| Using Reference Conditions |
|
|
|
Fire scars, such as this pair from pine trees in Durango, Mexico (left), and Mount Trumbull in the Uinkaret Moutains of northern Arizona (right), can help establish reference conditions. Arrows indicate rings with fire scarring; outermost dates indicate when the fire scars were collected. In Mexico, where livestock grazing was slight and fire suppression virtually absent, fires continued at regular intervals through the twentieth century. In northern Arizona regular fires ceased with the onset of large-scale livestock grazing, with the last fire occurring in 1869. Photograph by Dan Boone/Northern Arizona University. “Reference conditions” are those that existed before forest structure and function were altered by Euro-American settlers. These dynamic conditions were not unchanging, but they sustained themselves. In particular, southwestern ponderosa pine ecosystems were subject to frequent surface fires, some no doubt ignited by indigenous peoples and some by lightning (Allen 2002). Both types of fires had the same effect: they sustained forest structure by removing tree seedlings and cycling nutrients to plants. These conditions apparently were stable for a long time: soil analyses have shown that some grassy openings have existed in the same places for centuries, and perhaps much longer, while areas with clumps of pines were wooded for equally long periods (Kerns et al. 2001). Reference conditions are useful tools because they show what a site’s potential can be under self-sustaining conditions (Egan and Howell 2001). They can be determined by locating trees or tree remains that were present before Euro-American settlement, which generally include living pines or snags with yellowed bark, as well as large downed logs, stumps, and stump holes. Tree-ring records can help document past forest structure and fire history, as can historic photographs, Forest Service records, and other written records. Relatively undisturbed sites nearby can also aid in understanding what reference conditions may have existed on a site to be treated, though the great differences in stand density and structure that can exist on even adjacent sites must be taken into account. Using reference conditions to establish restoration objectives does not imply that the goal of restoration is to return a forest to precisely the structure and composition it had in the past. Due to irreversible changes such as invasive plant species and climate change, that’s not possible—and in many cases it may not even be desirable, thanks to ecological, social, and economic considerations. Instead, reference conditions simply provide us with the last, best information about what these forests were like before the unnatural changes brought about by Euro-American settlement. Restoration is all about “learning how to discover the past and bring it forward into the future” (Egan and Howell 2001). Understanding what forests were like in the past is one of our most important tools in creating a road map that details what we’d like them to be like in the future. To learn more, read about Establishing Reference Conditions. References:
|
