Forest floors often host a several-cm-thick layer of plant matter in various states of decomposition that intercepts and transiently stores precipitation to either percolate downward or evaporate (up to >20% of annual precipitation). While prior research has revealed that litter has important effects on vegetation via modulation of heat and moisture fluxes (among other factors), that research has not been systematically conducted across ecosystems to yield generalizable findings or concepts regarding litter effects on terrestrial water fluxes. In fact, a review of literature reveals seemingly paradoxical inferences of litter layers driving (seemingly) countering hydrological effects. Litter can insulate soils from energy influxes and resist vapor effluxes (as mulch provides in a watered garden bed), reducing soil water loss. However, litter also intercepts precipitation such that it may evaporate before it reaches the soil. Thus, litter can have dichotomous effects on evaporation: (a) thicker litter layers imply more water storage and potential interception losses; while (b) thicker litter can also limit soil moisture losses through its insulative ‘mulching effect’. An unresolved challenge lies in understanding the ecosystem, climate, and litter conditions where the mulching effect (reducing soil evaporation) outweighs the litter interception effect (reducing precipitation inputs to the soil) and vice versa.
We propose to test hypotheses to advance our understanding of the relative roles of insulation and interception on soil moisture and evaporation across ecoregions represented in the National Ecological Observatory Network (NEON). The project will comprise four major research tasks. In Task 1, we will quantify litter-specific storage capacities and drainage parameters using litter collected from 32 NEON sites and an in-lab precipitation generator. In Task 2, at a subset of NEON sites we will measure soil and litter traits as well as in-situ surface evaporation rates to understand their interplay. In Task 3, we will use an ex-situ replicated plot-scale field study using litter collected from a subset of NEON sites to continuously monitor litter effects on soil water dynamics. In addition to using statistical analyses to test our hypotheses, in Task 4 we will also use the data generated from each prior task to constrain evaporation modeling parameters and to train and validate a soil water-balance model; that model will be applied to test meteorological controls over litter increasing interception losses versus reducing soil evaporation.
Task 1, conducted by NEON field crews, and Task 2, conducted by our team, are proposed to take place at Harvard Forest.