Modeled Leaf Assimilation Increases Under Rising CO₂ in Anatolian Forest Trees Despite Warming and Drying, While Thermal Threshold Exceedance Increases

Rising atmospheric CO₂ suppresses photorespiration, whereas warming and drying can limits leaf carbon gain. We coupled a Farquhar-von Caemmerer-Berry photosynthesis model with an ATP/NADPH budget, soil-plant hydraulics, and leaf energy balance, assigning photosystem II thermal thresholds based on leaf habit. Using ISIMIP3b forcing from five CMIP6 models, we simulated eight tree species across 27 species-site combinations at six locations in Türkiye, comparing the periods 2020–2030 and 2070–2080 under the SSP5-8.5 scenario. With soil moisture reset each season, the mean seasonal net assimilation increased by 130.3 gC m⁻² of leaf area (95% CI: 76–187 gC m⁻²). The forcing-channel contributions were +219.5 gC m⁻² from CO₂, −66.7 gC m⁻² from temperature/VPD, −38.8 gC m⁻² from precipitation, and −2.8 gC m⁻² from radiation (residual: +19.1). Species and sites accounted for 81% of the fitted variance, compared to 3.1% for GCMs. Under year-round soil water tracking, the mean increase was 106.5 gC m⁻² (95% CI: +43 to +167), with declines in 11 of 135 cells. Days with peak leaf temperature exceeding assigned thermal thresholds increased from 1.5 to 12.2 per season, primarily in broad-leaved species. These counts were sensitive to thermal parameters and stomatal assumptions and do not directly quantify tissue damage. Within the tested configurations, higher modeled leaf assimilation coexisted with more frequent threshold exceedance. Neither outcome alone establishes stand productivity or forest resilience.