Boreal conifers maintain carbon uptake with warming despite failure to track optimal temperatures

Terrestrial photosynthesis is a crucial carbon sink, offsetting a significant fraction of anthropogenic CO₂ emissions. However, global change drivers like warming and drought could significantly alter photosynthetic rates, impacting atmospheric CO₂ accumulation. Current Terrestrial Biosphere Models (TBMs) used in global climate models rely on data primarily from young trees grown in greenhouses under ambient CO₂, which may not accurately reflect the responses of mature trees in natural field conditions, especially under elevated CO₂. Photosynthesis's temperature response is non-linear, peaking at a thermal optimum (*T*_optA) before declining at higher temperatures. Plants can acclimate to long-term warming by increasing *T*_optA, but the extent of this acclimation varies widely among species. Studies on boreal conifers, mostly conducted on seedlings, show varying acclimation capacities. It remains unclear whether mature, field-grown conifers can adjust *T*_optA to compensate for rising temperatures and elevated CO₂. Elevated CO₂ stimulates photosynthesis initially, but long-term effects are often reduced due to biochemical acclimation and sink limitations. Elevated CO₂ also suppresses photorespiration, potentially affecting *T*_optA. Most studies on thermal sensitivity of photosynthesis have focused on ambient CO₂-grown plants, leaving the impact of elevated CO₂ on thermal acclimation poorly understood. This study aims to assess the thermal acclimation of photosynthesis in mature tamarack and black spruce trees under both ambient and elevated CO₂ conditions combined with whole-ecosystem warming.

Previous research on the thermal sensitivity of photosynthesis largely utilized data from seedlings grown under controlled conditions in greenhouses or growth chambers, primarily under ambient CO2 levels. These studies reported a wide range of increases in *T*_optA (0.16-0.78 °C per 1 °C of warming), highlighting varying acclimation capacities among species. Some boreal conifers demonstrated *T*_optA acclimation, while others did not. The limited sample sizes in these studies raise questions about the representativeness of these findings. Furthermore, these studies lacked assessments on whether *T*_optA increases matched growth temperature increases. One three-year field warming study on seedlings showed that while *T*_optA shifts occurred, they were considerably smaller than increases in growth temperature. The impact of elevated CO2 on thermal acclimation has also been sparsely explored, with limited research investigating the interplay between elevated CO2 and temperature acclimation on mature trees under natural field conditions. This gap in knowledge necessitates the current study focusing on mature trees under field conditions to determine the thermal responses to combined warming and elevated CO2.

This study was conducted at the Oak Ridge National Laboratory's SPRUCE (Spruce and Peatland Responses Under Changing Environments) project site in Minnesota, USA. The site consists of a 50-year-old boreal peatland forest dominated by black spruce (*Picea mariana*) and tamarack (*Larix laricina*). The experiment employed ten large open-top enclosures, five with ambient CO₂ and five with elevated CO₂ (+430 to +500 ppm above ambient). Five temperature treatments were established (ambient +0, +2.25, +4.5, +6.75, and +9 °C). Warming treatments began in August 2015, with CO₂ treatments starting a year later. Field measurements were conducted in June and August 2017. Gas exchange measurements were performed on sun-exposed branchlets from randomly selected trees within each plot using portable photosynthesis systems (Li-COR 6400 XT). Net CO₂ assimilation rates (A) were measured at a saturating light intensity and eleven different CO₂ concentrations to generate A-Cᵢ curves at five leaf temperatures (15, 25, 32.5, 40, and 45 °C). The Farquhar, von Caemmerer, and Berry (FvCB) C₃ photosynthesis model and the *fitacis* function from the *plantecophys* R package were used to derive maximum Rubisco carboxylation rate (*V*_cmax) and maximum electron transport rate (*J*_max) from A-Cᵢ curves. Temperature sensitivity parameters of *V*_cmax and *J*_max (*T*_opt^V, *T*_opt^J, *E*_a^V, *E*_a^J) were derived using a modified Arrhenius function. The temperature response of A at growth CO₂ was fitted using a quadratic equation to estimate *T*_optA and the breadth of the photosynthetic temperature response curve (b). Statistical analysis using mixed-effects regression models examined the effects of elevated CO₂ and warming on photosynthetic parameters. To assess the potential effects of stomatal conductance on *T*_optA shifts, net photosynthesis was recalculated at a specific Ci*/Ca ratio. Net CO₂ assimilation rates were estimated at growth temperature conditions using mean daytime air temperature for 10 days preceding each measurement and for the entire growing season of 2016 and 2017.

