Ponderosa Pine Fire Ecology

Introduction

The extensive ponderosa pine forests of Arizona and the Southwest have evolved over thousands of years. Over this time the tree has developed several adaptations which help it survive in it's dry, often warm habitat. A once common occurrence in these forests which has shaped the pine's particular ecological adaptations is fire. Recent studies indicate that the ponderosa pine forests of this region were subjected to low-intensity ground fires perhaps every 2-12 years over historical time. However, beginning in the early 1900's this pattern of fire drastically changed. A fire suppression policy implemented by the United States Forest Service and other land management agencies at this time greatly decreased the occurrence of fire in these forests. The absence of reoccurring fire, coupled with widespread logging and grazing of forest lands, has led to unforeseen changes in forest structure and ecology. Today's forest is often characterized by dense "dog-hair" thickets of young pines with a thick accumulation of litter on the forest floor. Previously, many pine forests of the region were open stands of large, old ponderosa pine underlain by an understory of native grasses. Small fires maintained this open structure by killing seedlings and encouraging growth of grasses. Some ecologists recognized this change in the nature of these pine forests as a possible problem as early as the 1930's, but changes in forest management did not occur until the 1970's. Fires in many of today's ponderosa pine forests are no longer low-intensity ground fires but rather catastrophic, stand-replacing crown fires. How will the pines and other related plants and organisms recover after these large, high-intensity fires which were absent or rare during the long development of these forests? A study now underway is looking at the reconstruction of ponderosa pine forests following these catastrophic fires. Headed by Will Moir of the United States Forest Service's Rocky Mountain Research Station in Flagstaff, the study will include a number of researchers from several different organizations. Julie Zimmerman of Northern Arizona University and several researchers from the United States Forest Service, including Jill Dwyer, Sean Kyle, and George Robertson, are currently involved in the project though more may participate.

Some scientists feel that years of fire suppression in ponderosa pine ecosystems has led the forests into an unpredictable and critical state. The graph below helps illustrates this point.

Graph showing amount of acres burned by wildfires in Arizona and New Mexico per year over the last 90 years (small triangles).

From about 1910 to approximately 1990, the amount of acres burned by wildfire in Arizona and New Mexico oscillated between a few thousand acres to 60,000 acres annually. This yearly amount is dependent on local factors such as weather and fuel loads on the forest floor. However, beginning in 1992 the amount of acres burned between the two states has skyrocketed, with over 180,000 acres burning in 1997. Prior to fire suppression, the fires in the pine forests of the region behaved in a somewhat predictable manner determined by years of evolution and natural processes. The forest ecosystem of today, in contrast, has possibly reached a point of unstable criticality. A lightning strike may lead to a few trees burning, a few acres burning, or a catastrophic stand-replacing fire sweeping over thousands of acres of forest. Land managers and scientists are no longer able to predict with much confidence what direction fires in the ponderosa pine forests of the Southwest might take.

Fire control personnel with the United States Forest Service and other land management agencies are concerned that more fires might be dangerous, catastrophic fires until fuel loads are reduced below the critical threshold. Extensive tree-thinning projects and prescribed burning are two steps forest managers are taking to try to decrease the danger of high-intensity fires as well as restore the ponderosa pine forests of the region to a more "natural" state.

Methods

Currently two sites in northern Arizona have been chosen for study; the Hochderffer-Horseshoe burn just west of the San Francisco Peaks near State Highway 180 and the Bridger-Knoll burn on the Kaibab Plateau north of Grand Canyon National Park. These fires occurred in 1996 and have both high-intensity crown fire areas as well as moderate-intensity fire sites.

The overall objective of the study is to document the ecologic recovery or reconstruction of the sites during the early years after the burn. Sample plots are set up to monitor species composition and colonization. Despite the bleak appearance of charred black sticks following a major crown fire, native organisms and plants often quickly invade the site and recovery is underway. However, in many areas following these burns invasive species are able to establish themselves crowding out native species. A component of this study will look at this colonization by non-native species. The pre-fire seedbank in the soil of the site will also be studied as it may play a strong role in post-fire succession.

Preliminary Results

Preliminary observations suggest that seedbanks are important in post-fire reconstruction of heavily-burned sites. New pine seedlings and lamb's quarter are growing at the sites from seeds previously stored in the soil.

Important Terms

Ground Fire- a low-intensity fire which sweeps through the forest burning only shrubs and grasses on the forest floor, charring the lower bark of trees but NOT reaching into their branches (crown)

Moderate-Intensity Fire- a moderate-intensity fire burns the understory and partway into the crown of the trees. Many trees survive by replacing burnt foliage over time.

Crown Fire- a high-intensity fire which sweeps through the forest killing most trees. A crown fire's flames can be tens of feet high and reach up into the crown of the tree.

Invasive- a non-native species which has not evolved over time within the regional ecology of an area. Sometimes invasive species can severely disrupt the ecology of an ecosystem, as in the case of the introduced toadflax (Linaria dalmatia) and ponderosa pine forests.

Succession- the predicatable recolonization of an area by plants and animals following a major disturbance such as fire.

Seedbank- a "bank" of ungerminated seeds preserved in the soil which may sprout if environmental conditions change

Reconstruction- the process of establishing new food webs in a forest following a catastrophic fire. Reconstruction occurs when a situation develops which has not yet been seen in the evolution of an ecosystem, such as high-intensity crown fires in ponderosa pine forests.