Linking Soil CO2 Efflux to Individual Trees: Size-Dependent Variation and the Importance of the Birch Effect

Patterns and determinants of soil CO2 efflux in major forest types of Central Himalayas, India

Soil CO₂ efflux (F_soil) is a significant contributor of labile CO₂ to the atmosphere. The Himalayas, a global climate hotspot, condense several climate zones on account of their elevational gradients, thus, creating an opportunity to investigate the F_soil trends in different climate zones. Presently, the studies in the Indian Himalayan region are localized to a particular forest type, climate zone, or area of interest, such as seasonal variation. We used a portable infrared gas analyzer to investigate the F_soil rates in Himalayan tropical to alpine scrub forest along a 3100-m elevational gradient. Several study parameters such as seasons, forest types, tree species identity, age of trees, distance from tree base, elevation, climatic factors, and soil physico-chemical and enzymatic parameters were investigated to infer their impact on F_soil regulation. Our results indicate the warm and wet rainy season F_soil rates to be 3.8 times higher than the cold and relatively dry winter season. The tropical forest types showed up to 11 times higher F_soil rates than the alpine scrub forest. The temperate Himalayan blue pine and tropical dipterocarp sal showed significant F_soil rates, while the alpine Rhododendron shrubs the least. Temperature and moisture together regulate the rainy season F_soil maxima. Spatially, F_soil rates decreased with distance from the tree base (ρ =  - 0.301; p < 0.0001). Nepalese alder showed a significant positive increase in F_soil with stem girth (R² = 0.7771; p = 0.048). Species richness (r, 0.81) and diversity (r, 0.77) were significantly associated with F_soil, while elevation and major edaphic properties showed a negative association. Surface litter inclusion presented an elevation-modulated impact. Temperature sensitivity was exorbitantly higher in the sub-tropical pine (Q₁₀, 11.80) and the alpine scrub (Q₁₀, 9.08) forests. We conclude that the rise in atmospheric temperature and the reduction in stand density could enhance the F_soil rates on account of increased temperature sensitivity.