ESRM 401, Spring 2010
Spring Comes to the Cascades
Instructors: Tom Hinckley & Julie Combs
| Home & Critical Page
Student Issues Teaching Team Background material on:
Field Trips Historical
|
Fire Ecology of the Cascade Mountains Anna E. Hohl, ESRM 401, Spring 2006 Wildland fire is an important ecological process and a powerful evolutionary force. Biodiversity, animal populations, hydrologic cycles, aquatic ecosystems, and insect and pathogen cycles are all shaped by both the presence and absence of fire. Fire regimes are also closely tied to climate, elevation, and topography. The interplay between all these factors is exemplified by the differing fire regimes of the ESRM 401 field trip sites (spring 2006). Following is a brief description of the fire regimes characteristic of the western and eastern Cascades and a brief discussion of the connections between fire, human activities, climate change, and the general course themes of elevation, climate, and topography. Appended is a "Fire Fact Sheet" for the major conifer species seen at each field site. Factsheets are listed by species in the table below. I. Fire Regimes of the Cascade Mountains The West Side of the Cascade Mountains ( Tsuga heterophylla forests) The coastal forests of Washington are characterized by a mild maritime climate that limits the natural frequency and intensity of fires. The Douglas-fir/western hemlock/western red cedar forest of Lake Tradition is a typical example of the stands found throughout the low elevation areas of western Washington. Although fire return intervals for this forest type are not well defined, it is believed to have a return interval of 200-500 years. Studies indicate that these usually large, stand-replacing fires occurred in rare conditions of high winds and drought rather than as a regular, cyclic fire interval that is typical of drier, mixed conifer forests. The co-dominance of Douglas-fir ( Pseudotsuga menziessii ) is associated with an increase of fire activity in this forest type. The lack of fire activity in western hemlock forests has been attributed partially to its bordering forest types: moister Sitka spruce ( Picea sitchensis) and cooler Pacific silver fir ( Abies amabilis) forests, which burn less frequently. The rare weather and drought conditions that allow a fire to spread in this moist forest type usually result in high fire intensities that cause stand-replacing crown fires. The major species in this forest type are very sensitive to fires. Hence, even a fire of moderate intensity can cause high mortality (Agee 1993). Regeneration following fires is generally dependent on off-site seed sources. On drier sites, the stand is typically dominated by Douglas-fir for the first 100 years, and then western hemlock will establish itself in the understory. On moister sites, western hemlock and Douglas-fir tend to colonize the site at the same time. If there is sufficient overstory remaining following a fire, hemlock and western red cedar may successful regenerate while Douglas-fir will not (Agee 1993). Western hemlock is the common late successional dominant in this forest type. The dominance of Douglas-fir for many centuries prior to settlement was most likely the result of frequent fires. In the Tsuga heterophylla forest type, Douglas-fir is the most fire-adapted species. Fire also opens up the canopy, allowing the shade intolerant Douglas-fir to establish itself. As fires continue to decline, the proportion of Douglas-fir will decline, allowing western hemlock to become a more dominant species (Agee 1993). Mid-montane and Subalpine Forests of the Cascades (Abies amabilis , Abies lasiocarpa , and Tsuga mertensiana forests) Abies amabilis forests are characterized by a cool, temperate climate with moderate winter temperatures and substantial snowpack. At lower elevations, co-dominants are typically Douglas-fir and western hemlock. At higher elevations, mountain hemlock ( Tsuga mertensiana ) and Alaska yellow-cedar ( Chamaecyparis nootkatensis ) are more common. In the east Cascades, subalpine fir ( Abies lasiocarpa ) and lodgepole pine ( Pinus contorta ) are common co-dominants, as was seen on the North Fork of the Teanaway site. Similar to the west Cascades, these forest types are characterized by infrequent fires of high severity that usually occur under rare conditions of drought and east winds. They tend to be high intensity, stand replacing events. In moist Abies amabilis forests, fire return intervals range from 300 to 600 years. At lower, drier elevations, fire return intervals are generally 100 to 300 years. In the eastern Cascades, fires are most frequent in areas where silver fir exists only on north aspects and is surrounded by forests with warmer, drier environments (Agee 1993). As you move to higher elevations and into Tsuga mertensiana and Abies lasiocarpa forests, fires regimes become more varied and overall less frequent. Fires in these forests are usually erratic, weather driven events, but they are the main large-scale disturbance in these forests. The extent of these fires depends mainly on the distribution of forest vegetation. Subalpine forests are often patchy and do not allow fires to carry across rock or snow fields. Crown fires can occur in subalpine forests when foliar moisture is low and may be aided by lichens within the canopy. Restocking of