Positive Feedbacks to Fire-Driven Deforestation Following Human Colonization of the South Island of New Zealand

Altered fire regimes in the face of climatic and land-use change could potentially transform large areas from forest to shorter-statured or open-canopy vegetation. There is growing concern that once initiated, these nonforested landscapes could be perpetuated almost indefinitely through a suite of positive feedbacks with fire. The rapid deforestation of much of New Zealand following human settlement (ca. 750 years ago) provides a rare opportunity to evaluate the feedback mechanisms that facilitated such extensive transformation and thereby help us to identify factors that confer vulnerability or resilience to similar changes in other regions. Here we evaluate the structure of living and dead vegetation (fuel loading) and microclimate (fuel moisture) in beech (Nothofagaceae) forests and adjacent stands that burned within the last 60-140 years and are dominated by mAnuka (Leptospermum scoparium) or kAnuka (Kunzea spp.). We show that the burning of beech forests initiates a positive feedback cycle whereby the loss of microclimatic amelioration under the dense forest canopy and the abundant fine fuels that dry readily beneath the sparse mAnuka/kAnuka canopy enables perpetuation of these stands by facilitating repeated burning. Beech regeneration was limited to a narrow zone along the margin of unburned stands. The high flammability of vegetation that develops after fire and the long time to forest recovery were the primary factors that facilitated extensive deforestation with the introduction of human-ignited fire. Evaluating these two characteristics may be key to determining which regions may be near a tipping point where relatively small land-use- or climatically driven changes to fire regimes could bring about extensive deforestation.