群落物种及结构多样性对森林地上生物量的影响及其尺度效应:以巴拿马BCI样地为例

Scale dependent effects of species diversity and structural diversity on aboveground biomass in a tropical forest on Barro Colorado Island,Panama

随着全球物种多样性丧失速率的日益加剧,森林群落(特别是占陆地碳储量一半的热带森林)内物种多样性与地上生物量之间的关系愈加受到关注。一般认为森林群落的物种多样性能够促进地上生物量的累积。但随着研究的深入,研究人员发现物种多样性对地上生物量的促进作用可能会受到结构多样性和非生物环境因子的干扰或抵消,而且对于影响地上生物量的各类因素的作用强度是否会随空间尺度的不同而发生变化也还存在争议。本文基于巴拿马Barro Colorado岛(BCI)50 ha大型森林动态监测样地的调查数据,采用结构方程模型对影响其地上生物量的直接及间接因素进行分析。结果表明:随着空间尺度的增加,物种多样性对地上生物量的作用由不显著转为显著负作用,且强度逐渐增加;而结构多样性对地上生物量均具有显著正作用,且强度逐渐减弱。非生物因子与两种多样性之间均具有一定的相关性,并进而对地上生物量产生间接影响。这一结果暗示未考虑结构多样性而得到的物种多样性与地上生物量的正相关关系可能是由结构多样性介导或受非生物因素的间接作用所致,而在森林群落特殊的林窗及强烈的非对称性竞争中,物种多样性与地上生物量更可能呈现负相关关系。

With global decreases in species diversity, the effects of species diversity on aboveground biomass of forest communities, especially tropical forests which store nearly half of terrestrial carbon, have received much attention. It is commonly assumed that species diversity can promote the accumulation of aboveground biomass in forest communities. However, increasing evidence suggests that this positive effect of species diversity can be influenced or neutralized by structural diversity and various abiotic environmental variables. It is still far from clear whether scale dependent effects of various drivers of aboveground biomass exist. Using structural equation modeling, we examined direct and indirect drivers of aboveground biomass in a 50 ha tropical forest dynamics plot on Barro Colorado Island（BCI）, Panama. Our results showed that the effect of species diversity on aboveground biomass was non-significant at the smallest scale and became significantly negative at larger scales. The strength of this negative effect increased with spatial scales. Conversely, structural diversity had a significant positive effect on aboveground biomass and the strength of this effect decreased with scale. Abiotic variables had some relationships with both types of diversity and can indirectly affect aboveground biomass. These results suggest that positive relationships between species diversity and aboveground biomass in conventional studies may be caused by structural diversity. The relationship between species diversity and aboveground biomass is more likely to be negative, due to under the gap dynamics and strong asymmetric competition in forest communities.