A 13,100-year-long high-resolution pollen and charcoal record from Foy Lake in western Montana is compared with a network of vegetation and fire-history records from the Northern Rocky Mountains. New and previously published results were stratified by elevation into upper and lower and tree line to explore the role of Holocene climate variability on vegetation dynamics and fire regimes. During the cooler and drier Lateglacial period, ca 13,000 cal yr BP, sparsely vegetated Picea parkland occupied Foy Lake as well as other low- and high-elevations with a low incidence of fire. During the warmer early Holocene, from ca 11,000-7500 cal yr BP, low-elevation records, including Foy, indicate significant restructuring of regional vegetation as Lateglacial Picea parkland gave way to a mixed forest of Pinus-Pseudotsuga-Larix. In contrast, upper tree line sites (ca >2000 m) supported Pinus albicaulis and/or P. monticola-Abies-Picea forests in the Lateglacial and early Holocene. Regionally, biomass burning gradually increased from the Lateglacial times through the middle Holocene. However, upper tree line fire-history records suggest several climate-driven decreases in biomass burning centered at 11,500, 8500, 4000, 1600 and 500 cal yr BP. In contrast, lower tree line records generally experienced a gradual increase in biomass burning from the Lateglacial to ca 8000 cal yr BP, then reduced fire activity until a late Holocene maximum at 1800 cal yr BP, as structurally complex mesophytic forests at Foy Lake and other sites supported mixed-severity fire regimes. During the last two millennia, fire activity decreased at low elevations as modern forests developed and the climate became cooler and wetter than before. Embedded within these long-term trends are high amplitude variations in both vegetation dynamics and biomass burning. High-elevation paleoecological reconstructions tend to be more responsive to long-term changes in climate forcing related to growing-season temperature. Low-elevation records in the NRM have responded more abruptly to changes in effective precipitation during the late Holocene. Prolonged droughts, including those between 1200 and 800 cal yr BP, and climatic cooling during the last few centuries continues to influence vegetation and fire regimes at low elevation while increasing temperature has increased biomass burning in high elevations.
Bibliographical noteFunding Information:
The research described in this paper was supported by a grant from the National Science Foundation ( EAR-9906100 to Whitlock). Stephen Coleman funded several radiocarbon dates from the Foy 2002 core. Cathy Whitlock, Sherilyn Fritz, Jeffery Stone, Karlyn Westover, Bryan Shuman, and Josh Campbell assisted in fieldwork. Adrienne Vincent assisted with laboratory analysis. Contributors: M.J.P. designed the study, collected, analyzed, interpreted the data, and wrote the manuscript, C.W. and P.B. provided funding, assisted with study design, data analysis, and interpretation, and editing. Lora Stevens also assisted in study design, fieldwork, and developed the varve chronology. We also thank Andrea Brunelle for providing pollen and charcoal data from several records in the Northern Rocky Mountains. This paper is also a contribution to the ongoing work of the Global Palaeofire Working Group (GPWG) of the International Geosphere-Biosphere (IGBP) Cross-Project Initiative on Fire. We thank our colleagues and data contributors to the Global Charcoal Database.
Copyright 2011 Elsevier B.V., All rights reserved.
- Biomass burning
- Holocene climate
- Rocky mountains