Soil water depletion and recharge patterns in mixed and pure forest stands of European beech and Norway spruce

Abstract

Automated time domain reflectometry (TDR) measurements in high resolution over soil depth and over time were performed in a mixed beech-spruce and a spruce stand during two hydrologically contrasting seasons. Soil drying was more intensive and reached deeper soil layers in the mixed stand, which on the other hand allowed more stand precipitation, compensating for the higher evapotranspiration rates. These results were confirmed by a large number of spatially distributed TDR measurements along grids of different spacing, which additionally covered a beech stand. Spatial water depletion patterns of the topsoil in spring appeared to be largely congruent with tree species distribution and reflected the higher water consumption of fully foliated beech. Variability was highest in the mixed stand, where a spatial correlation within a range of about 7 m was observed. The pure stands lacked spatial correlation. The effect of the mixed stand on soil water depletion and recharge turned out to be non-additive as compared to the pure stands of beech and spruce: changes of soil water storage under the mixed stand almost equalled the values measured in the beech stand. During selected drying periods in 2000 average daily water extraction rates from the uppermost 60 cm of soil amounted to 1.65 mm in the beech as well as in the mixed stand, which is about 45% more than under pure spruce. Maximum differences of up to 84% occurred in periods with high evaporative demand. The over-proportionate evapotranspiration of the mixed stand was exclusively attributable to beech, which deepened and intensified its fine-root system in mixture, while spruce rooted more shallowly. The mixed stand extracted a higher percentage of water from deeper soil layers than the pure stands.