Title: Acclimation Capacity of Photosynthesis and Respiration to Temperature in Dominant Mature
Boreal Conifer Tree Species
Author(s): Mirindi Eric Dusenge1*, Joseph R. Stinziano1, Jeffrey M. Warren2, Eric Ward2,
Stan D. Wullschleger2, Danielle A. Way1,3
Affiliation: 1Dept. of Biology, The University of Western Ontario, 2Oak Ridge National Laboratory and
3Nicholas School of the Environment, Duke University
Boreal forests are often assumed to be temperature-limited, and warming is therefore expected to stimulate their carbon uptake. However, much of our information on the ability of boreal conifers to acclimate photosynthesis (carbon gain) and respiration (carbon losses) to increasing temperatures comes from seedlings. To address how the carbon fluxes of mature, field-grown trees respond to warming, we measured net CO2 assimilation rates (A) and dark respiration (R) at both a set temperature of 25 °C (A25 and R25) and at prevailing growth temperatures (Agrowth and Rgrowth) in black spruce (Picea mariana) and tamarack (Larix laricina) trees exposed to ambient, +2.25, +4.5, +6.75 and +9 °C warming treatments at the SPRUCE experiment in May, June and August, 2016. In spruce, R25 showed little thermal acclimation, leading to increases in Rgrowth in trees exposed to warming, although acclimation appeared to dampen the large expected stimulation of Rgrowth in the warmest treatments (+6.75 and 9 °C) in the warmest months. In all three months, A25 and Agrowth were similar across all the plots, such that carbon gain in black spruce was generally insensitive to temperature. In tamarack, there was also little evidence for thermal acclimation of respiration. Values of R25 were constant across the treatment, and Rgrowth was higher in the warmer treatments than in the ambient plots, especially in August. Across the measurement months, A25 was similar or greater in warmed trees compared to ambient tamarack, an effect that led to homeostasis or a slight increase in Agrowth. Our work suggests that photosynthesis thermally acclimated in both species, which implies that carbon gain may be unaltered (or even increase) with warming in these dominant boreal conifers. In contrast, there was little indication that respiration acclimated to elevated temperatures in either species, except in spruce at the most extreme degree of warming, which could lead to greater carbon losses from vegetation in future climates.
(MAX 350 words)
Keywords: Climate change, carbon balance, boreal forests.