Aridity drives clinal patterns in leaf traits and responsiveness to precipitation in a broadly distributed Australian tree species

Abstract

Aridity shapes species distributions and plant growth and function worldwide. Yet, plant traits often show complex relationships with aridity, challenging our understanding of aridity as a driver of evolutionary adaptation. We grew nine genotypes of Eucalyptus camaldulensis subsp. camaldulensis sourced from an aridity gradient together in the field for ~650 days under low and high precipitation treatments. Eucalyptus camaldulesis is considered a phreatophyte (deep-rooted species that utilizes groundwater), so we hypothesized that genotypes from more arid environments would show lower aboveground productivity, higher leaf gas-exchange rates, and greater tolerance/avoidance of dry surface soils (indicated by lower responsiveness) than genotypes from less arid environments. Aridity predicted genotype responses to precipitation, with more arid genotypes showing lower responsiveness to reduced precipitation and dry surface conditions than less arid genotypes. Under low precipitation, genotype net photosynthesis and stomatal conductance increased with home-climate aridity. Across treatments, genotype intrinsic water-use efficiency and osmotic potential declined with increasing aridity while photosynthetic capacity (Rubisco carboxylation and RuBP regeneration) increased with aridity. The observed clinal patterns indicate that E. camaldulensis genotypes from extremely arid environments possess a unique strategy defined by lower responsiveness to dry surface soils, low water-use efficiency, and high photosynthetic capacity. This strategy could be underpinned by deep rooting and could be adaptive under arid conditions where heat avoidance is critical and water demand is high.

Keywords: photosynthetic capacity; phreatophyte; plasticity; stomatal behavior; thermoregulation; water‐use efficiency.

FIGURE 1

(a) Mean (±standard error, n = 6) values of aboveground dry mass (DM) for nine Eucalyptus camaldulensis genotypes grown under high and low precipitation treatments in a common garden in relation to the genotype's home‐climate aridity (low aridity values = less arid, high aridity values = more arid). In (a), p_H is the p‐value for the relationship between aridity and aboveground DM under high precipitation, while p_L is the p‐value for the relationship between aridity and aboveground DM under high precipitation. (b) Genotype aboveground DM response to reduced precipitation calculated as Response (%) = 100 × ([X_L – X_H]/X_H), where X_L is the genotype mean under low precipitation and X_H is the genotype mean under high precipitation.

FIGURE 2

Mean (±standard error, n = 6) values for leaf hydraulic (a, c, and e) and gas‐exchange traits (g, i, and k) for nine Eucalyptus camaldulensis genotypes grown under high and low precipitation treatments in a common garden. These traits showed evidence of genotype × precipitation interactions. Means are plotted against the genotype's home‐climate aridity (low aridity values = less arid, high aridity values = more arid). Regression lines and coefficients of determination (r ²) are show when trait–aridity relationships were significant at p < .10. Genotype trait responses to reduced precipitation in relation to home‐climate aridity are also shown (panels b, d, f, h, j, and l). Genotype responses were calculated as: Response (%) = 100 × ([X_L – X_H]/X_H), where X_L is the genotype mean under low precipitation and X_H is the genotype mean under high precipitation.

FIGURE 3

Relationships between genotype means (±standard error, n = 12) for several economic (a, b, c, and d), hydraulic (e and f), and gas‐exchange (g and h) traits and genotype home‐climate aridity (low aridity values = less arid, high aridity values = more arid). These traits showed no evidence of genotype × precipitation interactions (unlike traits shown in Figures 1 and 2). Plotted means are average values across treatments and timepoints (in the case of V _cmax and J _max). Regression lines and coefficients of determination (r ²) are shown when trait–aridity relationships are significant at p < .10.

FIGURE 4

Relationships between genotype leaf trait (hydraulic, gas exchange) responses (% change) to reduced precipitation and genotype aboveground DM responses to reduced precipitation. Regression lines and coefficients of determination (r ²) are shown when relationships are significant at p < .10.