Biogeochemical changes in a subalpine ecosystem linked to high-severity fires over four millennia 1. Background and Motivation 5. Biogeochemical Impacts of Fire 4. Fire History Acknowledgments Funding was provided by the National Science Foundation (OISE , PEH), a National Park Ecological Research Fellowship (PEH), and a Stillinger Forest Science Fellowship from the University of Idaho (PVD). We thank Ryan Moran, Justin Ziegler, Maggie Keefe, and Shannon Pauli for laboratory assistance, the National Park Service for field assistance, and the U. Idaho Paleoecology and Fire Ecology Lab for feedback. PS 5-45 ESA, Methods Vegetation history was inferred from fossil pollen assemblages, quantified at ~200-yr intervals. Chickaree Lake has been surrounded by lodgepole pine forests for at least 4500 years. Fire history was reconstructed by identifying peaks in charcoal accumulation rate (CHAR) and magnetic susceptibility (MS) using CharAnalysis (Fig. 3). CHAR peaks coincident with MS peaks were interpreted as “high-severity catchment fires” and all other peaks were interpreted as “low-severity / extra local fires.” Biogeochemical proxies included sediment C, N, δ 15 N, δ 13 C, and bulk density, sampled at a median interval of 4 yr, including ~40 yr of continuous sampling before and after high-severity catchment fires and a subset of low-severity/extra local fires. Biogenic silica (BSi) was measured at lower resolution throughout. Biogeochemical response to fires was quantified using Superposed Epoch Analysis (SEA), Monte Carlo-derived confidence intervals (Fig. 4), and correlations between charcoal peak magnitude and proxy response (Fig. 5). Increasing fire activity in the western U.S. has raised concerns about feedbacks among climate, fire, and biogeochemical cycling. The nitrogen (N) cycle is particularly relevant within this context, as N limits forest productivity and carbon (C) storage and is susceptible to fire-induced losses. Current understanding of N cycle response to forest disturbance stems primarily from chronosequences and modern observations, despite processes that operate over 100s of years. Here, we take a paleoecological approach to study the biogeochemical impacts of fire at multi-decadal time scales in a subalpine watershed of Rocky Mountain National Park, CO. Using a high-resolution lake sediment record and multiple proxies, we reconstructed the timing, nature, and ecosystem impacts of fire over the past 4200 years. Subalpine forest in the Colorado Rocky Mountains. Paul V. Dunnette, Philip E. Higuera*, Kelly M. Derr College of Natural Resources, University of Idaho, Moscow, ID Corresponding author: 3. Environmental Context Figure 1. Study site. Chickaree Lake is an 8 m deep, ~ 3 ha lake surrounded by a lodgepole-pine dominated subalpine forest, at 2800 m elevation. Figure 2. Chronology. The chronology is based on Pb and C samples. The 8-m, 6300-yr sediment core has a median sample resolution of ~5 yr. The 1782 stand-originating fire, identified by tree-ring dating (4), was clearly detected in the fire history record. Charcoal and MS analysis revealed 34 fire events and 20 erosion events. Eleven fire events coincided with erosion events, defining the high-severity catchment fires used in the SEA. Values of and correlations among biogeochemical variables suggest significant terrestrial subsidies to the lake and relatively high rates of aquatic primary productivity (1-3). Figure 4. Superposed Epoch Analysis. Time series (left) were interpolated to the median resolution of 5 yr, 500-yr trends (thick black lines) were removed, and residual data around each fire were averaged across events to produce a composite time series. The composite residual response (right) illustrates average values for 50 yr before and yr after high-severity catchment fires (red lines) and low- severity / extra local fires (blue dashed lines). Figure 5. Magnitude of proxy response. Pearson correlation (r) for comparisons between response variables (residuals, y-axis) and charcoal peak magnitudes (x-axis) from high-severity catchment fires (left) and low-severity / extra-local fires (right). Y-axis values are the residual means from 5 to 20 years after fires, matching periods of peak response in the SEA. Relationship to Fire Size / Severity Figure 3. Fire history. CHAR and MS were interpolated to 10-yr intervals, background values (gray and black lines) were estimated with a 500-yr locally weighted regression, and residuals were calculated by subtracting background values from interpolated data. Residuals exceeding the local threshold (99 th percentile of noise distribution, A-B, red line) were identified as fire and erosion events, respectively. References 1. P. A. Meyers, Organic Geochemistry 34, 261 (2003). 2. J. L. Teranes, S. M. Bernasconi, Limnology and Oceanography 45, 801 (2000). 3. L. Bunting, P. R. Leavitt, R. P. Weidman, R. D. Vinebrooke, Limnology and Oceanography 55, 333 (2010). 4. J. S. Sibold et al., Ecological Applications 17, 1638 (2007). 5. P. Boeckx, L. Paulino, C. Oyarzún, O. V. Cleemput, R. Godoy, Isotopes in Environmental and Health Studies 41, 249 (2005). 6. P. Hogberg, New Phytologist 137, 179 (1997). 7. K. K. McLauchlan, J. M. Craine, W. W. Oswald, P. R. Leavitt, G. E. Likens, PNAS 104, 7466 (2007). 8. J. E. Compton, T. D. Hooker, S. S. Perakis, Ecosystems 10, 1197 (2007). 9. J. M. Craine et al., New Phytologist 183, 980 (2009). Chickaree Lake CONCLUSIONS  High-severity fires left distinct sediment signatures reflecting ecosystem impacts  Post-fire trends were consistent with forest chronosequences, suggesting lake sediments recorded successional biogeochemical changes  Ecosystem recovery within ~75 yr was robust to environmental variability over four millennia Pollen abundance (%) Vegetation history B. MS and inferred erosion events A. CHAR and inferred fire events 1782 AD stand- replacing fire Lodgepole pine forests around Chickaree Lake originated in a 1782 stand- replacing fire (4), clearly recorded in the lake-sediment record. © P. Higuera (2010) Two parallel, overlapping sediment cores were collected from the center of Chickaree Lake in 2010 using a modified Livingstone-type piston corer. © P. Higuera (2010) © P. Higuera (2007) Low-severity / extra-local fires High-severity catchment fires (, = coincident with fire ) (+)(+) Average Response to Fire Events Composite residual response and 95% (-) and 99% (--) CI High-severity catchment fire (-) Low-severity / extra-local fire (--) Not analyzed in SEA ( ) Calibrated years BP High-severity catchment fires left distinct disturbance signatures, reflecting physical and ecosystem impacts: Increased δ 15 N is consistent with elevated N availability and losses. Increased bulk density suggests an influx of 15 N- enriched soil following combustion of the forest floor (5-7). Decreasing δ 15 N ~20-60 yr after fire is consistent with decreasing N availability during forest aggradation (7-9). Low C and C:N for decades after fire indicate a dramatic reduction in terrestrial subsidies to the lake. Increasing C and C:N ~25-75 years after fire suggest forest biomass accumulation and recovery of terrestrial inputs (1). Ecosystem impacts varied with disturbance size / severity: Biogeochemical and physical proxies were significantly correlated with the magnitude of high-severity catchment fires. Biogeochemical and physical proxies were uncorrelated with the magnitude of low severity / extra local fires.