Stand Response to Western Spruce Budworm and Douglas-Fir Bark Beetle Outbreaks, Colorado Front Range

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Canadian Journal of Forest Research

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The montane forests (i.e. below about 2900 m altitude) of the Colorado Front Range have experienced repeated outbreaks of western spruce budworm (Choristoneura occidentalis) and Douglas fir bark beetle (Dendroctonus pseudotsugae), both of which locally attack Douglas fir (Pseudotsuga menziesii). The effects were studied of (historically documented) outbreaks of these insects on succession, stand structure, and radial growth of host and non-host species in Rocky Mountain National Park; the most recent budworm (1974-85) and bark beetle (1984-present) outbreaks resulted in the most severe and widespread disturbance of these forests since the late 19th century. Studies were carried out during 1987 and 1988 in 11 stands representing a wide range of environmental conditions, and dominated by ponderosa pine (Pinus ponderosa) and Douglas fir, in order to investigate the responses of stands to repeated outbreaks of these pests. The stands were either old (>250 yr) with relatively open understories or young (yr), dense, even-aged post-fire stands. Cores were taken from some Douglas fir, ponderosa pine and lodgepole pine (Pinus contorta) for dendrochronological analyses. Young, vigorous post-fire stands showed minimal budworm defoliation, and in these stands only remnant trees from the pre-fire generation appeared to be susceptible to beetle-caused mortality. Dense stands exhibited higher budworm-induced mortality, which hastened the natural thinning process and shifted dominance towards the non-host species. The stands most severely disturbed by these insects were multi-storied stands with high host densities and a wide range of stem sizes. The stand response to these disturbances included the growth release of shade-intolerant, seral species, and in some cases, a higher survivorship among mid-sized individuals of Douglas fir. The net result of the combined insect outbreaks is the temporary slowing of the successional trend towards a steady-state Douglas fir forest. Fire suppression, by increasing the density of suppressed Douglas fir, has previously been shown to favour increased outbreak severity of western spruce budworm in the N. Rocky Mountains. However, in the Front Range, recent increases in outbreak severity and their synchroneity may also be the result of large areas of forest, (burned during the late 19th century during European settlement) simultaneously entering structural stages susceptible to insect outbreak.