Climatic and Human Influences on Fire Regimes of the Southern San Juan Mountains, Colorado, Usa

Abstract

Fire severity, frequency, and extent are expected to change dramatically in coming decades in response to changing climatic conditions, superimposed on the adverse cumulative effects of various human-related disturbances on ecosystems during the past 100 years or more. To better gauge these expected changes, knowledge of climatic and human influences on past fire regimes is essential. We characterized the temporal and spatial properties of fire regimes in ponderosa pine forests of the southern San Juan Mountains of southwestern Colorado by collecting 175 fire-scarred tree samples from nine sites across a wide range of topographic settings. All tree rings and fire scars were dated using standard dendrochronological techniques. Fire-free intervals were statistically modeled using the Weibull distribution to provide quantitative measures that characterized the historical range of variation in pre-EuroAmerican fire regimes.Fires during our reference period were more frequent in the low elevation ponderosa pine forests (6-10 yr) than in the high elevation, mixed conifer forests (18-28 yr). Fires at lower elevations were predominantly low-severity, isolated fires. Fires during some years (e.g., 1748) were spatially extensive throughout the entire mountain range. Intervals that delimited significantly long fire-free periods ranged from 10-19 yr (low elevation) to 27-50 yr (high elevation). Fire histories were similar between the eastern and western portions of the mountain range, although we found significant evidence of topographic isolation on fire regimes at one site. Pre-1880 fires primarily occurred in the dormant season, and we found no temporal changes in past fire seasonality. We found no compelling evidence that Native Americans influenced fire regimes in our study sites.We found a hiatus in fire occurrence between 1750 and 1770 that we believe was likely related to weakened El Niñ o-Southern Oscillation activity, an extended series of cool-phase Pacific Decadal Oscillation events, and weakened monsoonal moisture, all possibly entrained in an invasive air mass typical of locations that are more northerly. In addition, pre-1880 fires occurred during years of severe drought, conditioned by above average moisture conditions in preceding years. The 20th century is characterized by a near complete absence of fires (fire-free interval of Ͼ100 yr), suggesting future wildfires may be more widespread and ecologically severe compared to pre-1880 fires.

Wildfire frequency varies with the size and shape of fuel types in southeastern France: Implications for environmental management

“…However, our FRI values for shrublands were intermediate between those of many MTEs including the Portuguese shrublands (12-16 yrs., Fernandes et al 2010), South Africa fynbos (10-13 yrs., Van Wilgen et al 2010) or grasslands (2-10 yrs., Archibald et al, 2011), and those of southern California chaparral (33-42 yrs., Moritz 2003;Moritz et al 2004) or shrublands of south-western Australia (47 yrs., O'Donnell et al, 2011). Fire intervals in forests were much higher than those of ponderosa pine forests of Colorado (2-20 yrs., Grissino-Mayer et al, 2004) but much lower than those of Australian forests (310 years, (O'Donnell et al, 2011). They fall within the range for many MTEs (20-50 years, In Diaz-Delgado et al, 2004).…”

“…The Weibull model (see Johnson and Gutsell, 1994) has been proved flexible and able to provide superior fit to fire history data than most other distributions (see Grissino-Mayer et al, 2004). Using this model, the distribution of fire intervals can be described in two complementary forms: F(t) gives the probability of fire occurrence before or at a time t, and f(t) is the probability density function reflecting the frequency of burning in a given time interval (Moritz, 2003;Moritz et al, 2009) as follows:…”

Historical Fire Regimes in Ponderosa Pine and Mixed-Conifer Landscapes of the San Juan Mountains, Colorado, USA, from Multiple Sources

“…Nine fire-scar studies provided data about fire frequency, particularly for low-severity fires, in these forests [10,[26][27][28][29][30][31][32][33]. Data are only available for the southern and western part of the San Juan Mountains, except one site at Hot Creek on the eastern slope of the San Juan Mountains (Figure 5a).…”

Modeling human-caused forest fire ignition for assessing forest fire danger in Austria

“…In this context, the probability of human-caused fire ignition is the result of the direct or indirect presence of human activity in the landscape (Martinez et al 2009). International studies indicate that roughly 90% of forest fires are human-caused, whereas only a small percentage of forest fires have natural causes, i.e., lightning (Cardille et al 2001, Grissino-Mayer et al 2004, Mollicone et al 2006, Vacik et al 2011. In Europe, human activities account for the majority of fire ignition (Leone et al 2002, Catry et al 2009, Martinez et al 2009).…”

“…In Europe, human activities account for the majority of fire ignition (Leone et al 2002, Catry et al 2009, Martinez et al 2009). Various goods and services provided by forests such as water supply, carbon sinks, recreation and protection services are most likely to be impacted by wildfires (Wotton et al 2003, Grissino-Mayer et al 2004, Brown et al 2004, Fried et al 2004, Catry et al 2009, Dumas et al 2008, Weibel et al 2009). Especially in the densely populated European mountain forests the danger of fire ignition is of high significance for the maintenance of its ecosystem goods and services as these ecosystems are very sensitive to environmental changes (Steininger & WeckHannemann 2002, Lindner et al 2010.…”