Wednesday, August 4, 2010

PS 52-49: CANCELLED - Top-down and bottom-up control of fire regimes in montane grasslands of the Valles Caldera, New Mexico, USA

Jacqueline J. Dewar1, Donald A. Falk1, Craig D. Allen2, Robert R. Parmenter3, Thomas W. Swetnam1, and Christopher H. Baisan1. (1) University of Arizona, (2) Jemez Mountains Field Station, (3) Valles Caldera Trust

Montane grasslands are widely distributed across the western United States, including the southern and central Rocky Mountains, but little is understood about their historic fire regimes. These ecosystems provide habitat for numerous species of flora and fauna, while benefiting local economies through livestock grazing and recreation. Land managers require specific information about past fire regimes in these systems, including the frequency, extent, and seasonal timing of fires, their spatial complexity, and the role of climate variability. To provide this historical perspective, we reconstructed the historic fire regimes in ponderosa pine and mixed conifer forests growing on volcanic domes which are surrounding valles grasslands in the Valles Caldera National Preserve (VCNP) of the Jemez Mountains in northern New Mexico.  To do so, we employed dendrochronological methods to reconstruct temporal and spatial patterns of historic fire occurrence recorded in fire-scarred ponderosa pine and mixed conifer species. Using a spatially-explicit sampling design, we collected 610 fire-scarred samples from predominately relict wood in 27 sites surrounding the perimeter of the eight valles. Fire-scarred samples date back to 1479 AD with appropriate sample size from about 1700 AD, suggesting an adequate period of time to assess patterns of synchrony both within and amongst the valles

We identified 687 fire events representing 181 separate fire years spanned from 1479 to 1900 AD. For this period, the mean fire interval for the entire VCNP is 2.33 years/fire. Preliminary results confirm the pre-1900 historic occurrence of high frequency, low-severity surface fires over multiple centuries. In some fire years, such as 1748 and 1752, synchronous fires burned across the grasslands and into the surrounding forests over the majority of the VCNP (greater than 35% of total sites burning).  These large fires were typically occurring at 10.5 year intervals on average. However, fires in other years burned in a relatively small proportion of total sites (between 10 and 35% of total sites), creating asynchronous burn patches throughout the VCNP. The smaller, patchier fires were typically occurring at 6 year intervals on average. One hypothesis for widespread synchronous fires is the suggestion of top-down climate control of fire occurrence in years with regional drought conditions, whereas asynchronous fires indicate stronger bottom-up control driven by fuels, fuel moisture, short-term weather conditions, topography and fire spread. The relative influence of top-down and bottom-up regulation of fire regimes has important implications for forest policy and management.