Rainfall partitioning through a mixed cedar swamp and associated C and N fluxes in Southern Ontario, Canada

Abstract

Partitioning of rainfall through a forest canopy into throughfall, stemflow, and canopy interception is a critical process in the water cycle, and the contact of precipitation with vegetated surfaces leads to increased delivery of solutes to the forest floor. This study investigates the rainfall partitioning over a growing season through a temperate, riparian, mixed coniferous-deciduous cedar swamp, an ecosystem not well studied with respect to this process. Seasonal throughfall, stemflow, and interception were 69.2%, 1.5%, and 29.3% of recorded above-canopy precipitation, respectively.Event throughfall ranged from a low of 31.5 ± 6.8% for a small 0.8-mm event to a high of 82.9 ± 2.4% for a large 42.7-mm event. Rain fluxes of at least 8 mm were needed to generate stemflow from all instrumented trees. Most trees had funnelling ratios <1.0, with an exponential decrease in funnelling ratio with increasing tree size.Despite this, stand-scale funnelling ratios averaged 2.81 ± 1.73, indicating equivalent depth of water delivered across the swamp floor by stemflow was greater than incident precipitation. Throughfall dissolved organic carbon (DOC) and total dissolved nitrogen (TDN) averaged 26.60 ± 2.96 and 2.02 ± 0.16 mg L −1 , respectively, which were~11 and three times above-canopy rain levels. Stemflow DOC averaged 73.33 ± 7.43 mg L −1 , 35 times higher than precipitation, and TDN was 4.45 ± 0.56 mg L −1 , 7.5 times higher than rain. Stemflow DOC concentration was highest from Populus balsamifera and TDN greatest from Thuja occidentalis trees.Although total below-canopy flux of TDN increased with increasing event size, DOC flux was greatest for events 20-30 mm, suggesting a canopy storage threshold of DOC was readily diluted. In addition to documenting rainfall partitioning in a novel ecosystem, this study demonstrates the excess carbon and nitrogen delivered to riparian swamps, suggesting the assimilative capacity of these zones may be underestimated.

Event Scale Relationships of DOC and TDN Fluxes in Throughfall and Stemflow Diverge From Stream Exports in a Forested Catchment

“…Mean (±standard deviation) flux‐based DOC enrichment ratios observed in this study for sugar maple and yellow birch stemflow relative to incident precipitation (ER S,P = 110 ± 88 and 122 ± 102, respectively) were higher than those observed in a mixed cedar swamp in Ontario (ER S,P = 16 to 68) (Duval, 2019) and higher than those of an evergreen broad leafed stand in Japan (68) (Chen et al., 2019). Mean flux‐based TDN enrichment ratios of stemflow relative to precipitation in this study (range = ER S,P = 8.0 to 16) were more similar to those reported by Duval (2019) (4.2–16). Stemflow ER S,P for total nitrogen from a variety of research sites globally ranged from three in a tropical Costa Rican lowland rainforest to 44 in a South African Eucalyptus plantation (Levia and Germer, 2015).…”

“…Low variability of the exponential decay model slopes ( c in Equation ) fit to DOC and TDN concentrations for both throughfall and stemflow suggests a shared process (e.g., dilution by rainfall) drives the variability in DOC export (Figure 5). Exponential decay slopes modeled in this study (∼0.07 mm −1 ) are higher than those reported by Duval (2019) (∼0.03 mm −1 ) but lower than those reported in Van Stan et al. (2017) (∼0.1 mm −1 ).…”

“…Stemflow ER S,P for total nitrogen from a variety of research sites globally ranged from three in a tropical Costa Rican lowland rainforest to 44 in a South African Eucalyptus plantation (Levia and Germer, 2015). Mean stemflow enrichment ratios relative to throughfall (ER S,T ) for DOC and TDN in this study (range = 17 to 20) were also higher than those reported by Duval (2019) (1–10). Stemflow ER S,T for total nitrogen reviewed by Levia and Germer (2015) ranged from 3 to 39.…”

“…In contrast to streams, throughfall and stemflow DOC concentrations generally decreased with precipitation event size (Comiskey, 1978; Duval, 2019; Van Stan et al., 2017). This pattern has been described more widely for inorganic ions, although increases in concentrations of individual ions have been observed during events potentially driven by variable precipitation intensity and other factors (Hansen et al., 1994).…”

Economic Evaluation of Water Supplying Function of a Mountain-forest Area of Turkey: Forest Rotation Impacts

“…Shading interception rate (α) depends on the type of forest, shade density, and leaf surface width (Wang et al, 2012). In studies, the shading interception rate was found between 18.33% and 26.38% (McDowell et al, 2020;Duval, 2019;Biao et al, 2010). It was stated that this ratio is between 23.50% and 26.38% in pine species and forest areas with mixed trees.…”