Throughfall variability between oak and beech trees in a mountainous Mediterranean catchment

Forests and trees are integral part to the global water cycle and therefore vital for water security. Forest and mountain ecosystems serve as source areas for more than 75% renewable water supply, delivering water to over half the world’s population.

Throughfall generally represents about 70% of bulk precipitation, with a much smaller portion, less than 5%, delivered to the forest floor along tree trunks (i.e., stemflow), and the remainder (~25%) intercepted by the forest canopy and evaporated back to the atmosphere.

The partitioning of water into these three pathways is largely controlled by seasonality, precipitation characteristics, meteorological conditions in addition to physiological and morphological traits related to forest composition.

This study aims to determine the spatial and seasonal variability of throughfall in oak and beech trees growing on two hillslopes of contrasting aspect in the Ussita stream basin (44 km²), Apennine Mountains, central Italy.

Throughfall was measured during 30 sampling periods between July 2022 and December 2022 at four locations by means of gutters connected to tipping buckets. characterized by different land cover, e.g., beech trees and oak trees. Specifically, two monitoring plots are located on a hillslope facing south and the monitoring two stations are located on a hillslope facing north. Moreover, two meteorological stations provide open-area precipitation measurements.

The measurements show that the leafed canopy phase reduced the amount of throughfall in all four experimental sites. In particular, beech trees exhibited the largest inter seasonal differences in throughfall partitioning. This is mainly related to the rapid defoliation characterizing the beeches’ sites starting from September.

The volumetric throughfall was higher during medium and severe rainfall events, while during low rainfall the forest canopy was found intercepting most of the precipitation. On the contrary, during severe events, the forest canopy storage capacity was saturated and most of the rainfall occurring after the saturation was converted into throughfall.

The measurements carried out during medium rainfall events indicate that the differences between canopy structure in oak and beech trees, such as the number of canopy layers and branches orientation, can strongly affect the rainfall partitioning. Oak trees, with high number of canopy layers, low seasonal defoliation and roughness of the bark, have higher canopy storage values than beech trees and are able to generate less throughfall.

Additional data to be collected during the next months will allow us to extend the results achieved in the first phase of analysis.