Climatic and Landscape Influences on Fire Regimes from 1984 to 2010 in the Western United States

RESEARCH ARTICLE Climatic and Landscape Influences on Fire Regimes from 1984 to 2010 in the Western United States Zhihua Liu*, Michael C. Wimberly Geospatial Sciences Center of Excellence, South Dakota State University, Brookings, South Dakota, United States of America * Abstract OPEN ACCESS Citation: Liu Z, Wimberly MC (2015) Climatic and Landscape Influences on Fire Regimes from 1984 to 2010 in the Western United States. PLoS ONE 10(10): e0140839. doi:10.1371/journal.pone.0140839 Editor: Lucas C.R. Silva, University of California Davis, UNITED STATES Received: May 16, 2015 Accepted: September 29, 2015 Published: October 14, 2015 Copyright: © 2015 Liu, Wimberly. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. An improved understanding of the relative influences of climatic and landscape controls on multiple fire regime components is needed to enhance our understanding of modern fire regimes and how they will respond to future environmental change. To address this need, we analyzed the spatio-temporal patterns of fire occurrence, size, and severity of large fires (> 405 ha) in the western United States from 1984–2010. We assessed the associations of these fire regime components with environmental variables, including short-term climate anomalies, vegetation type, topography, and human influences, using boosted regression tree analysis. Results showed that large fire occurrence, size, and severity each exhibited distinctive spatial and spatio-temporal patterns, which were controlled by different sets of climate and landscape factors. Antecedent climate anomalies had the strongest influences on fire occurrence, resulting in the highest spatial synchrony. In contrast, climatic variability had weaker influences on fire size and severity and vegetation types were the most important environmental determinants of these fire regime components. Topography had moderately strong effects on both fire occurrence and severity, and human influence variables were most strongly associated with fire size. These results suggest a potential for the emergence of novel fire regimes due to the responses of fire regime components to multiple drivers at different spatial and temporal scales. Next-generation approaches for projecting future fire regimes should incorporate indirect climate effects on vegetation type changes as well as other landscape effects on multiple components of fire regimes. Data Availability Statement: Data and R code are available at: https://github.com/liuzh811/ FireRegimeWestUS.git. Funding: Financial support for this work was provided through Research Work Order Number G12AC20295 from the United States Geological Survey (http://www.usgs.gov/) and Grant Number NNX11AB89G from National Aeronautics and Space Administration (http://www.nasa.gov/). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Introduction Fire is an integral component of the earth system and plays a key role in regulating vegetation structure and ecosystem function [1–3]. Understanding the relative influences of multiple controlling factors on fire regimes is one of the fundamental objectives of fire ecology, and this knowledge is critical for improving our ability to anticipate future fire regime changes. Climatic variability is a major driver of fire in many terrestrial ecosystems, as reflected in Bradstock’s PLOS ONE | DOI:10.1371/journal.pone.0140839 October 14, 2015 1 / 20 Controls of Fire Regimes in the Western US Competing Interests: The authors have declared that no competing interests exist. conceptual model of four climatic ‘switches’ that influence fire regimes by controlling fuel amount, fuel moisture, and fire weather at contrasting temporal scales [4]. However, fire regimes are also affected by other controls such as landscape-scale patterns of vegetation, topography, and human activities [5]. For example, recent analyses in boreal Canada found that vegetation and fuels influenced the spatial and temporal patterns of fires, even in systems where climate was considered the most limiting factor [6, 7]. Topography also influences fire regimes through its effects on fuel loads and fuel moisture via site productivity and microclimate [8]. Humans can modify fire regimes by changing ignition patterns [9] and by altering fuel amount and continuity [10]. Therefore, understanding how fire regimes respond to landscape controls in addition to climatic shifts is critical in this era of unprecedented global change, and will require research that explores the effects of multiple, interacting drivers of fire regimes [11]. Fire regimes are typically described by statistical distributions of frequency, size, severity, and seasonality in a particular area during a given time period. Thus, the environmental determinants of fire regimes can be assessed by exploring how environmental drivers operating over a range of scales affect the spatial and temporal patterns of these fires (Fig 1). The behavior and effects of an individual wildfire emerge over days to weeks as a result of weather interacting with fine-grained spatial variability in fuels and vegetation. However, these interactions are also constrained by biogeographic drivers that vary over broader spatial and temporal scales. Climate, for example, is connected to fires at two distinct temporal scales [12]. Short-term climatic anomalies (months to years) affect fires by modifying vegetation growth and fuel moisture before the fire and by influencing weather during the period of fire spread. In addition, climate has more indirect, long-term (decadal or longer) effects on the distributions of major vegetation types, which in turn constrain the landscape-scale mosaics of fuels and vegetation. Topography provides a relatively stable physical template that influences fire through direct interaction with fire spread and indirect effects on vegetation, fuel amounts, and fuel moisture. Humans can affect fire through a variety of pathways including ignition, suppression, and alteration of fuels and vegetation [13]. These human impacts are in turn strongly influenced by variability in human population density, land ownership, and the resulting patterns of land use and natural resource management activities. Because climate, vegetation, topography, and human activities interact with fire behavior and effects at different spatial and temporal scales [14, 15], they are likely to have distinctive effects on fire occurrence, size, and severity. These multiple fire regime components interact with climate along with other biophysical and human drivers to form characteristic fire regimes in different geographic settings [1]. Studies conducted at a global scale have shown that fire frequency and bur (...truncated)