Vegetation response to wildfire and climate forcing in a Rocky Mountain lodgepole pine forest over the past 2500 years
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Abstract:
Wildfire is a ubiquitous disturbance agent in subalpine forests in western North America. Lodgepole pine ( Pinus contorta var. latifolia), a dominant tree species in these forests, is largely resilient to high-severity fires, but this resilience may be compromised under future scenarios of altered climate and fire activity. We investigated fire occurrence and post-fire vegetation change in a lodgepole pine forest over the past 2500 years to understand ecosystem responses to variability in wildfire and climate. We reconstructed vegetation composition from pollen preserved in a sediment core from Chickaree Lake, Colorado, USA (1.5-ha lake), in Rocky Mountain National Park, and compared vegetation change to an existing fire history record. Pollen samples ( n = 52) were analyzed to characterize millennial-scale and short-term (decadal-scale) changes in vegetation associated with multiple high-severity fire events. Pollen assemblages were dominated by Pinus throughout the record, reflecting the persistence of lodgepole pine. Wildfires resulted in significant declines in Pinus pollen percentages, but pollen assemblages returned to pre-fire conditions after 18 fire events, within c.75 years. The primary broad-scale change was an increase in Picea, Artemisia, Rosaceae, and Arceuthobium pollen types, around 1155 calibrated years before present. The timing of this change is coincident with changes in regional pollen records, and a shift toward wetter winter conditions identified from regional paleoclimate records. Our results indicate the overall stability of vegetation in Rocky Mountain lodgepole pine forests during climate changes and repeated high-severity fires. Contemporary deviations from this pattern of resilience could indicate future recovery challenges in these ecosystems.Keywords:
Subalpine forest
Fire ecology
Fire history
Fire regime
Mountain pine beetle
Bark beetles and their associated fungi are among the greatest natural threats to conifers worldwide, but the degree to which host stored resources influence tree-beetle-fungal interactions has not been investigated. In western North America, the range of the mountain pine beetle (Dendroctonus ponderosae) has expanded from lower elevation Pinus contorta (lodgepole pine) forests into high elevation Pinus albicaulis (whitebark pine), a presumed superior host. I investigated whether stored resources in tree sapwood change after D. ponderosae attack, and whether this change relates to fungal colonization and beetle performance. I also studied how phloem and sapwood resources vary with elevation and tree diameter and examined the effect of tree species and diameter on D. ponderosae host selection. Following beetle attack and fungal colonization, sapwood non-structural carbohydrates (NSC), lipids, and phosphorus declined in attacked trees relative to un-attacked trees. Resource declines were related to the degree of fungal colonization, suggesting a direct benefit to fungi in both host species. In P. contorta, beetle performance was also positively related to stored resources. The concentration of stored resources was generally higher in P. albicaulis than in P. contorta and increased with elevation and tree diameter, suggesting a potential increase in host quality for D. ponderosae and/or fungi. Beetles preferred larger diameter hosts, and although stored resources did not affect beetle performance in P. albicaulis, beetles were more likely to attack P. albicaulis even when larger P. contorta were available. In a parallel system in Norway, phloem NSC and sapwood lipids also declined in Picea abies trees inoculated with the fungus Ceratocystis polonica relative to trees attacked by the bark beetle Ips typographus (which vectors C. polonica) or control trees, again indicating that stored resources enhance fungal colonization. Overall, my results suggest that host stored resources influence the interaction between bark beetles, fungi, and conifers primarily by enhancing fungal growth. Fungal access to stored resources may also benefit beetles in some host tree species. A better understanding of the trophic interactions between beetles, fungi, and conifers may improve our ability to predict bark beetle dynamics and range expansion.
Mountain pine beetle
Dendroctonus
Picea engelmannii
Abies lasiocarpa
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The persistence and fall rate of snags (standing dead trees) generated during bark beetle outbreaks have consequences for the behavior, effects, and suppression of potential wildfires, hazard tree and timber salvage operations, wildlife habitat, and numerous ecosystem processes. However, post-beetle snagfall dynamics are poorly understood in most forest types. We tagged standing live and dead lodgepole pine (Pinus contorta), subalpine fir (Abies lasiocarpa), and Engelmann spruce (Picea engelmannii), including beetle-killed pine snags following the peak of a recent mountain pine bark beetle outbreak in watersheds at the Fraser Experimental Forest in northcentral Colorado and sampled snagfall 10 and 12 years later. Bark beetle attacks began in 2003, peaked by 2006, and killed 78% of overstory lodgepole pine in 133 plots distributed across a range of stand and site conditions. Of those snags, only 17% fell between 2007 and 2018. Most snags broke at ground level, due to butt rot, and were oriented downhill. In contrast, snags that tipped up or snapped off above the ground were oriented with the prevailing winds. Equal numbers of snags fell singly and in multiple-tree groups, and equal numbers remained elevated rather than in contact with the ground. Lodgepole pine snagfall was 1.6-times higher on steep slopes (>40%) where dead pine density was higher, compared to flatter sites. Based on our findings and previous research, we estimate that one-half the beetle-killed lodgepole pine in high-elevation forests such as those at Fraser may fall within 15-20 yr of beetle infestation, but that some pine snags are likely to persist for decades longer. Post-outbreak snagfall dynamics create a multiple-decade legacy of bark beetle outbreaks that will persist longer in high-elevation compared to lower-elevation forests.