a burned site is largely a function of seed source and climate. Although cool and moist, post-fire environments at these elevations may be too harsh for seedling survival of some tree species. Open sites may have increased frost and increased daytime maximum temperatures. Western white pine and noble fir can survive on frost-prone and drier areas while Douglas-fir will be dominant on the drier, warmer sites (Emmingham and Halverson 1981). Pacific silver fir may depend on at least partial shade for successful establishment on recently burned sites. None of the tree dominates in the higher elevations are adapted to grow well in open, recently burned environments, allowing shrubs and herbs to dominate a post-fire plant community for a century or more (Agee 1993). The East Side of the Cascades ( Pinus ponderosa forests) Ponderosa pine forests have the most altered fire regimes of the Pacific Northwest's forests. Historically, frequent, low intensity fires maintained an open, park like appearance and rarely scorched the crowns of mature trees. Fires of higher intensity occurred during longer than normal fire return intervals that allowed high fuel build-ups, were often fueled by large insect outbreaks, and/or were accompanied by unusual weather events, such as drought or high winds. Forest species on the eastside of the Cascades are well adapted to fire. Ponderosa, Douglas-fir, and western larch have high crowns and thick bark that allow them to survive frequent, low intensity surface fires. With the advent of successful fire exclusion and grazing that decreased fine fuel loads, these forests have become denser with more shade-tolerant, fire-sensitive species such as grand fir. The result is a shift to a fire regime of lower frequency and higher severity (Wright 2004, Everett et. al. 1999, Volland 1981). Ponderosa pine stands often suffer post-fire insect attack. Scorched trees are more susceptible to various species of beetles. However, fire may help control dwarf mistletoe infestation by pruning dead branches and consuming tree crowns with low-hanging witches' brooms as well as decreasing stand density, which decreases susceptibility to insect and pathogen infestations (Agee 1993). Although a significant amount of work has been done on the Ponderosa pine forests of the Southwest, relatively little research has been done on the Ponderosa forests of the Pacific Northwest. Research suggests that Ponderosa forests in the Pacific Northwest may have burned less frequently than similar stands in the Southwest (Agee 1993). II. Climate, Topography, and Elevation Interactions with Fire Climate, topography, and elevation are powerful forces that shape a forest community. Through both their effects on vegetation and their direct effects on fire behavior, these factors play a similarly important role in determining fire regimes and the impacts of fire on a forest community. .The Role of Climate (precipitation, wind, and temperatures) Climate influences fire regimes by determining the vegetation available for fires to burn through, through the direct connections between wind, temperature, and humidity on fire intensity, and through the season of natural ignitions. Generally, increases in wind and temperature and decreases in precipitation, whether between climates or seasonal variations, will increase fire intensity. Climate is a significant factor in determining the species composition of a forest. The type and extent of vegetation ultimately determine the intensity and extent of a fire. The cool, moist climate of the Cascades has a relatively long fire return interval because high humidity and high foliar moisture decrease fire intensity. Many of the dominant tree species adapted to the Cascades are very fire sensitive, which increases the severity of even low intensity fires. At higher elevations, the colder, harsher environments limit the extent of vegetation, which limits the extent of fire spread. Harsher environments also influence the pattern of vegetation reestablishment following a fire. It can take centuries for trees to reestablish a burn site in the higher elevations of the Cascades. Some species will reestablish relatively quickly in the moister climate of the West Cascades versus the drier environments of the East Cascades. At the higher elevations, a persistent snowpack shortens summer drought and favors Pacific silver fir seedlings, which can shed the accumulated litter in the melting snowpack much better than western hemlock seedlings (Thornburgh 1969). A defining characteristic of a fire regime is the season fires typically occur. The overriding determinant of fire season is climate. The season of burn is one of the most important factors determining the fire effects on a plant (Whelan 1995). Summer burns will generally be more intense due to higher temperatures and lower moisture. A fire in the spring will generally be more damaging than a burn in other seasons due to the damage to buds and new foliage. The season and climate pattern also influence the natural pattern of ignition. An area that has frequent dry lightening storms in the summer months will have a higher frequency fire regime than an area with either less lightening or higher precipitation. Climate change is not an insignificant factor in terms of fire's role in natural landscapes. The forests of the Cascades, with