Landscape Influences on Occurrence and Spread of Wildfires in Patagonian Forests and Shrublands

Spatial heterogeneity of vegetation types and the abiotic environment can influence the occurrence and spread of wildfires, but in some landscapes the importance of these effects varies under conditions of severe fire weather. In the northern Patagonian landscape of forests and shrublands we examined the effects of vegetation type (tall forest vs. tall shrubland) and abiotic factors (elevation, topography, and precipitation) on fire occurrence at a broad scale and on fire spread at a fine scale. We used satellite images (1985–1999) and aerial photography (1950–1999) to map fires in relation to pre-burn vegetation type and abiotic factors. Fire extent is greatest at intermediate elevations and locations of intermediate precipitation. Fire extent is limited by lack of fuel quantity at the lower end of the precipitation gradient and by infrequent or insufficient fuel desiccation at the upper end. Tall shrublands are proportionally more affected by fire than are adjacent mesic forests of Nothofagus dombeyi and N. pumilio. Patches of subalpine forests often tend to serve as natural fire breaks, except under the most severe fire weather. Tall shrublands are dominated by species that resprout vigorously so that fuels quickly recover. In contrast, forests are dominated by species dependent on seed reproduction that sometimes fails after severe fires so that shrublands tend to replace burned forests. The greater propensity of shrublands to burn is a positive feedback that is favorable to fire and that accelerates the replacement of forest by shrublands. Infrequently occurring severe weather is important in the burning of otherwise relatively fire-resistant subalpine forests. Past burning, associated both with drought and early forest clearing, expanded shrublands at the expense of forests so that in the modern landscape an increase in anthropogenic ignitions and the positive feedback of fire and shrublands are synergistically accelerating conversion from forest to shrubland.