Process-based modeling of jack pine productivity and climatic thresholds in the Canadian boreal forest

Abstract

ABSTRACT Boreal forests are important for carbon storage and timber production, yet their growth responses to climate change remain uncertain, particularly in northern regions where warming may both enhance and constrain productivity. Jack pine (Pinus banksiana Lamb.), a dominant boreal species in Ontario, is expected to be sensitive to these changes. Here, a process-based forest growth model (3-PG) was calibrated and validated for jack pine in Ontario’s Nipigon (3W) Ecoregion using provincial growth and yield data, and applied to assess productivity responses under four CMIP6 Shared Socioeconomic Pathways (SSP1–2.6, SSP2–4.5, SSP3–7.0, SSP5–8.5). Model performance was evaluated using diameter growth, and nonlinear climate responses were analysed using segmented regression. The calibrated model reproduced observed DBH–age relationships with high accuracy (pseudo-R² = 0.86) and performed well across independent validation stands (pseudo-R² = 0.81–0.87). Simulations indicated increased productivity under moderate warming, while higher forcing scenarios showed growth saturation and decline. Threshold analysis identified a mean annual temperature breakpoint at approximately 4.2 °C, beyond which net primary productivity decreased, and a precipitation threshold near 482 mm, above which productivity increased. These results highlight the value of process-based models for evaluating climate impacts and informing adaptive forest management.