Climate change and the effects pine beetles have on the forests.

Introduction

The ever growing effects of natural and man-made climate change are having a wide-spread effect on many mixed and coniferous forest ecosystems. Particularly as average annual temperatures increase, the habitat for the mountain pine beetle, Dendroctonus ponderosa, has expanded, because the beetle is able to occupy new habitats at higher elevations, which were previously too cold for it. The mountain pine beetle habitat extends from Mexico through the Western United States and up to Canada. The damage caused to cone-bearing trees by the mountain pine beetle is threatening the strength of the North American mixed and coniferous ecosystems at every trophic level, as well as affecting the mountain forest carbon cycle and watershed hydrology. This damage has combined with the effects of human logging practices in the coniferous forest to drastically alter these ecosystems. Though these areas have been severely degraded, there are solutions that can slow or reverse the damage that has already been done.

Background

Human-caused climate change is having a pronounced effect on many different ecosystems. One of these effects is the spread of mountain pine beetles through the forests of the Western United States and Canada. Mountain pine beetles inhabit many species of coniferous trees such as jack pine, whitebark, lodgepole, Scotch, ponderosa, and limber pines. Pine beetles typically attack and inhabit trees at lower elevations, but the effects of climate change have allowed them to inhabit ever higher elevations. With mild winters and warmer summers, the pine beetles have been able to infest mature pine trees that could resist the beetles before the change in average temperature (Carroll et al. 2003). There were many pine beetle outbreaks in Canada and the U.S. Rockies since the 1940’s but they are considered mild compared to the more recent outbreaks. The earlier outbreaks were partly contained by human intervention and were ultimately ended by severe winter conditions, the natural regulator of pine beetle populations. As climate change undermines this natural regulation system, the current infestations are much larger in scope and much harder to contain through human intervention (Ono. 2003). Mountain pine beetles spend their life cycle in four stages: egg, larva, pupa, and adult. In the summer the adults migrate and infest new trees. During the stages of their life they live under the tree bark where they feed and lay their eggs (Carroll et al. 2003). The mountain pine beetles’ life cycle and survival rate is regulated by the temperature of their habitat. Temperature at a particular time of year helps regulate the life cycle stage of the beetles, determining in part when eggs are laid, when the pupas become adults, when they migrate, and ultimately when the beetles die. A rapid decrease in temperature can kill an entire population of beetles. Other factors for survival include the dryness of the tree and the amount of snow insulation, but mainly the change (drop) in temperature is what keeps beetle populations in control (Creeden et al. 2013).

Effects on the Ecosystem

Changing climate and weather conditions in the higher elevations and their forest ecosystems are expanding the survivable habitat for the mountain pine beetle. The climate change occurring in these areas is weakening the overall strength of mixed and coniferous forest ecosystems. Hotter summers with less rainfall have caused many tree species to lose their natural defenses. Drought conditions have long been an indicator of previous outbreaks and are now a predictor of new pine beetle outbreaks. In turn, extended drought can also have a negative effect on the pine beetles if the drought lasts too long and the number of available hosts drops. Rising temperatures creates tree loss from drought and fires that hinders pine beetle migration (Creeden et al. 2013). At the same time, climate change exposes beetle infested conifers to a greater risk of fire and drought, reduces their resistance to both, and threatens both the beetles and the trees. Climate change is not only affecting the pine beetles, but species at every trophic level. One of the biggest food sources in the forest is new tree growth and seeds from mature trees. With changing climate conditions trees are not able to reproduce at the pre-climate-change rates. This is especially true in drought areas where the mountain pine beetle has infested the trees. The pine beetles weaken and destroy trees and reduce the rate of seed production and new tree growth, which affects small mammals such as squirrels that rely on the seeds pines produce. Decreasing food supply for smaller animals affects the population size of these species, which in turn affects their larger predators on higher trophic levels (Bartos et al. 1990). Climate change and the spread of pine beetle habitat are having such large effects on mountain ecosystems that it can change the diversity of species in the forests. The mountain pine beetle is no longer just an important species in the forest ecosystems they normally in habitat, they have become an important indicator species for ecological problems areas they normally do not inhabit. The expansion of their habitat demonstrates that climate change is having an effect on the entire ecosystem. Moreover, climate change is having an immediate effect on biodiversity and the ecosystem of conifer forests, which is increasing and accelerating (Logan et al. 2003). This is why it is important to keep track of the health of the ecosystem and make quick decisions when detrimental changes are observed.

