Closing the life cycle of forest trees: The difficult dynamics of seedling‐to‐sapling transitions in a subtropical rainforest

• Published in: The Journal of ecology 2021-07, Vol.109 (7), p.2705-2716
• Main Authors: Chang‐Yang, Chia‐Hao, Needham, Jessica, Lu, Chia‐Ling, Hsieh, Chang‐Fu, Sun, I‐Fang, McMahon, Sean M.
• Format: Article
• Language: English
• Subjects: Biology; community assembly; Demographics; Demography; Dynamic structural analysis; Dynamics; Fushan forest dynamics plot; Growth rate; Height; hierarchical Bayesian model; Life cycle; Life cycles; Life history; life‐history strategy; Mortality; non‐random ecological process; Observational studies; Plant species; Population biology; Probability theory; Rainforests; Seedlings; Species; species coexistence; stochastic simulation; Structure-function relationships; Survival; Temporal variations
• ISSN: 0022-0477; 1365-2745
• DOI: 10.1111/1365-2745.13677

Experimental and observational studies on seedling dynamics posit mechanisms that can influence forest diversity, structure and function. However, high mortality and slow growth of seedlings make it difficult to evaluate the importance of this life‐history filter to total tree life history. Quantifying the duration and transition of the seedling phase would help us understand this ‘black box’ in tree population biology. We used a 16‐year dataset of comprehensive seedling‐to‐sapling demography from a subtropical rainforest to construct population models that capture temporal demographic fluctuations for eight major tree species. We used data‐driven demographic models and simulations to estimate the transition ratios from newly recruited seedlings to saplings of 2‐m height and the time taken to attain 2‐m height for a newly recruited seedling conditional on its survival. Projections among species estimated that as few as 57 to more than 40,000 seedlings (with a median of 2,087) were required to make a single 2‐m high sapling. Furthermore, it would take 22–200 years (with a median of 47) for a newly recruited seedling to become a 2‐m high sapling. We found that temporal variation in demographic rates could greatly reduce the number of seedlings per established sapling, but not passage times (PTs). We also identified the importance of consistently fast growth rates for seedlings to escape the high mortality of early stages. Synthesis. Our findings demonstrate that high mortality in the very early seedling stage severely limits the probability that a newly recruited seedling will transition to the sapling stage. Although the PTs vary, we found this to be true across species with a range of life‐history strategies. Only seedlings with consistently fast growth rates are expected to pass through this life‐history filter. Findings from seedling studies should consider how short‐term studies of seedling demography might capture the rare exceptional individuals and exceptional conditions that might define the dynamics of this seedling bottleneck. 摘要 許多小苗的動態監測與實驗研究均假定此階段的動態變化會影響森林的多樣性、結構與功能。然而，受限於小苗的高死亡率與低生長率，我們很難評估小苗階段的篩選對於樹木整體生活史動態的重要性。具體量化樹木個體在小苗階段的停留時間與轉換比例，可以幫助我們了解這個樹木族群動態中的「黑盒子」。 我們使用台灣福山亞熱帶雨林長達16年的小苗動態監測資料，來建構該森林中8種主要樹種的族群模式，此一模式可以忠實呈現樹木小苗生長率、死亡率隨時間變化之特性。我們透過此一族群模式進行數值模擬，估算樹木從剛萌發的新增苗成長到2 m高小樹之轉換比例，以及若小苗可以順利存活，其成長到2 m高所需的時間。 依據模式所推估的結果，有的樹種平均只需要57株新增苗就能有一株能長成2 m高的小樹，但也有樹種需要超過40,000株新增苗才能有一株小樹 (中位數為2,087株小苗)。而樹木個體在這個階段的停留時間，則介於22到200年 (中位數為47年)。當模式中納入小苗存活率與生長率之年間