The amplifying effects of humans on fire regimes in temperate rainforests in western Patagonia Andrés Holz ⁎, Thomas T. Veblen Department of Geography, University of Colorado, Boulder, CO 80309 USA abstract article info Article history: Received 11 June 2011 Received in revised form 14 August 2011 Accepted 19 August 2011 Available online 26 August 2011 Keywords: Land-use changes Human-ignition Burning Logging Pilgerodendron uviferum Patagonia Global warming Climate variability During European colonization and settlement of southern hemisphere temperate ecosystems, historical fire regimes were often dramatically altered by either burning vast areas to create farmland, or reducing fire fre- quencies by suppressing fires or by eliminating aboriginal populations that formerly set fires. To determine the historical range of variability of wildfire and the potential human influences on wildfire activity in tem- perate rainforests of western Patagonia, we used tree rings to reconstruct fire history over the past ca. 400 years. Over a 6° latitudinal range, we examined spatiotemporal changes in fire history and compared it to ethnohistorical evidence of human activities. Time series of fire years were developed from fire-scars at 27 sites for comparison with a priori defined periods of land use in each of six areas of homogeneous land- use history. We also examined the influence of climate variability to discriminate the relative roles of human ignitions and decadal-scale climate variability on fire activity. Fires were relatively common in the forest-bog habitats sampled in our study, at least two centuries prior to any likely impact from Euro-Chilean settlers, implying that fires set by the indigenous peoples in this rainforest climate were much more common (and sometimes even widespread) than previously known. Our results also show that coincident with Euro- Chilean settlement, fire regimes shifted into long-lasting regimes of substantially higher fire frequency. How- ever, decadal-scale climate variability, also clearly was a driver of shifts in fire regimes, and the second half of the 20th century has been a time of increased temperatures and drought throughout the region. Overall, our findings indicate that although in both pre-historic and modern times climate variability is the dominant control on years of widespread fires, aboriginals and Euro-Chilean settlers have amplified fire activity (particularly dur- ing the 20th–21st centuries) and shifted the region's fire regimes to new behaviors. © 2011 Elsevier B.V. All rights reserved. 1. Introduction Factors that discriminate wildfire activity in different biomes in- clude the nature of vegetative fuels to burn, environmental condi- tions (largely weather) that promote burning, and availability of human versus natural sources of ignition (Pausas and Keeley, 2009). There is much controversy about the relative importance of humans and lightning as sources of ignition in particular biomes and during particular time periods (Denevan, 1992; Swetnam, 1993; Vale, 1998; Keeley, 2002; Pausas and Keeley, 2009), and the relative roles of humans and climate remain difficult to discriminate and generalize in broad-scale syntheses of centennial and millennial scale fire records (Marlon et al., 2008; Power et al., 2008). In some bi- omes with abundant fuels to burn such as the southern temperate forests of New Zealand where lightning is rare, primacy is attributed to ignitions by humans in explaining historical variation in wildfire activity (Ogden et al., 2003; McWethy et al., 2009, 2010). In similar forests occurring in the west-coast maritime climate of southwest- ern South America, where lightning activity is also relatively rare, there has been considerable debate about the relative effects of humans and climate on fire regimes (Heusser, 1987, 1994; Haberle et al., 2000; Huber et al., 2004). The goal of the current study is to im- prove our understanding of the effects of humans, both Native Amer- icans and people of European origin, on fire regimes in the temperate rainforests of western Patagonia (i.e. southern Chile from ca. 42 to 48°S) by comparing tree-ring fire histories to changes in land use change and climate. Paleoecological studies of fire history in other temperate ecosystems of southern Chile and Argentina (i.e. the Andean–Patagonian region south of ca. 36°S) have varied widely in their interpretations of the rela- tive roles played by humans versus lightning in accounting for long-term variations in wildfire activity. Some authors have stressed the importance of climatic control and assumed that lightning ignitions were sufficiently frequent to account for most of the variability in wildfire activity during the Holocene (Markgraf and Anderson, 1994; Moreno et al., 2001). Others instead, have associated fire activity primarily with prehistoric human activities (Heusser, 1987, 1994; Haberle et al., 2000). The propor- tion of pre-historic fire ignitions due to lightning as opposed to humans is Palaeogeography, Palaeoclimatology, Palaeoecology 311 (2011) 82–92 ⁎ Corresponding author. Tel.: + 1 303 492 4785; fax: + 1 303 492 7501. E-mail address: holzc@colorado.edu (A. Holz). 0031-0182/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.palaeo.2011.08.008 Contents lists available at SciVerse ScienceDirect Palaeogeography, Palaeoclimatology, Palaeoecology journal homepage: www.elsevier.com/locate/palaeo