Mountain pine beetle
Abies lasiocarpa
Dendroctonus
Subalpine forest
Picea engelmannii
Coarse Woody Debris
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Mountain pine beetle
Dendroctonus
Aerial imagery
Aerial photos
Loblolly pine
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Reconstructing fire history of lodgepole pine on Chagoopa Plateau, Sequoia National Park, California
Information on fire’s role in pre-twentieth-century lodgepole pine forests of the southern Sierra Nevada is limited. It has generally been assumed that fire plays only a minor role in lodgepole’s dynamics unlike in other portions of its range. This assertion was examined by sampling fire-scarred trees and reconstructing fire history in monospecific stands of lodgepole pine (Pinus contorta var. murrayana [Grev. & Balf.] Engelm.) on Chagoopa Plateau in the Kern River drainage of Sequoia National Park. Using dendrochronological methods 17 fire events were dated between A.D. 1385 and 2000. Prior to 1860 and Euro-American settlement, fire event dates showed mixed degrees of synchronization among sites with a number of widespread fires of the plateau. Mean fire return interval among sites was 45.4 yr, ranging from 31 to 74 yr by site. The frequency of past fire occurrence on the plateau indicates fire had a strong influence on this ecosystem, which continues through the present. These findings differ significantly from the generally held notion that fire does not play an important role in lodgepole ecosystems in the Sierra Nevada. Also of interest was a cluster of 1880s fire dates at sites near Sky Parlor Meadow suggesting burning around meadows by Euro-American shepherds.
Sequoia
Fire history
Fire ecology
Fire regime
Colorado plateau
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Dendroctonus
Mountain pine beetle
Ophiostoma
Abies lasiocarpa
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A fire history investigation was conducted for three forest community types in the Absaroka Mountains of Yellowstone National Park, Wyoming. Master fire chronologies were based on fire-initiated age classes and tree fire scars. The area's major forest type, lodgepole pine (Pinus contorta Dougl. var. latifolia) ecosystems, revealed a predominant pattern of stand replacing fires with a 200 year mean interval-nearly half the length estimated in previous studies of lodgepole pine on less productive subalpine plateaus in YNP. High elevation whitebark pine (P. albicaulis Engelm.) forests had primarily stand replacing fires with >350 year mean intervals, but some stands near timberline also occasionally experienced mixed severity- or non-lethal underburns. Before nearly a century of effective fire suppression in Yellowstone's northern range, lower elevation Douglas-fir (Pseudotsuga menziesii [Mirb.] Franco.) communities adjacent to Artemesia tridentata (Nutt.) grasslands experienced primarily non-lethal underburns at 30 year mean intervals. While short interval fire regimes have been altered by longterm fire suppression, fire exclusion apparently had only limited influence on the area's infrequently burned ecosystems prior to widespread stand replacement burning in 1988.
Abies lasiocarpa
Fire regime
Fire ecology
Fire history
Picea engelmannii
Subalpine forest
Elevation (ballistics)
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The recent mountain pine beetle (Dendroctonus ponderosae Hopkins; MPB) outbreak throughout North America has affected thousands of hectares of lodgepole pine (Pinus contorta Douglas var. latifolia Engelmann) in northern Colorado. This severe disturbance in subalpine forests has created conditions supportive of advance regeneration and new recruitment of the shade-tolerant species Engelmann spruce (Picea engelmannii) and subalpine fir (Abies lasiocarpa). On the other hand, a variety of factors including canopy opening and litter depth limit lodgepole pine seedling regeneration in MPB-attacked lodgepole pine stands. Significantly higher densities of subalpine fir and Engelmann spruce seedlings following MPB outbreak suggest that severe MPB outbreak accelerates the succession of lodgepole pine and mixed subalpine stands toward a canopy codominated by more shade-tolerant species. SUBALPINE FOREST REGENERATION 3
Abies lasiocarpa
Picea engelmannii
Mountain pine beetle
Subalpine forest
Dendroctonus
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Dendroctonus
Mountain pine beetle
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We examined the development of lodgepole pine (Pinus contorta Dougl.) in uneven-aged stands in the Interior Douglasfir (IDF) biogeoclimatic zone of central of British Columbia (B.C.), which are currently undergoing a massive outbreak of the mountain pine beetle (Dendroctonus ponderosae Hopkins; MPB). Using historical ecological approaches, dendrochronology, and stand measurement data, we determined the roles MPB and fire disturbances have played in the ecological processes of lodgepole pine in an Interior Douglas-fir zone. We found that multiple mixed-severity fires created patchy uneven-aged stands dominated by lodgepole pine. Since fire suppression in the 20 th century, multiple MPB disturbances have maintained the structural complexity of the stands and favoured regeneration of lodgepole pine in the understory despite the absence of fire, resulting in self-perpetuating multi-age lodgepole pine stands. Analysis of the stand structures remaining after multiple MPB outbreaks showed that, even with high overstory mortality, the sample stands contained several MPB-initiated cohorts, consisting of younger and smaller-diameter lodgepole pine. These surviving lodgepole pine layers, which are less susceptible to beetle, will provide important ecological legacies, and could play an important role in the mid-term timber supply chain. We concluded that, in the absence of fire, the MPB plays a more frequent role in directing stand dynamics and structure in uneven-aged lodgepole pine stands resulting in selfperpetuating complex stands in the central interior. We compared and contrasted these findings with those obtained in “even-aged” lodgepole pine stands, also in the Interior Douglas-fir zone in the southern interior, which were investigated in an earlier study. Key words: lodgepole pine, mountain pine beetle, dendroecology, complex stands, mixed-severity fire regime
Mountain pine beetle
Fire history
Dendroctonus
Understory
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