Rainfall partitioning and associated chemical alteration in three subtropical urban tree species

Rainwater is altered hydrologically and chemically as it passes through tree canopies to reach the soil surface, but this aspect of urban forest ecosystems has received less attention than that of natural ecosystems. Hydrological partitioning and changes in solute composition of rainfall were examined in an urban forest within Guangzhou city, in southern China, with reference to three typical subtropical urban tree species (Roystonea regia, Ficus microcarpa, and Lagerstroemia speciosa) with large but morphologically distinct canopies. Three components of rainfall, namely throughfall, stemflow, and interception, were measured and sampled from September 2018 to September 2019. A canopy budget model was used for estimating the contribution of canopy exchange and dry deposition to net rainfall (throughfall + stemflow) solute flux. Being a palm, R. regia recorded the highest percentages of throughfall (72.60%) and stemflow (8.68%) whereas interception was higher in F. microcarpa (33.48%) and L. speciosa (30.54%). The concentrations of almost all the solutes (Ca2+, K+, Na+, Mg2+, Cl, NO3- , and SO42-) in throughfall and stemflow were higher than those in rainfall, suggesting that water was enriched with these solutes as a result of passing through the canopy. The model of canopy budget also showed that whereas all the three species were similar in terms of the leaching of macronutrient cations, the species differed in term of anions: whereas R. regia and L. speciosa generally absorbed or retained anions on foliage, twigs, or trunks, F. microcarpa leached Cl and NO3- (presumably because of its unique aerial root). These results can inform the design of urban forests. For example, the lack of NO3- uptake by F. microcarpa suggests that it would be less suitable for mitigating the problems of N deposition which are significant in southern China than the other two species; on other hand, the rainfall partitioning and canopy exchange characteristics of L. speciosa make it more suitable for intercepting rainwater as well as for alleviating the problem of N deposition. The findings of this study therefore have significant implications for the selection of urban tree species in similar subtropical regions worldwide.