Forest-floor litter and deadwood cycle significant amounts precipitation

Forests modulate precipitation and evapotranspiration fluxes. One important – yet often overlooked - component in the forest water cycle is the forest-floor litter layer. Leaves and deadwood retain significant amounts of annual precipitation and enhance subcanopy humidity. At the “Waldlabor Zurich” ecohydrology field site we conducted numerous experiments to quantify the water fluxes from and to the forest-floor litter layer. We estimated the total retention capacities of needle, broadleaf and deadwood litter, assessed the litter water content before and after precipitation events, and measured soil moisture in litter-covered and litter-free plots. We used micro lysimeters to estimate evaporation from the litter layer and measured subcanopy humidity and temperature at different heights above the forest floor to assess the effect of evaporation on subcanopy microclimate.

Storage capacities of needle litter and broadleaf litter averaged 3.1 and 1.9 mm, respectively, with evaporation timescales exceeding 2 days, whereas deadwood stored ~0.7 mm of precipitation, and retained water for >7 days. Deadwood water retention increased with more advanced decomposition. Together the forest floor litter layer reduced soil water recharge, reduced soil evaporation rates, and insulated against ground heat fluxes thus impacting snowmelt patterns. Timeseries of deadwood water content revealed a diel cycle of stored water, water content increased during nighttime due to condensation of dew and fog and decreased during the day when vapor pressure deficit and evaporation were high. The water evaporating from the forest‐floor litter layer increased humidity, decreased temperature, and reduced vapor pressure deficit in the subcanopy atmosphere. Although, the absolute amounts of water storage in the forest-floor litter layer are relatively small, these storages were frequently filled and emptied with every precipitation event, thus effecting the overall soil water recharge. Overall, 18% of annual precipitation, or 1/3 of annual evapotranspiration, were retained in the forest-floor litter layer suggesting that overlooking litter interception may lead to substantial overestimates of recharge and transpiration in many forest ecosystems.