Influences of Secondary Disturbances on Lodgepole Pine Stand Development in Rocky Mountain National Park

Abstract

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.

Differences in litter cover and understorey flora between stands of introduced lodgepole pine and native Scots pine in Sweden

“…Also, successively increasing natural regeneration of Pc (Nyström, 1988), which can occur in a broad spectrum of habitats from wet to xeric (Despain, 2001), and was also observed in this study, would imply more Pc trees and a further increase of litter production. During dry conditions, litter on the ground serves as fuel which can increase the risk of forest fires, a disturbance commonly structuring stands of Pc (Sibold et al, 2007). More fires would have extensive effects on stand as well as landscape levels but are likely to pave the way for denser understorey vegetation (Bataineh et al, 2006) and a more pristine understorey flora (Laughlin et al, 2004).…”

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

“…1.5 ha surface area; 7.9 m depth) in a lodgepole-pine-dominated subalpine forest in Rocky Mountain National Park. The even-aged forest surrounding the lake regenerated after a high-severity (i.e., stand-replacing) fire in 1782 CE (common era) (Sibold et al, 2007). The fire regime in subalpine forests of Rocky Mountain National Park is characterized by infrequent, high-severity crown fires (ca.…”

“…DayCent submodels associated with tree physiological parameters, site characteristics, soil parameters, and disturbance events were modified using available site-specific observations (Dunnette et al, 2014;Sibold et al, 2007), values from the literature (Kashian et al, 2013;Turner et al, 2004), and publically available climate and soil databases. Climate data required for DayCent include daily minimum and maximum temperature and precipitation, which were obtained for a 30-year period from DAYMET (Thornton, 2012).…”

“…First, DayCent was spun-up and equilibrated to soil C and NPP levels characteristic of mature lodgepole pine stands in the region with a constant return interval of 145 years between high-severity fires with erosion, replicating the estimated fire rotation period (and mean fire return interval) for the broader study area (Sibold et al, 2007). This spin-up period lasted for 2000 years, and it represents what would be done for model use, in the absence of the long-term fire history information from the paleo record.…”

“…A constant 145-year fire return interval was used for the equilibrium run. The vertical red line indicates the most recent stand-replacing fire (1782 CE), reconstructed from the tree-ring record (Sibold et al, 2007).…”