Rising CO2 Increases Modeled Leaf Assimilation in Anatolian Forest Trees, but Thermal Threshold Exceedance Also Rises

Rising atmospheric CO2 suppresses photorespiration, whereas warming and drying can limit leaf carbon gain. We coupled a Farquhar–von Caemmerer–Berry photosynthesis model with soil–plant hydraulics and a 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 2020–2030 with 2070–2080 under SSP5-8.5. With soil water tracked year-round, the mean seasonal net assimilation increased by 106.5 gC m−2 of leaf area (95% CI: +43 to +167), with declines in 11 of 135 cells; with soil moisture reset each season, it increased by 130.3 gC m−2 (95% CI: +76 to +187). In the reset configuration, the contributions were +219.5 gC m−2 from CO2, −66.7 from temperature and the associated vapor pressure deficit (VPD), −38.8 from precipitation, and −2.8 from radiation (residual: +19.1). Species and sites accounted for 81% of the fitted variance, compared to 3.1% for GCMs. Days with peak leaf temperature exceeding assigned thermal thresholds increased from 1.5 to 12.2 per season, primarily in broadleaved species. This number depends on the assumed thresholds (16.9 days with a common threshold of 44 °C), energy balance parameters, and stomatal behavior, and does not 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.