HOLOCENE FIRE, CLIMATE & EROSION

Transcription

1 HOLOCENE FIRE, CLIMATE & EROSION IN THE JEMEZ MOUNTAINS, NM NATURAL AND ANTHROPOGENIC CONTROLS Erin Fitch 1 & Grant Meyer 2 1 University of Hawai i at Mānoa 2 University of New Mexico 2011 Las Conchas Fire: Photo by Kristin Honig, US Forest Service

2 Fire severity in the Western US has increased in the past three decades Increased forest stand density associated with fire-suppression As we learn more about how forests respond to climate change, we will have more insight into the future risk of fire in the Western US

3 Jemez Mountains How do forests respond to climate change? Identify correlations between late Holocene climate change and fire activity Northern New Mexico has not yet been studied beyond shortertimescale tree-ring chronologies and this work aimed to fill that gap Alluvial studies Record of post-fire erosion Estimate fire severity Longer record than tree rings but lower resolution Useful where natural lakes are absent John Nelson, IDV Solutions

4 The Jemez Mountains: diverse ecosystem, natural resources, and cultural and scientific sites Fire in the Jemez since ~1900 AD Ponderosa pine and mixedconifer forests have been ravaged by extensive severe fires in the last two decades Almost 1000 km 2, roughly 30% of this middle-elevation range Management of this region would greatly benefit from a long-term historical perspective of fire and climate

5 Tree-ring fire history reconstructions Low-severity fire regime characterized the ca. 400 years before Euroamerican settlement Fuel buildup from fire suppression and grazing increased fire severity in recent years frequent, low severity 1900 AD less frequent higher severity Therefore, in order to better understand natural variability, climatic influences, and erosional effects of wildfire activity since ~5000 cal yr BP We identified and radiocarbondated fire-related alluvial deposits in the Jemez Mountains

6 This study was carried out in the 2002 Lakes Fire burn area Post-fire hillslope erosion and alluvial-fan sedimentation and incision Exposed fan stratigraphy Processes commonly associated with fire in the Jemez Mountains FAN INCISION FAN DEPOSITION

7 Douglas-fir dominated, north-facing slopes less steep than ponderosa pine dominated south-facing slopes SLOPE & BEDROCK EXPOSURE WITH ASPECT Microclimate differences, due to solar insolation differences, may influence geomorphic processes SOUTH-FACING NORTH-FACING (mostly burned) Therefore, this work also uses fan stratigraphy and geomorphic measurements to test whether post-fire erosion and sedimentation patterns in the Jemez Mountains reflect differing microclimates associated with north and south slope aspects

8 Alluvial fans in small mountain tributaries contain: Debris-flow Hyperconcentrated-flow Streamflow deposits Alluvial fans in the study area Fires tend to promote erosion: Runoff-generated debris flows and flash floods during summer convective storms of moderate to high intensity These fire-related flows carry burned material from soil surfaces on steep slopes abundant macroscopic angular charcoal, charred litter, and ash that will be preserved within the resulting deposit

9 We described 18 fan sections Identified firerelated deposits Performed grainsize analysis We radiocarbon dated 54 fragments of charcoal Provides deposit age Estimated fire severity based on deposit thickness

10 Fire-related sedimentation events make up 57% of the total thickness of fans North-facing alluvial fans are dominated by fire-related deposits 77% of the total thickness of alluvial fans South-facing fans are dominated by non-fire related deposits 61% of the total thickness of fans No particular flow process (e.g., debris flow) dominates either slope aspect North-facing slopes may: Have a thicker cover of relatively permeable, weathered colluvium Be anchored by a thicker, more mesic forest and understory structure Not be as susceptible to erosion unless these slopes are impacted by fire South-facing slopes may: Have sparser vegetation, and a thinner cover of colluvium and more exposed bedrock Produce substantial runoff and sedimentation during intense storms even in the absence of fire Fire has an important influence on hillslope erosion and fan sedimentation in the Jemez Mountains

11 Probability Radiocarbon dating results in a probability distribution for each sample EXAMPLE SINGLE DISTRIBUTION Age We sum the sample age probabilities for each year, resulting in an annual cumulative distribution Clear peaks in fire event probability More likely that fire activity occurred during these peaks Fading record effect Based on the number of samples that contribute to different parts of our record, the most recent 2000 years of our record are best represented

12 Most late Holocene firerelated erosional events were relatively minor Consistent with the lowseverity burns that dominate the tree-ring record Larger debris flows also occurred Suggesting at least small areas of high-severity fire

13 Peaks in fire-event probability correspond with severe regional multidecadal droughts ca and 375 cal yr BP The late MCA peak in fire-event probability overlaps a severe multidecadal drought years BP in the Four Corners area

14 High fire-event probability between 2000 and 500 cal yr BP : generally occurs during periods of large-scale climatic variance as well as periods of frequent climatic shifts

15 The lack of recorded fire activity during much of the Medieval Climatic Anomaly may have been the result of: Dampened precipitation variance A change to a more open forest structure and lower-severity fire regime Severe fire in the previous millennia Frequent, severe drought in the Four Corners area Regime Change?

