台灣植群之平均科年齡與譜系結構沿海拔梯度之模式

Abstract

由於環境和歷史過程的多層次影響，要研究群落集合的機制是一項挑戰。然而，分析群落中物種的親緣關係可以提供關於群落如何形成及保留的資訊。兩個主要的假設解釋了沿著環境梯度的物種多樣性模式：1）熱帶生態位保守主義（TNC）和2）熱帶地區（OOT）假設。 TNC預測隨著溫度降低，平均群落的年齡往往更年輕，因為生態棲位受溫度限制，導致某些譜係不斷適應溫帶氣候。相反，OOT假設意味著隨機分佈的譜系快速遷移到較涼爽的氣候，在那裡它們緩慢多樣化，因此缺乏較年輕的譜系。台灣有年輕的地質年代、多樣的氣候條件和陡峭的海拔梯度，是一個適合觀察集合過程的模型系統。在這項研究中，我們專注於確定台灣木本被子植物的潛在機制。通過分析3000多個植被群落圖，我們發現隨著海拔的提升，溫度降低，木本被子植物群落中的親緣關係相關性趨於降低，而平均家庭年齡（MFA）在裸子植物森林影響之前的中等海拔高峰時達到峰值。這種模式與OOT假設是一致的，並且進一步得到了MFA與最冷月最低溫度等因素相關的研究結果的支持。通過將親源分析的方法應用於群落集合，提供有關不同海拔植物群落中物種的生物地理和演化來源的新資訊，我們的結果更多地告訴我們台灣的群落集合機制。

The mechanisms of community assembly are a challenge to investigate due to the multi-level influences from both environmental and historical processes. However, analyzing the phylogenetic relationships of species in a community may provide some insights into how communities are formed and retained. Two leading hypotheses that explain species diversisty patterns along environmental gradients are the 1) Tropical Niche Conservatism (TNC) and 2) Out of Tropics (OOT) hypothesis. TNC predicts that as niches are temperature constrained, when temperature decreases along the altitude only certain adapted lineages can continuously diversify to temperate climate, the mean community clade age tend to be younger. In contrast, the OOT hypothesis signifies a rapid migration of randomly distributed lineages to cooler climates where they diversified slowly, therefore lacking younger lineages. Taiwan is a suitable model system for the observation of the assembly process due to its young geological age, diverse climatic conditions and steep elevation gradient. In this study, we focus on determining underlying mechanisms for the woody angiosperm of Taiwan. By analyzing more than 3000 vegetation community plots, we found that with decreasing temperatures toward higher elevation, phylogenetic relatedness within the woody angiosperm communities tend to decrease while the mean family age (MFA) peaks at mid elevations just before gymnosperm forests influence. This pattern is coherent with the OOT hypothesis and is further supported by findings that show MFA correlates to factors such as lowest minimum temperature of the coldest month. By applying phylogenetic methods to community distributions, our results tell us more about Taiwan’s community assembly mechanisms, providing new information about the biogeographical and evolutionary source of species in plant communities at different elevations.