Elevation, aspect, and climate shape blue pine (Pinus wallichiana) biomass and carbon dynamics in the Bhutanese Himalayas

Background

Forests cover over two-thirds of Bhutan and are constitutionally protected, enabling national carbon neutrality. Yet, climate change, increasing temperatures, variable precipitation, and heightened wildfire risk threaten carbon storage capacity and species distributions. Blue pine is a fast-growing, socio-economically important species dominating inner dry valleys between roughly 2200–3000 m, making it an ideal model to understand how elevation and slope aspect regulate biomass accumulation and carbon sequestration under changing climate conditions.

Goals and Methods

This study quantifies how elevation, slope aspect, and climate shape annual above-ground biomass and carbon accumulation in blue pine and identifies climatic thresholds that limit growth. Using existing tree-ring data from stratified plots across 200 m elevation bands between 2200 and 3000 m on north and south aspects, researchers detrended ring widths to generate indices. They related these indices to monthly temperature and precipitation through response-function and regression-tree analyses. Radial growth was converted to biomass by reconstructing annual diameters from ring sums and applying a species-specific allometric equation calibrated from 52 destructively sampled trees. Carbon was estimated as 47 percent of the above-ground biomass. Growth patterns were modelled with generalized additive models to capture non-linear responses to age and elevation.

Conclusions and Takeaways

This research shows that blue pine biomass and carbon accumulation peak at lower elevations, with an optimum near 2350 m, where trees add roughly 24.5 kg of biomass and 11.5 kg of carbon annually. South-facing slopes support higher accumulation than north-facing ones. Growth responds strongly to April precipitation and late-season minimum temperatures, with clear thresholds in spring rainfall and in August–September minimum temperatures that enhance ring formation. Alternatively, warmer late-season conditions at higher elevations can suppress ring formation. These findings suggest that maintaining Bhutan’s forest carbon sink will depend on targeted protection of lower-elevation, south-facing blue pine forests, alongside the systematic integration of identified climatic thresholds into fire management, restoration strategies, and anticipatory range-shift planning under projected warming scenarios.