Stand structure and disturbance history of old-growth blue pine (Pinus wallichiana) forests in the Bhutan Himalaya

Blue pine (Pinus wallichiana) forests are of significant ecological, economic, and cultural importance in the Himalayas. We used dendrochronological methods to investigate the role of natural and human disturbance in shaping the development of these forests. Analyses of the age structure and growth patterns of blue pine populations over the period 1760–2020 at two different sites in central Bhutan revealed that blue pine tends to establish as single-cohort stands following relatively intense disturbances and as multi-cohort stands after low-to-moderate severity disturbances. Shifting cultivation, which was common across the region, likely led to the establishment of single-cohort stands, particularly near human settlements, whereas natural disturbances are likely responsible for the development of multi-cohort stands. Tree-ring records revealed an acute change in recruitment patterns in the early 1970s associated with the 1969 Forest Act of Bhutan, which limited traditional practices, such as firewood collection and grazing, within the forests. This led to a sudden and sustained increase in the recruitment of broadleaf tree species and effectively curtailed blue pine regeneration over the past half century due to thick understory and midstory vegetation reducing the amount of light reaching the forest floor. These results highlight the role of disturbances, both human and natural, in driving forest stand dynamics in Himalayan forests and how forest policy and traditional practices can alter those dynamics.