Spatial and temporal variation in historic fire regimes in subalpine forests across the Colorado Front Range in Rocky Mountain National Park, Colorado, USA

Abstract

Aim The historical variability of fire regimes must be understood in the context of drivers of the occurrence of fire operating at a range of spatial scales from local site conditions to broad-scale climatic variation. In the present study we examine fire history and variations in the fire regime at multiple spatial and temporal scales for subalpine forests of Engelmann spruce-subalpine fir (Picea engelmannii, Abies lasiocarpa) and lodgepole pine (Pinus contorta) of the southern Rocky Mountains.Location The study area is the subalpine zone of spruce-fir and lodgepole pine forests in the southern sector of Rocky Mountain National Park (ROMO), Colorado, USA, which straddles the continental divide of the northern Colorado Front Range (40°20¢ N and 105°40¢ W).Methods We used a combination of dendroecological and Geographic Information System methods to reconstruct fire history, including fire year, severity and extent at the forest patch level, for c. 30,000 ha of subalpine forest. We aggregated fire history information at appropriate spatial scales to test for drivers of the fire regime at local, meso, and regional scales. ResultsThe fire histories covered c. 30,000 ha of forest and were based on a total of 676 partial cross-sections of fire-scarred trees and 6152 tree-core age samples. The subalpine forest fire regime of ROMO is dominated by infrequent, extensive, stand-replacing fire events, whereas surface fires affected only 1-3% of the forested area.Main conclusions Local-scale influences on fire regimes are reflected by differences in the relative proportions of stands of different ages between the lodgepole pine and spruce-fir forest types. Lodgepole pine stands all originated following fires in the last 400 years; in contrast, large areas of spruce-fir forests consisted of stands not affected by fire in the past 400 years. Meso-scale influences on fire regimes are reflected by fewer but larger fires on the west vs. east side of the continental divide. These differences appear to be explained by less frequent and severe drought on the west side, and by the spread of fires from lower-elevation mixed-conifer montane forests on the east side. Regional-scale climatic variation is the primary driver of infrequent, large fire events, but its effects are modulated by local-and meso-scale abiotic and biotic factors. The low incidence of fire during the period of fire-suppression policy in the twentieth century is not unique in comparison with the previous 300 years of fire history. There is no evidence that fire suppression has resulted in either the fire regime or current forest conditions being outside their historic ranges of variability during the past 400 years. Furthermore, in the context of fuel treatments to reduce fire hazard, regardless of restoration goals, the association of extremely large and severe fires with infrequent and exceptional drought calls into question the future effectiveness of tree thinning to mitigate fire hazard in the subalpine zone.

Assessing Watershed-Wildfire Risks on National Forest System Lands in the Rocky Mountain Region of the United States

“…Mixed confiner forests make up the remaining majority of study area watersheds, with those in the higher elevations comprised of lodgepole pine, douglas fir (Psuedotsuga menziesii) and ponderosa pine (Pinus ponderosa), and those in lower elevations having a stronger ponderosa pine component. These characterizations are consistent with literature describing the forest types found in the Region [38,39]. There are three dominant fire regimes in this study area: (1) replacement severity with 35-200 year fire return interval; (2) low and mixed severity with 35-200 year fire return interval; and (3) low and mixed severity with ≤35 year fire return interval.…”

Fire-regime variability impacts forest carbon dynamics for centuries to millennia

“…The fire regime in subalpine forests of Rocky Mountain National Park is characterized by infrequent, high-severity crown fires (ca. 100-300-year mean return intervals) associated with severe seasonal drought (Sibold et al, 2006). The mean monthly temperature is −8.5 • C in January and 14 • C in July, and the average total annual precipitation is 483 mm (Western Regional Climate Center 1940-2013 observations from Grand Lake, CO).…”

Effects of stand density on ecosystem properties of subalpine forests in the southern Rocky Mountains, USA

“…For example, prior to significant human influence, the mixed-coniferous subalpine forests of the Rocky Mountains experienced relatively infrequent (up to many centuries), high-intensity fires (Schoennagel et al, 2004;Sibold et al, 2006). However, selective logging in subalpine forests is currently widespread, in part to reduce perceived fire hazard, but also to control outbreaks of forest pathogens and support other desirable attributes, such as growth of aspen (P. tremuloides; Colorado Division of Forestry, 2005).…”