| The karst seasonal rain forest in the south-west Guangxi,which grows on the carbonate bedrock in the climatic conditions of tropical northern margin,is the most unique type of karst forests in the world.It supports very high level of endemic species of plants,vertebrates and invertebrates and exerts important ecological functions.An increase of biological knowledge is urgently needed to better preserve its huge diversity and its important ecological functions.Forest gap is prevalent in forest ecosystems,which drives the development of forest regeneration and affects the stand structure and dynamics.However,little is known about the gap pattern and characteristics of karst seasonal rain forests,since large-scale studies with continuous coverage are lacking.In this study,high-spatial-resolution Worldview-3 satellite imagery was coupled with field data to assess gap disturbance regime and its pattern in a 15-ha northern tropical karst seasonal rain forest dynamic plot in Nonggang National Nature Reserve,Guangxi.Furthermore,the influence on gap microclimate and effect on seedling recruitment of gaps were also discussed in this study.Total 305 gaps were identified,ranging in size from 10 ? to 371 ? and with mean size 52.3 ?.The canopy gap fraction was 10.6%,and gap density was about 20 per hectare.The spatial pattern of gaps was clustered at scale 0-50 m.Significant association was found between gaps and big trees(DBH?10cm),and null associations between gaps and small trees(DBH<10cm)and between gaps and standing dead trees.The prevailing ecological factors had different influence on gap distribution and gap characteristics.Convexity,altitude above channel(VDC)and tree species richness had significant effect on the gaps distribution.Gap size was significantly influenced by slope,mean elevation and tree species richness.Gap perimeter was mainly affected by mean elevation,rock-bareness rate(RBR),topographic wetness index(TWI),VDC and tree species abundance.The air temperature of gaps was significantly higher than that of gap edge from 11:00-15:00.The average daily temperature range of gap and gap edge was 5.1 ? and 4.4 ?,respectively,and the difference was significant between them.The soil surface temperature was slightly higher than that of gaps,but the difference was insignificant at all-time points.The illumination in gaps was significantly higher than that at gap edge from 8:00-17:00.However,the relative air humidity in gap was lower significantly than that at gap edge at all-time points.The difference among air temperature,soil surface temperature,illumination and relative air humidity of different positions in gaps were significant.In contrast,the microclimate differed insignificant in different positions of gap edge.The seedlings of 72 tree species were observed in the 150 sampling plots of gaps and non-gaps.Among the 72 species,54 of them were documented in gap plots,and 66 species in non-gaps plots.6 species were only found in gaps and 18 species were only found under closed canopy,and there were 48 species were common in gaps and non-gaps.The Jaccard index of seedling species was 0.667 between gaps and non-gaps,indicating the response of different species to gap was inconsistent.Whether in the dry season and rainy season,the density and death density of seedlings was significantly lower in gaps than in non-gaps.In the dry season,there was no significant difference in recruitment density between gaps and nog-gaps.However,the recruitment density was lower in gaps than in non-gaps in the rainy season.The seedling mortality was prominently lower in gaps than in non-gaps,but showing insignificant difference in the rainy season.The seedling recruitment rate was higher in gaps than in non-gaps in the rainy season,and showing insignificant difference in the dry season.The occurrence of gaps and its basic characteristics had significant influence on the survival probability of the seedlings.The occurrence of gaps increased the survival probability of seedlings.The gap characteristics influenced on the survival of seedlings was not consistent,the survival of seedlings was negative correlation with gap size as well as positive correlation with gap perimeter.In addition,several topographical factors,i.e.,convexity,mean elevation and RBR had significant effects on survival of seedlings.Among the three topographical factors,convexity had positive effect on survival of seedlings,and mean elevation and RBR had negative effect on survival of seedlings.The above results suggested that gap disturbance was prevailing in the karst seasonal rainforest.The topographic differentiation of spatial distribution and characteristics of gaps reflected the combination effects of different processes shaping forest structure,i.e.,special karst geological conditions,habitats and the biological characteristics of karst forest.Gaps influenced significantly on temporal and spatial variations of microclimate and was an important driver contributed to the habitat heterogeneity.The results also showed that seedling regeneration was significantly influenced by gap disturbance and topographical factors,indicating that seedling survival in karst seasonal rainforest was affected by a variety of factors... |
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