HOLOCENE CLIMATE, WILDFIRE AND VEGETATION IN A FOREST-STEPPE ECOTONE AT THE CITY OF ROCKS NATIONAL RESERVE, IDAHO

In many parts of the world, ecosystem response to climate change will entail plant migrations and altered wildfire regimes. However, the relationships among climate, vegetation and fire are not well understood, especially along sensitive ecotones. This study uses alluvial charcoal exposed in incised stream banks and alluvial fans to develop a long-term (10³-10⁴ yr) chronology of fire in a semi-arid, forest-steppe ecotone in the City of Rocks National Reserve (CIRO) in southern Idaho. Fire and fire-related sedimentation events are compared to a highly-resolved vegetation reconstruction from nearby woodrat middens.

Modern woodlands at CIRO are dominated by single-needle pinyon (P. monophylla) in association with Utah juniper (J. osteosperma), Rocky Mountain juniper (J. scopulorum) and an occasional limber pine (P. flexilis). Pinyon likely established its dominance at this northernmost, isolated post in its distribution within the last 1000 years. Its colonization was probably assisted by the multi-decadal droughts, climate variability and large-scale ecological disturbances (e.g. fire, insect outbreaks, drought-related tree die-offs, etc.) of the Medieval Climatic Anomaly (MCA) ~1050-650 cal yr BP. Preliminary results (n=17) indicate that fires burned between ~11,500-9,900 cal yr BP and correspond to the shift from the Younger-Dryas to warmer, drier conditions. Regional paleo-climate records support fluctuating climate between ~3,600-1,500 cal yr BP when fires burned during periods of drought. Late Holocene fire peaks correspond to the MCA and the Little Ice Age (LIA); (~650-100 cal yr BP). Although characterized by glacial advances, the LIA also experienced climate variability and these younger fire events are better preserved in the stratigraphic record. Preliminary midden analysis suggests a colder climate ~13,000 cal yr BP, as indicated by midden assemblages dominated by limber pine with occurrences of subalpine fir (A. lasiocarpa) and common juniper (J. communis).

The study will elucidate millennial-scale feedbacks among climate, vegetation and fire in a semi-arid ecotone. Furthermore, the study allows us to evaluate the role of synchronous fires and ecological disturbances in equally synchronous migrations along the northern periphery of plant distribution in the western U.S.