Carbon and water relations of contrasting Arctic plants: implications for shrub expansion in West Greenland

Abstract

Abstract The recent expansion of deciduous shrubs is a common observation throughout the Arctic. However, we lack a complete understanding of how physiological differences between deciduous shrubs and coexisting species may confer competitive advantages to shrubs. We combined leaf gas exchange and stable isotope analyses of two important species, Betula nana and Poa pratensis, to elucidate the processes governing seasonal carbon (C) gain in West Greenland. We tested two competing hypotheses. On one hand, we anticipated the cooler, drier soils beneath the Betula canopy could result in greater drought sensitivity in this species. Alternatively, because Poa tends to occupy sites with wetter soils, we hypothesized that it may be more sensitive to drought. Our results revealed greater drought sensitivity in Poa, which displayed reductions in Amax and gs during periods of high atmospheric demand and dry soils. Additionally, leaf Δ13C and Δ18O were negatively correlated in Poa, suggesting strong stomatal influence on Δ13C. Conversely, there was no relationship between gs and canopy or soil microclimates and no correlation between leaf Δ13C and Δ18O in Betula, indicating that variation in Δ13C may have been driven by variation in photosynthesis. Our results suggest that Poa is more susceptible to drought than Betula, whereas Betula was able to maintain steady, yet conservative, C gain. These differences in C‐H2O relations may confer a competitive advantage to Betula in a warmer, drier climate.

Keywords

• ∆13C
• ∆18O
• Arctic
• Betula nana
• dual isotope model
• photosynthesis