Estimating forest-floor litter evaporation from above- and below-canopy flux tower data

Forests modulate precipitation and evapotranspiration fluxes. One important – yet often overlooked – component in the forest water cycle is the forest-floor litter layer. Organic matter on the forest floor retains significant amounts of annual precipitation (i.e., throughfall), subsequent evaporation from these forest-floor litter layers enhances below-canopy humidity, thereby potentially reducing atmospheric water demand in closed canopy stands. Evaporation fluxes from the forest floor are often attributed to transpiration, because partitioning of evaporation and transpiration is difficult and thus typically has large uncertainties. Here, we hypothesize that current partitioning estimates that do not account for forest-floor evaporation overestimate forest transpiration rates.

Previous measurements at our “WaldLab Forest experimental site” in Zurich and additional litter sampling in ~400 plots across the European Alps showed that needle and broadleaf litter retained up to 18% of annual precipitation or on third of annual evapotranspiration (ET), leading to substantial overestimates of recharge and transpiration in Alpine forest ecosystems. Here, we compare these results with temporally high-resolved water vapor flux data measured above- and below-canopy at the Swiss FluxNet sites Lägeren (CH-Lae; mixed deciduous forest) and Davos (CH-Dav; evergreen coniferous forest). We estimated the potential contribution of litter-layer evaporation to total below-canopy ET, by calculating half-life storage decay in the litter layer. The maximum water retention capacity of the forest-floor litter layer was estimated from soil moisture measurements at 5 cm depth, and the litter-layer retention timescales were estimated from changes in below-canopy ET after precipitation events. Overall, we found that roughly 60% of below-canopy ET at the Lägeren and Davos sites can be attributed to litter-layer evaporation, thereby suggesting overestimation of transpiration in water balance estimates and potential underestimation of tree water use efficiency.