The Effect Of A Rare And Extreme Ice Storm On Density Dependence In Forest Species Coexistence

Both density dependence and environment filtering play important roles in biodiversity maintenance and species coexistence. An increasing frequency and intensity of occasional extreme climatic events happen under climate changes. We know little about the effect of extreme climate events on forest biodiversity maintenance. Based on a seedling dataset collected from year 2006 to 2013 in GTS forest, China. This study estimate the relative importance of density dependence and environment filtering between pre-and post-ice storm in February 2008, which is a rare and extreme ice storm. The results showed that seedling density and Simpson diversity of seedlings greatly increased after the ice storm compared to years pre-ice storm, while Seedling mortality rate and species richness have no significant response after the storm. The results indicated that ice-storm did not dramatically damage the structure of seedling community, but could promote seedling recruitment and increase evenness of seedling community.Previous studies indicated that the strength of negative density dependence may change substantially over census time, particularly in fluctuate environments and may accumulate over time. Thus, this study tested how census intervals affected the contributions of negative density dependence on seedling community. Census interval may bias the effect of negative density dependence. In this study,1-2 years were the best census interval for detecting density dependence in subtropical forest such as GTS forest. Seedling neighbors significantly affect seedling survival in short term (less than 1 year), while larger neighbors significant affect seedling survival in long term (more than 1 year).This study evaluated the relative importance of density dependence and environment filtering on biodiversity maintenance and species coexistence between pre- and post-ice storm. Biotic variables determined seedling survivorship during pre-ice storm, while both abiotic and biotic variables determined seedling survivorship during post-ice storm. Thus, density dependence may be more important than environment filtering on maintaining biodiversity and species coexistence in GTS forest pre-ice storm. The species-level effects of conspecific seedling density was significantly negative for 100% and> 97% of species for cohort 2006 (pre-ice storm) and 2010 (post-ice storm), separately. The results greatly support the standpoint that negative density dependence is pervasive in tropical and subtropical forest. Negative density dependence driving by larger neighbors was detected in cohort 2006, pre-ice storm, but was not detected right after ice storm in cohort 2008 and 2009. However, negative density dependence driving by seedling neighbors was detected in cohort 2010 after two years recovery following ice-storm. The results suggested that a rare and extreme climate event such as ice storm significantly affect negative density dependence, and also significantly change the driving factors of negative density dependence. The phylogenetic negative density dependence was not found here as previous studies. Results support the viewpoint that species favour coexistence of close phylogeny relatives.Shade-tolerance is often a key trait in determining seedling survivorship in forest understorey. We categorized species as shade tolerant, intermediate, or light-demanding species to test whether shade tolerance affect species’ response to ice storm. Negative density dependence was detected among three plant functional groups during pre-ice storm. This suggested that negative density dependence regulated population dynamics and provided space and resource via density-dependent mortality to promote species coexistence. The results was consistent with results from community level in this study. However, in post-ice storm, negative density dependence was only detected among intermediate species post-ice storm. Our results suggested that light-demanding species could benefit from ice storm, and overcome density-dependent mortality, which would promote specie coexistence.Although ice storm could significantly alter neighborhood interaction on seedling survival in short term (< 2 years), the effects diminished in the longer terms. However, ice storm increased the density and diversity of seedling community, which would increase the forest tree diversity in the long term. Our study suggests that extreme climate event such as ice storms could also benefit forest biodiversity maintenance and species coexistence. Our findings highlight the importance of census interval and shade-tolerance ability in quantifying the contribution of density dependence and environmental to tree species coexistence.