Grazing exclosures have been promoted as an effective and low-cost land management strategy to recover vegetation and associated functions in degraded landscapes in the tropics. While grazing exclosures can be important reservoirs of biodiversity and carbon, their potential in playing a dual role of conservation of biodiversity and mitigation of climate change effects is not yet established. To address this gap, we assessed the effect of diversity on aboveground carbon (AGC) and the relative importance of the driving biotic (functional diversity, functional composition and structural diversity) and abiotic (climate, topography and soil) mechanisms. We used a dataset from 133 inventory plots across three altitudinal zones, i.e., highland, midland and lowland, in northern Ethiopia, which allowed local- (within altitudinal zone) and broad- (across altitudinal zones) environmental scale analysis of diversity-AGC relationships. We found that species richness-AGC relationship shifted from neutral in highlands to positive in mid- and lowlands as well as across the altitudinal zones. Structural diversity was consistently the strongest mediator of the positive effects of species richness on AGC within and across altitudinal zones, whereas functional composition linked species richness to AGC at the broad environmental scale only. Abiotic factors had direct and indirect effects via biotic factors on AGC, but their relative importance varied with altitudinal zones. Our results indicate that the effect of species diversity on AGC was altitude-dependent and operated more strongly through structural diversity (representing niche complementarity effect) than functional composition (representing selection effect). Our study suggests that maintaining high structural diversity and managing functionally important species while promoting favourable climatic and soil conditions can enhance carbon storage in grazing exclosures.

提倡放牧作为一种有效且低成本的土地管理策略，可在热带地区退化的景观中恢复植被和相关功能。尽管放牧的禁猎物可能是生物多样性和碳的重要储存库，但它们在发挥保护生物多样性和减轻气候变化影响的双重作用方面的潜力尚未建立。为了解决这一差距，我们评估了多样性对地上碳（AGC）的影响以及驱动生物（功能多样性，功能组成和结构多样性）和非生物（气候，地形和土壤）机制的相对重要性。我们使用了来自埃塞俄比亚北部三个高海拔地区（即高地，中部和低地）的133个清单地块的数据集，这允许对多样性（AGC）关系进行局部（在垂直区域内）和广泛（在整个垂直区域内）环境规模分析。我们发现物种丰富度-AGC关系从高地的中性转变为中低地以及整个海拔区域的正向。结构多样性始终是物种丰富度对海拔区域内和跨海拔区域的AGC产生积极影响的最强媒介，而功能成分仅在广泛的环境规模上将物种丰富度与AGC联系起来。非生物因子通过生物因子对AGC具有直接和间接影响，但它们的相对重要性随海拔区域而​​变化。我们的结果表明，物种多样性对AGC的影响取决于海拔高度，并且通过结构多样性（代表生态位互补效应）比功能组成（代表选择效应）更有效地起作用。我们的研究表明，保持较高的结构多样性和管理功能重要的物种，同时促进有利的气候和土壤条件，可以提高放牧场所的碳储量。