Comparison of Rainfall Partitioning and Estimation of the Utilisation of Available Water in a Monoculture Beech Forest and a Mixed Beech-Oak-Linden Forest

Monoculture forests formed by Fagus sylvatica L. belong to one of the most sensitive forest ecosystems, mainly at low altitudes. Cultivation of this species in mixed stands should reduce its sensitivity to drought in the vegetation period, which is why we researched the water balance in one pure-beech (i.e., monoculture) and one beech–oak–linden (i.e., mixed) forest. This research was carried out in Drahanská vrchovina in the Czech Republic in the period 2019–2021. The total precipitation was measured, together with its partitions (i.e., throughfall and stemflow), and the crown interception was also calculated. The total forest transpiration was calculated from the values measured on the sample trees. The values of each rainfall partition and transpiration (and their percentages) were compared. The rainfall partitions in the monoculture forest differed from those in the mixed forest. While, on average, the annual percentages of the throughfall, stemflow and crown interception in the monoculture forest were 63%, 6% and 31%, respectively, these partitions in the mixed forest were 76%, 2% and 22%, respectively. The crown interception was greater in the monoculture (31% of precipitation) and the effective precipitation (i.e., the sum of throughfall and stemflow) was greater in the mixed forest (78% of precipitation). The greatest differences (in each rainfall partition) between the monoculture and mixed forest were in the summer and winter. The throughfall was greater in the mixed forest (ca. 22% in the summer and ca. 12% in the winter), and the stemflow was greater in the monoculture forest (ca. 66% in the summer and ca. 51% in the winter). The mean annual transpiration was 318 (±52) mm in the monoculture and 451 (±58) mm in the mixed forest, i.e., about 99 (±65) mm more in the mixed forest than in the monoculture forest. The transpiration, in comparison with the effective precipitation, made up, on average, 70% of the effective precipitation in the monoculture forest. On the other hand, the transpiration reached 71% (in 2019), 74% (in 2020) and even 100% (in 2021) of the effective precipitation in the mixed forest. Our results show that an oak–beech–linden mixed forest can manage water better than a beech monoculture because more precipitation leaked through the mixed forest onto the soil than through the monoculture, especially via the throughfall in the summer. On the other hand, the amount of water that transpired was greater in the mixed forest than in the monoculture. However, the utilisation of the effective precipitation by trees was very similar in the monoculture in all three years, while, in the mixed forest, the utilisation of the effective water by trees increased, which may have been caused by the saturation of the deeper soil layers with water in the first two years of measurement. We can, Therefore, say that, at lower altitudes, it will be more suitable in the future to cultivate beech in mixed forests because of the assumed lack of water (mainly in early spring and summer).