What happened when a Mediterranean woodland stayed wetter in summer?

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Abstract. Precipitation patterns in the Mediterranean region are expected to change over the coming decades, with less total rainfall and more extreme events projected.

Researchers manipulated precipitation in a Mediterranean woodland dominated by Arbutus unedo L. to study effects on above-ground net primary production (ANPP) and soil carbon dynamics, specifically plant-derived carbon input to soil and soil respiration (SR). Experimental plots received either a 20% reduction in throughfall or added water intended to keep soil water content above a minimum of 10% v/v. Both treatments were compared with control plots receiving ambient precipitation.

Compared with controls, enhanced summer soil moisture increased annual stem primary production, litter fall, SR and net annual plant-derived carbon input to soil by an average of 130%, 26%, 58% and 220%, respectively. In contrast, the 20% reduction in throughfall, equivalent to a 10% reduction in precipitation, did not significantly change soil moisture at the site and therefore did not significantly affect ANPP or SR.

The researchers concluded that minor changes in precipitation amount, around a 10% reduction, are unlikely to significantly affect ANPP or soil carbon dynamics in Mediterranean woodlands. If summer rain increases, however, carbon cycling will significantly accelerate, while soil carbon stocks are unlikely to change in the short term. They called for more studies modelling long-term carbon dynamics to predict whether the estimated increases in soil carbon input under wet conditions will persist and whether labile carbon is being substituted for stable carbon, negatively affecting long-term soil carbon stocks.