Landscape Heterogeneity and Disturbance Interactions in a Subalpine Watershed in Northern Colorado, USA

Abstract

Three major disturbances affect subalpine forests in the Rocky Mountains: blowdown, insect outbreaks, and fire. These disturbances may influence one another temporally and spatially, creating a mosaic of disturbance patches. In 1997, a severe windstorm blew down trees on over 10,000 ha of subalpine forest in northern Colorado. Did previous disturbances influence the spatial pattern of blowdown in a representative part of the blowdown in the Middle Fork Elk River watershed? Dendrochronological evidence and a geographic information system were used to reconstruct the disturbance history, and the relationship between disturbance history (i.e., patch age) and the 1997 blowdown was examined. The fire regime varies temporally and spatially, with an area-weighted mean fire interval/fire rotation of 108 to 195 years. Stand-replacing fires appear to have had the most impact on the fire regime, but evidence of non-stand-replacing fires was also found. South-facing, lower-elevation slopes were the settings most likely to burn. Low-severity spruce-beetle disturbance was found in the mid-1700s and mid- to late 1800s, possibly related to regional outbreaks. Sheep-grazing and fire suppression may have influenced tree-regeneration and fire-frequency patterns. Each disturbance event occurred with varying severity across the landscape, influenced, in part, by the patterns of severity of previous disturbances. Patch age contributed to the pattern of the 1997 blowdown, but the relationship is complicated. The vegetation mosaic influenced spatial patterns of blowdown, resulting in new complexity and maintaining landscape heterogeneity.

Key Words:

• blowdown
• disturbance interactions
• fire ecology
• subalpine forest

Notes

Note: The number of stands (origin) is a count of stands that originated in that time period. Number of stands with peaks and with survivors are counts of stands with peaks or survivors coinciding with the origin age-cohort and the mapped polygon for each event. Fire scars and rate of initial growth were used to help interpret the disturbance agent associated with each event—that is, fire, beetles, or wind (scars that were not coincident with the event are not included). Initial growth was characterized as fast (F) or slow (S) based on the dominant trend in the trees in each stand that represented the postdisturbance cohort. Numbers in parentheses under Initial Growth are the number of stands containing fast and slow initial growth. Area is the approximate original extent of the disturbance (within the study area), rounded to the nearest 100 ha.

Note: The 1580–2000 period has two calculations of fire rotation: the first includes the incomplete interval of time from the last fire to the present, and the second uses the fire-to-fire interval only. Only stand-replacing fires are included in the fire-rotation calculation, which estimates the time necessary to burn an area equal to the study area (below treeline only). Fire rotation is also calculated with the exclusion of the 1640 disturbance event; since no fire scars were found, the disturbance agent is ambiguous. Areas are rounded to the nearest 100 ha. Burned area is within the approximately 3,700 ha area below treeline for stand-replacing and non-stand-replacing fires.