Assessing the Ecological Significance of Throughfall in Forest Ecosystems

Abstract

Throughfall is the incoming precipitation that passes through a vegetative canopy. Because it represents the major fraction of incident precipitation that is delivered to the forest floor, it is central to understanding forest hydrology. The quantity of throughfall in a forest stand is related to canopy architecture and leaf morphology, because canopy structure can enhance or reduce interception losses. Meteorological conditions also affect throughfall generation, as interception is greater for a given total annual precipitation regime when storms are small, frequent, and of short duration. The hydrology of throughfall generation and appropriate methods of throughfall collection are well studied, with the literature suggesting that an extensive network of collectors is often required to accurately assess total throughfall inputs. Throughfall is enriched in many solutes relative to incoming precipitation, with the biggest increases noted for dissolved organic carbon, some nitrogen species, and potassium. Much of the interest in throughfall biogeochemistry for the past 40 years has been spurred by its importance in measuring dry deposition and documenting the effects of acidic deposition on forest ecosystems. In contrast, the fundamental ecological and evolutionary implications of temporal and spatial variability in throughfall chemistry are not well understood. Literature from other disciplines suggests that allelopathy should be considered as one factor contributing to the variability in throughfall biogeochemistry among tree species. Future work should focus on developing long-term records of throughfall quantity and chemistry for all major solutes, assessing variation in throughfall across biomes, and understanding its potential ecological and evolutionary significance.