for 426 fire management and ecology - university of idaho...random event, it is driven by the fuels...

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FOR 426 Fire Management and Ecology

Ecology and Fire Ecology of Ponderosa Pine Forests

Chad Hoffman, Penny Morgan, and Leigh Lentile

This presentation focuses on the fire ecology and fire management of ponderosa pine forest ecosystems, as they were historically, and how they are now. We have chosen to include ponderosa pine in the course because it is the focus of so much fire ecology work and so much discussion about fuels management, because it is an example of an ecosystem that once burned very frequently (and many of the ideas can be applied to other long-needled pine ecosystems around the world that also burned frequently), and because there are many different well-documented efforts to restore native fire regimes and forest structure and composition in ponderosa pine forests.

We hope you’ll enjoy learning about it, and that you’ll finish with more questions than when you began – more thoughtful and more in-depth –

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Ponderosa pine (Pinus ponderosa)Ponderosa pine (Pinus ponderosa)

Dave Powell, USDA Forest Service,

www.forestryimages.orgTerry Spivey, USDA Forest Service, www.forestryimages.org

On the best sites, ponderosa pine trees can reach heights of > 100 feet, and diameters up to 3 feet, but sizes varies greatly with site conditions, stand history, age of the tree, and other factors. Mature boles are ordinarily symmetrical and clear for one half or more of their lengths, short conical or flat toped crowns are characteristic of older trees. This species can grow on dry sites in combination with junipers. However its maximum development tends to occur on relatively moist well-drained sites. It can be found growing in soils developed from igneous, metamorphic, and sedimentary rocks. Soils in ponderosa pine forests vary in texture, pH, nutrient levels, moisture holding and release capabilities, compactness, depth and other characteristics. The bark is brown to black and deeply furrowed on young trees and yellowish brown to cinnamon red on older trees. Large trees also tend to have a vanilla odor.

The tree is generally classified as shade intolerant, with seedlings being able to germinate under a canopy or in full sunlight as long as moisture and temperature are adequate. Growth rates are highly variable. Ponderosa pine trees commonly live for 150 to 300 years, while some individuals can survive much longer.

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Range of ponderosa pineRange of ponderosa pine

• Extensive area• Diverse soils, plant

associates, and environmental conditions

• Great genetic variation, including 2 varieties and several races

from D. Little. 1966. Trees of North America

Ponderosa pine forests are very widespread through western US and into Mexico and Canada. This tree ranges on elevations from sea level up to 9900 feet, and can be found in pure stands or in mixed conifer stands.

These forests are diverse to say the least. They are found on many different parent materials and soils as I just mentioned, though almost always on sites with well-drained soils. This species can also withstand significant drought conditions.

Ponderosa pine is found growing with many different species of trees, shrubs, grasses, and forbs. There are at least 17 ponderosa plant habitat type identified in the southwestern US and many more habitat types in which the tree can be found. Here in Idaho there are another 4 ponderosa pine habitat types identified with a host of other types in which it can be found.

As one might expect these forest support many different wildlife species as well. Ponderosa pine is genetically diverse within stands and across the range of this species. There are 2 varieties of ponderosa pine and several races which have been identified. Pinus ponderosa variety ponderosa is found in the mountains of the Pacific Coast from southern British Columbia to southern California and western Nevada, and variety scopulorum is found in the rocky mountains from southwestern North Dakota, Montana, and Idaho south to Arizona, New Mexico, western Texas and into Mexico.

In many parts of its range, ponderosa pine is an important commercial timber

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BiodiversityBiodiversity

• Ponderosa pine forests provide habitat for many animals (at least 250 species of vertebrates), plants, invertebrates, and microbes

• Many rare, sensitive and declining species, e.g. northern goshawk and flammulated owl

• Habitat alteration and fragmentation affects invertebrates and soil organisms that are critical to ecosystem function

In Idaho, ponderosa pine forests are one of the most important vegetation types for neotropical migrant birds. As many as 250 different kinds of vertebrates live at least part of their lives in ponderosa pine forests. We probably don’t even know the name of many of the plant, invertebrate and microbial species that inhabit ponderosa pine forests. In multiple areas, particularly in the southwestern US, special management guidelines have been developed for ponderosa pine forests that restrict management options to help protect the many rare and sensitive species that

It is likely that the degree to which ponderosa pine forests have been altered by land use, including roads, fire exclusion, and logging, has altered their value as habitat for critters and plants as well as changed the fire regime. We’ll talk more about this later.

