Foliar Age and Season Affect Photosynthetic Temperature Response in Black Spruce
Black spruce trees at the southern edge of the vast boreal forest are being exposed to progressive increases in temperature due to climate change. Temperature increases could change the balance between photosynthetic uptake of carbon dioxide (CO2) and respiratory release of CO2, which could further affect climate change. Since black spruce trees retain their needles for several years, the different age classes may have different responses to temperature increases. Thus, to understand and model how the boreal forest will function in the future, seasonal- and age-specific photosynthetic and respiratory temperature response functions must be measured. From 2011 to 2014, research was undertaken in a nutrient-limited black spruce and Sphagnum bog forest in northern Minnesota in the United States. Measurements were collected seasonally on different needle age classes from mature trees and included photosynthetic capacity, foliar respiration (Rd), and leaf biochemistry. Scientists from Oak Ridge National Laboratory used the results to model the predicted total annual carbon uptake by the trees under normal and elevated temperature scenarios. Temperature responses of key photosynthetic parameters were dependent on season and less responsive in the developing new needles (Y0) as compared with 1-year-old (Y1) or 2-year-old (Y2) needles. Each process initially increased with temperature, peaking between 19 °C and 38 °C, then declined at higher temperatures. Different age classes differed in their leaf structure and photosynthetic capacity, which resulted in 64% of modeled total annual carbon uptake from the older Y1 and Y2 needles (56% of the tree leaf area), and just 36% from Y0 cohorts (44% of tree leaf area). Under warmer climate change scenarios, the contribution of young needles was even less, just 31% of annual carbon uptake for a modeled 9 °C rise in summer temperature. Results suggest that net annual carbon uptake by black spruce could increase under elevated temperature and become more dependent on the older needle age classes. This study illustrates the physiological and ecological significance of different leaf ages, and indicates the need for seasonal- and leaf age-specific model parameterization when estimating carbon uptake capacity of boreal forests under current or future temperatures.
Jensen, A. M., J. M. Warren, P. J. Hanson, J. Childs, and S. D. Wullschleger. 2015. “Needle Age and Season Influence Photosynthetic Temperature Response in Mature Picea mariana Trees,” Annals of Botany 116, 821–32. DOI: 10.1093/aob/mcv115.