Effects on the Carbon Cycle

Climate change and pine beetle outbreaks are also having an adverse effect on the carbon feedback cycle. Currently in British Columbia, Canada the outbreak of mountain pine beetles is so large that Canadian conifer forests have turned from carbon sinks to sources of carbon. Because of the combined effects of mountain pine beetle infestation, logging, and forest fires, large sections of forest are being damaged, which increases the amount of carbon dioxide. It is estimated that 435 million trees have been lost to the combined effects of climate change. This is having a big economic effect on the timber industry. The industry has responded by increasing the rate of harvesting and moving into areas that have not previously been logged. (Kurz et al. 2008). In these area loggers are stripping the biomass of the forest and turning it into, among other things, wood pellets as an energy source for Europe. Combining the damage from the mountain pine beetles, increased forest fires, drought, and all the commercial uses, these forests will release more carbon dioxide than they absorbs. This will increase both the causes and effects of climate change and worsen the situation years from now (Lamers et al. 2014). Kurtz modeled the effects of the mountain pine beetle in a test area of 374,000 km² during the years 2000 to 2020. The study estimated that 270 megatons of carbon would be released during the pine beetle outbreaks in the test area. The model showed that if an area was untouched by pine beetle infestation but had moderate timber harvesting and fires, then the test area was a slight carbon sink; in the control scenario the area averaged 1 to 5 megatons of carbon release per year between 2007 and 2020. Two test scenarios were modeled, one in which the forest was infested with mountain pine beetles and one that included both infestation and additional timber harvesting. The scenario with mountain pine beetle infestation and normal harvesting showed the forest to be a net carbon source ranging from 10 to 20 megatons of carbon per year during 2007-2020. The scenario with the section of forest infested with mountain pine beetles and having additional harvesting due to timber damage showed that the forest was a net source ranging from 10 to 25 megatons of carbon per year for the years 2007 to 2020 (Kurz et al. 2008). This model shows that in the forests of British Columbia mountain pine beetle infestation combines with natural disasters and timber harvesting to help drive climate change (See fig 1).

Effects on Watershed Hydrology

Increased destruction of forests by the mountain pine beetle is causing a large effect on the hydrology of pines forests and the watershed. The increase in dead trees is having an effect on evapotranspiration as less of the sunlight evaporates water from live trees and instead heats the surrounding surface, raising local temperatures. This is turn is causing a change in hydrology locally and in areas downstream. The damage to the trees is also having an effect on the water quality and the biochemistry of the area (Mikkelson et al. 2013). The snow packs are also being affected; there has been an increase in canopy transmittance and a decrease in the amount of snow that is stored in the canopy. As more trees die solar radiation has been able to penetrate farther causing an increase in evaporation and a change in albedo (Winkler et al. 2012). Once an area of forest is affected, it takes several years for the changes in the canopy cover to effect a complete change. It takes an average of two to three years for the needles of the pine trees to change from green to red. During this time only a small portion of the canopy cover is lost and results in only a small change in interception. A few years after the needles turn red is when the trees turns gray and the majority of the needles are lost to the forest floor. During this stage pine needles and branches fall to the forest floor and eventually the entire tree falls and decays. This increases the amount of nutrients in the soil such as carbon, phosphorus, and nitrogen, which leads to nitrification of the water supply (Mikkelson et al. 2013). These changes alone will cause noticeable changes in the quantity and quality of the water. Adding more numerous forest fires and increased timber harvesting can have a drastic effect on the local watershed (See Fig. 2). Rita Winkler and her colleagues studied the effects of snow accumulation, forest structure, and snow surface albedo in the Rocky Mountains after a mountain pine beetle infestation. Over a seven-year period they studied these effects in areas that were clear cut, mixed, or young pine forests. Their study showed that areas that were primarily infested young pines completely lost their canopy within six years. Due to the loss of their canopy the snow accumulation decreased while transmittance and snow surface albedo increased when trees turned from green to gray. The study found that as trees turned from green to gray the rate of snow water melt increased. In areas that had mixed species of trees the effects were not as drastic as the areas with only young pines. The primary reason that snow accumulation and surface snow albedo were not as affected was due to a large diversity of tree species. In these areas there was also a more developed understory that reduced the effects seen in areas that were primarily young pine. Moreover, while the effects increased from mixed species areas to young pine areas, the affects in neither area compared to the far greater affects in areas that were clear cut (Winkler et al. 2012). It is easy to see from this study that the type of forest, level of beetle infestation, and timber harvesting practices will have a significant or large effect on the water cycle and the hydrology of the surrounding watershed. In the Rocky Mountains this can have a particularly extreme effect on the quality and quantity of river flow in this region since the Colorado River is supplied largely by snow melt. If this trend continues, the amount of water coming out of the Colorado River will decrease, which could lead to increased water shortages in the Southwest.