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Ponderosa pine forests are shaped byPonderosa pine forests are shaped by

• Frequent fires• Episodes of tree

regeneration• Insect and pathogen

infestations• Regional climatic

events, such as droughts

• Human use

Ponderosa pine forests have been shaped by many interacting processes and disturbances. In general, ponderosa pine forests have historically been shaped by frequent fires, episodes of high tree germination driven by regional climate, and the interaction of insects and pathogens. Other disturbance factors such as human use have since become major agents of change in these forests.

Historically, many ponderosa pine stands were open, with park-like conditions with a vigorous and abundant herbaceous under-story. Although many of us are familiar with this description, some parts of some ponderosa pine forests were dense (See Woolsey 1911). Prior to Euro-American settlement, mosaic of conditions were found in ponderosa pine forests, with large areas of open park-like conditions and areas with dense younger ponderosa pine stands, and on many sites a mix of other trees. This view is supported in part due to the regeneration tendencies of ponderosa pine which regenerates episodically, with an overabundance of seedlings followed by a high rate of juvenile mortality. The large patches of yellow-barked ponderosa pines were most likely survivors which emerged from these dense patches that were established during the rare episodes of reproduction.

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Stand development Stand development

Terry Spivey, USDA Forest Service, www.forestryimages.org

Dave Powell, USDA Forest Service, www.forestryimages.org

Open stands of ponderosa pine were capable of supporting a productive, diverse understory. Studies have reported that these stands could produce between 200 to 300 pounds per acre of herbaceous materials and that production could exceed 1000 pounds per acre, but this no doubt varied greatly with tree density, site conditions, disturbance history, and location. High levels of production were often a result of frequent surface fires which increased nutrient cycling and reduced competition from trees. Needle cast and litter from the previous year’s growth formed a highly flammable fuel bed that dried out quickly in the spring and readily ignited during a long fire season. Frequent fires killed small trees, promoting the open park-like conditions, with many grasses, forbs, and shrubs surviving. Large trees often survived surface fires. With their thick bark, and high, open crowns, large ponderosa pine trees are very resistant to surface fire unless there fuels have accumulated.

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Fire ecology of ponderosa pineFire ecology of ponderosa pine

• Historically, fires were frequent (every 2-30 yr) and predominantly nonlethal

Dave Powell, USDA Forest Service,

www.forestryimages.org

Many of the fire history studies that have been done are from ponderosa pine forests because the fire-scarred stumps, logs and standing trees are very resinous. Sometimes we find only the fire-scarred face of old stumps in the forest – all the sap wood and other parts of the wood have rotted away. We know from such studies that fires were frequent, sometimes scarring the same tree multiple times.

The range in fire frequency is in general from 2 to 30 years, but most of the fire history studies suggest that fires occurred every 5-15 years.

Early fire history work from the southwestern US reported fire return intervals of between 4 and 12 years (Weaver 1951), while more recent studies (e.g. Swetnam 1990 and Kitzberger et al. 2006) have found the mean return interval to range between 1.9 and 6.4 years. Studies in the Pacific Northwest have found fire return intervals ranging from 11 to 16 years (Weaver 1959), to 3 to 36 years (Soeriaatmadja 1966). We can conclude from these studies and many other studies that fires occurred wherever ponderosa pine was found, and that fires were relatively frequent. The severity of fires was likely highly variable depending on stand structure and burning conditions. In all fires, there were patches that burned with stand-replacing effects, and in some years those patches may have been large. Unfortunately, we cannot infer fire severity from the fire history studies.

