The Effect of Tree Decline Over Soil Microclimate Largely Controls Soil Respiration Dynamics in a Mediterranean Woodland

Abstract

As drought-induced tree defoliation and mortality (i.e. tree decline) in the Mediterranean is expected to worsen with ongoing climate change, it is of paramount importance to understand how, why, and when tree decline affects soil respiration (Rs) and its relationship with soil microclimate and other potential controls. We carried out a novel study exploring the interacting effects of climatic variability and tree decline on soil microclimate and Rs temporal variability in a Mediterranean holm oak woodland. The study further explores the effects of tree decline on the main controls of Rs at the stand scale. We monitored Rs, soil temperature (Tsoil), and soil volumetric water content (SWC) under the canopy of 30 holm oak trees with different defoliation degrees (healthy, affected, and dead) during two years of contrasting precipitation patterns. We estimated different plant structural variables (e.g. DBH, height, and canopy diameter) on those selected trees under whose canopies we also collected soil samples to analyze different soil physicochemical variables. Our results stress the important role of tree health as a modulator of the response of Rs to SWC, with stronger responses of Rs to variations in the amount and distribution of precipitation under healthy than under declining trees. They also suggest that tree decline can significantly increase SWC and decrease Rs but largely depending on the declining stage, the year, and the season. Finally, tree decline also affected the relationship of Rs with soil microclimatic variables, particularly SWC, and the relative importance of the different drivers of Rs, with microclimate variables gaining importance as trees defoliate and die. Altogether, our results point towards a negative impact of drought-induced tree decline on soil C content and cycling, particularly under forecasted climate change scenarios with dryer and more intense precipitation regimes.