Interactions of past human disturbance and aridity trigger abrupt shifts in the functional state of Mediterranean holm oak woodlands

Abstract Empirical evidence of the vulnerability of dryland ecosystems to suffer abrupt changes in response to global change is highly needed to assess the applicability of threshold models, understand the underlying mechanisms and anticipate the onset of abrupt shifts. We study the onset of abrupt changes in Mediterranean holm oak (Quercus ilex) woodlands, by analyzing the combined effect of past human disturbance (mainly deforestation) and aridity on the functional state of these ecosystems. We determine ecosystem state in terms of herbaceous/woody species ratio, species richness and soil properties related to carbon and nitrogen cycling and microbial activity. The influence of both drivers was determined along spatial gradients of past human disturbance (from lightly to highly disturbed areas) and aridity (three levels: sub-humid, dry-transition, semi-arid). Our results show a strong interaction between aridity and past human disturbance on soil and vegetation properties. As disturbance increased, soil function decreased linearly in semi-arid and dry-transition conditions, but the trend was non-linear in sub-humid conditions. The latter showed a sharp decline at low-to-moderate disturbance intensity followed by a gentle recovery at higher intensities. These patterns were consistent for all soil properties. Structural changes in vegetation along the disturbance gradient, from open woodlands to shrublands in semi-arid and dry-transition conditions and from dense woodlands to grasslands in sub-humid conditions accompanied the linear vs non-linear patterns in soil function, respectively. Our results evidenced the existence of a critical climatic threshold in the boundary between sub-humid and dry-transition conditions, where the ecosystem switched from a fertile grassland to a highly degraded shrubland at moderate-to-high levels of disturbance (shrub encroachment), losing its capacity to buffer the effects of increasing disturbance. We propose a conceptual model illustrating the ecological mechanisms that may be underlying the onset of such abrupt shift.