Using stable isotopes to quantify water sources for trees and shrubs in a riparian cottonwood ecosystem in flood and drought years

Abstract

Riparian cottonwood forests in dry regions of western North America do not typically receive sufficient growing season precipitation to completely support their relatively high transpiration requirements. Water used in transpiration by riparian ecosystems must include alluvial groundwater or water stored in the potentially large reservoir of the unsaturated soil zone. We used the stable oxygen and hydrogen isotope composition of stem xylem water to evaluate water sources used by the dominant riparian cottonwood (Populus spp.) trees and shrubs (Shepherdia argentea and Symphoricarpos occidentalis) in Lethbridge, Alberta, during 3 years of contrasting environmental conditions. Cottonwoods did not exclusively take up alluvial groundwater but made extensive use of water sourced from the unsaturated soil zone. The oxygen and hydrogen isotope compositions of cottonwood stem water did not strongly overlap with those of alluvial groundwater, which were closely associated with the local meteoric water line. Instead, cottonwood stem water δ 18 O and δ 2 H values were located below the local meteoric water line, forming a line with a low slope that was indicative of water exposed to evaporative enrichment of heavy isotopes. In addition, cottonwood xylem water isotope compositions had negative values of deuterium excess (d-excess) and line-conditioned (deuterium) excess (lc-excess), both of which provided evidence that water taken up by the cottonwoods had been exposed to fractionation during evaporation. The shrub species had lower values of d-excess and lc-excess than had the cottonwood trees due to shallower rooting depths, and the d-excess values declined during the growing season, as shallow soil water that was taken up by the plants was exposed to increasing, cumulative evaporative enrichment. The apparent differences in functional rooting pattern between cottonwoods and the shrub species, strongly influenced the ratio of net photosynthesis to stomatal conductance (intrinsic water-use efficiency), as shown by variation among species in the δ 13 C values of leaf tissue. K E Y W O R D Shydrogen and oxygen isotopes, Populus sp, Shepherdia argentea, Symphoricarpos occidentalis

Hydraulic traits that buffer deep‐rooted plants from changes in hydrology and climate

“…The overarching contrast between the two is that, by definition, an obligate phreatophyte must remain in contact with groundwater at all times, whereas a facultative phreatophyte can extract water from both phreatic and vadose zones of the soil profile. Unlike obligate phreatophytes, the groundwater dependency of facultative phreatophytes may vary dramatically in both time and space (Snyder & Williams, ; Flanagan, Orchard, Tremel, & Rood, ). Some facultative phreatophytes are connected to groundwater throughout their life span but may only acquire a relatively small percentage of their water through groundwater sources.…”

Multiple processes contribute to methane emission in a riparian cottonwood forest ecosystem

“…For example, cottonwood trees were included among the first studies that demonstrated methane production in stem heartwood (Bushong, 1907; Zeikus & Ward, 1974; Wang et al ., 2016; Yip et al ., 2019). In addition, cottonwood trees rely on alluvial groundwater to supply a significant fraction of water for their transpiration requirements in these semi‐arid regions (Flanagan et al ., 2017, 2019; Yang et al ., 2019), and the alluvial groundwater may contain dissolved methane that could be released to the atmosphere during transpiration (Nesbit et al ., 2009; Covey & Megonigal, 2019). Finally, methane production and emission from a river (Stanley et al ., 2016) may interact with and influence ecosystem–atmosphere exchange processes in the adjacent riparian forest ecosystem, a topic considered in this study.…”

Flows for floodplain forests: Conversion from an intermittent to continuous flow regime enabled riparian woodland development along a prairie river

“…Flows of the LBR fell substantially through the 1920 summer but the exceptionally heavy spring rains would have saturated the river valley, reducing drought stress through that first growth season (Flanagan, Orchard, Tremel, & Rood, 2019). Flows of the LBR were generally around 1 m 3 /s throughout the 1921 growth season (not shown) and…”