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Fire frequency and environmental gradientsFire frequency and environmental gradients

The historical mean fire return interval likely varied across environmental gradients. Fires were relatively more frequent in dry ponderosa pine forests than in mixed-conifer forests at higher elevations and in comparison to the sagebrush steppe vegetation at lower elevations. This pattern is not a random event, it is driven by the fuels and climatic conditions of ponderosa pine forests.

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People have long used warm, dry forestsPeople have long used warm, dry forests

• Indians extensively used ponderosa pine forests

• Forests were homes, a source of food for people and animals, and many sites were culturally important

• Euro-Americans logged, grazed, and mined these forests

Indians peeled the bark from this tree to reach the inner bark which they used for food and ceremonies

Photo by Penny Morgan, S. Fork of Salmon River, Idaho

Pinus ponderosa forests are generally at the edge of grasslands and have been utilized for many purposes. The picture you see here is a ponderosa pine tree peeled by Native Americans who harvested the inner bark for food, medicinal and ceremonial purposes.

Long before Euro-Americans, Native Americans were using ponderosa pine forests. They lived in them, hunted and gathered in them, sometimes grew crops in them, and otherwise made these forests important to their culture. Euro-Americans have logged, grazed, mined, and used these forests intensively since they first settled in the western US. Even today, there are many contentious battles over logging, land use, fuels management, and wildlife habitat fought over ponderosa pine forests. The large-diameter, ponderosa pine trees are especially valued as timber, habitat for many different birds, spiritual values, and others important ecological and cultural values. With the heavy use by Native Americans and by Euro-Americans, fires have often been ignited by humans, some on purpose and others not.

Ignition does not seem to be the limiting factor in this ecosystem. Certainly, Native Americans used fires and did so in ponderosa pine ecosystems. Yet, ponderosa pine forests also often experience enough lightning to explain the frequency of fires recorded in fire scar records. This sets up one of the challenges to interpreting historical fire frequency. Should we consider the fires ignited by Native Americans and other people natural? For most ponderosa pine forests, we cannot know the fire frequency in the absence of people, and the frequency of fires likely varied through time as people’s attitudes about fire and use of ponderosa pine forests changed.

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Changing ecological processes & conditions Changing ecological processes & conditions

Figure adapted from: An assessment of forest ecosystem health in the southwest: USDA Forest Service RM-GTR-295

One of the earliest changes in ponderosa pine forests occurred as a result of the introduction of domestic livestock. The effect of grazing on ponderosa pine forests is particularly evident in the southwestern US where by 1880, cattle herds were estimated at 172,000. Reports from western rangelands indicated that over-grazing was depleting the range conditions. By 1890, cattle numbers had increased to more than 1.5 million head and an additionally large number of sheep were being grazed. In 1893, the governor of Arizona said that nearly all districts had an increase in weedy species over high quality grasses as a result of over-grazing (Baker 1988). This pattern continued and by 1912 it is reported that livestock pressure had reached the most remote, timbered and mountainous regions.

Although grazing and browsing pressure has changed over time, it has directly and indirectly affected ponderosa pine forests. Where grazing was very intensive, cattle and sheep ate much of the grass that had helped to carry fires through ponderosa pine forests. This explains why fire frequency often decreased before there was effective, organized fire suppression. Note, however, that fire suppression also started early in ponderosa pine forests, as soldiers and early forest rangers sought to put out fires.

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Changing ecological processes & conditionsChanging ecological processes & conditions

With the reduction of understory vegetation by VERY intensive grazing by livestock and an increase in the number of roads and trails in ponderosa pine forests, fuels became much less abundant and much less continuous, so fires spread less readily. Fires were less extensive and less frequent in the mid- to late 1900s than they were prior to 1850. This native, characteristic fire regime was disrupted by fire exclusion (fire suppression, grazing, logging, roads, settling of the valleys and other land uses). In many areas fuels have accumulated where fires have been less frequent. The build up of fuels on the forest floor further altered the natural fire regime. Logging sometimes added heavy fuels to the mix in the form of limbs, tree tops and cull logs, especially where large trees were preferentially removed and the stands of younger trees weren’t thinned.

