Effect of Substrate Type and Bryophyte Presence on the Early‐Stage Performance of Chamaecyparis obtusa var. formosana Seedlings Under Different Drought Scenarios

Taiwan hinoki ( Chamaecyparis obtusa Sieb. & Zucc. var. formosana (Hayata) Rehder) is one of the dominant coniferous species in subtropical montane cloud forests in Taiwan, with a high affinity for fog immersion. With the changing climate, the species is expected to face new environmental challenges. Field observations indicate that seedlings are mainly establishing on coarse woody debris (CWD) rather than forest soil, and on surfaces covered by bryophytes rather than those without. To test this “safe site” hypothesis together with vulnerability to climate change, we conducted a factorial growth‐chamber experiment. We quantified seedling performance and functional traits under varying substrate types (forest soil vs. CWD), presence/absence of bryophyte carpets, and three increasingly short‐duration drought regimes. Contrary to the prediction of the hypothesis, CWD alone resulted in poor seedling performance due to severe nutrient limitations. However, bryophyte carpets acted as a critical facilitator; the humus layer accumulated beneath bryophytes provided high concentrations of N, P, and K, allowing seedlings on CWD to achieve growth rates comparable to those on forest soil. While drought was confirmed as a significant stress factor, neither CWD nor bryophytes provided strong evidence for the expected hydrological buffering capacity. These findings suggest that the regeneration success observed on CWD in the field is driven by bryophyte nutrient facilitation rather than the wood substrate itself. Consequently, while bryophytes can ameliorate nutrient stress, the seedlings' high susceptibility to drought indicates that increasing dry intervals under future climate scenarios poses a critical threat to early‐stage regeneration.