The asymmetric relationships of the distribution of conspecific saplings and adults in forest fragments

Aims With the increase of land-use change,habitat fragmentation has become a major factor affecting plant diversity.Generally,when the adult density is high,the survival rate of conspecific saplings may decline and provide more resources and space for other species to maintain a high local species diversity.Therefore,the spatial relationship of conspecific saplings and adults(SRCSA)can regulate plant diversity.However,very few studies have assessed SRCSA within fragmented forests,and we still largely unknown the relationship between SRCSA and species diversity in fragmented landscapes.Methods We calculated the mean strength of SRCSA using the spatial distribution data of saplings and adults with a log-transformed hierarchical offset-power model on 27 islands in a land-bridge island system.The higher strength of SRCSA reflects a more negative relationship between the density of conspecific saplings and adults on an island(i.e.the looser pattern of SRCSA).We tested the relationships among island attributes(i.e.island area,the distance to the mainland—MD,and the distance to the nearest island—ND),SRCSA and species diversity.Important Findings The strength of SRCSA increased with ND.Meanwhile,the species diversity significantly increased with the strength of SRCSA,and island area and the strength of SRCSA independently explained 26%and 6%of variation of species diversity,respectively,and their interactions explained 8%.Shade-intolerant and low-abundant species showed looser patterns of SRCSA.Our study suggests that SRCSA may have the effect of excluding species with a relatively low abundance in isolated island forests,which illustrates the importance of biotic interactions in maintaining plant diversity in fragmented forests.Our results also emphasize that we should consider forest connectivity when testing the conspecific negative density dependence.

随着土地利用方式变化的加剧,生境片段化已成为影响植物多样性的主要因子之一。通常,当成年树个体的密度越高,其周边同种幼树个体的存活率可能会下降,从而为其它物种提供了空间和资源,进而可以维持较高的局域物种多样性。因此,同种成年树和幼树个体的空间分布格局关系和作用强度可以调节植物多样性。然而,对于在片段化森林中,同种成年树和幼树个体空间分布关系的研究却很少报道,迄今尚不清楚片段化景观中同种个体的空间分布关系与物种多样性之间的联系。本研究选择千岛湖陆桥岛屿系统中的27个岛屿,基于岛屿上幼树和成年树个体的空间分布数据,利用混合效应模型分析它们之间的作用强度。同种幼树和成年树个体的空间作用强度越大,说明它们之间的负相互作用越强,即幼树和成年树个体空间分布越分散。此外,本研究分析了岛屿属性(岛屿面积,与大陆的距离和与最近岛屿的距离)与同种个体空间作用强度及物种多样性之间的关系。结果表明,同种个体的空间作用强度随着与最近岛屿距离的增加而增加。同时,物种多样性随着同种个体的空间作用强度的增加而显著增加,且岛屿面积和同种个体的空间作用强度分别解释了岛屿间物种多样性差异的26%和6%,共同解释了8%。耐阴种和非常见种比非耐阴种和常见种的同种幼树和成年树的空间分布更为分散。本研究表明,同种个体的空间分布可能会影响多度较低物种在片段化森林中的生存,反映了生物相互作用对于维持片段化森林中的植物多样性具有重要作用。本研究也强调在检验同种密度制约时应考虑森林之间的连接度。