Interannual variability of surface energy exchange depends on stand age in a boreal forest fire chronosequence



[1] Understanding how changes in the boreal fire regime will affect high latitude climate requires knowledge of the sensitivity of the surface energy budget to shifts in vegetation cover. We measured components of the surface energy budget in three ecosystems that were part of a fire chronosequence in interior Alaska for 3 years. Our sites were within the perimeter of stand-replacing fires that occurred in 1999, 1987, and ∼1920 (hereafter referred to as the 1999-burn, the 1987-burn, and the control). Vegetation cover consisted primarily of sparse short grasses at the 1999-burn, aspen and willow (deciduous trees and shrubs) at the 1987-burn, and black spruce (evergreen conifer trees) at the control. Averaged over the 3 years of our study, annual net radiation decreased by approximately 25% at the 1999-burn and 30% at the 1987-burn, relative to the control. Sensible heat decreased by an even larger amount, by approximately 57% for the 1999-burn and 44% for the 1987-burn as compared with the control. Climate during spring and summer varied considerably among the 3 years. The three stands responded differently to this climate variability with consequences for surface energy exchange. As a result of earlier snow cover loss in 2003 and 2004, net radiation during spring increased substantially in the recently disturbed stands, but not in the control. In response to a sustained summer drought in 2004, latent heat decreased more in the 1987-burn during August than in the control. Our results imply that a shift in plant functional types expected to accompany increases in boreal fire activity may amplify interannual climate variability during both spring and summer.