The disruption of the natural fire regime in ponderosa pine forests has decreased the diversity of stands across the landscape and allowed the establishment of young trees under the older stands, providing ladder fuels which can carry fire into the canopies of the large, old trees. Once their crowns burn, even old, large veterans of multiple fires will die.

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Fire exclusionFire exclusion

• When was the last surface fire that scarred this tree?

• What was the average number of years between fires that scarred this tree prior to the last extensive surface fire?

Partial cross-section from a fire-scarred tree from Arizona, Photo courtesy of Tom Swetnam

The effects of grazing, fire exclusion and other factors on the fire regime are seen on many fire-scarred trees. Generally, we see frequent fires until the late 1800s and than a decrease in the frequency of fires. The picture shown here is from an area in Arizona where fires were very frequent. After 1898 there are no fires recorded in fire scars on this tree. The disruption of the fire regime is obvious.

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Forest structure has changedForest structure has changed

• Fewer large trees and snags –these are ecologically, economically, and socially more important than small trees

• More trees that are less fire resistant

• Unnaturally dense stands of suppressed young trees now threaten the remaining large trees through competition and by fueling crown fires

• Degree of forest change varies greatly from place to place

As a result of fire exclusion (not just fire suppression!) many forests once dominated by ponderosa pine have changed in structure and composition. One of the most obvious changes is that there are fewer large (>16” DBH) trees and snags, and it is the larger trees that are often the source of conflicts about whether they should be cut or not. Also we see that there are more trees present today than in the past. The increase in density is expressed in several ways:

First, the increase in tree density produces more shade which impacts the abundance and diversity of understory plants.

Second, since most of the increase in in density is in smaller trees, there is an increase in ladder fuels

Third, increased tree density lowers tree vigor and allows for more successful bark beetle attacks

Fourth, the dense multistoried stands provide suitable conditions for rapid spread and intensification of dwarf mistletoes

Lastly, the increased density results in lower water yields which has a negative effect on riparian areas

Be very aware that although there has been an increase in stand density, number of small trees, and increased proportion of shade-tolerant tree species in ponderosa pine forests overall, the degree to which this has happened varies greatly from one location to another even within a given watershed. Also, it is much easier to reconstruct whether large trees were present in an area historically, but much more difficult to reconstruct how many small trees were present.

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Ecosystem composition & function changesEcosystem composition & function changes

• Old-growth is relatively rare• Forest structure is more

homogeneous & tree vigor is reduced

• Lower native plant and animal abundance and diversity

• Stagnant nutrient cycles• Fewer meadows• More stand-replacing fires

The effects of these changes in tree density have indirectly affected other emergent properties of ponderosa pine forests. For example, old growth stands are believed to have declined in extent throughout the range of ponderosa pine since the start of the 20th century. In addition, the forest structure has become more homogenous and tree vigor has been reduced primarily through competition. These changes, as well as lower native plant and animal diversity and abundance, stagnant nutrient cycles, forest intrusion into meadows, and a change in fire behavior and fire regimes all point to the importance of our alterations to this ecosystem. Wally Covington and others feel that there is a forest health crisis in ponderosa pine forests.

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Fuels accumulate when fires are less frequentFuels accumulate when fires are less frequent

• Fuels accumulate on the forest floor (as duff, litter, woody debris) and in the crowns of trees

• Increased crown fuel loading • Fuels are more continuous

horizontally• Fuels are more continuous

vertically• Fire size and intensity

increases• Crown fires are more likelyNot to the same degree everywhere!

So, at this point we should discuss what exactly is driving the shift from a fire regime of frequent, mostly low-intensity fires to a regime of relatively infrequent fires that are more likely to be stand-replacing over large areas. The key change that has affected this shift is changes in forest fuels. Specifically, with fewer fires, surface and canopy fuels have increased above levels found historically. In many stands, scientists think that canopy fuels are more continuous and that the canopy base heights are lower than they were historically. Evidence comes from modeling, fire history studies, comparing historical to current conditions as documented by observers and as reconstructed from stand conditions.

