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 induced seedling mortality and investigate how different ecological factors influence early-stage safe site requirements of the seedlings. 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 survey on both regional and local scale. We identified two alternative ways of seedling death, by environmental-induced mortality and by 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 causes to optimize their growing conditions. On a regional scale, we observed 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 is one of the key mechanisms of environmental-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 significantly hindered seedling establishment. We argue that to safeguard the regeneration of Chamaecyparis obtusa var. formosana population, preserving CWD in the forest floor as a safe site for the seedlings after tree-replacing disturbance in natural forests is essential, particularly under ongoing climate change where more frequent and prolonged drought events are predicted. HighlightsO_LIOn a local scale, decayed coarse wood debris (CWD) provides "safe site" for the establishment of Chamaecypairs obtusa var. formosana seedlings. C_LIO_LIRegional climatic variables had limited effects on seedlings, but all regions had similar seasonal patterns of seedling mortatlity. C_LIO_LIFacilitation effect of small-stature bryophytes and litterfall avoidance might be the underlying mechanisms behind CWD safe sites. C_LIO_LIPreserving CWD for seedlings is important in the context of predicted prolong drought events under ongoing climate change. C_LI