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Topmost trees and foremost species underlie tropical forest structure, diversity and biomass through opposing mechanisms
Forest Ecology and Management ( IF 3.7 ) Pub Date : 2020-10-01 , DOI: 10.1016/j.foreco.2020.118299
Arshad Ali , Eskil Mattsson , S.P. Nissanka , Li-Qiu Wang

Abstract Tropical forests play a main role in the global carbon cycle due to their higher exchange capacity of carbon dioxide with the atmosphere than any other forest type on the Earth. In this study, we aimed to explore the relative importance of foremost species and topmost trees in shaping forest structure, diversity and biomass in natural tropical forests. We hypothesized that topmost trees promote but foremost species decline tropical forest structure, diversity and biomass in a changing environment (i.e. the ‘tree overtopping hypothesis’). We formulated three questions to address the proposed hypothesis: (1) Are forest structure, diversity and biomass affected by both foremost species and topmost trees, and what is the magnitude and direction of each relative effect? (2) Are foremost species and topmost trees influenced similarly by multiple environmental factors? (3) How do foremost species and topmost trees mediate the feedbacks of forest structure, diversity and biomass to environmental factors? Using 189 plots data from Sri Lanka, we quantified 16 environmental (9 climate and 7 soil) factors, two indices of the topmost trees (i.e. top 1% large-diameter, and tall-stature) and their combination, four indices of foremost species (i.e. top 1% species' importance value index or each of its three components including either relative basal area, relative frequency or relative density), rarefied species richness, and stand density. We used structural equation modeling to test the proposed hypothesis. Strong positive effects of topmost trees whereas negative to negligible positive effects of foremost species shaped tropical forest structure, diversity and biomass through opposing mechanisms, i.e., the promoting part of big trees and suppressing part of dominant species, respectively. Moreover, forest structure promoted biomass directly and indirectly via forest diversity. Environmental factors (i.e. high climatic water and low soil fertility) increased forest structure, diversity and biomass indirectly via topmost trees but decreased via foremost species. The main novelty or contribution of this study determines that the adverse effects of few foremost (i.e. dominant or abundant) species shaped forest structure, diversity and biomass in tropical forests when simultaneously considered the positive effects of topmost trees. Hence, encouraging topmost trees while managing foremost species might necessarily increase complementarity resource-use within a forest community, leading to positive forest diversity – structure – biomass relationships. We argue that both topmost trees and foremost species might have important influences on forest carbon stock in the context of global climate change.

中文翻译:

最顶端的树木和最重要的物种通过相反的机制构成热带森林结构、多样性和生物量的基础

摘要 热带森林在全球碳循环中发挥着重要作用,因为它们与大气的二氧化碳交换能力高于地球上任何其他森林类型。在这项研究中,我们旨在探索最重要的物种和最重要的树木在塑造天然热带森林的森林结构、多样性和生物量方面的相对重要性。我们假设,在不断变化的环境中,最顶层的树木促进了热带森林的结构、多样性和生物量,但最重要的物种会下降(即“树木越过假说”)。我们制定了三个问题来解决提议的假设:(1) 森林结构、多样性和生物量是否受到最重要的物种和最重要的树木的影响,每个相对效应的大小和方向是什么?(2) 最重要的物种和最上面的树木是否受多种环境因素的影响相似?(3) 最上层树种和最上层树如何介导森林结构、多样性和生物量对环境因素的反馈?使用来自斯里兰卡的 189 个地块数据,我们量化了 16 个环境因素(9 个气候和 7 个土壤)、两个最顶层树木的指数(即前 1% 的大直径和高个子)及其组合、四个最重要物种的指数(即前 1% 物种的重要性价值指数或其三个组成部分中的每一个,包括相对基面积、相对频率或相对密度)、稀有物种丰富度和林分密度。我们使用结构方程模型来检验提出的假设。最上层树木的强正效应,而最前种树种的负面效应至可忽略不计的正面效应分别通过相反的机制塑造了热带森林结构、多样性和生物量,即分别促进部分大树和抑制部分优势物种。此外,森林结构通过森林多样性直接和间接促进生物量。环境因素(即高气候水和低土壤肥力)通过最顶端的树木间接增加了森林结构、多样性和生物量,但通过最重要的物种而减少。本研究的主要新颖性或贡献决定了少数最重要(即优势或丰富)物种的不利影响塑造了热带森林中的森林结构、多样性和生物量,同时考虑了最顶层树木的积极影响。因此,在管理最重要的物种的同时鼓励最高的树木可能必然会增加森林社区内的互补资源利用,从而导致积极的森林多样性 - 结构 - 生物量关系。我们认为,在全球气候变化的背景下,最顶层的树木和最重要的物种都可能对森林碳储量产生重要影响。
更新日期:2020-10-01
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