| The dynamic of canopy structure is a manifestation of the growth dynamics of canopy,and the heterogeneity of canopy structure plays an important role in forest community productivity,ecosystem function,community renewal and succession.Accurate quantification of forest community canopy structure and undergrowth radiation characteristics provides a unique perspective for studying the forest ecosystem functions.The canopy structure of forest has an important influence on the growth and development of undergrowth plants by redistribution light,water and heat resources,and creating the microclimate environment under the forest.Therefore,it is very important to study the characteristics of community canopy structure and the undergrowth radiation dynamics,as well as the response of undergrowth plants to light environment.In this paper,five different forest types in the Pearl River Delta region were studied,including Rhodoleia championii forest,Cinnamomum camphora plantation,Cunninghamia lanceolate forest,Coniferous-broad-leaved mixed forest and Moso bamboo forest.The solar radiation-canopy structure-undergrowth vegetation was treated as a unified continuous system to explore the interaction among the light-undergrowth plant-canopy structure.The main conclusions are as follows:(1)A total of 3846 individual undergrowth plants were recorded in this study,belonging to 81 families,140 genera and 194 species,and there were differences among families,genera and species in different community types.(2)There were significant differences in the number of individual,species number,Simpson index,Shannon-Wiener diversity index and Berger-Parker dominance of different forest types,but there was no significant difference in evenness index.There was significant difference in the functional diversity of undergrowth plants under different forest types.There was a significant positive correlation between the species richness and functional diversity indexes of undergrowth plants,and there was a significant positive correlation between the individual number of undergrowth plants and the functional richness and Shannon functional diversity index.(3)There were significant differences in canopy structure,undergrowth radiation and radiation reduction rate among different forest types.The monthly dynamic changes of transmitted direct radiation,transmitted diffuse radiation and transmitted totals radiation were very significant,showing a single peak distribution pattern.There was a high intensity negative correlation between canopy openness and leaf area index,and the canopy structure had different effects on both undergrowth radiation and radiation reduction rate.(4)The canopy openness has an important influence on the diversity of undergrowth plants,and there were significant differences in the number of individual,species number,Shannon-Wiener diversity index and Simpson index under different canopy openness gradients,but there was no significant difference in evenness index and Berger-Parker dominance.The correlation between undergrowth radiation and the diversity index of undergrowth plants was weak,but there was a significant negative correlation between the transmitted direct radiation and the individual number of undergrowth plants.The site environmental factors such as aspect,slope and elevation also affect the diversity of undergrowth plants by controlling light and precipitation,which has an important influence on the distribution pattern of undergrowth plants.(5)Canonical correspondence analysis by using species composition data matrix combined with the environmental factors showed that the canopy openness and elevation has strong intra-set correlations with ordination axis,while transmitted direct radiation,transmitted diffuse radiation and transmitted totals radiation has weak correlation with ordination axis.Results of canonical correspondence analysis by using data matrix of understory plants functional groups combined with the environmental factors showed that the canopy openness,leaf area index,elevation and transmitted diffuse radiation all exhibit high strength intra-set correlation with ordination axis,but the correlation between transmitted direct radiation,transmitted totals radiation and the sorting axis was weak.(6)The results of random forest analysis showed that elevation,aspect,forest types,transmitted direct radiation,leaf area index and canopy openness are important environmental factors affecting undergrowth plant diversity.Fourth-corner analysis using the lottery model showed that the canopy openness,leaf area index,transmitted direct radiation,elevation and slop were correlated with undergrowth plants functional group in varying degrees,but the correlation between transmitted totals radiation and the functional groups was weak.(7)The indicator species analysis showed that there were 19 species of undergrowth plants with significant indicative effect on different canopy openness gradients,and there were 30 species of undergrowth plants which have significant indicative effect on different slope gradients,and the appearance of these plants can accurately indicate the habitat conditions of different canopy openness gradients and different aspect gradients. |
