State Key Laboratory of Vegetation and Environmental Change, Institute of Botany, Chinese Academy of Sciences, Beijing, China.
PLoS One. 2013 Sep 5;8(9):e74560. doi: 10.1371/journal.pone.0074560. eCollection 2013.
The Janzen-Connell hypothesis is among the most important theories put forward to explain species coexistence in species-rich communities. However, the relative importance of Janzen-Connell effects with respect to other prominent mechanisms of community assembly, such as dispersal limitation, self-thinning due to competition, or habitat association, is largely unresolved. Here we use data from a 24-ha Gutianshan subtropical forest to address it. First we tested for significant associations of adults, juveniles, and saplings with environmental variables. Second we evaluated if aggregation decreased with life stage. In a third analysis we approximately factored out the effect of habitat association and comprehensively analyzed the spatial associations of intraspecific adults and offspring (saplings, juveniles) of 46 common species at continuous neighborhood distances. We found i) that, except for one, all species were associated with at least one environmental variable during at least one of their life stages, but the frequency of significant habitat associations declined with increasing life stage; ii) a decline in aggregation with increasing life stage that was strongest from juveniles to adults; and iii) intraspecific adult-offspring associations were dominated by positive relationships at neighborhood distances up to 10 m. Our results suggest that Janzen-Connell effects were not the dominant mechanisms in structuring the spatial patterns of established trees in the subtropical Gutianshan forest. The spatial patterns may rather reflect the joint effects of size-dependent self-thinning, dispersal limitation and habitat associations. Our findings contribute to a more comprehensive understanding of the relative importance of Janzen-Connell effects in influencing plant community structure under strong topographic heterogeneity.
詹森-康奈尔假说 (Janzen-Connell hypothesis) 是解释物种丰富群落中物种共存的最重要理论之一。然而，相对于其他突出的群落组装机制（如扩散限制、竞争导致的自疏或栖息地关联），詹森-康奈尔效应的相对重要性在很大程度上仍未得到解决。在这里，我们使用来自 24 公顷古田山亚热带森林的数据来解决这个问题。首先，我们测试了成虫、幼体和树苗与环境变量之间是否存在显著关联。其次，我们评估了聚集度是否随生命阶段而降低。在第三次分析中，我们近似地排除了栖息地关联的影响，并全面分析了 46 种常见物种的种内成虫和后代（树苗、幼体）在连续近邻距离下的空间关联。我们发现：i）除了一个物种外，所有物种在至少一个生命阶段都与至少一个环境变量相关，但显著的栖息地关联频率随着生命阶段的增加而降低；ii）聚集度随生命阶段的增加而降低，从幼体到成虫降低最为明显；iii）种内成年-后代的关联主要由近邻距离在 10 米以内的正相关关系主导。我们的结果表明，詹森-康奈尔效应不是塑造古田山亚热带森林中已建立树木空间格局的主要机制。空间格局可能反映了大小依赖自疏、扩散限制和栖息地关联的综合影响。我们的发现有助于更全面地理解在强烈地形异质性下，詹森-康奈尔效应在影响植物群落结构方面的相对重要性。
