The effect of elevated [CO2] and elevated temperature on tree growth and physiology

Abstract

Increasing atmospheric CO2 concentration ([CO2]) is predicted to increase mean air temperature, which in turn is anticipated to affect precipitation. Expected changes in [CO2], temperature, and soil moisture availability may have a substantial effect on tree physiology and species distribution. Changing environmental conditions, especially temperature, may have a variable effect on physiological processes depending on where an individual is located within the species distribution. The hypothesis that an increase in temperature will boost growth in sub-optimal temperature environments, while an increase in temperature in supra-optimal temperature environments will reduce growth was tested. To test this hypothesis two studies were conducted. The first study investigated the effect of elevated temperature, elevated [CO2] and decreased soil moisture availability on loblolly pine physiology and growth at sites located near the northern and southern edge of the species range. Based on the findings from the loblolly pine study, a second study was conducted to investigate the combined effect of elevated temperature and [CO2] on northern red oak seedlings grown near the southern edge of its distribution.

Net photosynthesis and biomass accumulation of one-year-old loblolly pine seedlings increased with exposure to elevated [CO2] and temperature at both the northern and southern sites within the species distribution. A decrease in soil moisture availability significantly reduced accumulation, regardless of growing location or growing conditions. These findings are in conflict with assumptions of models that utilize environmental conditions to predict species distribution. These models predict that an increase in temperature will result in a northward shift in species distributions. Our study suggests that changes in climatic conditions may increase growth across the loblolly pine distribution and may result in an expansion of the species distribution. Northern red oak, unlike loblolly pine, was negatively affected by elevated temperature. The response of oak to elevated temperature suggests that an increase in growing temperature has the potential to negate the boost that elevated [CO2] may have on net photosynthesis and growth at ambient temperatures. The different responses of these two species to predicted future climatic conditions suggest that models utilized to predict future distributions require species specific verification.