Leaf-level photosynthesis and respiration can be highly sensitive to both short-term and longer-term environmental change, and how these processes respond with phenological transitions and daily temperature variability dictate how carbon is first assimilated and lost. Here, we examine the short-term temperature response of photosynthesis and respiration in three dominant canopy species at Harvard Forest across the 2015 growing season. A main objective of this study was to quantify the light inhibition of dark respiration in leaves from these species at different measurement temperatures at different points of the growing season, and use this information to better inform daytime carbon assimilation and efflux estimates. To do this, we used the Kok-method of high-resolution light-response curves and measured leaves from Quercus rubra, Fagus grandifolia, and Acer rubrum at three measurement temperatures (15, 25, and 35˚C) each month between late May and late September. We also sampled and measured leaves from Quercus rubragrown under long-term warming (7+ yr) conditions to address how elevated soil temperatures may impact these processes. Leaf-level physiological data was complemented by leaf nitrogen and physical trait values from measured leaves as well as and data from visible band and near-infrared band camera images that track phenology via canopy color indices.
Maximum photosynthesis values varied between species and measuring month, with mid-growing season rates highest. Dark respiration (measured at a common temperature) declined between late May and late August, and increased slightly by September. A similar trend, though reduced in the degree of the flux, is observed in respiration in the light, with highest values in the early season, reduced respiration in mid-season, and increased values by late September. Light significantly reduced dark respiration across all species and seasons. The proportion of photosynthesis to respiration in the light increased across the growing season, implying higher rates of daytime carbon use efficiency during mid- and late-summer due to the lower rates of respiration. Leaf nitrogen (area) increased concomitantly with the increase in leaf mass-per-area through the growing season. In Quercus rubra grown under long-term warming, rates of photosynthesis declined, though rates of respiration in the light increased, suggesting potential higher carbon loss in trees grown in elevated temperatures. The results from this study can be used to better inform how carbon is exchanged at the canopy-scale through a correction of eddy-covariance estimates of daytime net ecosystem exchange and as a result, gross primary productivity.