Data from: Successional status, seed dispersal mode and overstorey species influence tree regeneration in tropical rain-forest fragments in Western Ghats, India

The effects of fragmentation and overstorey tree diversity on tree regeneration were assessed in tropical rain forests of the Western Ghats, India. Ninety plots were sampled for saplings (1–5 cm diameter at breast height (dbh); 5×5-m plots) and overstorey trees (>9.55 cm dbh; 20×20-m plots) within two fragments (32 ha and 18 ha) and two continuous forests. We tested the hypotheses that fragmentation and expected seed-dispersal declines (1) reduce sapling densities and species richness of all species and old-growth species, and increase recruitment of early-successional species, (2) reduce the prevalence of dispersed recruits and (3) increase influence of local overstorey on sapling densities and richness. Continuous forests and fragments had similar sapling densities and species richness overall, but density and richness of old-growth species declined by 62% and 48%, respectively, in fragments. Fragments had 39% lower densities and 24% lower richness of immigrant saplings (presumed dispersed into sites as conspecific adults were absent nearby), and immigrant densities of old-growth bird-dispersed species declined by 79%. Sapling species richness (overall and old-growth) increased with overstorey species richness in fragments, but was unrelated to overstorey richness in continuous forests. Our results show that while forest fragments retain significant sapling diversity, losses of immigrant recruits and increased overstorey influence strengthen barriers to natural regeneration of old-growth tropical rain forests.

本研究针对印度西高止山脉（Western Ghats）的热带雨林，评估了森林片段化与上层林木多样性对林木自然更新的影响。我们在2个森林片段（面积分别为32公顷与18公顷）以及2片连续森林中设置了90个样方，分别对幼树（胸径（diameter at breast height，dbh）1~5厘米，样地规格5×5米）与上层林木（胸径>9.55厘米，样地规格20×20米）开展调查采样。本研究验证了三项假说：其一，森林片段化与预期的种子传播衰减会（1）降低所有物种与老龄原生种的幼树密度及物种丰富度，并提升早期演替物种的定植率；（2）降低传播定植个体的占比；（3）增强本地上层林木对幼树密度与物种丰富度的影响。整体而言，连续森林与森林片段的幼树密度及物种丰富度并无显著差异，但片段内老龄原生种的密度与丰富度分别下降了62%与48%。森林片段中外来幼树（指因附近无同种成年个体、推测为通过传播迁入样地的个体）的密度与丰富度分别降低39%与24%，其中依赖鸟类传播的老龄原生种的外来幼树密度下降了79%。在森林片段中，幼树的物种丰富度（整体与老龄原生种）随上层林木物种丰富度的提升而增加，但在连续森林中二者并无关联。研究结果表明，尽管森林片段仍保留了可观的幼树多样性，但外来定植个体的流失与上层林木影响的增强，进一步加剧了热带雨林老龄原生种自然更新的障碍。