Responses of canopy hydrometorological parameters to oak dieback in the Mediterranean sparse forest, Iran

The potential for sudden tree dieback exists when there is significant variation in perturbation frequency and intensity, which can alter canopy-atmosphere interactions, like canopy rainfall partitioning. In the context of close-to-nature silviculture practices, dieback trees can endure for several decades in specific environments. Therefore, it is warranted to explore the interaction between tree dieback and rainfall partitioning, including throughfall, stemflow, and rainfall interception. The primary aims of this study were to (a) measure the partitioning of incident rainfall into throughfall, stemflow, and rainfall interception, beneath Quercus brantii (Lindl; Brant's oak) canopies in various dieback states in a Mediterranean region of Iran, and (b) quantify canopy and trunk ecohydrological parameters in the various treatments. To do this, we randomly selected 30 trees in three crown dieback states (i.e., healthy, moderate dieback, and severe dieback classes), with two diameters at breast height (DBH) classes (small and medium DBH). The rainfall partitioning, as well as the underlying canopy ecohydrological parameters, differed meaningfully across the oak dieback classes. This is most likely driven by changes in forest canopy structure as oak dieback progresses. The greatest changes included the increase in throughfall and a decrease in interception with leaf mortality. Regarding stemflow input, we observed significant stemflow variability among canopy dieback states-with stemflow percentage comprising 7.6 % of rainfall for healthy small DBH oaks, decreasing by half in severely declined ones-and it was notable that the precipitation thresholds required for stemflow initiation increased from healthy to severely declined trees. However, it should be noted that while decreased intercepted water by declined oak trees may occur, this does not necessarily translate to a significant increase in the proportion of rainfall transformed into stemflow. After the decline of oak trees, there were significant alterations to the ecohydrological parameters of both the canopy and trunk. Notably, canopy ecohydrological parameters (e.g., crown saturation points and canopy storage capacity), which are integral to the interception and storage of rainwater, experienced a marked decrease. In contrast, trunk ecohydrological parameters had not a constant trend among treatments, and it would require further study. These findings can help clarify rainfall partitioning and subsequent ecohydrological parameters dynamics and driving factors across crown dieback treatments, in addition to that offer parameters for hydrological simulations specifically tailored to Mediterranean forests. Studying how rainfall is distributed and the resulting changes in ecohydrological parameters across various crown dieback states, phenological scales, and DBH classes can enhance our comprehension and predictive abilities concerning the contribution of forests to hydrological recharge processes.

Automated summary

The degree of dieback in oak trees has a significant impact on rainfall partitioning and ecohydrological parameters, with more severe dieback resulting in higher throughfall and lower rainfall interception. Furthermore, dieback of oak trees also leads to significant changes in canopy and trunk ecohydrological parameters.