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).