Wind
One type of biodisturbance on hillslopes in the Front Range is from wind knocking trees down, i.e., windthrow. Windthrow, or blowdown, is known as an abiotic disturbance which accounts for 11 percent of tree mortality in Colorado (Macdonald and Stednick, 2003). Subalpine zones are more susceptible to windthrow because of poorly drained soils and vegetation patterns that constrict and accelerate wind. Waterlogged soils increase the susceptibility. After a stand develops for more than 300 years the scale and frequency of windthrow steadily increase. Trees with root diseases have an increased susceptibility to windthrow.
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Insect Type
The most common categories of insects found in the Colorado Front Range are bark beetles, defoliators and various pathogens. The table to the right describes the different insect and pathogens, the trees they attack, what makes the trees susceptible and the commonality of attacks.
Bark beetles tend to attack trees with diameters greater than 10 to 20 cm. Attacks by bark beetles are normally lethal from both the beetles’ excavations through the bark, where they lay eggs, and the fungi they carry with them. This results in blockage of water- and nutrient-conducting tissues.
Defoliators lay eggs in the tree buds and the hatched caterpillars eat the leaves in spring and early summer. Four different types of defoliators are described in Table 1, but none of these have common outbreaks in the Front Range.
Pathogens tend to weaken trees and make them more susceptible to other types of attack, e.g., mountain pine beetle.
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History of Outbreaks in the Front Range
Bark beetles are a natural part of the forest ecology and are a constant presence in the ecosystem. The Front Range has a long history of outbreaks for bark beetles and defoliators. Data have been gathered from tree-rings and observations of bark beetle and defoliator attacks in the 19th and 20th century. For many of these insects tree-ring studies have not been conducted in the Front Range pre-20th century (Veblen and Donnegan, 2006).
Tree ring data indicate that Western Spruce Budworm has coexisted with Douglas fir for over 500 years (Veblen and Donnegan, 2006). The current range of WSB outbreaks is not outside the historic range of variability (Veblen and Donnegan, 2006).
In the mid-1970s there were high levels of ponderosa pine mortality from a mountain pine beetle infestation. Populations of mountain pine beetle have been increasing since 1991 across Colorado (Negròn and Popp, 2004).
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Insects and Fire
The relationship between bark beetle outbreaks and fire is complex and the focus of much debate and research. The combination of climate, geography and disturbance history makes it difficult to isolate the cause and effect relationship of fire, fuel and beetles. Few quantitative studies show an increase in fuel quantity and quality following bark beetle outbreak (Romme et al., 2006; Jenkins et al., 2008). Jenkins et al. (2008) found that 20 or more years are needed before the majority of beetle-killed trees fall down. Initially, the fine dead fuels increase for 2 to 3 years after the outbreak in the canopy. Understory shrubs and herbaceous plants may increase, which would decrease the potential for surface fires. In high elevation systems the large amount of fuel present may not represent abnormal conditions. Changes in fuel composition are significant when weather conditions are favorable for fire. The likelihood of crown-fire initiation increases with high fuel loading under moderate fire weather (Jenkins et al., 2008).
Bark beetle outbreaks following fire have been noted in some areas in the intermountain west, but are not common. Many factors contribute to the potential for an outbreak of bark beetle following fires (Jenkins et al., 2008):
1) The stand susceptibility pre- and post- burn.
2) The extent and severity of the fire
3) The population levels of bark beetle in the area prior to the fire.
4) Climatic controls
The interaction of beetles and fire also depend on the forest type. Douglas-fir stands are increasingly susceptible to Douglas fir beetle activity as the stand age increases. Beetle activity may increase after a western spruce budworm outbreak, or after low-intensity surface fires damage trees and make them more susceptible to beetle colonization.
Lodgepole pine stands are most susceptible to mountain pine beetle infestation when they are 60 years or older. They are particularly susceptible at mid-elevations where surface fires are more common. In the absence of fire these stands would then most likely be replaced by Engelmann spruce and subalpine fir at higher elevations and Douglas fir at lower elevations (Jenkins et al., 2008).
Spruce-fir forests have a low frequency of wildfires, with return intervals of 100 years or more. Therefore, spruce-beetles have an important influence on the vegetative structure of spruce-fir forests. After severe outbreaks, it can take several hundred years for a new spruce-dominated community to develop. Once overstory spruce trees are killed, subalpine fir are more likely to dominate the stands for up to 200 years, especially at lower elevations. These trees will eventually succumb to disease or windthrow, which creates gaps where spruce seedlings can re-establish and mature (Jenkins et al., 2008).
Jenkins et al. (2008) found that fine woody fuels increased in epidemic stands of all the above conifer types. The fine woody fuel reached the highest levels immediately following the epidemic. Litter increases as well from needles that fall from the dead trees. The large woody fuel increases with time after an epidemic as beetle-killed trees fall. The fuel loads then decrease again once most of the dead trees are on the ground as a result of decomposition and compaction. As water and sunlight availability increase for the understory, the herbaceous fuel load increases and eventually shrubs begin to grow and out-compete the herbaceous species. Shrub growth does not increase as much in the dry pine sites as in the mesic spruce and Douglas fir sites. As fuel loads change with time, so does the fire behavior. Jenkins et al. (2008) found that initially after the epidemic the spread rate and fireline intensity would decrease and then increase again decades later as fuel composition changes.
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Insects and Drainage Basin
The decreased amount of interception and transpiration from trees can increase the peak flow as well as the water yield to the streams. Studies in Colorado and Wyoming that focused on the impacts of timber harvest on stream channels have been extrapolated to hypothesize what might occur in areas with beetle kill trees. A drainage area has to have greater than 50 cm of precipitation and 15 – 20% of the canopy removed before any measurable increase in streamflow is detected (Romme et al., 2006).
Large differences exist between areas with beetle-killed trees versus clear-cuts: in beetle kills, trees remain standing and continue to intercept precipitation, infiltration rates do not change, all trees are never killed, and re-growth is rapid. A few studies have investigated increases in streamflow following beetle attacks. Bethlalmy (1974) showed that streamflow increased over a 25 year interval. The impact of this increase on stream channels is unknown. The potential changes in the channel are bank instability, change in grain size, change in amount of wood, and in extreme cases channel incision. It is possible that the increase in streamflow is not adequate to cause any changes in a stream channel.
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