Stem CO2 efflux characteristics and their response to environmental factors in a boreal Betula platyphylla secondary forest in China

Stem CO₂ efflux is an important component of forest carbon emission. The response of stem CO₂ efflux rate (Es) in Betula platyphylla secondary forests at different phenological stages and environmental factors, however, has been inadequately explored. We used an LI-6800 gas analyzer to measure Es from Betula platyphylla secondary forests in northern China in four diameter classes (D₁:6–10, D₂:10–14, D₃:14–18, and D₄:18–22 cm) throughout the year and during different phenological periods. The study reveals that daily Es across all diameter classes exhibited single-peak trends per phenological stage, peaking at 13:00–19:00 (1.95–0.08 μmolm⁻² s⁻¹) and bottoming at 23:00–07:00 (0.81–0.01 μmolm⁻² s⁻¹). The Es values of D₃ and D₄ showed a unimodal trend throughout the year, with peaks at the full leaf period (1.34 and 1.48 μmolm⁻² s⁻¹), while D₁ and D₂ showed a fluctuating downward trend, peaking at the leaf spreading stage (0.87 and 0.93 μmolm⁻² s⁻¹), and then reaching a minimum at the leafless period. Air temperature (Ta) and photosynthetically active radiation (PAR) were the dominant drivers of Es across diameter classes, while soil volumetric water content (VWC) exhibited weaker effects. The responses of Es to Ta and PAR differed phenologically: in leaf spreading and leaf fall periods, Es increased with Ta and decreased with PAR; during full leaf period, Es was strongly temperature-dependent but PAR-insensitive; in leafless period, Es showed a quadratic response to PAR but minimal temperature sensitivity. These findings suggest that stem CO₂ efflux in boreal Betula platyphylla forests will increase under warming climates and extended leafy periods, with secondary modulation by soil moisture dynamics.