The Gap Light Changes And Effects On Understory Vascular Vegetation In Evergreen Broad-Leaved Forest

Light largely drives plant growth and development and is one of the major limiting factors understory plants. Vascular ground vegetation is an important part of forest ecosystems in nutrient cycling and maintenance of forest productivity, forest ecosystem diversity and ecosystem function, etc. Light in forest has seasonal dynamics and spatial heterogeneity. Analyzing characteristics of understory light and its influence on understory plants distribution, species diversity can provide basic data for testing light variation in subtropical evergreen broad-leaved forests and evidence for testing forest diversity.The 2 ha permanent sample plots were established in Xiangtoushan National Nature.We censused understory vegetation and determined understory light condition in subtropical evergreen broad-leaved forest and analyzed species composition, light condition and their relationships. The main results of the study are as follows:(1) We recorded 180 understory vascular species, from 78 families 140 genera. Among these species, dicotyledonous was the dominant species in understory vascular plants.According to life-form, the tree seedlings accounted for the largest proportion and no gymnosperms tree seedlings were recorded, which indicated that the two plots are not at the early succession stage.(2) Understory light was the lowest in January and highest in April and July.Transmitted total solar radiation was significantly correlated with canopy openness and upper canopy light. However, upper canopy light had a greater influence on transmitted total solar radiation than did canopy openness. Total radiation attenuation rate was highly significantly correlated with canopy structure. For different month, canopy canopy was significantly correlated with total amount of radiation, but there was no significant in therate of reduction of total light. Transmitted direct solar radiation had a greater contribution to transmitted total solar radiation than did transmitted diffuse solar radiation. In this study,canopy structure had a stronger correlation with transmitted diffuse solar radiation than with transmitted direct solar radiation. Simulations of seasonal variation in transmitted direct solar radiation and transmitted total solar radiation performed well, but simulations of seasonal variation in transmitted diffuse solar radiation performed poorly.(3) Multi-response Permulation Procedure for grouping of four gap light factors showed significant differences among groups in canopy openness and transmitted direct solar radiation and non-significant differences in transmitted diffuse solar radiation and transmitted total solar radiation. Indicator Species Analysis detected that tree seedlings were more indicative of low canopy openness(Itea chinensis, Pithecellobium clypearia,Cryptocarya chinensi, etc.). Herbs and herbaceous vine has better indication for large canopy openness. Indicator species of transmitted direct solar radiation and transmitted diffuse solar radiation performed well in indicating gradient 3 of light condition. Canonical Correspondence Analysis showed that among four gap light factors, the canopy openness had most influence on understory vascular vegetation distribution.(4) Linear regression of gap light factor and vascular ground vegetation diversity suggested that in the study area differences in gap light factor had little effect on species richness and diversity, and has strong influence on the evenness and individuals number. In the study area, the distribution of the vascular ground vegetation tends to be uniform with increasing canopy openness and transmitted diffuse solar radiation.(5) In the study area, variation in gap light factors impacted species diversity of herb more significantly than that of shrub. However, the effect of variation in gap light factor on evenness of shrub was stronger than on evenness of herb.