Importance of rainfall partitioning in a northern mixed forest canopy for soil water isotopic signatures in ecohydrological studies

Abstract

The forest canopy can play a significant role in modifying the amount and isotopic composition of water during its passage throughout the near‐surface critical zone. Here, partitioning of gross rainfall into interception, throughfall, and stemflow and its implications for the amount and isotopic composition of soil water was studied for red oak, eastern white pine, and eastern hemlock trees in a northern hardwood‐conifer forest in south central Ontario, Canada. Stemflow production was greatest for red oak as a result of its upward‐projecting branches and least for eastern white pine due to its horizontal branches and rougher bark. These stemflow contributions to the near‐bole soil surface failed to produce consistently wetter soils relative to distal locations from the bole for all tree species. There was also no consistent evidence of isotopic enrichment of throughfall and stemflow relative to gross rainfall or of stemflow relative to throughfall for red oak or eastern hemlock. However, there was isotopic enrichment of both throughfall and stemflow for eastern white pine with increasing maximum atmospheric vapour pressure deficit, which may reflect the potential for evaporative fractionation as a result of retention and detention of water moving through the canopy by the rougher bark of this species. Dry soil conditions limited sampling of mobile soil water during the study, and there was no consistent evidence that either throughfall or stemflow fluxes controlled temporal changes in the isotopic signature of soil water beneath the tree. Thus, the potential for throughfall and stemflow fluxes in northern hardwood‐conifer forests to modify the isotopic composition of water taken up by the tree via transpiration remains an open question.

Co-evolution of xylem water and soil water stable isotopic composition in a northern mixed forest biome

“…Total water depth in the upper 0.5 m of soil at 0.1 and 1 m from He, Or and Pw tree boles showed similar trends from early June to late October 2016 (Fig. A1): gradual draining through June into early August, a marked increase following 123 mm of rain between 9 and 17 August and relatively high SWCs until the end of monitoring (see Snelgrove et al, 2019, for greater detail). SWCs were similar at 0.1 and 1 m from the boles of trees of a given species.…”

“…2b), particularly deeper samples and those taken post-senescence in 2015 and 2016. Snelgrove et al (2019) also found limited evaporative enrichment of mobile soil water and weak correspondence between isotopic composition of mobile soil water and that of P g , throughfall or stemflow inputs to the soil. Most bulk soil water samples also plotted along the LMWL.…”

“…This bulk soil water was assumed to represent all water stored within the soil, including both mobile and more tightly held soil water. Mobile soil water was sampled from tension lysimeters installed at 0.1 and 0.4 m depths at 0.1 and 1 m from the tree bole in a randomised direction for three He, three Or and three Pw trees sampled for xylem water (Snelgrove et al, 2019). Tension lysimeters were manufactured using Soil Test TM 2 bar ceramic cups and PVC tubing.…”

“…Measurements at each depth were made three times per access tube and averaged to obtain mean SWC at each depth. These values were used to estimate the total depth of water held in the upper 0.5 m of soil as described in Snelgrove et al (2019).…”

“…Most throughfall and stemflow samples for He, Or and Pw trees fell on the LMWL, indicating limited isotopic enrichment of P g as it passed through the forest canopy (Snelgrove et al, 2019).…”