Rocky Mountain bristlecone pine (Pinus aristata) and limber pine (Pinus flexilis) are important but little-studied high-elevation pines of the southern Rockies. Both species and the communities they form are forecast to decline due to the recent spread of white pine blister rust (Cronartium ribicola) into this region. Proactive management strategies to promote the evolution of rust-resistance and maintain ecosystem function require an improved understanding of 1) the role of disturbance on the population dynamics of both species, and 2) environmental conditions that favor seedling establishment. We examined patterns of bristlecone and limber pine regeneration across the perimeters of three, 29-year old, high-severity burns in Colorado: the Ouzel burn (northern Front Range), Badger Mountain (southern Front Range), and Maes Creek (farther south, in the Wet Mountains).
Results/Conclusions
Bristlecone pine regeneration was concentrated near burn edges and beneath surviving seed sources. This spatial pattern is consistent with limitations incurred by wind-dispersal, also borne out by the low occurrence of seedling clusters. Overall, the relative abundance of bristlecone pine decreased in burns due to greater increases by spruce and aspen. Limber pine regeneration pattern varied between sites: highest in the burn interior at Ouzel, in burn edges at Badger, and in unburned stands at Maes. The poor establishment of limber pine in burn interiors in the southern sites is suggestive of a latitudinal shift in establishment niche, as opposed to changes in dispersal mechanism. Clark’s Nutcracker dispersal of limber pine in each study area was indicated by high seedling distance from possible seed sources and high frequencies of clustered stems. Except in the Ouzel burn interior, limber pine regeneration was also dwarfed by that of other tree species. Across sites, establishment by both species was boosted by nearby nurse objects (rocks, fallen logs, and standing tree trunks) a relationship that extended out at least as far as the closest three such objects, usually found within 50 cm. Fire increased cover by Ribes species (the alternate host of white pine blister rust) — potentially increasing future pathogen exposure— though only one species, R. cereum, was positively associated with either pine species. We conclude that bristlecone pine populations may benefit from management generating small (0-15 m) canopy and ground layer disturbances; limber pine should benefit from large disturbances in northern subalpine forests but will not necessarily respond similarly farther south. Management that creates microtopographic structure (e.g., abundant fallen logs) will promote regeneration by both species.
