Master thesis at the Institute of Ecology and Evolutionary Biology, National Taiwan University (March 2022)

https://doi.org/10.6342/NTU202300650

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

臺灣扁柏為臺灣重要之珍貴樹種，具有生態與經濟上的重要價值。本研究利用三種不同的取樣方法與分析方式，探討扁柏小苗的建立、存活與生長表現在大尺度氣候因子、小尺度的微棲地因子與生物間交互作用中如何形塑扁柏小苗早期建立的安全域（safe sites），並透過串連野外調查的結果與溫室實驗，進一步建立野外觀察中的重要因子（短期乾旱逆境、基質類型與蘚苔覆蓋）與小苗早期建立之間的因果關係。

野外調查結果顯示，環境選汰與草食獸取食為小苗早期建立的主要死亡原因。區域性氣候因子在研究區域間的變異度較小，對小苗的建立與存活影響有限，但皆呈現類似的季節性趨勢。研究推論，環境選汰的死亡原因可能為梅雨過渡到颱風季前的短期乾旱逆境所造成；另一方面，腐朽程度較高的粗木質殘體因有較高的蘚苔覆蓋與較少枯落物覆蓋等因素，因此可以促進小苗的建立。溫室實驗證實短期乾旱逆境對於小苗有負面影響，而蘚苔可透過腐植層的積聚提供較好的養分環境，因此在小苗建立與存活上扮演重要角色。同時，枯落物會阻礙小苗的根系生長與覆蓋小苗個體，對小苗的建立與存活有負面影響。此外，我們發現小苗可能需要權衡不同死亡原因所造成的影響來最大化其生長。

綜上所述，本研究證實林業界對於扁柏小苗可在倒木上建立的觀察，並指出此現象可能為粗木質殘體相較於其他基質類型有不同的微環境，尤其是透過不同程度的蘚苔與枯落物累積的影響，並發現小苗的早期建立不僅受到環境選汰，也同時受到草食壓力的影響。

關鍵字：粗木質殘體、苔蘚、案例對照研究、競爭風險模型、裂區實驗、溫室實驗、小苗存活