Final Report Summary - ECOTRI (Ecophysiological Aspects of Tree Ring Isotopes)
The objectives of this project were to advance our understanding of the factors controlling the isotopic composition of tree-rings. In this regard, the focus was on the leaf-level physiological factors affecting the isotopic composition of the substrates of ring wood, i.e. the products of photosynthesis. Specifically, it aimed to use controlled environment experiments to improve our understanding of the enrichment of leaf water isotopic composition and investigate the response of carbon isotope discrimination to global change processes.
To this end, a series of experiments were conducted to identify the relative importance of advection and diffusion processes on leaf water isotopic enrichment through detailed analysis of the relaxation kinetics of leaf sections during light to dark transitions. There were clear patterns of increasing enrichment along the monocot leaf blade, but with no clear trend over time between the different samples and leaf sections. However, the values of the total leaf water did show a progressive depletion, indicating a diffusive mixing with the un-enriched vein water. High resolution infra-red thermography leaf temperature measurements also revealed that leaf temperature gradients from the central mid-vein (warm) to the edge of the leaf (cooler) were stronger than along the length of the leaf. This data was used to test an advection-diffusion model of leaf water enrichment that incorporates aspects of leaf morphology, and accurate simulations indicated the importance of leaf temperatures and the within leaf pathway tortuosity.
The response of plants to increasing atmospheric CO2 concentrations will have important consequences for plant physiology, with the increase in CO2 to date already having had an effect that has been observed in tree-ring 13C composition. In the context of the project, leaf level physiological responses to CO2 concentration of small (1.5 m) Pinus nigra trees maintained at three different CO2 concentrations (380 ppm, 500 ppm, 800 ppm) were investigated, allowing potential whole-plant feedback effects to be incorporated into the responses. The response of gas exchange parameters to leaf level changes in CO2 was measured at each ambient CO2 concentration, allowing the instantaneous response to be compared to the 'acclimated' response. Assimilation rates and stomatal conductance both showed rapid acclimation, with the result that the acclimated intrinsic water-use efficiency increased by 125 % with CO2, or 20 % more than predicted from the instantaneous measurements. Measurements of the chamber air CO2 and leaf sugar isotopic composition also indicated decreasing photosynthetic discrimination against 13CO2, consistent with the increase in water use efficiency. Transpiration did not show a clear trend with CO2 due to leaf temperature feedbacks. These results possibly indicate interactions with stomatal conductance that result in constant total plant water use, but increasing transpiration ratio with increasing CO2. Likewise, there was little increase in the evaporative enrichment of leaf water isotopic composition (18O/16O), despite the changes in stomatal conductance.
Dendrological studies have a well-established history of use in climate studies, but the use of tree-ring isotopes to enhance the physical and morphological parameters has been hindered by an incomplete understanding of the relationship between climate drivers, physiological responses and the resulting isotopic signals. The final results of this work will aid our understanding of tree-ring isotopic composition and improve their utility for both paleoclimate studies and for inferring the responses of vegetation to global change. This in turn will enable us to both constrain our predictions of responses to future changes and enable better informed policy decisions relating to necessary adaptation strategies in the face of a rapidly changing environment.
To this end, a series of experiments were conducted to identify the relative importance of advection and diffusion processes on leaf water isotopic enrichment through detailed analysis of the relaxation kinetics of leaf sections during light to dark transitions. There were clear patterns of increasing enrichment along the monocot leaf blade, but with no clear trend over time between the different samples and leaf sections. However, the values of the total leaf water did show a progressive depletion, indicating a diffusive mixing with the un-enriched vein water. High resolution infra-red thermography leaf temperature measurements also revealed that leaf temperature gradients from the central mid-vein (warm) to the edge of the leaf (cooler) were stronger than along the length of the leaf. This data was used to test an advection-diffusion model of leaf water enrichment that incorporates aspects of leaf morphology, and accurate simulations indicated the importance of leaf temperatures and the within leaf pathway tortuosity.
The response of plants to increasing atmospheric CO2 concentrations will have important consequences for plant physiology, with the increase in CO2 to date already having had an effect that has been observed in tree-ring 13C composition. In the context of the project, leaf level physiological responses to CO2 concentration of small (1.5 m) Pinus nigra trees maintained at three different CO2 concentrations (380 ppm, 500 ppm, 800 ppm) were investigated, allowing potential whole-plant feedback effects to be incorporated into the responses. The response of gas exchange parameters to leaf level changes in CO2 was measured at each ambient CO2 concentration, allowing the instantaneous response to be compared to the 'acclimated' response. Assimilation rates and stomatal conductance both showed rapid acclimation, with the result that the acclimated intrinsic water-use efficiency increased by 125 % with CO2, or 20 % more than predicted from the instantaneous measurements. Measurements of the chamber air CO2 and leaf sugar isotopic composition also indicated decreasing photosynthetic discrimination against 13CO2, consistent with the increase in water use efficiency. Transpiration did not show a clear trend with CO2 due to leaf temperature feedbacks. These results possibly indicate interactions with stomatal conductance that result in constant total plant water use, but increasing transpiration ratio with increasing CO2. Likewise, there was little increase in the evaporative enrichment of leaf water isotopic composition (18O/16O), despite the changes in stomatal conductance.
Dendrological studies have a well-established history of use in climate studies, but the use of tree-ring isotopes to enhance the physical and morphological parameters has been hindered by an incomplete understanding of the relationship between climate drivers, physiological responses and the resulting isotopic signals. The final results of this work will aid our understanding of tree-ring isotopic composition and improve their utility for both paleoclimate studies and for inferring the responses of vegetation to global change. This in turn will enable us to both constrain our predictions of responses to future changes and enable better informed policy decisions relating to necessary adaptation strategies in the face of a rapidly changing environment.