The thermal optimum of net photosynthesis (*T*_optA) increased with warming in both tamarack and black spruce (0.26–0.35 °C per 1 °C warming). This increase was similar for both ambient and elevated CO₂ treatments. *T*_optA was 3 °C higher in elevated CO₂ tamarack compared to ambient CO₂ tamarack, while no CO₂ effect was observed in black spruce. Warming-induced increases in *T*_optA correlated with increases in the thermal optima of *V*_cmax and *J*_max. There was no acclimation of the activation energy for *V*_cmax, but in black spruce, the activation energy of *J*_max declined non-linearly with warming in elevated CO₂ trees. Neither stomatal conductance nor respiration correlated with *T*_optA shifts. Under ambient CO₂, mean daytime growth temperature exceeded *T*_optA across all warming treatments. Elevated CO₂ reduced this difference in tamarack at +2.25 °C warming, but had weak or no effect on black spruce. Elevated CO₂ increased the parameter 'b' (representing the sensitivity to short-term temperature fluctuations) in both species, suggesting increased short-term temperature sensitivity. Net photosynthetic rates at *T*_optA were constant across warming treatments in tamarack, but increased with warming in elevated CO₂ black spruce. Net photosynthetic rates at growth temperature remained constant or increased across warming treatments, indicating maintained carbon uptake at growth temperatures.

This study provides the first field-based assessment of the short-term temperature sensitivity of photosynthesis in response to long-term whole-ecosystem warming and elevated CO₂ in mature trees. The results reveal that while *T*_optA increased with warming, the increase was insufficient to fully compensate for the rise in air temperatures. The observed *T*_optA shifts are at the lower end of the range reported in previous experimental studies, suggesting that mature trees may exhibit less acclimation potential than seedlings. The strong correlation between *T*_optA and the thermal optima of *V*_cmax and *J*_max highlights the role of photosynthetic biochemical processes in thermal acclimation. The lack of significant effects of elevated CO₂ on the thermal optima or activation energies of *V*_cmax and *J*_max suggests that the temperature response functions currently used in TBMs, derived mostly from ambient CO₂-grown plants, may accurately represent carbon uptake under both current and projected elevated CO₂ conditions for boreal conifers. However, the study reveals that elevated CO₂ significantly affects the overall thermal sensitivity of net photosynthesis, represented by parameter 'b'.

This study demonstrates that although thermal acclimation of *T*_optA in mature boreal conifers is limited and does not fully match temperature increases, carbon fixation is maintained at growth temperatures due to photosynthetic acclimation and changes in the instantaneous temperature response of photosynthetic processes. These findings offer improved parameters for modeling photosynthesis in TBMs by suggesting the possibility of ignoring elevated CO₂'s impact on the thermal sensitivity of key photosynthetic biochemical parameters. However, the effects of elevated CO₂ on *T*_optA and the overall thermal sensitivity of net photosynthesis should be incorporated into TBMs. Future research should expand to other biomes and plant functional types to validate these findings.

The study was limited to two boreal conifer species at a single site. Generalizing these findings to other species and biomes requires further research. Leaf and air temperatures were assumed to be similar, which may not always hold under all conditions. Mesophyll conductance was not measured, limiting the ability to fully assess the mechanisms behind the differences observed between the species in their response to elevated CO2. The study focused on leaf-level processes; understanding whole-ecosystem carbon balance requires considering other factors, such as autotrophic respiration, which may be affected differently by warming.