Trading water for carbon in the future: Effects of elevated CO2 and warming on leaf hydraulic traits in a semiarid grassland

Abstract

The effects of climate change on plants and ecosystems are mediated by plant hydraulic traits, including interspecific and intraspecific variability of trait phenotypes. Yet, integrative and realistic studies of hydraulic traits and climate change are rare. In a semiarid grassland, we assessed the response of several plant hydraulic traits to elevated CO₂ (+200 ppm) and warming (+1.5 to 3°C; day to night). For leaves of five dominant species (three graminoids and two forbs), and in replicated plots exposed to 7 years of elevated CO₂ , warming, or ambient climate, we measured: stomatal density and size, xylem vessel size, turgor loss point, and water potential (pre-dawn). Interspecific differences in hydraulic traits were larger than intraspecific shifts induced by elevated CO₂ and/or warming. Effects of elevated CO₂ were greater than effects of warming, and interactions between treatments were weak or not detected. The forbs showed little phenotypic plasticity. The graminoids had leaf water potentials and turgor loss points that were 10% to 50% less negative under elevated CO₂ ; thus, climate change might cause these species to adjust their drought resistance strategy away from tolerance and toward avoidance. The C4 grass also reduced allocation of leaf area to stomata under elevated CO₂ , which helps explain observations of higher soil moisture. The shifts in hydraulic traits under elevated CO₂ were not, however, simply due to higher soil moisture. Integration of our results with others' indicates that common species in this grassland are more likely to adjust stomatal aperture in response to near-term climate change, rather than anatomical traits; this contrasts with apparent effects of changing CO₂ on plant anatomy over evolutionary time. Future studies should assess how plant responses to drought may be constrained by the apparent shift from tolerance (via low turgor loss point) to avoidance (via stomatal regulation and/or access to deeper soil moisture).

Keywords: carbon dioxide; drought; intraspecific; plant functional type; species; stomata; turgor loss point; warming; water potential; xylem.

FIGURE 1

Volumetric water content of surface soils (5–25 cm) during the growing season in which leaf sampling occurred. Panel (a) shows the daily trajectory of soil moisture prior to and between sampling events for leaf water potential, which occurred on day of year (DOY) 164 and 191 (dashed vertical lines). Weekly to biweekly measures of plot greenness are also shown in panel (a) using larger, open symbols (Zelikova et al., 2015). Panel (b) shows mean soil moisture integrated over two different time periods relevant to plant sampling. Error bars are based on standard error and are shown only for select dates in panel (a). Mixed models that account for spatial and temporal autocorrelation of daily VWC during the two periods show evidence of an interaction between the eCO₂ and warming treatments (p = .01 for both time periods). For DOY 164 and 191 only, the p values for the CO₂*temperature interaction were 0.02 and 0.11.

FIGURE 2

Mean leaf water potentials (pre‐dawn) for different treatments, species, and sampling periods. The bottom panel shows how much pre‐dawn water potentials declined from the first (June) to the second (July) sampling period, mirroring the decline of soil moisture observed in all treatments (Figure 1). The left panel (a) shows treatment means separately for each species, while the right panel (b) shows treatment means averaged across all species. See Table 1 for the modeled effects of species and treatments and their interactions.

FIGURE 3

Mean values of leaf traits for different species and treatments. The panels on the left (a) show treatment means separately for each species, while the panels on the right (b) show treatment means averaged across three to five species. Xylem area was measured as the cross‐sectional area of individual vessels. See Table 1 for the modeled effects of species and treatments and their interactions.