Fire Ecology of Pacific Northwest Forests

James K Agee

• Publisher: Island Press
• Year: 2013
• ISBN:
  9781610913782
• Topic: Biological Sciences
• Subtopic: Science General
• Index: Biological Sciences

The structure of most virgin forests in the western United States reflects a past disturbance history that includes forest fire. James K. Agee, an expert in the emergent field of fire ecology, analyzes the ecological role of fire in the creation and maintenance of natural western forests, focusing primarily on forest stand development patterns. His discussion of the natural fire environment and the environmental effects of fire is applicable to a wide range of temperate forests.

CHAPTER 1

THE NATURAL FIRE REGIME

DISTURBANCE IS AN INTEGRAL PROCESS in natural ecosystems, and management of forest ecosystems must take into account the chance of natural disturbance by a variety of agents. In some situations, such as park or wilderness management, natural disturbance may be required by law or policy to maintain natural ecosystems. In others, natural disturbance may wreak havoc with specific management goals, such as wood production or maintenance of a specific wildlife habitat. Fire is a ubiquitous disturbance factor in both space and time, and it cannot be ignored in long-term planning. Its effects can be integrated into land management planning through an understanding of how fire affects the site and the landscape.

Today’s plant communities reflect species assemblages in transition, each reacting with different lag times to past changes in climate, and each migrating north or south, up or downslope. Many species have not closely coevolved with the other species they are found growing with today, because of differential rates of migration over past millennia. Each species, however, may have coevolved for much longer periods with particular processes associated with it. Fires have been associated with most species of angiosperms and gymnosperms through much or all of their evolutionary development.

THE PALEOPYRIC IMPERATIVE

Fire is by no means a recent phenomenon. As long as plant biomass has been present on the earth, lightning has ignited fires, and the myriad ecological effects have been repeated time and again. The history of fire extends well back into the Paleozoic Era, hundreds of million years before the present and long before the angiosperms existed on earth. The Carboniferous Period, so named because of the extensive coal deposits formed during that time, have extensive amounts of fusain (Komarek 1973, Beck et al. 1982). Fusain is a fossil charcoal produced by fires that is almost completely inert, allowing it to survive through the geologic eras (Harris 1958). Fusain has little volatile content and glows on combustion, in contrast to coalified plant tissue, which burns with a smoky flame (Harris 1981). Wildfire was probably a regular occurrence on the earth during and since the Mesozoic (Cope and Chaloner 1985), when gymnosperms dominated the earth and angiosperms developed.

Fire may have been associated with the extinction of dinosaurs. A catastrophe following a large meteorite striking the earth is now a widely accepted theory for the significant deposition of iridium at the Cretaceous-Tertiary (K-T) boundary, also associated with significant peaks in carbon content (10²—10⁴ above background levels; Wolbach et al. 1988). The carbon is mostly soot, and the ejecta from the hypothesized collision lie on top of the soot, implying that the soot was created rapidly and was deposited before the weeks- to months-long deposition of the remainder of the mineral ejecta. A single massive global fire or a series of forest fires occurring around the globe would have been necessary to explain the amount of carbon found in these deposits (Wolbach et al. 1988). Whether such fires were simply another effect of the meteorite impact or whether they were in fact a co-primary cause of biological extinction is a question that may be debated for decades. The magnitude of such a potential event makes the Yellowstone fires of 1988 seem no more than a minor spark on the landscape.

Ecosystems with substantial presence of fire almost always contain species that are able to take advantage of it to survive as individuals or species. Plant adaptations, which will be discussed further in chapter 5, such as thick bark, enable a species to withstand or resist recurrent low intensity fires while less well-adapted associates perish. Some pine species have serotinous (late-opening) cones, which have changed little since the mid-Miocene (Axelrod 1967). While closed, these cones hold a viable seedbank in the canopy that remains protected until the trees burn. After a fire, the cone scales open and release seed into a freshly prepared ashbed. Other species maintain a similar seedbank in the soil, which lies dormant until heated. Many species have the ability to sprout after being burned, either from the rootstock or from the stem. The adaptations of plant species to fire are more widespread and common than animal adaptations, but they are less spectacular than the adaptation of the Melanophila beetle.

The Melanophila beetles are flat-headed borers, found worldwide, which usually breed in fire-damaged pines. Eggs are deposited below the bark, where in larval form the beetles feed on the cambium of newly killed trees and later emerge as adult beetles. Adults are known to be stimulated by heat and/or attracted to smoke. Linsley (1943, p. 341) noted that at University of California football games, with 20,000 or so cigarettes ablaze at any time (remember, this was the 1940s), a haze of tobacco smoke would hang over Memorial Stadium. Melanophila beetles would “annoy patrons by alighting on the clothing or even biting” during a big game, which was more disturbing to fans than a Stanford touchdown. Linsley found that the beetles had sensory pits on their bodies and could somehow sense heat or smoke. Later these pits were determined to be infrared detectors that allowed beetles to find burned areas where newly damaged trees were likely to be found and where the highest probability existed of successfully rearing a brood. This adaptation can only be interpreted as a direct attraction to the presence of fire to increase species fitness, an adaptation that must have taken millennia to evolve.

