Tree species mixing affects throughfall in a young temperate forest plantation

Abstract

Rainfall partitioning is a key process in forest ecosystems, affecting evaporation, transpiration and water storage in the canopy, litter and soil. Although rainfall partitioning has been well studied in forest ecosystems, less is known about how rainfall partitioning is modified by tree species diversity. To disentangle the effects of tree species identity and diversity on rainfall partitioning, we installed 128 rain gauges in an experimental plantation, located in Zedelgem (Belgium), part of a large, multi-site forest diversity experiment in Belgium (FORBIO). With these rain gauges, we monitored throughfall for a full year, with a temporal resolution of two weeks. Based on these data, we analysed identity (i.e. whether throughfall is species-specific), non-additive (i.e. whether throughfall in mixtures is different from the expected amounts based on monocultures) and richness effects (i.e. how species richness affects throughfall). First of all, we found that rainfall partitioning highly differed between tree species, due to pronounced species identity effects. The presence of Pinus sylvestris [pine] was always associated with lower throughfall. Year-round throughfall was, for example, the lowest in mixtures of pine and Tilia cordata [lime] (43.8 ± 14.5% of the open-field rainfall) and monocultures of pine (48.6 ± 11.8%). Additionally, non-additive effects were significant, leading to a 10% decrease of rainfall reaching the forest floor in mixtures compared to the expected throughfall based on measurements in monocultures. Tree species richness effects on rainfall partitioning acted indirectly, mostly via an increased basal area in mixed stands. Our findings suggest that mixing tree species leads to a higher than expected reduction in throughfall, which lowered soil moisture in mixed plots. Whether this reduced throughfall and soil moisture affects ecosystem functioning remains to be quantified.