Changes in rainfall pH after passing through the forest canopy: Increase in throughfall but decrease in stemflow

Abstract

Acid rainfall has the potential to harm the structure and function of forest ecosystems. Forest canopy, which is the first surface of contact for rainfall entering the forest ecosystem, may play a role in filtering acid and increasing pH, acting as a natural protective layer for the understory communities in the forest. However, there is currently a lack of global quantitative assessments on the impacts of forest canopy on rainfall pH. To better understand and predict the relationship between forest canopy and rainfall pH that reaches the forest floor (i.e., throughfall and stemflow), we systematically combined data from 244 published articles, including 2563 paired observations for throughfall and 1431 for stemflow, to investigate whether forest canopy has a buffering effect on rainfall pH. We found that (1) throughfall pH significantly increased by an average of 0.14 after passing through forest canopy, while stemflow pH significantly decreased by 0.74; (2) understory rainfall (throughfall and stemflow) ΔpH was higher in broadleaved compared to needleleaf forests, while stemflow ΔpH was lower in evergreen forests and plantations than in deciduous, primary or secondary forests; (3) throughfall ΔpH was mainly influenced by tree height, rainfall K+ concentration and leaf type, while stemflow ΔpH was mainly affected by stemflow ΔMg2+, ΔNO3- and leaf phenology. Our results showed that forest canopy has a significant impact on the pH of rainfall entering forests, which can be influenced not only by rainfall partitioning, but also by vegetation type and rainfall characteristics. Our findings have important implications for predicting understory rainfall pH in the context of global climate change and will contribute to a better understanding of nutrient cycling associated with hydrological processes in forests.