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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, U.S.A. (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.

Wildfire is a ubiquitous disturbance agent in Rocky Mountain subalpine forests.Lodgepole pine (Pinus contorta), a dominant tree species in subalpine forests of western North America, is largely resilient to high-severity fires. However, the resilience of lodgepole forests may be compromised with predicted changes to climate and moisture availability. While the modern post-fire dynamics of these systems are well studied, less is known about post-fire responses of lodgepole forests over the past few centuries and millennia. Thus, I investigated fire occurrence and post-fire vegetation change in a lodgepole forest over the past two millennia to understand ecosystem responses to variability in wildfire activity and climate. I reconstructed vegetation composition over the past 2,500 years in the small basin of Chickaree Lake, Colorado, U.S.A., in Rocky Mountain National Park. Pollen samples (n=52) were analyzed to characterize both broad-scale (centennial to millennial) trends in pollen assemblages associated with climate, and short-term (decadal) changes associated with multiple high-severity fire events previously reconstructed through charcoal analysis. Pollen assemblages were dominated by Pinus throughout the record. The primary broadscale change in pollen composition characterized by an increase Artemisia and Rosaceae, and a decrease in mean Pinus and extra local pollen (Quercus, Salix, and Sarcobatus), which occurred around 1,155 calibrated years before present (cal yr BP). This change is coincident with a shift towards increased winter precipitation which characterizes modern climate identified from δO 18 and lake level data. Wildfire occurrence resulted in significant decreases in Pinus pollen following the zone break at 1,155 cal yr BP, suggesting that impacts of fire on the composition of lodgepole pine forests may depend on underlying climate conditions. Throughout the past 2,500 years, vegetation composition returned to pre-fire conditions within 75 years, indicating overall resilience of Rocky Mountain lodgepole forests to fire activity despite some variability in post-fire vegetation recovery. v

<p><strong>Abstract.</strong> The essential elements for the structure and function of forest ecosystems are found in relatively predictable proportions in living tissues and soils; however, both the degree of spatial variability in elemental concentrations and their relationship with wildfire history are unclear. Quantifying the association between nutrient concentrations in living plant tissue and surface soils within fire-affected forests can help determine how these elements contribute to biogeochemical resilience. Here, we present elemental concentration data (C, N, P, K, Ca, Mg, S, Fe, Mn, Zn) from 72 foliar and 44 soil samples from a network of 15 sites located in the fire-prone subalpine forests of the northern Rocky Mountains, USA Plant functional type is strongly correlated with carbon (C) and nitrogen (N) &amp;ndash; C concentrations are highest in coniferous needles, and N concentrations are highest in broadleaved plant species. The average N&amp;thinsp;/&amp;thinsp;P ratio of foliage among samples is 9.8&amp;thinsp;±&amp;thinsp;0.6 (μ&amp;thinsp;±&amp;thinsp;95&amp;thinsp;% confidence). This suggests that N is the limiting nutrient for these plants, however several factors can complicate the use of N&amp;thinsp;/&amp;thinsp;P ratios to evaluate nutrient status. Average C concentrations in organic soil horizons that were burned in regionally extensive fires in 1910 or 1918&amp;thinsp;CE are lower than those from sites that burned prior to 1901&amp;thinsp;CE (p&amp;thinsp;<&amp;thinsp;0.05). This difference suggests that wildfires reduced the pool of soil C and that the legacy of these fires can be measured a century later. Our results help aid in modeling how changing wildfire regimes will influence biogeochemical cycling in subalpine forests.</p>

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