Assessing stand water use in four coastal wetland forests using sapflow techniques: annual estimates, errors and associated uncertainties

Abstract

Abstract:Forests comprise approximately 37% of the terrestrial land surface and influence global water cycling. However, very little attention has been directed towards understanding environmental impacts on stand water use (S) or in identifying rates of S from specific forested wetlands. Here, we use sapflow techniques to address two separate but linked objectives: (1) determine S in four, hydrologically distinctive South Carolina (USA) wetland forests from 2009-2010 and (2) describe potential error, uncertainty and stand-level variation associated with these assessments. Sapflow measurements were made from a number of tree species for approximately 2-8 months over 2 years to initiate the model, which was applied to canopy trees (DBH > 10-20 cm). We determined that S in three healthy forested wetlands varied from 1.97-3.97 mm day À1 or 355-687 mm year À1 when scaled. In contrast, saltwater intrusion impacted individual tree physiology and size class distributions on a fourth site, which decreased S to 0.61-1.13 mm day À1 or 110-196 mm year À1 . The primary sources of error in estimations using sapflow probes would relate to calibration of probes and standardization relative to no flow periods and accounting for accurate sapflow attenuation with radial depth into the sapwood by species and site. Such inherent variation in water use among wetland forest stands makes small differences in S (<200 mm year À1 ) difficult to detect statistically through modelling, even though small differences may be important to local water cycling. These data also represent some of the first assessments of S from temperate, coastal forested wetlands along the Atlantic coast of the USA.

Evaporation and the subcanopy energy environment in a flooded forest

“…In upland closed-canopy forests, transpiration generally far exceeds evaporation (Kool et al, 2014); for examples, evaporation was 8% of ET during the growing season in a closed-canopy forest in the southeastern United States (Wilson et al, 2000) or evaporation averaged 0.19-0.38 mm/day in a temperate coniferous forest (Schaap & Bouten, 1997). If canopy transpiration in the present study was assumed to bẽ 2-3 mm/day, as observed for a reasonably similar ash-sweetgumbaldcypress forest in the south-eastern United States (Krauss et al, 2014), evaporation at our study site would be 22-36% of estimated ET in the growing season months (June-September). This proportion is more consistent with observations in sparser semi-arid (e.g., Baldocchi, Xu, & Kiang, 2004Raz-Yaseef et al, 2010;Scott et al, 2003) or boreal forests with more radiation reaching the forest floor (reviewed by Baldocchi & Ryu, 2011;Iida et al, 2009).…”

“…Although evaporation is often a small component of terrestrial water budgets (Baldocchi & Ryu, 2011), particularly in closed canopy forests, it could be reasoned that wetlands having higher evaporation could result in high total evapotranspiration (ET). Wetland forest structure is highly variable (Allen, Whitsell, & Keim, 2015), so it is difficult to generalize across field studies, but even closed-canopy wetland forests have not been shown to have particularly high transpiration rates (Bosch, Marshall, & Teskey, 2014;Krauss, Duberstein, & Conner, 2014) compared to upland forests in the same region (e.g., Domec et al, 2012). Given heat conduction and storage differences between water and soil, the influence of floodwater on proportional contributions of evaporation versus transpiration may distinguish wetland energy and water budgets from those of upland environments.…”

Why Do We Need to Document and Conserve Foundation Species in Freshwater Wetlands?

“…High salinity causes osmotic stress and decreased stomatal conductance [126], as indicated by decreased water use efficiency from 100 kg/day on a freshwater site to 23.9 kg/day on a saline water site [127,128]. In tidal forested wetlands, increased soil water salinity negatively reduces nutrient uptake in bald cypress [128][129][130][131][132], which in turn may decrease nitrogen and P burial [133].…”

Evapotranspiration Characteristics Distinct to Mangrove Ecosystems Are Revealed by Multiple‐Site Observations and a Modified Two‐Source Model

“…Such upscaling approaches depend crucially on quadrat surveys (Granier et al, 1996). Although we have done our best to sample representative quadrats within the eddy tower footprint, errors in the quadrat survey and variations in wind direction may cause uncertainties in estimations of stand transpiration using the sap flow method (Köstner et al, 1998;Krauss, Duberstein, et al, 2015). Our calculations of T strictly follow the standard sap flow method; we therefore assume that they are reliable within that context.…”