Effects of elevated temperature and elevated CO2 on leaf carbon fluxes in boreal conifers: lab and field studies

Mirindi Eric Dusenge, The University of Western Ontario

Abstract

Rising CO₂ may warm northern latitudes up to 10 °C by the end of the century. However, responses of plant physiological processes (such as photosynthesis and respiration) and growth to climate change remain uncertain. Seedlings and mature trees of tamarack (a deciduous species) and black spruce (an evergreen species), North America dominant conifers, were exposed to combined warming (up to +9 ˚C) and elevated CO₂ (up to +300 ppm). In seedlings, stomatal conductance (g_s) tended to increase with warming in tamarack seedlings, while g_sdeclined with warming in spruce. In both species, CO₂ had weak effect on g_s. Photosynthetic capacity (maximum rates of Rubisco carboxylation, V_cmax and of electron transport, J_max) was reduced in warm-grown seedlings, while it was not affected by high CO₂. As a result, photosynthetic rates (A) remained constant in tamarack while they declined in warm-grown spruce seedlings. In mature trees, there was a slight increase in g_swith warming in tamarack, while it decreased in spruce. However, g_s was not affected by growth CO₂ in both species. A was slightly stimulated by warming in mature tamarack, but similar across warming in spruce trees. A was also increased by elevated CO₂in tamarack but not spruce trees, a result that correlated with strong CO₂-induced reductions in V_cmaxand J_maxin spruce. In both seedlings and mature trees, the temperature sensitivity parameters of V_cmaxand J_max responded strongly to warming, with few CO₂ effects. Similarly, thermal optimum of A (T_optA) increased with warming with little CO₂ effect. Therefore, T_optA was largely correlated with temperature sensitivity parameters of V_cmax and J_max. In seedlings, leaf respiration (R_d) measured at a common temperature decreased with warming. In contrast, in mature trees, R_d was constant across warming treatments. Differential responses of these physiological processes to the treatments resulted in different growth between species. In seedlings, moderate warming increased biomass in tamarack, while warming reduced biomass in spruce. However, in mature tamarack, growth was not affected by warming while it decreased in mature spruce. Overall, my findings largely suggest that warming-induced productivity expected in higher latitudes in future climates may be species-dependent.