Warming interacts with environmental conditions to alter seedling carbon assimilation

Background/Question/Methods

Climate predictions for the southeastern United States include higher temperatures, reduced precipitation, and prolonged droughts.  Tree growth rates are largely expected to increase with warming conditions, but responses are idiosyncratic, due to interactions between warming, drought, and competition.  For example, increased drought could reduce photosynthetic rates, without reducing the respiratory load.  In 2008, a manipulative warming experiment was implemented in North Carolina to determine plant-level responses to changing climatic conditions.  The experiment utilizes a factorial design for warming (control, ambient, +3-5°C), and two light levels (gap and understory) for a total of 24 plots.  Individual physiological, demographic, and phenological responses were tracked and modeled based on environmental variation.  A mixed-effects-model and light-response-model were used to determine the impact of warming, light levels and their interaction on the balance between carbon gain and carbon loss for four representative species of the eastern deciduous forest.  We quantified the sensitivity of each species to warming, how warming effects interacted with changing drought status over the course of the experiment, and how environmental effects rates may shift the competitive balance between species.   

Results/Conclusions

For all species tested, warming treatments consistently increased the maximum rate of assimilation while simultaneously higher temperatures elevated respiration rates.   Under ideal conditions of high moisture and full sunlight, net assimilation was enhanced by warming.  During drought, respiratory rates remained high as assimilation rates declined, making individuals more vulnerable to carbon starvation.  This effect was most severe in warmed plots, allowing us to quantify the warming/drought interaction for each species.  These same general trends held in light limited understories, but the small margin between assimilation and respiration amplified the negative drought-warming interaction.  When net assimilation is aggregated across the season based on environmental conditions, the implication of small shifts in temperature/moisture sensitivities becomes apparent, as some species change rank in terms of the amount of carbon they assimilate.  Overall, higher temperatures yield a higher seasonal net carbon gain in full sunlight conditions, but when resources (light or moisture) are limiting respiration becomes a more dominant factor, which ultimately leaves individuals more vulnerable to carbon starvation.