Aridity Induces Nonlinear Effects of Human Disturbance on Precipitation-Use Efficiency of Iberian Woodlands

The effects of ecosystem degradation are pervasive worldwide and increasingly concerning under the present context of global changes in climate and land use. Theoretical studies and empirical evidence increasingly suggest that drylands are particularly prone to develop nonlinear functional changes in response to climate variations and human disturbance. Precipitation-use efficiency (PUE) represents the ratio of vegetation production to precipitation and provides a tool for evaluating human and climate impacts on landscape functionality. Holm oak (Quercus ilex) woodlands are one of the most conspicuous dry forest ecosystems in the western Mediterranean basin and present a variety of degraded states, due to their long history of human use. We studied the response of Iberian holm oak woodlands to human disturbance along an aridity gradient (that is, semi-arid, dry-transition and sub-humid conditions) using PUE estimations from enhanced vegetation index (EVI) observations of the Moderate-Resolution Imaging Spectroradiometer (MODIS). Our results indicated that PUE decreased linearly with disturbance intensity in sub-humid holm oak woodlands, but showed accelerated, nonlinear reductions with increased disturbance intensity in semi-arid and dry-transition holm oak sites. The impact of disturbance on PUE was larger for dry years than for wet years, and these differences increased with aridity from sub-humid to dry-transition and semi-arid holm oak woodlands. Therefore, aridity may also interact with ecosystem degradation in holm oak woodlands by reducing the landscape ability to buffer large changes in vegetation production caused by climate variability.

This study was supported by a project (INDICAR, CGL2013-42213-R) funded by the Spanish Ministry of Economy, Industry and Competitiveness (MEIC). MMdlH research was supported by a Beatriu de Pinós fellowship (2014 BP-B00111) co-funded by the Generalitat de Catalunya and the European Commission, and a Juan de la Cierva fellowship (IJCI-2015-26463) funded by the MEIC. We are grateful to the NASA, IGN and AEMET for granting access to the MODIS EVI data, high-resolution PNOA orthophotos and precipitation records, respectively, that were applied in this study. We also thank two anonymous reviewers and the subject-matter editor, Brandon Bestelmeyer, for their helpful comments, and Jesús Romero for language corrections.

Peer reviewed