Search

Photosynthesis supports life on earth and is highly temperature dependent. Extreme temperatures can inhibit photosynthesis and damage the photosystem machinery, potentially limiting future productivity and plant survival. With increasing risk of extreme temperature exposure under climate change, plants may be pushed to the edge of their thermal limit, but at what point is a complex question. Temperatures that cause substantial damage to photosystems, encapsulated by heat tolerance thresholds, help to answer this question. On hot days leaf temperatures can spike multiple times, yet what we know of the variability of heat tolerance often comes from tests that vary in only one dimension – temperature. In Chapter 2, I demonstrated that varying combinations of heat characteristics can accumulate as heat doses and reveal multiple heat tolerance thresholds. By varying the heat dose, the thermal sensitivity of tolerance can also be examined, which is a first in plants. In Chapter 3, I followed the temporal effects of multiple exposures to extreme high temperatures, which potentially both reduced and delayed the capacity for repair to Photosystem II (PSII) with sustained photoinhibition present on the following day. Examining plants in less obviously extreme environments, alpine summers are predicted to be warmer and longer under climate change, potentially increasing heat stress for alpine plants. In Chapter 4, I explored the scarcely studied effect of elevated and extended growth temperature on the heat tolerance of Australian alpine species. While alpine plant species maintained surprisingly high photosynthetic heat tolerance, they only marginally increased their tolerance in response to warming, suggesting increased vulnerability to heat stress with long term climate change. The application of plant physiological heat tolerance in assessing future vulnerability to increasing temperatures under climatic change, however, is complicated. As I showed in Chapter 5, water avai

1. The rate of elongation and thickening of individual branches (shoots) varies across plant species. This variation is important for the outcome of competition and other plant-plant interactions. Here we compared rates of shoot growth across 44 species from tropical, warm temperate, and cool temperate forests of eastern Australia. 2. Shoot growth rate was found to correlate with a suite of traits including the potential height of the species, xylem-specific conductivity, leaf size, leaf area per xylem cross-section, twig diameter (at 40 cm length), wood density and modulus of elasticity. 3. Within this suite of traits, maximum plant height was the clearest correlate of growth rates, explaining 50 to 67% of the variation in growth overall (p < 0.0001), and 23 to 32% of the variation (p < 0.05) in growth when holding the influence of the other traits constant. Structural equation models suggest that traits associated with ‘hydraulics’, ‘biomechanics’, and the ‘leaf economics spectrum’ represent three clearly separated axes of variation, with the hydraulic axis exhibiting the strongest alignment with height and largest independent contribution to growth (in the case of branch thickening). However most of the capacity of these axes to predict growth was also associated with maximum height, presumably reflecting coordinated selection on multiple traits that together influence life histories. 4. Growth rates were not strongly correlated with leaf nitrogen or leaf mass per unit leaf area. 5. Correlations between growth and maximum height arose both across latitude (47%, p < 0.0001) and from within-site differences between species (30%, p < 0.0001). Covariation between growth and maximum height was driven in part by variation in irradiance across sites as well as among canopy positions within sites (23%, p < 0.0001). A significant fraction of this shared variation was ind

1. The rate of elongation and thickening of individual branches (shoots) varies across plant species. This variation is important for the outcome of competition and other plant-plant interactions. Here we compared rates of shoot growth across 44 species from tropical, warm temperate, and cool temperate forests of eastern Australia. 2. Shoot growth rate was found to correlate with a suite of traits including the potential height of the species, xylem-specific conductivity, leaf size, leaf area per xylem cross-section, twig diameter (at 40 cm length), wood density and modulus of elasticity. 3. Within this suite of traits, maximum plant height was the clearest correlate of growth rates, explaining 50 to 67% of the variation in growth overall (p < 0.0001), and 23 to 32% of the variation (p < 0.05) in growth when holding the influence of the other traits constant. Structural equation models suggest that traits associated with ‘hydraulics’, ‘biomechanics’, and the ‘leaf economics spectrum’ represent three clearly separated axes of variation, with the hydraulic axis exhibiting the strongest alignment with height and largest independent contribution to growth (in the case of branch thickening). However most of the capacity of these axes to predict growth was also associated with maximum height, presumably reflecting coordinated selection on multiple traits that together influence life histories. 4. Growth rates were not strongly correlated with leaf nitrogen or leaf mass per unit leaf area. 5. Correlations between growth and maximum height arose both across latitude (47%, p < 0.0001) and from within-site differences between species (30%, p < 0.0001). Covariation between growth and maximum height was driven in part by variation in irradiance across sites as well as among canopy positions within sites (23%, p < 0.0001). A significant fraction of this shared variation was ind