Fire-climate linkages in western North America and southern South America
Climate, fire, and vegetation are closely related components of forest ecosystems in western North and South America. The strength and nature of the linkages among components vary depending on the time scale of interest. On intra-annual time scales, weather conditions determine the likelihood of ignition and low fuel moisture necessary for fire events. On inter-annual time scales, climate variability is important in that the sequence of wet and dry years or decades indirectly influences fire occurrence. Vegetation composition and structure on inter-annual scales also exert an influence on inter-annual fire activity by determining the nature and distribution of fuels. Fire, in turn, causes mortality that shapes forest succession and creates landscape patterns that set the stage for subsequent disturbance events. On centennial-to-millennial time scales, climate controls the long-term variations in fire frequency, size, and severity and also in vegetation composition and species distributions. Vegetation characteristics on long time scales also exert a positive feedback to fire regimes in terms of shaping the nature of the fuel biomass. Although the linkages on particular time scales seem reasonably clear, we lack a conceptual model that explains how the relationships on one time scale influence those at another. Existing fire history records, for example, reveal periods of higher-than-present fire frequency in the past, but whether these arise from the persistence of particular atmospheric circulation patterns that induce fire, persistence of particular vegetation types, or persistence of climate conditions that affect fuel moistures is not known.
This research seeks to understand the climate-fire-vegetation linkages in the temperate forests of western North and South America through a collaborative effort between paleoecologists who work on sediment-based records of fire and vegetation change and paleoclimatologists who study the link between large-scale features of climate and fire-weather conditions. The two regions offer interesting comparisons in terms of their present climate, vegetation, and fire regime and their environmental history. They feature different vegetation types but have experienced similar climate conditions on annual-to-millennial time scales. Both regions show similar climate histories, but they arise from different responses to large-scale controls of climate (i.e., variations in summer insolation) in the Holocene. This study thus benefits from the array of similar and contrasting conditions in the two regions, because they provide a set of natural experiments not possible in a single region. For example, both regions experience higher-than-present fire frequency in the early Holocene, even though summer insolation, considered to be the ultimate long-term control of drought and fire occurrence in northwestern North America, was at its lowest in South America then.
The research takes two approaches: One is to build on current efforts to develop long high-resolution fire-history records in the western Americas. These efforts represent collaboration among U.S. and foreign researchers. The second approach is to discern the hierarchy of climate controls that either promote or suppress fire on particular time scales. Modern climate data are examined to determine the large-scale climate anomalies that influence fire regimes in the study areas.
