RESUMEN
The effects of forest vertical structural variables (crown index, tree height, and large and small tree proportions) and density variables (basal area, adult tree density, shrub density, shrub coverage, and Excentrodendron hsienmu proportion in the forest canopy) on the natural rege-neration of the typical E. hsienmu populations in the karst mountains of Southwest Guangxi, China were studied. The results showed that the average regeneration density of E. hsienmu populations was 1742-3861 trees·hm-2 and relatively low. Forest vertical structure and density variables did not significantly affect the number of E. hsienmu seedlings, but significantly affected seedling ground diameter and height to some extent. Among the vertical structure variables, a significant nega tive correlation was observed between crown index and seedling height, but the association between crown index and seedling ground diameter was not significant. The large tree proportion and tree height were significantly negatively correlated with both seedling ground diameter and height. However, the proportion of small trees was significantly positively related to both seedling ground diameter and height. Among the density variables, adult tree density showed a significant positive correlation with both seedling ground diameter and height. A significant negative association was observed between the proportion of E. hsienmu and seedling ground diameter. Multiple regression analysis indicated that the model based on stand structure factors showed a poor fitness with regard to the number of E. hsienmu seedlings (P>0.05), and the quantitative distribution of E. hsienmu seedlings was affected to some extent by vertical structural variables. The stand structure factor model fitted well for the diameter and height of the seedlings (P<0.01). Seedling ground diameter was mainly affected by adult tree density, and seedling height was mainly determined by the combined effects of adult tree density and basal area.
Asunto(s)
Bosques , China , Plantones , ÁrbolesRESUMEN
Community invaded by Praxelis clematidea in karst mountainous area of Pingguo, Guangxi Province was investigated including 4 land types, i.e. abandoned cropland, young plantation, roadside and shrub. Meta analysis, analysis of covariance, linear mixed effect model and recursive partitioning were used to analyze their herb diversity and richness in relation to environmental variables. The results showed that summary effects were positive and significant, effects of groups were positive, effect of abandoned cropland was negative, and effects of other three land types were positive, and shrub had high weight in the analysis. Analysis of covariance indicated that importance value of P. clematidea, distance to driveway and soil were the main numeric factors influencing herb diversity and herb species richness, and plots invaded had higher herb diversity and richness than those uninvaded. Land types had extremely significant impacts on herb diversity but had nothing on herb species richness. Linear mixed effect model partitioned the explained variances of random effects, factor P. clematidea present or absent and residuals respectively contributed 50.9% and 49.1% in Shannon model, while 39.0% and 61.0% in richness model, groups and land types contributed zero to the random effects. P. clematidea invasion could be classified into 5 levels, and modest invasion caused higher herb diversity and richness. When P. clematidea importance value was more than 23.6%, the critical threshold, both herb diversity and richness declined.