Potential for significant precipitation cycling by forest-floor litter
and deadwood
Abstract
The forest-floor litter layer can retain substantial volumes of water,
thus affecting evaporation and soil-moisture dynamics. However, litter
layer wetting/drying dynamics are often overlooked when estimating
forest water budgets. Here we present field and laboratory experiments
characterizing water cycling in the forest-floor litter layer, and
outline its implications for subcanopy microclimatic conditions and for
estimates of transpiration and recharge. Storage capacities of spruce
needle litter and beech broadleaf litter averaged 3.1 mm and 1.9 mm
respectively, with drainage/evaporation timescales exceeding 2 days.
Litter-removal experiments showed that litter reduced soil water
recharge, reduced soil evaporation rates, and insulated against ground
heat fluxes that impacted snowmelt. Deadwood stored ~0.7
mm of water, increasing with more advanced states of decomposition, and
retained water for >7 days. Observed daily cycles in
deadwood weight revealed decreasing water storage during daytime as
evaporation progressed and increasing storage at night from condensation
or absorption. Water evaporating from the forest-floor litter layer
modulates the subcanopy microclimate by increasing humidity, decreasing
temperature and reducing VPD. Despite the relatively small litter
storage capacity (<3.1 mm in comparison to ~10
2 mm for typical forest soil rooting zones) the litter
layer alone retained and cycled 18% of annual precipitation, or 1/3 of
annual evapotranspiration. These results suggest that overlooking litter
interception may lead to substantial overestimates of recharge and
transpiration in many forest ecosystems.