Photosynthetic acclimation and trade-offs in biomass allocation have not been widely incorporated into physiological and ecosystem models, suggesting that models may over- or underestimate how species might respond to air temperature (Ta) increases and how ecosystem productivity could be altered under global warming. Eastern cottonwood (Populus deltoides) is a fast growing, widely distributed North American tree species used for bioenergy and bioproducts. Since future warming is likely to have significant direct effects on poplar carbon allocation, ecophysiology and productivity, we designed an experiment to examine their responses and test mechanistic representation in a terrestrial biosphere model. We grew well-irrigated P. deltoides saplings at three different Ta cycles including 24/18 °C (day/night), 32/26 °C and 40/34 °C for five months. We quantified changes in net photosynthesis (Anet), dark respiration (Rd), water use, growth and biomass allocation. Subsequent thermotolerance was tested by exposing plants to short-term heat waves (reaching Tmax 47, 48 and 51°C over a period of 3 days) followed by photosynthetic measurements.
Results/Conclusions
Compared to 24/18°C, exposure to 32/26 °C or 40/34°C treatments reduced Anet@25°C by 38% with a concomitant decrease in Rd@25°C by 61%. Though whole-plant transpiration initially peaked and remained higher in elevated Ta plants, both leaf-level and whole-plant based transpiration showed significant draw-down with increased plant height and canopy development when compared to 24/18°C counterparts. We observed a significant decrease in leaf mass per area (LMA) and increase in stem wood density (DW) with increasing growth Ta. The anatomical and biochemical investigations for the observed changes in DW and LMA are ongoing. Increased Ta increased whole plant biomass with more allocation to roots and leaves as compared to stem, showing tissue-specific allocation trade-offs that will be important to consider during modeling efforts. Growth temperature had a modest impact on thermotolerance during the first heat wave, with a 69% loss in Anet in 24/18°C and ~63% loss for 32/26 °C and 40/34°C plants. However, recurring exposure to heat waves improved net photosynthesis irrespective of growth T, but respiration remained significantly higher in 24/18°C plants than compared to plants grown at higher Ta. Analyses, including leaf temperature dependent A-Ci and Rd response curves is ongoing, and data will be applied to the E3SM Land Model. Results will be used to improve mechanistic understanding of native poplar responses to future warming conditions, including net primary productivity, water use and allocation.