Climate and humans interact to shape the fire regime of a chir pine forest in eastern Bhutan

Abstract Background: Chir pine (pinus roxburghii Sarg.) forests are distributed in the dry valleys of Bhutan Himalaya. These forests are heavily influenced by biotic activities such as grazing, resin tapping and NWFP (non-wood forests produce) collection. Understanding the relationship between past and present climate and the frequency and severity of forest fires is important for assessing the relative importance of potential drivers on fire occurrence and extent. This study aimed to reconstruct the fire history of a forested landscape dominated by chir pine in eastern Bhutan using crossdated fire scars disc samples. Results: The chir pine fire chronology extended to the early 1900s. Between 1900 and ~1970 fires were typically small and limited to a small number of sites within the larger study area. After 1970 a distinct shift in fire activity occurred with fires in 1985, 1989, 1996, 2000, and 2013 burning >90% of sample plots. Fire activity was positively associated with La Niña conditions (wetter, cooler) in the preceding year, which likely contributed to a greater accumulation of fuels on the forest floor. Conclusions: Historical reconstruction of fire activity from a site in eastern Bhutan highlights the interaction between human activity and climatic conditions. Prior to 1970 fire activity was dominated by patchy, low-intensity fires. After 1970 fires became larger and more damaging. This shift is associated with the enactment of the Bhutan Forest Act in 1969, which regulated grazing and implemented a policy of strict fire exclusion in government-reserved forests. This led to large build-ups of fuels, particularly after La Niña years. The ubiquity of ignition sources and strong rainfall seasonality ensured that fire could occur in most years. However, the change in forest policy and local community activities in the forests altered the amount and connectivity of fuels within the landscape, reshaping the fire regime in the chir pine forests of eastern Bhutan.