Throughfall and stemflow: The crowning headwaters of the aquatic carbon cycle

Abstract

Dissolved organic matter (DOM) is a master variable that modulates the form and function of many ecosystems. Approximately, half of the mass of DOM is carbon. Fluxes of DOM transfer carbon and other vital elements between ecosystems and between organisms (e.g., trees to bacteria) and components (e.g., vegetation to soil) within ecosystems. The DOM flux out of trees and understory plants to the forest floor is a poorly studied component of the carbon and nutrient budgets of forest ecosystems. In freshwater systems, studies of DOM transport through terrestrial systems usually start at the stream. However, the interception of rainwater by vegetation marks the beginning of the terrestrial hydrological cycle making plant canopies the crowning headwaters of terrestrial aquatic carbon cycling. Rainwater interacts with canopies picking up DOM, which is then exported from the plant in stemflow and throughfall, where stemflow denotes water flowing down the plant stem and throughfall is the water that drips from and through the leaves, branches, and epiphytes of the canopy. As nearly all studies of vegetation-derived DOM to date report DOM derived from tree canopies (tree-DOM), in this chapter we discuss the quality, potential sources, and potential fates of tree-DOM. We then describe and discuss the drivers of variation of quantitative fluxes of tree-DOM and place these quantitative fluxes in biogeochemical and ecological contexts at scales ranging from the individual tree, forest, and watershed to global trends.