University of Colorado at Boulder

Colloquia Event

Friday September 20, 2013. 03:30 pm. GUGG 205

Post-fire vegetation dynamics and feedbacks on fire susceptibility in Nothofagus forests of New Zealand

Alan Tepley

University of Colorado, Department of Geography

Abstract: As one of the last large land masses to be colonized by people, New Zealand provides a unique opportunity to learn about the effects of human fire use in forested landscapes. Studies of pollen and charcoal in lake sediments suggest fire was extremely rare prior to human arrival. However, Māori colonization of New Zealand in the 13th century brought a new ignition source, resulting in the rapid transformation of nearly one-half of the forested area to grasslands and shrublands. To characterize the mechanisms that enabled this rapid transformation by a relatively small human population, we sampled vegetation composition and structure (fuel loading) and microclimate (fuel moisture) in 15 paired burned and unburned stands. Even though the native southern beech (Nothofagus) forests have a high fuel loading, we demonstrate that they are not easily burned due to their cool, moist microclimate. By contrast, the vegetation that develops after fire has a high density of small-statured woody shrubs with thin, papery bark, abundant fine fuel near the ground, and a substantially warmer and drier microclimate than that in adjacent unburned forests. Thus, the initial burning of native forests initiates a positive feedback loop whereby high flammability of the post-fire vegetation contributes to a cycle of repeated burning and expansion of shrublands rather than recovery of the initial beech forests. Understanding how these feedbacks have played out under the well-documented case of abrupt landscape transformation in response to the recent introduction of human-ignited fire has important implications for identifying similar feedback loops in other systems and characterizing which types of landscapes are most susceptible to broad-scale changes in response to climate- or human-induced changes in disturbance regimes.