The ecological characteristics of the safe sites for early-stage establishment of Chamaecyparis obtusa var. formosana seedlings in Taiwan.

Chamaecyparis obtusa var. formosana is an ecologically and economically important species in Taiwan, with a high affinity for fog immersion. Our study aims to identify possible stress factors that induce seedling mortality and investigate how different ecological factors influence seedlings' early-stage safe site requirements. We focused on the effect of large-scale climatic variables, small-scale microhabitat conditions, and biotic interactions on seedling survival and establishment by applying seasonal seedling survival monitoring and establishment surveys on both regional and local scales. We identified two alternative ways of seedling death, by environment-induced mortality and as a result of herbivory. Opposite effects of the same environmental factors on different causes of mortality showed that seedlings might need to balance the risks posed by both mortality causes to optimize their growing conditions. On a regional scale, we observed a limited effect of regional climatic variables (namely fog frequency) on seedlings' establishment and survival but noted a similar seasonal survival pattern among regions. We hypothesize that short-duration droughts during the transition from Plum rain to typhoon season are one of the key mechanisms of environment-induced mortality. On a local scale, we found that decayed coarse wood debris (CWD) facilitates seedling establishment by providing a "safe site", likely due to increased colonization of small-stature bryophytes and decreased litterfall accumulation. The effect of bryophytes on seedling establishment varies depending on their thickness, with thicker ones having stronger negative effects. Aside from the bryophytes, the accumulation of litter hindered seedling establishment. We argue that to safeguard the regeneration of Chamaecyparis obtusa var. formosana population in natural forests, preserving CWD in the forest floor as a safe site for the seedlings after tree-replacing disturbance is essential, particularly under ongoing climate change where more frequent and prolonged drought events are predicted. • CWD provides "safe site" for the establishment of the seedlings at local scale. • All regions encompassed similar seasonal patterns of seedling mortality. • Bryophyte facilitation and litterfall avoidance drive CWD as safe sites. • Preserving CWD is vital for the seedlings under predicted prolonged droughts. [ABSTRACT FROM AUTHOR]