Master thesis at the Institute of Ecology and Evolutionary Biology, National Taiwan University (March 2022)
Author: Wei Shuo (魏碩)
Advisor: David Zelený
Chamaecyparis obtusa var. formosana is an ecologically and economically important species in Taiwan, with a high affinity for fog immersion. Our study aims to investigate how different ecological factors influence the early-stage establishment and survival of the seedlings, focusing primarily on the effect of large-scale climatic variables, small-scale microhabitat conditions, and biotic interactions. Also, we linked the field observation to a controlled experiment to identify the causal relationship between seedlings’ performance and key ecological factors, namely short-duration drought, substrates, and bryophyte cover.
We utilized three sampling and analysis schemas to quantify seedlings’ early-stage establishment, survival, and performance, respectively. Our field survey identified two primary causes of death for seedlings: environmental filtering and herbivory. On a regional scale, we observed limited variation in regional climatic variables, demonstrating no effect on seedlings’ establishment and survival. However, we noted a similar seasonal survival pattern among regions and hypothesized that short-duration droughts occurring during the transition from Mei-yu to typhoon season might be the underlying mechanism. On a local scale, we found that decayed coarse wood debris (CWD) can facilitate seedling establishment by providing a “safe site” for the seedlings, likely due to increased bryophytes colonization and decreased litterfall accumulation. Our controlled experiment showed that short-duration drought could stress the seedlings and that bryophyte carpets could be beneficial by providing additional nutrient input via humus accumulation. We also found that litter dynamics significantly hindered seedling establishment and survival. Additionally, we found that seedlings might need to optimize their growing conditions by compensating for risks posed by both causes of death.
Our study verified the common belief of foresters in Taiwan that seedlings of Chamaecyparis obtusa var. formosana can establish on CWD in forests in their early-stage. We suggest that this phenomenon might be due to different microhabitat conditions between CWD and other substrates, particularly thanks to the effect of bryophytes and litter. Also, not only environmental filtering but also herbivore stress influences the final fate of the seedlings.
Keywords: Bryophytes, case-control study, coarse wood debris, competing-risk Cox proportion hazard (PH) model, greenhouse experiment, seedlings survival, split-plot design