Boreal Forest Sensitivity to Temperature Is Jointly Regulated by Ecosystem Productivity and Seasonality: A Synthesis Based on Carbon Fluxes From FLUXNET Sites

Boreal forests are experiencing rapid global warming, which may amplify the impact of temperature on their carbon sink function. However, the factors influencing the sensitivity of the carbon balance to temperature in boreal forests are still not well understood, especially over short timescales. Leveraging the FLUXNET dataset, we calculated the sensitivities of carbon fluxes to temperature anomalies (T_s) across boreal forest (24 sites) on the seasonal scale. Here, a positive T_s value indicates that the carbon flux increases with temperature anomaly, while a negative value suggests the opposite. Results showed that T_s of NEP in early spring was positive for evergreen needleleaf forests but negative for deciduous broadleaved forests. Although T_s of gross ecosystem production (GEP) remained positive, it tended to decrease with increasing temperature and vapour pressure deficit in spring and summer. Associated with this decrease were relatively small changes in T_s of ecosystem respiration (R_e), so that T_s of NEP decreased with increasing temperature and vapour pressure deficit in spring and summer. Both T_s of GEP and R_e increased with ecosystem productivity (represented by annual GEP) in spring, with T_s of GEP being more sensitive, indicating that carbon sinks in sites with lower productivity tended to negatively respond to higher temperature. Young-aged and low-nutrient forests tended to respond negatively to increasing temperatures. Ecosystem productivity and seasonality played an essential role in regulating T_s of NEP. Our results suggest that significant declines in ecosystem productivity may exacerbate the negative response of NEP to elevated temperatures, further compromising carbon sequestration in boreal forests. This understanding is vital both for improving the predictive accuracy of carbon-climate models and for formulating targeted strategies to bolster resilience in the most vulnerable boreal ecosystems.