16 Lack of exposed and dated deposits older than 5000 cal yr BP Middle Holocene may have seen more severe fires and erosional activity Erosional response after the Lakes Fire was at least locally greater than at any time in the last 5000 yr BEDROCK Expansion of this small study area would help us better understand: Local fire-climate-erosional linkages in the Jemez Mountains The degree to which modern climatic warming and anthropogenic impacts have heightened severe fire activity

17 We would like to thank the Jemez Ranger Station and the Forest Service fire crews for their assistance Funding provided by: University of New Mexico New Mexico Geological Society Geological Society of America 2011 Las Conchas Fire: Photo by Kristin Honig, US Forest Service

18 Southwestern paleoclimate studies Tree-ring studies & fire-related deposits Allen, C. D., Fire and vegetation history of the Jemez Mountains. In: Johnson, P. S. (Ed.). Water, Watersheds, and Land Use In New Mexico: Impacts of Population Growth on Natural Resources, Santa Fe Region Socorro, NM: NM Bureau of Mines and Mineral Resources, pp Allen, C. D., Savage, M., Falk, D. A., Suckling, K. F., Swetnam, T. W., Schulke, T., Stacey, P. B., Morgan, P., Hoffman, M., Klingel, J. T., Ecological restoration of Southwestern Ponderosa pine ecosystems: a broad perspective. Ecological Applications 12 (5), Cannon, S. H., Debris-flow generation from recently burned watersheds. Environmental and Engineering Geoscience 7, Cannon, S. H., Bigio, E. R., Mine, E., A process for fire-related debris flow initiation, Cerro Grande fire, New Mexico. Hydrological Processes 15, Meyer, G. A., Wells, S. G., Jull, A. J. T., Fire and alluvial chronology in Yellowstone National Park: Climatic and intrinsic controls on Holocene geomorphic processes. Geological Society of America Bulletin 107, Touchan, R., Allen, C. D. and Swetnam, T. W., Fire History and Climatic Patterns in Ponderosa Pine and Mixed-Conifer Forests of the Jemez Mountains, Northern New Mexico, In: Allen, C. D. (tech. ed.), Fire Effects in Southwestern Forests: Proceedings of the Second La Mesa Fire Symposium. USDA Forest Service Gen. Tech. Rep. RM-GTR-286. Fort Collins, CO, pp Medieval Climatic Anomaly and Little Ice Age: Lamb, H. H., Climate: Past, Present, and Future. Methuen, London. Mann, M. E., Zhang, Z., Hughes, M. K., Bradley, R. S., Miller, S. K., Rutherford, S., Ni, F., Proxy-based reconstructions of hemispheric and global surface temperature variations over the past two millennia. Proceedings of the National Academy of Sciences 105 (36), Southwestern droughts: Cook, E. R., Woodhouse, C. A., Eakin, M., Meko, D. M., Stahle, D. W., Long-Term Aridity Changes in the Western United States. Science 306, Cook, E. R., Seager, R., Cane, M. A., Stahle, D. W., North American drought: Reconstructions, causes, and consequences. Earth-Science Reviews 81, Jimenez-Moreno, G., Fawcett, P. J., Anderson, S. A., Millennial- and centennial-scale vegetation and climate changes during the late Pleistocene and Holocene from northern New Mexico (USA). Quaternary Science Reviews 27, Stahle, D. W., Cook, E. R., Cleaveland, M. K., Therrell, M. D., Meko, D. M., Grissino-Mayer, H. D., Watson, E., Luckman, B. H., Tree-ring data document 16th century megadrought over North America. Eos 81 (12), El Niño Southern Oscillation records: Conroy, J. L., Overpeck, J. T., Cole, J. E., Shanahan, T. M., Steinitz-Kannan, M., Holocene changes in eastern tropical Pacific climate inferred from a Galápagos lake sediment record. Quaternary Science Reviews 27, Tree-ring drought record (Southern Colorado): Routson, C. C., Woodhouse, C. A., Overpeck, J. T., Second century megadrought in the Rio Grande headwaters, Colorado: How unusual was medieval drought?. Geophysical Research Letters 38, doi: /2011gl Speleothem precipitation record (Central New Mexico): Rasmussen, J. B. T., Polyak, V. J., Asmerom, Y., Evidence for Pacific modulated precipitation variability during the late Holocene from the southwestern USA. Geophysical Research Letters 33, doi: /2006gl