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Changes in canopy fuelsChanges in canopy fuels

Figure adopted form Fule et al. 2004

To emphasize how forest fuels have changed over time we will look at work that was done at the North Rim of the Grand Canyon by Fule et al. (2004). In this paper, the authors used forest reconstruction methods and simulated forest stand growth for a 160-year period beginning in 1880 and ending in 2040. The simulations were judged reasonably accurate based on a comparison of model predictions with field data collected in 2000.

The results show that tree canopy biomass increased on all sites, rising an average of 122% at lower elevation ponderosa pine sites and 279% at higher elevations mixed-conifer sites. The canopy bulk density of the stands increased at approximately the same rate. Potential crown fire behavior was modeled in relative terms using the crowning index as a metric. The crowning index is the wind speed needed to support a crown fire through the forest canopy. Crowning index decreased from between 23 to 80% over the model period.

At a threshold wind speed of 45 km/hr only 6% of the landscape was susceptible to crown fire in 1880, but 33% was susceptible by 2000. Thus, in relative terms, it appears that the potential for crown fire has increased by about 25% across this landscape. It is important to remember that the changes shown here are not going to be the same everywhere, but they do provide some indication to the processes that explain the changes in fire behavior that have been experienced since 1880 in many forests once dominated by ponderosa pine.

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Disturbances affectDisturbances affect

• Species composition• Forest structure• Ecosystem function

Other disturbance agents such as bark beetles and pathogens have long interacted with fire in ponderosa pine forests. These disturbance agents influence forest species composition, structure and function. Primarily, these disturbance agents have acted on fine, local scales, thus creating gaps within the tree canopy and increasing heterogeneity in forest structure. However, these other disturbance agents also interact with fire. This interaction is a two-way street with fire influencing the population levels of the other disturbance agents, and insects and diseases influencing fuel abundance and distributions, and ultimately fire behavior. In ponderosa pine forests, two of the major disturbance agents are bark beetles and dwarf mistletoe. It is thought that changes in forest structure in ponderosa pine forests have increased the susceptibility of trees to bark beetle attacks and that the continues canopy has allowed dwarf mistletoes to spread at a increased rate. Therefore any affects these disturbance agents may have might have on fire behavior may be increased compared to pre-settlement conditions.

Note: we really do not have a good understanding of the role dwarf mistletoes or brake beetles played din pre-settlement forests, but it is assumed that they have increased.

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Fire and bark beetlesFire and bark beetles

• Post-fire mortality and bark beetles• Beetle-caused mortality can influence the

risk of severe wildfires

Darren Blackford, USDA Forest Service,

www.forestryimages.org

Mark McGregor, USDA Forest Service, www.forestryimages.org

Let’s now discuss the interaction of bark beetles and fire in ponderosa pine forests. We will start by discussing the role of bark beetles in post-fire mortality of ponderosa pine trees and then discuss the effects of bark beetle mortality on fuels and fire behavior.

There are many studies of the influence of post-fire insect attack on tree mortality. It is commonly thought that some bark beetle species, particularly Dendroctonus species and Ips species will invade burned trees and then infest nearby trees, causing mortality. In one study of physiological responses, trees with high levels of crown scorch were more susceptible to insect attacks due to lower resin availability and production (Wallin et al. 2003). Insect attacks after fire can affect tree mortality (McHugh and Kolb 2003). However, it is not clear from these and other studies and others how bark beetle activity influences post-fire survival of trees because results are highly variable. For a more detailed discussion, please see Wallin et al. (2003).

Another common belief is that insect outbreaks set the stage for severe forest fires. However, there is little scientific proof that this actually occurs. Of the few studies that have been conducted, only a few report any effect of beetle infestation on fire behavior, and only a small effect at that. It is possible that insect activity could decrease or increase uncharacteristic wildfire behavior depending on how the forest insects alter the amount, distribution and structure of the fuels complex. The effect of the beetle outbreak on fire behavior is likely a function of the time since outbreak and might follow something like the pattern explained below.

