Determining the controlling factors for carbon sequestration in two contrasting forests in the Boreal and semi-arid Mediterranean regions (Part II)

Evergreen needleleaf forests span a wide climatic range, yet their carbon sequestration is increasingly constrained by climate-driven environmental limits. Building on results presented at EGU 2025, which showed a shift in the dominant seasonal control on productivity from atmospheric moisture in the Boreal Hyytiälä forest (Finland, HYY) to soil moisture in the semi-arid Yatir forest (Israel, YAT), we identify the environmental boundary conditions underlying this contrast.

Using PAR-saturated conditions to isolate eco-physiological controls on productivity, we derive key climatic and hydrological thresholds from SHAP-based analyses. In YAT, productivity is strongly constrained by deep soil water availability, with a clear threshold at ~15.8 %vol in the deepest measured soil layer (~45 cm). This threshold reflected a seasonal transition where deep soil moisture shifts from limiting productivity (ineffective water retention and root resistance) to supporting shallow root water uptake and productivity during the wet season. This transition coincides with the seasonal minimum in soil temperature imposing peak root resistance, indicating a compounded control on the onset of productivity in this water-limited ecosystem.

In contrast, seasonal productivity in HYY is dominated by precipitation, which both sustains evapotranspiration, closely linked to net ecosystem productivity (R=0.96), and likely reflects a favorable cloud and radiation regime. The high historical ratio of diffuse to direct shortwave radiation in HYY (S_diff:S ~ 3:4) helps to buffer canopy conductance against high vapor pressure deficit (VPD), consistent with the high sensitivity observed at this site (negative productivity response at VPD > 1 kPa). Such atmospheric constraints are lacking in YAT, where diffuse radiation is limited  (S_diff:S ~ 1:4) and VPD shows an order of magnitude larger range.

Despite adaptation to such contrasting environments, both forests exhibit a similar optimal air temperature range for productivity (14–20 °C), which highlights a shared physiological optimum across the divergent environmental limitations. Overall, our results demonstrate that carbon sequestration in these systems is not controlled by universal drivers, but by site-specific boundary conditions, such as deep soil water availability in semi-arid Mediterranean forests and precipitation-linked atmospheric regimes in Boreal forests.