The effects of precipitation, tree phenology, leaf area index, and bark characteristics on throughfall rates by urban trees: A meta-data analysis

Rainfall partitioning by urban tree canopies plays an important role in urban hydrological cycles and, potentially, on stormwater runoff. While the body of research to quantify rainfall partitioning processes by urban trees is growing, there has not yet been a concerted effort to quantitatively synthesize these data. This meta-data analysis attempts such synthesis to deepen understanding of urban tree throughfall processes in response to rainfall depth, leaf type and phenological periods, bark texture, and leaf area index (LAI). Nearly 400 precipitation partitioning events observed across 8 different studies were evaluated. Rainfall depth was a strong predictor for canopy throughfall, though tree characteristics also influenced throughfall response. Most notably, leaf type significantly affected throughfall rates, with the lowest rates observed for needleleaf evergreen trees (regression slope = 0.56). Deciduous leafless trees exhibited a lower threshold for throughfall generation than deciduous leafed or broadleaf evergreen trees; however, the slope of the regression line among these leaf types (0.8 to 0.86) was not significantly different. The effects of bark roughness were most evident for deciduous-leafless trees. Throughfall rates below rough-barked leafless trees were significantly lower than under their smooth-barked counterparts. Throughfall rates also tended to decrease as LAI increased; however, the effects of precipitation depth remained more influential across the range in precipitation events examine. Comparison to interception rates reported for rural forests suggests overlapping but potentially higher interception rates in urban tree canopies. By quantifying the influence of factors such as precipitation depth and tree characteristics on precipitation partitioning by urban trees, the outcomes of this synthesis can inform future efforts to more explicitly account for reductions in incipient rainfall and associated runoff by urban trees in stormwater management policies.