Implications for forest water balances
Together the forest canopy and the forest-floor litter layer modulate a
large fraction of the incoming precipitation at our site, as they likely
also do in many other forests around the world. Significant fractions of
annual precipitation are exchanged between the atmosphere, the canopy
and the forest-floor litter layer, without contributing to soil moisture
recharge, groundwater recharge, or runoff production (FigureĀ 10). This
also limits the water that is available to plants for transpiration.
This is important because these interception losses are not well
represented in many conventional water balances, or their magnitudes are
greatly underestimated. At our site, forest-floor interception losses
are similar to rates of canopy interception. Together they reduce the
available recharge by approximately 38 % on an annual basis (and by a
substantially higher percentage during summer when VPD is high, and a
lower percentage during winter when VPD is low). Thus at our site, and
in similar forests and similar climates, the fraction of summer
precipitation that actually infiltrates through the forest floor may be
very small. This could help to explain why during the summer months,
forest trees across Switzerland have been observed to transpire
precipitation from the previous winter (as shown in Allen et al., 2019).