Immediately following a bark-beetle outbreak, there may be an increase in the probability of crown fire initiation due to lower canopy fuel moisture as long as the dead needles remain in the tree crowns. However, there is likely no change in the potential for fires to spread from one tree canopy to the next. Once the needles fall off the trees, the likelihood of crown fire initiation and spread may actually decrease, since the dead trees create gaps in the forest canopy. And finally after the snags fall to the ground, crown fire initiation and spread may once again increase. This could occur through two mechanisms. First, if fires occur, the increase in surface fuels could contribute to a more intense wildfire. Second, an increase in the number of small trees will result in ladder fuels and a more continuous forested canopy. Again, this scenario is only theoretical in nature and more studies need to be done to make more conclusive statements about the affects of beetle infestation on stand and landscape-level fire behavior.

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Fire and pathogensFire and pathogens

Figure Adapted from Hoffman et al. 2007

One of the most important forest pathogens in ponderosa pine forests is dwarf mistletoe. Current estimates suggest that this pathogens affects over 1 million hectares of ponderosa pine stands in the western US. Much like bark beetles, it has been widely believed that this pathogen increases fuel loadings and affects fire behavior. Hoffman (2005) looked at the effects of dwarf mistletoe on fuels and fire behavior in ponderosa pine stands and found that surface fuel loadings tend to be about 4 times higher in stands severely-infested with dwarf mistletoe when compared to uninfected stands. However, he did not find any significant differences in canopy fuels, although the amount of canopy fuels decreased as dwarf mistletoe infection rates increased. The figure shown here shows the average surface fuel loadings by dwarf mistletoe infestation class for 12 stands in northern Arizona.

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Fire BehaviorFire Behavior

Figure Adapted from Hoffman et al. 2007

The table shown here gives the predicted fire type, rate of spread, fire line intensity, flame length, torching index and crowning index for 4 levels of mistletoe infestation under the 97.5th percentile weather conditions. Lets focus on two aspects of fire behavior. First notice that there were no statistical differences in the crowning index between the infestation classes. This implies that crown fire can spread through all infestation classes at the same relative ease. However when we look at the torching index we see that it decreases as infestation levels increase. This implies that severely infested stands will allow a surface fire to transition into the crown with less wind speed than non-infested stands (relatively speaking).

Another important interaction of dwarf mistletoe and fire is the ability of fire to act as a controlling agent of the pathogen. It is thought that since pre-European settlement the severity of infection by dwarf mistletoes has increased. This is due to changes in stand conditions (more dense and multi-layered stands) and a decrease in fire-caused mortality of the dwarf mistletoe plants growing in ponderosa pine tree crowns. If increased severity of dwarf mistletoe infestations have increased the probability of experiencing uncharacteristic fires in ponderosa pine stands, it is likely that the effect has been indirectly cause by changes in the fire regime of this system.

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Forests have become less sustainable thruForests have become less sustainable thru

• Fire suppression• Livestock grazing• Logging, especially of

bigger trees and pines• Road construction• Predator control• Exotic species

introductions

In retrospect, it is clear that fire suppression, grazing, logging, road construction, predator control and the introduction of exotic species have lead to great changes in ponderosa pine forest ecosystems. While these changes have not occurred to the same degree everywhere, all ponderosa pine forests have been affected to some degree. These changes have lead many to speculate that the current conditions are not sustainable. In light of such possibilities, many have argued that we should seek to do active forest management and restoration to help increase the long-term sustainability of ponderosa pine forests. In our next lecture, we will discuss this very idea. Specifically, we will talk about fuels treatments and forest restoration, but along the way we will briefly cover other topics such as prescribed fire and forest policy.

Between now and the next lecture, we would like you to think about other ecosystems which share a similar historical fire regime to ponderosa pine and think about what factors have affected those ecosystems. What is similar about fire effects and changes in fire regimes? Can you generalize to other long-needled pine forests?

for 426 fire management and ecology - university of idaho...random event, it is driven by the fuels...

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