| Summary: | Temperature plays a key role in plant growth and development, because it affects metabolic rates of every physiological process. With growing concern over global warming, the responses of forests to increasing temperature has become an important topic in recent decades. If we are to predict the fate of global forests in a future warmer world, we need to understand and quantify the mechanisms of the temperature response of plant growth. Predictions of global warming impacts on forest growth are widely made using terrestrial biosphere models. Terrestrial biosphere models use environmental data and physiological parameters to predict carbon, water, energy, and nutrient fluxes in terrestrial ecosystems. Realistic model representation and integration of processes that are upstream of plant growth is critical in model predictions. In this thesis, my broad aim is to improve the current representation of the temperature response of plant growth in terrestrial biosphere models by identifying the important mechanisms that account for the overall response. This thesis is divided into five stand-alone, interrelated research chapters to achieve the above objective. Chapters 2 and 3 cover the response of photosynthesis to temperature. In chapter 2, I compiled a global dataset of photosynthetic CO2 response curves measured at different leaf temperatures, including data from 141 C3 species from tropical rainforest to Arctic tundra. I utilised this dataset to quantify and model key mechanisms responsible for photosynthetic temperature acclimation and adaptation and developed a summary model to represent photosynthetic temperature responses in terrestrial biosphere models. Chapter 3 investigates the triose phosphate utilisation limitation (TPU) of leaf net photosynthesis at the global scale using the dataset compiled in chapter 2. In this chapter, I demonstrate that TPU does not limit leaf photosynthesis at the current ambient atmospheric CO2 concentration. In addition, I showed that instantaneous temperature responses of TPU are distinct from temperature responses of the maximum rate of Rubisco carboxylation. Chapter 4 concerns scaling from leaf level to canopy level photosynthesis, using a unique dataset from whole-tree chambers where whole-tree photosynthetic flux was measured at high time-resolution over several seasons. The dataset demonstrates that the optimal temperature for canopy photosynthesis is at least 6 °C lower than that for leaf photosynthesis. I tested several hypotheses to explain this difference, using models of canopy radiation interception and photosynthesis parameterized with leaf-level physiological data and estimates of canopy leaf area. In this chapter, I identify the influence of non-light saturated leaves as a key determinant of the lower optimal temperature of canopy photosynthesis. Further, I demonstrate the importance of accounting for within-canopy variation and seasonal acclimation of photosynthetic biochemistry in determining the temperature response of canopy photosynthesis. Chapters 5 and 6 connect temperature responses of individual plant processes to the overall response of plant growth. In chapter 5, I separate the indirect effect of temperature via increased water limitation from the direct effect of temperature on plant growth. I ran an experiment in which Eucalyptus tereticornis tree seedlings were grown in an array of six growth temperatures spanning from 18 to 35.5 °C with two watering treatments: i) water inputs increasing to match plant demand at all temperatures, isolating the direct effect of temperature; and ii) water inputs constant for all temperatures, matching demand for coolest grown plants, such that water limitation increased with growth temperature. I show that warming without a concomitant increase in water inputs decreases the temperature optima for both photosynthesis and growth by ~3 °C compared to warming with increasing water inputs. The results indicate that the indirect effect of increasing water limitation strongly modifies the potential response of tree growth to rising global temperatures. In Chapter 6, I apply a data assimilation framework to a seedling warming experiment to quantify the important mechanisms that determine the temperature response of tree seedlings growth. I show that the overall tree growth response to increasing temperature is determined not only by the effect of temperature on photosynthesis and maintenance respiration but also several other C balance processes, including changes in the utilisation rate of new photosynthate, growth respiration and C allocation to different tissues. Further, I quantify how the temperature response of each of these processes contribute to the temperature response of tree seedling growth. As a whole, this thesis identifies a number of key process representations that terrestrial biosphere models should incorporate for the robust quantification of forest growth in future warmer climates.
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