under current and future scenarios using a maximum entropy（Max Ent）model in this study.And the data of the locations of P.subaequalis were all obtained from our field survey.The main results are as follows:（1）Eighteen 400 m²plots were studied for importance value,niche breadth and overlap in the tree layer of P.subaequalis community to address the niche characteristics between P.subaequalis and its main associated tree species.The results showed that P.subaequalis had the greatest importance value and niche breadth in tree layer.For the first 10 dominant species,a considerable difference was detected by the Levins index and Shannon-Wiener index,but they were largely the same in order.Thirty-nine out of the 45 species pairs in Mt.Wanfo（WF）had the value of niche similarity(C_ik)indices>0.5;41 species pairs had the value of niche overlap(O_ik)indices>0.5.Niche overlap between P.subaequalis and its associated tree species was high in WF,the average value with C_ikand O_ikwere 0.71 and 0.81.In Mt.Dalongxikan（DL）,23 species pairs with C_ik>0.5 and 32 pairs with O_ik>0.5.And the average value with C_ikand O_ikbetween P.subaequalis and its neighbors were 0.59 and0.68.There were 17 pairs with C_ik>0.5 and 24 pairs with O_ik>0.5 in Mt.Longwang（LW）.The average value between P.subaequalis and other trees were 0.57 and 0.68.Based on the results of ecological niches,we found that the niche characteristics of P.subaequalis were basically the same across the 3 sites.The larger niche width of P.subaequalis in the community indicated that this tree has stronger environmental adaptation,which is consistent with the phenomenon that P.subaequalis can grow in the area with poor environmental conditions according to our observation in field.The average value of niche overlap among dominant species in the tree layer of the community was high,which indicated that these species had similar requirements for environmental resources,and most of them occupied the similar resource digit.This result may be ascribed to an unstable state of the community.Moreover,when environmental resources are insufficient,more intense interspecific competition is likely to occur.With the development of succession,P.subaequalis has no advantage over its main associated tree species in terms of the competition for environmental resources.（2）In order to understand the growth status of P.subaequalis population and the relationship with its neighboring tree species,we set up a 1600 m²plot in WF,a 900m²plot in DL and a 1600 m²plot in LW respectively.We calculated the Mingling index（M_i）,Tree-tree interval index（D_i）,Diameter differentiation index（TD_i）and height differentiation index（HD_i）by“structural group of four”.The results showed that the neighboring trees of P.subaequalis were mainly non-Parrotia subaequalis in light of the main value with M_i=0.67 and M_i=1.The average distance between P.subaequalis and the three nearest-neighboring trees was mostly concentrated within 1-3 m,of which the relative frequency of the average distance of 1-2 m was higher.Across the 3 plots,the average diameter and height of P.subeaqualis was 0-30%larger than their neighbors.Thus,we found that the growth status of P.subeaqualis in3 sites were similar.P.subeaqualis is surrounded by neighbors,which were the dominant species in the community.These nrighbors have strong competitive ability.The average distance between most P.subeaqualis and their neighbors was small,suggesting that P.subeaqualis may face intence interspecific competition from neighbors.In the study,we found that P.subeaqualis had a slight advantage in the horizontal and vertical plane according to the results of diameter and height differentiation,but this advantage was not considerable.P.subeaqualis has weak advantages over its neighbors in terms of the growth status,and is likely to be replaced in the near future.This may inhibit the growth and population regeneration of P.subeaqualis.（3）In this study,the prediction accuracy was considered excellent as AUC values were more than 0.9（AUC=0.995）,which showed the models have excellent performance for simulating the distribution of the species.The suitability of P.subeaqualis was divided into four grades,namely highly suitable habitat,moderately suitable habitat,marginally suitable habitat and unsuitable habitat.The potential suitable habitats of P.subeaqualis were mainly located in in Jiangsu Province,Anhui Province,Zhejiang Province,Henan Province and Hubei Province.The total habitat area was 59,512 km²,accounting for 6.19‰of China’s total area.Among them,highly suitable habitat was mainly distributed in Anhui and Zhejiang with 1,952 km²;the moderately suitable habitat distributed in Anhui,Zhejiang and Jiangsu which covering an area of 14,134 km².The predicted results are consistent with the actual geographical habitat of P.subeaqualis.The sum of contribution rate of the first 4environmental factors reached 87.8%,including precipitation of driest quarter（contributing 59.5%）,mean temperature of coldest quarter（contributing 27.3%）,annual mean temperature（contributing 6.4%）and mean temperature of wettest quarter（contributing 2.3%）.The optimal precipitation of driest quarter and mean temperature of coldest quarter of P.subeaqualis ranged from 110 to 150 mm and from 1 to 4℃,respectively,while the annual mean temperature and mean temperature of wettest quarter ranged from 12 to 16℃and from 22.5 to 28℃,respectively.（4）In this paper,we used BCC-CSM global climate change modeling data according to the RCP2.6-2050,RCP8.5-2050,RCP2.6-2070,and RCP8.5-2070scenarios for the future climate scenarios.RCP2.6 is a warming mitigation scenario,while RCP8.0 is a high greenhouse gas emission scenario,which corresponds to an increase in the mean global temperature of 1.0 and 2.0℃,respectively.The results showed that the potential suitable habitat of P.subeaqualis would be fragmented.More importantly,the area of suitable habitat,especially the highly suitable habitat,would significantly decrease.The area of the highly suitable habitat was the smallest especially under RCP8.5 with 194 km²,accounting for 0.02‰of China’s total area.It should be noted that such factors as geographic barriers,dispersal distances and limitations,and biotic interactions,which may influence population distribution over time,were not included in this model.Given that most of the current populations of P.subeaqualis are within the scenic areas that are generally subject to human disturbance,therefore,further climate change could cause its actual suitable habitat to be narrower than predicted.（5）Based on the above research results,together with the biological characteristics of P.subeaqualis,four suggestions were proposed to protect this species:（i）Reduce the niche overlap between tree layers of P.subeaqualis community and ensure that P.subeaqualis can obtain sufficient environmental resources;（ii）Reduce interspecific competition by thinning neighboring trees which had strong competitiveness and adaptability to enlarge the forest gap for the growth of P.subeaqualis;（iii）Maintain the current conservation class of this species and strengthen protection of P.subeaqualis populations and their habitats,with particular attention to the protection of the highly and moderately suitable habitat;（iv）Future climate change should be considered in the conservation strategy of P.subeaqualis.