Lawrence B. Flanagan

In semiarid regions, the growth and survival of cottonwoods (riparian Populus species) depend on river water supplementing the limited precipitation. Indicators of growth and productivity are needed to assess how altered streamflow regimes on regulated rivers impact cottonwood trees and the riparian forest ecosystems they support. We used satellite imagery from the Landsat program to make historical (1984–2020) assessments of ecosystem productivity in a riparian cottonwood forest along a regulated prairie river in southern Alberta, Canada, with an environmental flow regime that increased the minimum flows implemented in 1993. A version of the near‐infrared reflectance of vegetation scaled with incoming sunlight (NIRvP) was calculated from Landsat images to provide a proxy for primary production. Near‐infrared reflectance of vegetation scaled with incoming sunlight was correlated strongly with gross primary production measurements from eddy covariance and basal area increment measurements from tree ring analyses, supporting its use as a practical proxy. The lowest NIRvP values occurred in drought years with low flows and dry weather during the growing season, while the highest values occurred in wet years with high flows, including floods. Across all years, NIRvP was positively correlated with streamflow and a weather‐driven soil moisture index. This indicated that ecosystem productivity was limited by water supply, which is sourced from river water and local precipitation. Subsequently, cottonwood forests in this region would be vulnerable to drought from declines in streamflow, due to climatic variations or human water withdrawals, and reductions in shallow soil moisture from limited local precipitation. This satellite proxy should be broadly applicable and can provide a diagnostic indicator for assessing riparian forest health and responses to varying weather, changing climate, and streamflow regulation.

Summary We compiled hydrogen and oxygen stable isotope compositions (δ 2 H and δ 18 O) of leaf water from multiple biomes to examine variations with environmental drivers. Leaf water δ 2 H was more closely correlated with δ 2 H of xylem water or atmospheric vapour, whereas leaf water δ 18 O was more closely correlated with air relative humidity. This resulted from the larger proportional range for δ 2 H of meteoric waters relative to the extent of leaf water evaporative enrichment compared with δ 18 O. We next expressed leaf water as isotopic enrichment above xylem water (Δ 2 H and Δ 18 O) to remove the impact of xylem water isotopic variation. For Δ 2 H, leaf water still correlated with atmospheric vapour, whereas Δ 18 O showed no such correlation. This was explained by covariance between air relative humidity and the Δ 18 O of atmospheric vapour. This is consistent with a previously observed diurnal correlation between air relative humidity and the deuterium excess of atmospheric vapour across a range of ecosystems. We conclude that 2 H and 18 O in leaf water do indeed reflect the balance of environmental drivers differently; our results have implications for understanding isotopic effects associated with water cycling in terrestrial ecosystems and for inferring environmental change from isotopic biomarkers that act as proxies for leaf water.