Ecohydrological turnover in overstocked Aleppo pine plantations: Does the effect of thinning, in relation to water, persist at the mid-term?

Abstract

In Mediterranean pine plantations forest dieback and tree mortality are not only related to increased drought, but also to a lack of management, which intensifies inter-tree competition for available soil water. In this complex context simple but also difficult questions such as why, how and when manage forests should be directly responded and quantified by applied science. In this study we specifically analysed the forest-water relationships of an Aleppo pine plantation where experimental thinning was carried out ten years ago at three different intensities (H: high-, M: moderate- and L: low-thinning plots plus a control one, C). To this end, we again measured tree sap flow, soil water content and meteorological conditions. In addition, the relative importance (RI) of thinning intensity and environmental drivers when explaining tree/stand-water at the short-term were compared with those obtained in this study in order to elucidate how the role of thinning intensity may change on time. The impact of thinning on soil water content showed that significant differences were maintained after ten years (H > M > L > C), but that values between the different thinning intensities were closer than those observed at the short-term. In contrast, tree transpiration from the high-thinning plot was very similar to that from the moderate-thinning one (means of 13 and 14.7 l·day−1, respectively). These results support the idea that an excessive forest opening makes the understorey compete more strongly for water, thus counterbalancing the higher tree transpiration observed in the short-term. The combined analyses of thinning intensity and environmental drivers highlight how the role of thinning intensity in controlling tree and stand transpiration in the short-term was clearly replaced by soil water availability ten years after the thinning intervention (RI means from 13.1 to 39.5% for soil water availability and from 26.8 to 19.0% for thinning intensity). Our results support the need to study how the transpiration-soil water relationships progressively change over the distance in time from thinning in order to assess the impact of understorey properly and thus systematically calculate the ecohydrological turnover at every thinning intensity tested.