Influences of secondary disturbances on lodgepole pine stand development in Rocky Mountain National Park

Although high-severity fire is the primary type of disturbance shaping the structure of lodgepole pine (Pinus contorta) stands in the southern Rocky Mountains, many post-fire stands are also affected by blowdown, low-severity surface fires, and/or outbreaks of mountain pine beetle (MPB; Dendroctonus ponderosae). The ecological effects of these secondary disturbances are poorly understood but are potentially important in the context of managing for ecological restoration and fire hazard mitigation. We investigated the effects of blowdown, surface fires, and MPB outbreaks on demographic processes in post-fire lodgepole pine stands in Rocky Mountain National Park, Colorado, USA. We used dendroecological methods to reconstruct stand characteristics prior to and following secondary disturbances for paired stands with and without secondary disturbances. Surface fire events do not kill canopy trees or trigger pulses of recruitment and as such do not have detectable influences on stand development. In contrast, both MPB and blowdown kill canopy trees and trigger pulses of tree regeneration of lodgepole pine and subalpine fir (Abies lasiocarpa). The amount and species composition of post-disturbance regeneration is dependent on the severity of the disturbance and on the time since stand initiation. Secondary disturbances of higher severity (i.e., killing >50% of the canopy trees) that occur in younger post-fire stands favor new establishment of lodgepole pine. In contrast, secondary disturbances of lower severity in older stands (>250 years) trigger a pulse of establishment of subalpine fir. The results of this study demonstrate that the high tree densities characteristic of lodgepole pine stands in the southern Rockies (southern Wyoming to northern New Mexico) are the result of dense regeneration following stand-replacing fires and that surface fires had little or no thinning effect on tree densities. Thus, current high stand densities in the study area are not the result of suppression of surface fires. Moreover, the strong pulses of regeneration following forest thinning by MPB and blowdowns imply that, depending on the degree of thinning, thinning prescriptions to reduce fuels in the lodgepole pine forest type may have the unintended consequence of increasing ladder fuels 15-20 years following treatments.