The 3-PG physiological/mensurational hybrid model is a useful forest management tool capable of producing accurate growth results across a number of parameterised species. The temperature data used in the model are the average maximum and minimum values for photosynthesis above the compensation point (Landsberg and Sands 2011). There is a minimum temperature below which positive net CO₂ exchange will not occur, a maximum temperature above which it will not occur and an optimum temperature at which it is maximised. These parameters are used in the 3-PG physiological model of forest production. However, a species’ photosynthetic response to short-term variation may differ from one season to another as species acclimate to temperatures over periods of a few weeks. In this study, acclimation responses of four species of eucalypt and Sequoia sempervirens to long-term temperatures were studied over a wide range of short-term temperature changes in order to identify the minimum, optimum and maximum temperatures of CO₂ assimilation for physiological/mensurational hybrid modelling, and also to identify the sites for which the species would be best suited. In order to achieve the aims of this study, a growth chamber experiment was established. Seedlings of four eucalypt species and Sequoia sempervirens were grown at base-line day/night temperatures of 30/16, 22/12 and 10/5ºC in controlled environment chambers for three months and leaf gas exchange measurements were made of the species at seven short-term temperature levels (5, 10, 15, 20, 25, 30 and 35ºC). The optimum and the maximum temperatures for net photosynthesis increased with an increase in base-line temperature for all species. The highest optimum temperature and net photosynthetic rates recorded were in plants grown at 30/16ºC and the lowest were in those grown at 10/5ºC. The maximum rate of net CO₂ assimilation increased with the temperature at which plants were grown partly because of acclimation in key photosynthetic processes in the Calvin cycle. Responses of maximal carboxylation rate (Vcmax) and also the maximal light-driven electron flux (Jmax) to short-term temperature change varied with base-line temperature for all species studied. Net photosynthesis and photosynthetic parameters measured did not vary significantly with effects of nitrogen, phosphorus and their interaction (p = 0.1468). The ratio of Jmax to Vcmax decreased with increasing leaf temperatures for all species (p < 0.001). These results indicate that the species studied will adapt to long-run changes in temperature, and the parameters obtained from these studies can be used for models that simulate the physiology and growth of the species.

Junipers and pines exhibit contrasting patterns of growth decline and mortality with climate change-type warming and drought; yet, the underlying physiological mechanisms are not fully understood. Does warming exacerbate the effects of drought on gas exchange physiology and growth? Do the combined effects of drought and warming differ for pines and junipers? To what extent do isohydric vs. anisohydric responses to water limitation in pines and junipers constrain net leaf CO2 exchange and plant growth response to drought and warming? To address these questions, we compared responses of leaf gas exchange and growth in seedlings of juniper (Juniperus scopulorum, J. virginiana) and pine (Pinus edulis, P. taeda) species of contrasting arid and mesic origin in a study of combined warming (ambient, +1.8 °C) and enhanced summer drought (long-term mean, -40%). Warming and enhanced summer drought each reduced photosynthesis and growth and effects were largely independent, suggesting that warming exacerbates drought effects on growth. Enhanced summer drought and warming had distinct impacts on photosynthetic carbon gain that were differentially revealed depending upon soil water content. Warming reduced light-saturated net photosynthesis (Asat) under low soil water contents, whereas carry-over effects of drought treatment were evident under well-watered conditions. Short-term soil drying led to greater reduction of Asat in pines (-51%) rather than junipers (-30%). Under short-term water-limited conditions, Asat and gs were about two-times higher for junipers compared to pines. Relative growth rate of junipers declined with warming (-28%) and drought (-50%) treatments. In contrast, pine growth and Asat declined more with warming than drought. Only P. edulis exhibited increased mortality in response to warming and drought, reaching 75% in the combined warming and drought treatment. Diminished sensitivity of R to water limitations, coupled with steeper reductions in Asat with decreasing soil water content in isohydric pines compared to anisohydric junipers could account for the greater sensitivity of pines to warming and drought under climate change.

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