Assessing the impacting factors responsible for the variation in the throughfall rate of black pine trees in diverse urban climates

The contribution of trees in altering the hydrological cycle necessitates evaluating the effects of meteorological conditions, leaf cover, seasonal variations, and rainfall magnitude on throughfall beneath pine tree canopy across different climates. Understanding trees' rainfall interception characteristics is essential for effective urban greenery planning and stormwater management. Thus, this study aimed to examine the influencing factors that are responsible for the variation in the throughfall rate of black pine trees (Pinus nigra Arnold) in diverse urban climates. To achieve the aim of this study, we analyzed gross rainfall and throughfall at two research experimental sites in the city of Ljubljana, Slovenia, and the city of Sopron, Hungary. The measurement period spanned from September 2023 to September 2024, with 42 and 51 rainfall events recorded at the sites, respectively. Both manual and automatic meteorological measurements were conducted at each site.

The mean throughfall over the measurement period was 45% in Ljubljana and 50% in Sopron, with average rainfall intensities of 2.02 mm/h and 1.96 mm/h, respectively. Throughfall patterns were analyzed across phenological and calendar seasons and rainfall magnitude. Both sites exhibited similar seasonal trends, but Sopron consistently had higher throughfall than Ljubljana in both phenological periods, with throughfall percentages increasing as rainfall magnitude increased. Throughfall was higher in Sopron during autumn, winter, and spring, whereas Ljubljana had higher throughfall in summer. The difference between the leafed and leafless seasons was 2.0% in Ljubljana and 1.9% in Sopron, with higher throughfall recorded during the leafless period at both sites.

The impact of the meteorological variables and canopy characteristics on throughfall across the year, leafed, and leafless periods was investigated using the regression tree (RT) and boosted regression tree (BRT) models. Rainfall amount was the primary predictor in all cases, but secondary factors varied by site and season. RT analysis showed that relative humidity and leaf area index (LAI) impacted throughfall in Ljubljana, while relative humidity, LAI, and rainfall duration were significant in Sopron during the year. Seasonal variations affected these influences, with rainfall amount impacting throughfall only during the leafless period, while wind speed and relative humidity played key roles in the leafed season for Ljubljana and Sopron, respectively. BRT analysis further confirmed that relative humidity influenced throughfall year-round at both sites. Furthermore, rainfall intensity and wind speed became critical during the leafed season in Ljubljana and the leafless season in Sopron.

The 5% variance in the mean throughfall observed between the two locations underscores the effect of microclimatic conditions and canopy attributes on the interception of rainfall by a pine tree canopy. These findings strengthen the need for site-specific hydrological assessments to enhance tree-based stormwater management practices in urban environments.

Acknowledgment: This study is part of ongoing research entitled "Microscale influence on runoff" supported by the Slovenian Research and Innovation Agency (N2-0313) and National Research, Development, and Innovation Office (OTKA project grant number SNN143972). The work was also supported through the Ph.D. grant of the first author which is financially supported by the Slovenian Research and Innovation Agency.