Date of this Version
JFSP Project Number: 06-3-1-27
Montane grasslands are an important ecosystem type in Southwestern landscapes, occurring from the forest borders of piñon-juniper woodland to high elevation subalpine and alpine ecosystems. Thus, they often represent a mosaic element embedded in the forest landscape matrix, playing key roles in processing water and nutrients, and providing habitat for unique suites of species, often at higher levels of diversity than those found in surrounding uplands. Soils, local temperature and moisture gradients, and disturbance all play roles in determining the distribution and extent of montane grasslands. Of these factors, fire plays the most dynamic role in regulating the location and composition of the grassland-forest ecotone. In this project we undertook the first landscape-scale characterization of the historic fire regime of one of the major montane grassland-forest complexes in southwestern North America, the 36,400 ha (90,000 ac) Valles Caldera National Preserve (VCNP) in the heart of the Jemez Mountains of northern New Mexico. The VCNP is a post-volcanic landscape consisting of multiple forested resurgent domes within a matrix of high-elevation valle grasslands. Where forest meets grassland, an ecotonal zone occurs with moderate tree cover and high herbaceous productivity. We hypothesized that the ecotone represented the highest fire frequency and served as a pathway for fire spread among the valles. To test this idea we collected fire-scarred tree-ring samples from over 450 trees from the forest-grassland ecotone in all of the major valles in the VCNP, making this one of the most comprehensive landscape fire history collections in the United States. All sample locations were georeferenced to facilitate spatial as well as temporal patterns of historical fire occurrence. To allow stronger interpretation of the point fire-scar record as a proxy for fire spread, we modeled fire behavior in the same landscape using Minimum Travel Time (MTT) and other capabilities of the FlamMap system, using a model calibrated to local weather streams, ignitions, and fuels (Finney 2002). The resulting VCNP tree-ring record extends back to the early AD 1418, establishing a 591-yr period of record. We dated more than 2000 fire scars, with the earliest dating to 1501, representing 468 years with fire occurrence. Our fire history analysis shows high fire frequency in the forest-grassland ecotone, with average mean fire interval (MFI) of 6.9 yr in individual valles, and 1.3 yr between fires in the VCNP as a whole. Fire frequencies were highest in the largest valles and MFI was significantly associated with valle area. Historically, fires occurred in years of significantly low Palmer Drought Severity Index (PDSI); the most widespread fire years represented contingent combinations of low fire-year PDSI preceded by 2-3 years of unusually high PDSI (cool-wet conditions), indicating climate control on fine fuel production. More than 20 instances of back-to-back fire years occurred in the VCNP landscape, generally entrained by sequential low-PDSI years as well as spatial separation of burned areas. Fire modeling simulations identified the forest-grassland ecotone as the optimal pathway for fire spread: both more mesic, upslope closed forest and the mesic internal wet meadow conditions of the grasslands impede fire flow, whereas the relatively drier conditions with abundant fine fuels facilitate rapid fire spread across the VCNP landscape. These findings are being provided to managers of the VCNP to informal ecosystem management, including their fire management program.
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