Ecological and climatic controls of modern wildfire activity patterns across southwestern South America

Andrés Holz,Thomas T Veblen,Juan Paritsis,Thomas Kitzberger

doi:10.1890/es12-00234.1

Abstract

Understanding how patterns of wildfire activity across biomes are shaped by heterogeneity in biomass resources to burn and atmospheric conditions conducive to burning is a high research priority in the context of global environmental change. Along a latitudinal gradient (25 to 56° S) from semi‐arid scrublands through Mediterranean‐type vegetation to wet forests in southwestern South America (SSA) we analyzed influences of mean climate and interannual climate variability on fire activity using documentary fire records from 1984 to 2008. We identified large regions with common temporal variability in annual area burned, related this variability to local interannual climate variability and in turn to modes of the major tropical and extratropical climate drivers of the southern hemisphere—El Niño‐Southern Oscillation (ENSO) and the Antarctic Oscillation (AAO). Differences in fire activity response to interannual climate variability were related to the relative influences of available biomass to burn, and to weather effects on amounts of fine fuels and fuel moisture conditions. The pattern of average fire activity along this latitudinal moisture/productivity gradient corresponds well with the varying constraints model. This model predicts low fire activity towards the arid extreme due to reduced burnable biomass and again towards the humid extreme due to infrequent weather suitable for drying fuels, and predicts a broad zone of high fire activity at intermediate locations where resources to burn are abundant in all years and fuel moisture dries under reliably dry summer conditions. The dominant influence on interannual climate variability is AAO, which explained most of the variability in fire activity both by reducing seasonal precipitation in mesic and wet forests where fire is dependent on infrequent drought and by enhancing fine fuel production in Mediterranean‐type vegetation where fuel amount and continuity constrain fire activity. In the context of the drying and warming trends in SSA related to the continued positive anomaly in AAO, our results underscore the importance of the varying constraints on fire activity and modulation of fire‐climate relationships by different vegetation types, which is a much needed step toward developing fire projections under future climate.

Highlights

Understanding how climatic and biological systems interact to create spatial and temporal patterns of wildfire activity over broad regions is an urgent need in the context of global environmental change (Bowman et al 2009, Krawchuk et al 2009, Moritz et al 2012)
The objectives of our study are to: (1) determine the latitudinal pattern in average relative areas burned along the gradient from semi-arid Mediterranean-type ecosystems through cool temperate/ sub-Antarctic rainforests in southwestern South America (SSA), (2) identify regions with common temporal variability in annual area burned (AAB) and examine how this variability is associated with anomalies in both local climate and in major tropical and extratropical climate drivers indexed as El Nino Southern Oscillation (ENSO) and Antarctic Oscillation (AAO), and (3) assess the role of dominant fuel types in distinguishing these regions of common temporal variability in AAB and the respective relationships between local climate anomalies and major climate drivers
Woody vegetation.—Measures of central tendencies of fire activity for each fire-reporting district arranged from north to south along the latitudinal gradient exhibit lowest values at the dry and wet extremes (Fig. 2a)

Summary

Introduction

Understanding how climatic and biological systems interact to create spatial and temporal patterns of wildfire activity over broad regions is an urgent need in the context of global environmental change (Bowman et al 2009, Krawchuk et al 2009, Moritz et al 2012). Regional climate variability at interannual to inter-decadal time scales is often teleconnected to large-scale climate drivers such as El Nino Southern Oscillation (ENSO) that may synchronize fire activity over broad areas (Le Page et al 2008, Wooster et al 2012). Such relationships allow at least shortterm forecasts (months to one or two years) of fire risk (Le Page et al 2008, Prestemon et al 2008). Potential constraints on fire activity due to vegetation differences, both within climatically homogeneous regions and across different biomes, underscore the importance of considering ecological context in assessing effects of climate variability on past and future wildfire activity

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