Triple-funneling of trees? Intra-canopy preferential flow of water and elements induced by tree canopies

Trees affect the direction and distribution of crucial components of the hydrological cycle, which were mostly described by measurements on the quantity of precipitation, stemflow and throughfall (TF) collected underneath the canopy. However, due to poor accessibility of tree canopies, our knowledge on hydrological processes within canopies is limited. 

We propose that canopy structure shapes the spatial distribution of incoming rainfall (RF) within the canopy as well as the intra-canopy TF composition. The Leipzig Canopy Crane facility allows to (i) determine water fluxes from above the canopies (RF) and with TF at top, mid and bottom position within the canopy of three tree species – Quercus robur, Fraxinus excelsior, and Tilia cordata, and (ii) to determine the transport of dissolved and particulate organic carbon and nitrogen with TF. In total, 81 TF collectors were set up every month for a two-weeks-period from March to October 2021.

We found amplified water fluxes in TF collectors at top and mid canopy positions compared to incoming RF fluxes, while TF volumes at the bottom decreased. Dimensions of change appear related to RF amount and tree species. Moreover, stability plot analysis indicated that spatial “hot spots” of water fluxes within canopies were temporally persistent.

Our results raise the question whether the concept of a “double-funneling of trees” introduced by Johnson and Lehmann (2006) needs to be extended to a “triple-funneling” approach involving the intra-canopy preferential flow of water and elements occurring in upper to mid canopy positions. Canopy spots with higher water and matter accumulation will alter the chemical, biological, and hydrological heterogeneity in canopy habitat structures below, with strong implications for canopy-associated microbial communities and epiphytes and ecosystem functions.