Time series analysis of limber pine (Pinus flexilis) health in the U.S. Rocky Mountains in response to white pine blister rust (Cronartium ribicola) and bark beetles
From 2004-2007, 106 permanent limber pine monitoring plots were established and measured throughout the U.S. Rocky Mountains (MT, WY, CO) to characterize health trends in response to white pine blister rust (WPBR) and bark beetles (including mountain pine beetle, "MPB", and Ips spp., "Ips") over time. These plots were subsequently measured in 2011-2013 and again in 2016-17 to form a time series analysis of limber pine health. Data were gathered on 8,206 monumented trees (4,176 limber pine) and included measurements on various stand, ground cover, and landscape characteristics over the three time intervals. The overall percentage of live trees infected with WPBR was 29.4% in 2004-07 and 25.7% in 2016-17, with incidence decreasing in parts of Wyoming (Pole Mountain, Laramie Peak), increasing in southern Colorado (Sangre de Cristo Mountains), and stable in other subregions. However, of limber pines that were healthy during the first measurement, 22.2% were declining/dying and 21.1% had died by the end of the study period due to WPBR and/or bark beetle damages. Due to this, it is likely that new WPBR infections are occurring as the large number of live, infected trees dying during the survey may have masked newly infected trees in incidence calculations. In heavily WPBR-infected areas such as Pole Mountain, Wyoming, 65% of live trees were infected (in 2004-07), and of trees that began the study as healthy, 23% were declining or dying and 38% had died by the end of the study period (2016-17). Additionally, WPBR severity increased significantly from the beginning of the study with 4 previously uninfected sites gaining WPBR infections, 29 sites advancing to 'moderately infected' and 5 sites becoming 'heavily infected'. The overall average number of cankers per tree (3.5) was stable, but the number of infected limber pine with a canker in the lower 1/3 of the stem (18%) increased significantly (+4.2%, P = 0.001). When examining all limber pine in the study, 8%, 3% and 3% were killed by MPB/Ips., WPBR, and combined effects of these agents, respectively. Of the 887 live, but declining or dying limber pine, 52% had WPBR infections and 38% had damage from twig beetles (Pityophthorus spp., Pityogenes spp.) in 2016-17. Though all sites had ≥ 20% limber pine composition, 34% of sites had no limber pine regeneration and 7% had no regeneration of any tree species over the entirety of the study period. The results of this time series indicate that limber pine populations in the U.S. Rocky Mountains are declining due to effects from WPBR and MPB/Ips. Long-term surveys capture the effects of these damage agents on native tree populations and provide critical guidance for future management and restoration of these ecologically valuable species. Limber pine is at risk due to the various biotic and abiotic agents threatening their health. Thus, future directions involve restorative management practices for highly impacted areas where limber pine is a climax species and proactive management for healthy limber stands to promote resilience to likely damage agents. In highly impacted areas (WPBR incidence, mortality, or bark beetle damage on >50% of trees and low limber pine density and regeneration), where limber pine co-exists with other tree species, it may be favorable to allow the natural succession of other tree species to become dominant. However in xeric, harsh sites where limber pine is a climax species, these highly impacted areas are at-risk for losing all tree cover and should be considered for protective and restorative planting strategies. As natural resistance to WPBR occurs on the landscape, genetic screening and protection of mature limber pine carrying either complete or partial resistance to the pathogen should be pursued to preserve this genetic diversity. A priority should be to protect resistant against bark beetles and fire using established management practices. Additionally, seed-sourcing from resistant trees can allow for resistant progeny to be out-planted into high priority areas, thus buffering stands at risk for high WPBR mortality. Moreover management plans that promote diversification of age and diameter classes within stands can provide resilience against pest and pathogen attacks, as bark beetles vary in diameter preference and WPBR infections tend to cause higher mortality in smaller diameter trees. Lastly in healthy limber pine stands, proactive management of pest impacts to promote stand resilience is recommended as in Schoettle & Sniezko (2007) in order to preserve these healthy populations.
forest health management, mountain pine beetle, white pine blister rust, limber pine, forest health, Rocky Mountains
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