The response of pinon pine leaf respiration to different dark acclimation pre-treatments

Background/Question/Methods

Drought-induced tree mortality is a global occurrence, though the physiological mechanisms underlying documented mortality events are not well understood.  Understanding the controls on forest carbon budgets and their responses during drought is critical in informing vegetation models.  Leaf respiration is an important component of the overall carbon budget, but responses to drought are not well documented.  A common method of measuring daytime leaf respiration includes acclimating leaves to dark conditions in order to halt photosynthesis and measure CO₂ flux from respiration pathways only.  In this study, a greenhouse-grown, 40 year-old pinon trees were exposed to drought conditions and CO₂ flux was measured hourly on the leaves of six branches using clear polycarbonate rectangular chambers and a closed-path infrared gas analyzer.  The trees were not watered for three weeks prior to the experiment to simulate common short-term drought conditions.  The chambers of three of the branches were shaded by covering them with aluminum foil and cardboard prior to sunrise, and the remaining three branch chambers were exposed to ambient light conditions for four hours in the morning.  The latter three chambers were then covered in a similar manner to the former branch chambers for one hour and respiration was measured on both groups.

Results/Conclusions

Carbon fluxes were significantly different between the sunlight and shaded branches in the morning prior to the onset of the one-hour shading treatment, as expected from differences in light availability (analysis of variance, p-value = 0.023).  This resulted in substantial net carbon gain in the sunlit branches.  Despite the large differences in carbon balance prior to the one-hour shading treatment, there were no differences between the one hour and 14 hour shading treatments in respiration rates (analysis of variance, p-value = 0.278).  This suggests that a period of one hour of dark acclimation is adequate to achieve accurate respiration measurements unaltered by exposure to light.  Further analysis from field grown trees including temperature and precipitation manipulation impacts, along with shading treatments, will be discussed.