A conceptual framework for understanding semi‐arid land degradation: ecohydrological interactions across multiple‐space and time scales

Abstract

15Land degradation is a problem prolific across semi-arid areas world-wide. Despite 16 being a complex process including both biotic and abiotic elements, previous attempts 17 to understand ecosystem dynamics have largely been carried out within the disparate 18 disciplines of ecology and hydrology which has led to significant limitations. Here, an 19 ecohydrological framework is outlined, to provide a new direction for the study of 20 land degradation in semi-arid ecosystems. Unlike other frameworks that draw upon 21 hierarchy theory to provide a broad, non-explicit conceptual framework, this new 22 framework is based upon the explicit linkage of processes operating over the 23 continuum of temporal and spatial scales by perceiving the ecosystem as a series of 24 structural and functional connections, within which interactions between biotic and 25 abiotic components of the landscape occur. It is hypothesised that semi-arid land 26 degradation conforms to a cusp-catastrophe model, in which the two controlling 27 variables are abiotic structural connectivity and abiotic functional connectivity, which 28 implicitly account for ecosystem resilience, and biotic structural and function 29 connectivity. It is suggested therefore that future research must (1) evaluate how 30 abiotic and biotic function (i.e. water, sediment and nutrient loss/redistribution) vary 31 over grass-shrub transitions and (2) quantify the biotic/abiotic structure over grass-32 2 shrub transitions, to (3) determine the interactions between ecosystem structure and 1 function, and interactions/feedbacks between biotic and abiotic components of the 2 ecosystem. 3 KEY WORDS 5Ecohydrology, vegetation transition, structure, function, biotic, abiotic, connectivity, 6 cusp catastrophe 7 8 Introduction 9

Sediment connectivity: a framework for understanding sediment transfer at multiple scales

“…Sediment connectivity is based on the interplay of structural components (morphology) and process components (flow of energy/transport vectors and materials) that determine the longterm behaviour of the sediment flux which is manifest as a change in landform (Preston and Schmidt, 2003;Turnbull et al, 2008;Bracken et al, 2013). Thus, sediment connectivity is dependent not on individual processes, but on all aspects of the geomorphic system that control sediment flux -processes of detachment, entrainment and transport -but also the emergent characteristics of sediment deposition and sediment residence times (Preston and Schmidt, 2003;Sandercock and Hooke, 2011).…”

Effects of plant diversity on soil erosion for different vegetation patterns

“…This relationship is thus ambiguous under different environmental conditions (Bautista et al, 2007). For example, a positive correlation between plant diversity and soil erosion has frequently been observed; however, a negative correlation has also been found (Turnbull et al, 2008). After conducting research in the Netherlands, Martin et al (2010) found that different vegetation conditions give rise to different correlations between plant diversity and soil erosion.…”

Woody plant encroachment into grasslands leads to accelerated erosion of previously stable organic carbon from dryland soils

“…Combined with changes in hydrological function, changes in ecosystem structure may precipitate a potentially catastrophic shift to a much less stable ecosystem state [Turnbull et al, 2008a] with an increased vulnerability to fluvial erosion. Data from two woody landscapes in basin and montane ecotones, which may be analogous with large areas of the basin and range southwestern U.S., showed that not only more organic C was eroded per unit area than in pristine grasslands Journal of Geophysical Research: Biogeosciences 10.1002/2014JG002635 but that this included large quantities of previously stabilized organic C from legacy grasslands.…”

“…However, woody encroachment in drylands alters ecosystem structure and function [Barger et al, 2011;Turnbull et al, 2008a], potentially leading to degradation [Okin et al, 2009;D'Odorico et al, 2013]. The change in structure, following encroachment, is often characterized by a reduction in vegetation cover [Turnbull et al, 2010a] and increased heterogeneity in both soil and vegetation resources [Dickie and Parsons, 2012;Bhark and Small, 2003].…”