Influence of structural complexity of the Rumperöd forest on evapotranspiration

Abstract: The influence of Forest Structural Complexity (FSC) on evapotranspiration (ET) was investigated in a mixed forest of Southern Sweden where micrometeorological Eddy Covariance (EC) measurements were conducted for the years 2015 to 2022. For each year, the top 25% of Vapour Pressure Deficit (VPD) were selected to emphasize the response of ET to the driest atmospheric conditions. FSC was quantified using three indices. First, the Gini index represents the variation in basal area. Second, the Rumperöd Index (RI), specifically created for this study, considers the difference in tree height weighed by basal area, and includes stand density. The third index, named β, quantifies the proportion of coniferous species. These indices were calculated for eight transects around the EC tower. To investigate the influence of FSC on ET, fluxes were sorted by wind direction and compared to the indices per transect. To verify that the residuals of ET to VPD were dependent on the transects, a generalised linear model using a Wald Chi-Square test was performed. Species composition had a great influence on ET under increasing dry conditions: coniferous transects usually kept photosynthesizing at a higher VPD than deciduous. Species composition associated with high variation in basal area (Gini index) could explain higher annual ET. The ET response to a high complexity in RI resembled that of the Gini index, but high stand density and poor species diversity enhanced competition for water resources, hence a lower annual ET. The FSC indices together explain patterns of ET and show its dependence on FSC, which is supported by the statistical tests. This study shows the importance of structural complexity to sustain hydraulic functioning of forest stands under dry conditions.