Abstract
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.
Highlights
Grazing exclosures are the initiatives implemented on communal grazing lands to exclude them from human and animal interferences and promote the natural regeneration of the vegetation, thereby restoring the forest cover and associated biodiversity (Aerts et al, 2009; Birhane et al, 2017, 2007)
We explored how species richness and aboveground carbon (AGC) stock varied along the altitudinal gradients using generalized linear mixed-effects models (GLMMs), with altitudinal zones, i.e., highland, midland and lowland, as the explanatory variable and plot included as a random factor
There was a significant interaction between species richness and altitudinal zones on AGC stock (F= 21.12; p
Summary
Grazing exclosures are the initiatives implemented on communal grazing lands to exclude them from human and animal interferences and promote the natural regeneration of the vegetation, thereby restoring the forest cover and associated biodiversity (Aerts et al, 2009; Birhane et al, 2017, 2007). There has been evidence of neutral patterns in natural forests and human-modified ecosystems (Mensah et al, 2020a; Sullivan et al, 2017) This lack of consistency across biogeographical areas suggests that biodiversity and ecosystem functioning (BEF) relationships are controlled by an interplay of complex ecological processes, which may vary with environmental gradients, stand developmental stages, ecosystem type and structure and spatial scale (Grace et al, 2016; Hao et al, 2020; Mensah et al, 2018b, 2020d; Teixeira et al, 2020). While attempts have been made to explain the shifting biodiversity-ecosystem functioning (BEF) relationships across vegetation type (Mensah et al, 2020a), forest succession stages (Liu et al, 2018; Teixeira et al, 2020) and spatial scales (Chisholm et al, 2013; Sullivan et al, 2017), further insights from a wide range of altitudinal gradients might help resolve this question because controlling biotic and abiotic factors vary along altitudinal gradients
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