David D. Hanna’s research while affiliated with Florida Museum of Natural History and other places

Twentieth-century fire exclusion has produced unnatural and undesirable changes in vegetation structure and dynamics of many rangelands of western North America, but not all kinds of ecosystems have been so affected. A comparison of the historical and modern fire regimes, especially in peripheral populations that can be particularly vulnerable to climatic change, can help guide fire management planning with information on the degree to which a local area has been altered by past fire exclusion. Historical fire rotations in piñon-juniper (Pinus edlis Engelm.-Juniperus spp. L.) woodlands vary widely across woodland types, hence management applications should be specific to local historical and modern fire characteristics. We asked if the modern fire rotation is similar to or longer than the historical fire rotation before arrival of Euro-American settlers on the northern woodland boundary in northwestern Colorado and northeastern Utah. This study was initiated by managers from Dinosaur National Monument (DINO) concerned that lack of 20th-century fire may have allowed unnatural expansion of piñon-juniper woodlands into grasslands and shrublands. Fire history analysis using dendrochronol-ogy methods suggests a historical (pre-1900) fire rotation of ca. 550 yr, comparable with or longer than many other woodlands on the Colorado Plateau. In contrast, analysis of digital fire records reveals that the fire rotation between 1981 and 2010 was substantially shorter than historical; if only natural fires are considered, the piñon-juniper fire rotation was 364 yr, and if anthropogenic fires were included, the fire rotation was 233 yr. This shorter fire rotation supports a previously documented contraction in woodland extent in DINO during the past 90 yr. Our data support reducing the amount of fire in the landscape to preserve the integrity of the natural vegetation of this and other piñon-juniper woodlands, especially under projections of warmer and drier future climates.

Twentieth-century fire exclusion has produced unnatural and undesirable changes in vegetation structure and dynamics of many rangelands of western North America, but not all kinds of ecosystems have been so affected. A comparison of the historical and modern fire regimes, especially in peripheral populations that can be particularly vulnerable to climatic change, can help guide fire management planning with information on the degree to which a local area has been altered by past fire exclusion. Historical fire rotations in piñon-juniper (Pinus edlis Engelm.-Juniperus spp. L.) woodlands vary widely across woodland types, hence management applications should be specific to local historical and modern fire characteristics. We asked if the modern fire rotation is similar to or longer than the historical fire rotation before arrival of Euro-American settlers on the northern woodland boundary in northwestern Colorado and northeastern Utah. This study was initiated by managers from Dinosaur National Monument (DINO) concerned that lack of 20th-century fire may have allowed unnatural expansion of piñon-juniper woodlands into grasslands and shrublands. Fire history analysis using dendrochronology methods suggests a historical (pre-1900) fire rotation of ca. 550 yr, comparable with or longer than many other woodlands on the Colorado Plateau. In contrast, analysis of digital fire records reveals that the fire rotation between 1981 and 2010 was substantially shorter than historical; if only natural fires are considered, the piñon-juniper fire rotation was 364 yr, and if anthropogenic fires were included, the fire rotation was 233 yr. This shorter fire rotation supports a previously documented contraction in woodland extent in DINO during the past 90 yr. Our data support reducing the amount of fire in the landscape to preserve the integrity of the natural vegetation of this and other piñon-juniper woodlands, especially under projections of warmer and drier future climates.

In the last decade, piñon-juniper (Pinus edulis-Juniperus osteosperma) woodlands of southwestern Colorado have been heavily impacted by wildfires and insects, and the effects on fuel structures and future fire intervals have been unclear. In these piñon-juniper woodlands, pre-historic fire rotations of up to 400 years were documented, but in recent climatic conditions, large stand-replacing wildfires burned between 1989 and 2008. Since the 1990s, a period of drought and rising temperatures, portions of the post-fire landscape were targets for invasive Carduus nutans, muskthistle and Bromus tectorum, cheatgrass, whose presence create unprecedented fuels, may shorten fire intervals, and reduce native biodiversity. We predicted that extensive tree mortality would alter fuel properties and fire behavior in dense unburned woodlands. In his two-year study, we addressed changes in fuel structures due to recent fires and insect infestations and explored management implications of these changes in Mesa Verde National Park and adjacent portions of the Ute Mountain Ute Tribal land. Given the widespread nature of the mortality event of 2002-2004 we found few unaltered woodlands to serve as undisturbed controls. Therefore, also we re-evaluated nine fuel profiles originally sampled in 1993-1995 to provide pre-mortality fuel conditions. Woodland canopy and horizontal continuity of crown fuel has been significantly reduced. We found an increase in litter, 1to 100 hr fuels, and 1000 hr fuels following the mortality event. Woodland structural diversity as has decreased. However, the structure of post-mortality woodlands remains within the historic range of variability (HRV) for this region. Fire behavior has not changed significantly except for increased heat release. In contrast, fuel structures and predicted fire behavior vary significantly across a time-since-fire chronosequence. Higher spread rate and intensity values are modeled in mid-successional areas, but measures decline as piñon-juniper woodlands get older and are lowest in old-growth stands. Early successional stages (i.e. burns in last 25 years) support Bromus tectorum, cheatgrass, which presents unprecedented levels of continuous surface fuel. Post-fire mitigation sponsored by the Federal BAER program using native perennial grasses in 1996 and 2000 significantly reduced the abundance and cover of invasive plant species which will likely have a significant effect on frequency of future fires. Native species diversity and abundance were not reduced by post-fire mitigation, nor do we detect any changes in fire behavior due to the introduction of perennial grasses. Management implications of these trends are discussed in the final section of this report.