Drivers and consequences of an intensifying fire regime in northern high latitudes

The boreal region encompasses approximately 30 % of the world's forests and the arctic-boreal region stores about 35 % of the world's soil carbon. Climate warming is occurring more rapidly in these northern high latitudes than in the rest of the world. These warmer temperatures are causing the number of fires to rise.

This presentation highlights three aspects of a changing arctic-boreal fire regime: 1) warming-induced lightning ignitions as driver of increased fire activity, 2) holdover fires as an emerging phenomenon, and 3) preliminary field estimates of carbon combustion in larch ecosystems in northeast Siberia.

First, we show that warming-induced increases in lightning ignitions were a major driver of two recent large fire years in boreal North America, in the Northwest Territories in 2014, and in Alaska in 2015. We further append upon these case studies, by combining lightning observations from the Optical Transient Detector in combination with climate proxies for lightning from reanalysis to model contemporary and future lightning in the high latitudes. Results indicate a lightning-richer future for arctic-boreal regions, suggesting consequent increases in lightning fires.

Second, we found that the legacy of large fire years extends into the subsequent fire season through the phenomenon of holdover fires. Holdover fires are fires that seemingly stop at the end of the fire season when rain and snow arrive, yet these fires smolder in organic soils throughout winter to eventually reemerge in the subsequent spring under weather conditions that favor fire spread. We analyzed the spatiotemporal occurrence of holdover fires in relation to climatic and environmental variables and conclude that we may expect more holdover fires in the future.

Third, prior research has shown that fire regimes are different between the boreal continents: high severity stand-replacing fires dominate in North America, whereas low severity surface fires are more frequent in Eurasia. Significant progress has been made in quantifying carbon combustion in North American ecosystems, yet few measurements have been made in Eurasian ecosystems. We show preliminary carbon combustion estimates from a field campaign in larch ecosystems of northeast Siberia, and compare these with estimates from North America.