The earth has long been a fire environment. The fires of Indonesia in 1982 (Davis 1984) and northeastern China in 1987 (Salisbury 1989), each of which burned millions of hectares, are a testament that earth is still a fire environment. Smaller episodes like the Yellowstone fires of 1988 (570,000 ha) are not the first nor will they be the last to strike the northern Rocky Mountains (Christensen et al. 1989). Our approach to fire management in North America must accommodate fire (Pyne 1989a); we cannot be so bold as to think we can eliminate fire from the landscape. It has been with us so long precisely because it is an inevitable part of our environment.

THE RECENT QUATERNARY

Our knowledge of fire on the Pacific Northwest landscape improves as we approach the present, although much remains unknown. In particular, evidence since the last glaciation suggests a substantial interaction among vegetation, climate, and fire that continues to the present. Climate directly affects vegetation and influences the probability that the vegetation will burn. During periods of climatic change, when conditions at many sites will favor establishment of new species combinations, burning will increase the rate of expansion of shade-intolerant vegetation and decrease the spread of shade-tolerant, late successional species (Brubaker 1986).

Changes in species composition on a site may be inferred from pollen analysis of cores, usually drawn from peatland areas. Ages of the sequences within a core are determined from radiocarbon dating, and an index to fire activity can be determined from charcoal in the same layers. Pollen analyses can be used to reconstruct regional vegetation patterns during the Holocene (Fig. 1.1). In the western Cascades, the relationship between vegetation and fire activity was very dynamic (Tsukada et al. 1981, Cwynar 1987). Retreat of the Fraser glaciation resulted in a forest dominated by spruce and lodgepole pine (Picea and Pinus contorta) in the Puget Trough between 15,000 and 12,000 ybp. Western hemlock (Tsuga heterophylla ) entered at that time, suggesting a warming that was associated with increasing summer drought. Douglas-fir (Pseudotsuga menziesii) and bracken fern (Pteridium aquilinum) became dominants, with red alder (Alnus rubra) dominant in riparian settings (Barnosky et al. 1989). (Appendix B lists the common and scientific names of plants mentioned in the text.)

FIG. 1.1. Trends in species composition, based on pollen analysis, over the last 12,000 years in the Puget Trough, Washington. Dashed line is date of Mazama ash layer, about 6,600 ybp.

(From Brubaker 1991)

The period between 10,500 and 7,000 ybp was warmer and drier than today. Samples from that time period contain the greatest charcoal peaks, implying that fire was more prevalent during that dry, warm period. Over the past 5,000 years, charcoal peaks have declined from their maxima, and a more stable lowland vegetation, increasingly dominated by western hemlock and western redcedar (Thuja plicata), has persisted to the present. Although fire may have interacted with species such as Douglas-fir for hundreds of generations, it appears to have interacted with the species mix common to today’s mesic old-growth, Douglas-fir forests for perhaps 10—20 generations (5,000 years) of Douglas-fir (Brubaker 1991).

THE CURRENT MILLENNIUM

Fire evidence in the Pacific Northwest for the current millennium becomes more obvious, since many of the tree species can live for 500—1,000 years (Franklin and Waring 1979). These trees may provide, through forest age structure or fire scars, a direct record of fire activity (see chapter 4). Almost every forest type has experienced a fire in the current millennium, and some may have burned more than a hundred times. Although the evidence of fire is visible on today’s landscape, presence alone is an insufficient criterion by which to understand the effects of fire in forested ecosystems. Not only is there variability in fire frequency between forest types, but thi...

Table of contents

• ABOUT ISLAND PRESS
• Title Page
• Copyright Page
• Dedication
• Table of Contents
• PREFACE
• ACKNOWLEDGMENTS
• CHAPTER 1 - THE NATURAL FIRE REGIME
• CHAPTER 2 - THE NATURAL FIRE ENVIRONMENT
• CHAPTER 3 - THE CULTURAL FIRE ENVIRONMENT
• CHAPTER 4 - METHODS FOR FIRE HISTORY
• CHAPTER 5 - FIRE EFFECTS ON VEGETATION
• CHAPTER 6 - ENVIRONMENTAL EFFECTS OF FIRE
• CHAPTER 7 - SITKA SPRUCE, COAST REDWOOD, AND WESTERN HEMLOCK FORESTS
• CHAPTER 8 - PACIFIC SILVER FIR AND RED FIR FORESTS
• CHAPTER 9 - SUBALPINE ECOSYSTEMS
• CHAPTER 10 - MIXED-CONIFER/ MIXED-EVERGREEN FORESTS
• CHAPTER 11 - PONDEROSA PINE AND LODGEPOLE PINE FORESTS
• CHAPTER 12 - NORTHWEST WOODLANDS
• CHAPTER 13 - FIRE IN OUR FUTURE
• APPENDIX A - COMMON CONVERSION FACTORS
• APPENDIX B - NAMES OF PLANTS MENTIONED IN TEXT
• GLOSSARY
• REFERENCES
• INDEX
• ABOUT THE AUTHOR