their longer fire return intervals, will have return intervals interrupted by climate shifts more so than forests with shorter return intervals. Climate shifts can potentially interrupt any sort of "regular" fire return interval because the intervals happen on a time scale of centuries (Agee 1993). Increased stress on plants due to higher temperatures and possible drought conditions will increase fire caused mortality. As vegetation communities are altered in both structure and composition, the role fire plays in those communities will also change (Neilson 1992). The Role of Topography and Elevation Similar to climate, topography shapes the vegetation (i.e. fuel) available for fire to burn in. Differences in tree density and species composition on north/west versus south/east slopes define differences in fire intensity and species mortality. However, perhaps more significantly, topography is a powerful determinant of fire behavior and hence fire intensity. Steeper slopes and warmer, drier south aspects increase fire intensity. At high elevations, south facing slopes are much more likely to burn than north facing slopes (Agee and Smith 1984). Topographic features that influence the structure and distribution of forest communities can also act as natural fire barriers, limiting the extent of fires. Vegetation on broad slopes will burn more completely than slopes that contain ridgelines, water courses, or cliffs that interrupt fire spread. Sparsely vegetated areas such as talus slopes, avalanche tracks, and ridge tops can slow or stop fire spread all together (Everett 1999, Suffling 1993; Brown and Sieg 1996). Just as topography influences the pre-fire composition of forest communities, it influences the pattern of plant reestablishment following a fire. The heat of a south aspect or the exposure of high elevations can significantly limit the ability of certain species to recolonize a site following a fire, allowing species such as Douglas-fir to colonize a site before competition from other species becomes too great. Conversely, colder, wetter drainages favor Engelmann spruce regeneration on the eastside of the Cascades (Agee 1993, Everett 1999). The effects of landform and vegetation composition are more pronounced in low and moderate intensity fires than in high intensity fires. High intensity fires may be driven more by weather conditions than topographic and vegetation characteristics (Swanson 1978). Other Disturbances Due to the infrequency of fire in the forest types of the Cascades, other disturbances are important in shaping the fire regime of these areas. Any disturbance that causes mortality will increase fuel loads and hence the flammability of a stand. Pathogens such as white pine blister rust and spruce budworm create large patches of dead trees. Trees that are weakened or scarred by fire are often susceptible to insect or pathogen attacks. Wind throw, avalanches, and large floods can also increase the amount of dead ground fuels, significantly increasing fire intensity (Agee 1993). Human Impacts Through both introduction of and exclusion of fire, humans have altered the forests of the Cascade Mountains. Our understanding of the historical presence of fire comes from studies of vegetation, fire scars on trees, soil charcoal layers, the even-aged character of some of our forests, and recorded historical accounts. The long fire return intervals and stand replacing fires characteristic of the Cascade Mountains make it difficult to construct accurate fire histories. The role of Native American burning on the west side of the Cascades is unclear. It is also unclear exactly how much of an impact humans have actually had in altering fire regimes in the Cascades. However, as our understanding of ecological processes grows, we are finding more and more that many natural systems are dependent upon fire and are significantly altered by its exclusion (Agee 1993, Hessburg 2003). Increasingly, fire is being used as a tool to restore and maintain species and structural diversity that has been lost through fire exclusion. The rare-event conditions and large fires typical of the Cascades is a substantial obstacle to the reintroduction of fire to the landscape. Prescribed fir in these forest types is generally not a viable management option as adequate spread can not be attained under controllable conditions. Fires will generally increase fuel loading, since the majority of species in this forest type are highly sensitive to fires and high mortality with little consumption is a typical outcome of prescribed fires. Additional challenges include the increasing fragmentation of forests and the small size of many preserves, the increasing wildland-urban interface, and protection of endangered species habitat (e.g. the northern spotted owl) (Agee 1993). The question is often posed as to whether or not we can replace fire through mechanical or other treatments. If fire is viewed as a one dimensional, solitary disturbance agent, then the answer may be yes. However, fire, like other forest processes and disturbances, is part of a complex ecological web and is itself a complex process that is not fully understood. It is generally accepted that continued exclusion of fire from the forests of the Cascade Mountains will result in altered fire regimes and forest communities. Works Cited
|