Local neighborhood communities in the understory play a critical role by affecting regeneration niches and subsequent community assembly in a montane cloud forest

Abstract

Regeneration niches for tree seedlings are defined as possessing the biotic and abiotic requirements to successfully replace adults. However, two critical discrepancies obscure our understanding of the role of regeneration niches in forest community assembly: a weak association between seedling assemblages and environmental factors, and an uncoupling between seedling and adult-tree assemblages. However, understory plants, which may interact with microenvironments and seedlings, may be a better spatial descriptor of regeneration habitats. To test this, the spatial variations of seedling assemblages in a montane cloud forest of Taiwan were analyzed in terms of their association with neighborhood assemblages of understory plants, the shrub layer, and overstory trees, as well as environmental variables. We found that the understory plant spatial structure contributed most in explaining spatial variations of the seedling assemblages (especially for widespread Lauraceae and patchy Fagaceae) and facilitated the identification of patches of different regeneration habitats for specific seedling assemblages. Moreover, among these regeneration patches, tall herbs affected seedling density and diversity differently. We found segregation between tall herbs and Lauraceae seedlings, indicating that tall herbs shape seedling assemblages and uncouple the association between seedling and adult stages. However, positive covariations between seedlings/tall herbs and between seedlings/adults were found for Fagaceae and Pentaphylacaceae in different regeneration patches, suggesting that positive, neighboring effects and dispersal limitation may cause the patchy distribution of seedling assemblages and affect their coupling with adults. Thus, the understory plant spatial structure shapes seedling assemblages and provides a better link to spatial associations with regeneration habitats.