Localized Augmentation of Net Precipitation to Shrubs: A Case Study of Stemflow Funneling to Hummocks in a Salinity-Intruded Swamp

Abstract

The interception of precipitation by plant canopies can alter the amount and spatial distribution of water inputs to ecosystems. We asked whether canopy interception could locally augment water inputs to shrubs by their crowns funneling (freshwater) precipitation as stemflow to their bases, in a wetland where relict overstory trees are dying and persisting shrubs only grow on small hummocks that sit above mesohaline floodwaters. Precipitation, throughfall, and stemflow were measured across 69 events over a 15-months period in a salinity-degraded freshwater swamp in coastal South Carolina, United States. Evaporation of intercepted water from the overstory and shrub canopies reduced net precipitation (stemflow plus throughfall) across the site to 91% of gross (open) precipitation amounts. However, interception by the shrub layer resulted in increased routing of precipitation down the shrub stems to hummocks – this stemflow yielded depths that were over 14 times larger than that of gross precipitation across an area equal to the shrub stem cross-sectional areas. Through dimensional analysis, we inferred that stemflow resulted in local augmentation of net precipitation, with effective precipitation inputs to hummocks equaling 100–135% of gross precipitation. Given that these shrubs (wax myrtle, Morella cerifera) are sensitive to mesohaline salinities, our novel findings prompt the hypothesis that stemflow funneling is an ecophysiologically important mechanism that increases freshwater availability and facilitates shrub persistence in this otherwise stressful environment.

Shrub expansion in maritime forest responding to sea level rise

“…Among coastal shrubs, M. cerifera was ranked as more sensitive to salinity than B. halimifolia and I. frutescens (Young et al, 1994). But M. cerifera is more salt-tolerant than many forest understory species, due to its adaptations to tolerate salinity stress, such as closing its stomata gradually to prevent damage to the photosynthetic apparatus (Naumann et al, 2008) and using its wide canopy to gather critical freshwater resources from rain (Allen and Conner, 2021). The moderate salinity tolerance of M. cerifera explains why the species appears to be thriving in the lower salinity zones of the mid and high forest, but is less common and exhibits slower rates of growth in the low forest, where salinity levels have breached its tolerance threshold.…”