Managing Mountain Pine Beetle Outbreaks

As the population and habitat of the mountain pine beetle continues to expand several types of management practices have been tried. Currently Western Canada is experiencing one of the largest expansions of mountain pine beetles, with estimates that over 13 million hectares of conifer and mixed forest have been affected. There are two main methods to manage pine beetle outbreaks, or at least slow their expansion. The indirect control is a preventative method that tries to limit host trees through prescribed burning and forest thinning. The direct method tries to limit the population and growth rate of the beetle by destroying infected trees before the beetles emerge to migrate and attack new hosts (Wulder et al. 2009). There have also been studies done using chemicals on non-infected trees to try and limit the expansion of beetles to these areas. While this type of managing technique is effective, it is too costly to use on a large area of affected trees (Fettig et al. 2007). Coggins and his colleagues completed a study in Western Canada to test which management practices were the most effective. In their study, they used two different areas and selected 28 sites with each plot of trees having a radius of 30 meters. Each plot was selected due to the age of the trees, elevation, and the severity of beetle infestation. They broke their plots into two groups. Eighteen of the plots were not managed while management practices were implemented on the other ten plots. For the ten plots that were selected for mitigation they used tree removal techniques to remove the infected trees before beetle migration. In their study, they calculated that at the beginning of their study, the average expansion rate was 0.29 for non-managed plots and 0.12 for managed plots. Over a ten-year period they showed that the plots of unmanaged areas grew exponentially. While during this same period the managed plots went to zero infected trees after ten years with a 43% detection rate. The study found that the time to reach zero infected trees would be shorter if the detection rate was increased (Coggins et al. 2010). It can be concluded that managing practices will have an effect on the migration of the mountain pine beetle. The main problem with controlling their migration is detecting infected green trees. The problem is that they cannot be detected from the sky, so people have to actively go into the field to detect them. This causes a problem because some areas are inaccessible for a variety of reasons. Even with all of the problems associated with managing the mountain pine beetle, it is vital for the forest ecosystems of North America that these practices continue.

Conclusion

While the mountain pine beetles will continue to expand and inhabit new areas management practices need to be put into place. The main cause of the mountain pine beetle expansion is primarily due to climate change, particularly drier, hotter summers and shorter, warmer, drier winters. These insects in turn are also contributing to climate change. As their habitat expands, they are turning forests that were once net carbon sinks into net carbon sources. When infected trees die and decay, they release carbon dioxide and increase the amount of phosphorus and nitrogen in the ground. The damage the beetle is causing to forests is also changing the watershed in negative ways. Both the quality and quantity of water is being degraded, which affects the health of the ecosystem. All of these changes combined are having an extreme effect on the ecosystem and negatively affecting every species in it. At every trophic level there is some kind of effect as the mountain pine beetle expands and destroys the forests. More studies must be done to help mitigate mountain pine beetle expansion and more money needs to be invested in managing them. As a society we will take a major economic hit if the timber we need is destroyed by the mountain pine beetle. Also, if their expansion continues in the Rocky Mountains and damages the watershed there, it could affect a large portion of the drinking water for the Western United States. We need to manage the mountain pine beetle as we reduce carbon dioxide emissions to curb the effects of global warming.

Figure 1

Figure 2

Works Cited

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