Effects Of Neighborhood Tree Species Diversity On Growth,Nutrient Content And Water Use Efficiency Of Cunninghamia Lanceolata

Currently,the structure of plantation forest is inappropriate in China.Specifically,monoculture usually leads to biodiversity loss,which further threatens the ecosystem function.Studies on the effects of biodiversity on plant growth,nutrient and water use efficiency could help to further evaluate the mechanism of biodiversity impact on ecosystem functions.In our study,Chinese fir with different neighborhood species diversity(1,4,8,16,32)were selected as focal trees.We aimed to explore the impacts of neighborhood species richness,functional traits dissimilarity,neighborhood competition,and shading index on the growth,nutrient and water use efficiency of the focal trees.The main conclusions are as follows:1.The neighborhood species richness(NSR)significantly reduced the relative growth rate of Chinese fir,the dissimilarity in specific leaf area between focal trees and neighborhood trees(SLA＿diss).Neighborhood competition index(NCI)also significantly reduced Chinese fir growth.These results indicate that competition for resources such as nutrients and water may reduce resource availability between focal trees and its neighborhoods,which further lead to low growth rate of focal trees.Moreover,the interaction of NCI and SLA＿diss,the three-way interaction of NSR,NCI and SLA＿diss were all negatively correlated with Chinese fir growth,indicating that functional traits played an important role in regulating NSR and NCI during the growth of Chinese fir.2.The neighborhood tree species richness(NSR)had no significant effect on the leaf nutrient content of Chinese fir,but the neighborhood competition index(NCI)significantly decreased the leaf N content and N:P of Chinese fir.The dissimilarity in root tissue density between focal trees and neighborhood trees(RTD＿diss)significantly decreased the N content in leaves of Chinese fir.Specific root length(SRL＿diss)significantly increased the P content in leaves of Chinese fir.Both interactions between NCI and SRL＿diss,and between NSR and SRL＿diss significantly reduced the P content in leaves of Chinese fir.The interaction of NSR and neighborhood phylogenetic dissimilarity(NP＿diss)significantly increased the leaf N:P of Chinese fir.Our study showed that mixing with trees that have larger dissimilarity in functional traits significantly increased the P content of Chinese fir leaves,while the competition of neighborhood tree species decreased the N content of Chinese fir leaves.Tree species richness can help alleviate the adverse effects of interspecific competition on Chinese fir through niche complementation when mixing with multiple species,greater trait dissimilarity among species can beneficial to nutrient cycling and plant growth.3.The neighborhood height index(NHI)significantly decreased leaf intrinsic water use efficiency(i WUE),while the dissimilarity in specific root length between focal trees and neighborhood trees(SRL＿diss)significantly increased leaf i WUE,indicating that the positive effect of niche complementation on leaf i WUE was enhanced with the increase of NHI.Meanwhile,neighboring trees with different functional trait dissimilarity could reduce the negative effect of shade on leaf i WUE,when the shade intensity of neighboring trees around the target trees were higher.The interaction of NHI and SRL＿diss also had a significant positive effect on leaf i WUE.It was confirmed that functional trait dissimilarity plays a key role in the effect of NSR and NHI on leaf i WUE.In summary,NSR had a significant negative effect on relative growth rate,and no significant effect on nutrient and water use efficiency of Chinese fir.Also,NCI had a significant negative effect on Chinese fir growth,leaf nitrogen content and nitrogen-phosphorus ratio.NHI had a significant negative effect on leaf i WUE of Chinese fir.Different functional traits had different effects on Chinese fir,with root functional traits dissimilarity having a major impact on Chinese fir.Among the interaction between neighborhood species richness,functional trait dissimilarity and neighborhood competition index or neighborhood height index,the functional trait dissimilarity was the main influencing factor and played an important regulatory role to mitigate the negative effects of interspecific competition or shade on Chinese fir through ecological niche complementation.