There is some interesting new work carried out by researchers at Dartmouth College and the USDA Forest Service on the relationship between the Mountain Pine Beetle, major die-offs of forests in North America, and climate change.
The Mountain Pine Beetle (Dendroctonus ponderosae) is a kind of “bark beetle” (they don’t bark, they live in bark) native to western North America. They inhabit a very wide range of habitats and are found from British Columbia all the way south to Mexico. In British Columbia alone, the pine beetle, though a fairly complex process, has managed to destroy 16 of 55 million acres of forest. This epidemic of tree death is seen in mountain forest regions all across the western United States. The beetles affect a number of species of pine trees.
The beetle lays its eggs under the pine tree bark, and in so doing, introduces a fungus that penetrates adjoining wood. This fungus has the effect of suppressing the the tree’s response to the Pine Beetle’s larvae, which proceed to eat part of the tree. This suppressive effect blocks water and nutrient transport, together with the larvae eating part of the tree, quickly kills the host tree. The process can take just a few weeks. It takes longer for the tree to actually look dead (note the evergreen tree you cut and put in your living room for Christmas is dead the whole time it is looking nice and green and cheery). By the time the tree looks dead, i.e., the needles turn brown and fall off, it has been a dead-tree-standing for months and the Pine Beetles have moved on to find other victims.
It has long been thought that climate change has contributed to the western epidemic of Pine Beetles, as well as a similar epidemic in the Southeastern US (different species of beetles). The primary mechanism would be increasing winter extreme low temperatures. The very low temperatures would kill off the larvae, removing the threat of the beetle’s spread locally after that winter. Extreme winter temperatures have warmed by around 4 degrees C since 1960 across much of the beetle’s range. The lack of killing colds itself does not cause a beetle epidemic, but simply allows it, or produces a “demographic release.” If the beetles are already there, they have the opportunity to spread.
A recent study, just out, (see reference below) confirms this basic model but also adds a considerable degree of complexity. The study shows that there is not as strong of a correlation between raising winter temperatures above typical killing levels and the spread of the beetle. The study indicates that demographic release form an increase in extreme winter lows is part of the equation, but the situation is more complex and likely warming in general enhances beetle spread and reproduction during the summer part of its lifecycle, and may weaken the trees to make them more vulnerable to attack. In addition, other non-climate related factors probably play a role.
The study looked at several regions and assembled data on beetle frequency and spread over time, and various climate related data. From the abstract:
We used climate data to analyze the history of minimum air temperatures and reconstruct physio- logical effects of cold on D. ponderosae. We evaluated relations between winter temperatures and beetle abundance using aerial detection survey data… At the broadest scale, D. ponderosae population dynamics between 1997 and 2010 were unrelated to variation in minimum temperatures, but relations between cold and D. ponderosae dynamics varied among regions. In the 11 coldest ecoregions, lethal winter temperatures have become less frequent since the 1980s and beetle-caused tree mortality increased—consistent with the climatic release hypothesis. However, in the 12 warmer regions, recent epidemics cannot be attributed to warming winters because earlier winters were not cold enough to kill D. ponderosae…There has been pronounced warming of winter temperatures throughout the western US, and this has reduced previous constraints on D. ponderosae abundance in some regions. However, other considerations are necessary to understand the broad extent of recent D. ponderosae epidemics in the western US.
“This amount of warming could be the difference between pests surviving in areas that were historically unfavorable and could permit more severe and prolonged pest outbreaks in regions where historical outbreaks were halted by more frequent cold bouts,” says first author Aaron Weed, an ecologist at the National Park Service.
In the 11 coldest regions, winter temperatures cold enough to e lethal to D. ponderosae have become less frequent since the 1980s, and this is associated with an increase in tree mortality, confirming the link between warming conditions and increased parasite caused tree death. However, in the 12 regions with the warmest climate, recent epidemics are not clearly linked to warming winters simply because the earlier, colder, winters were already not cold enough to repress the tree-killing mountain pine beetle. This suggests that other factors may play a role in the epidemics in the western United States.
Evens so, the pattern of warming (including increase of minimum winter temperature) correlates to the demographic release of the mountain pine beetle. The authors note that “warming year-round temperatures that influence generation time and adult emergence synchrony … and drought effects that can weaken tree defenses …” are plausible explanations, but further note that a simple single explanation is not likely to be sufficient to explain the overall phenomenon. The link between warmer years, added number of generations per year, and the epidemic is explored here.
This is, in a sense, a numbers game. A cold winter does not kill off all of the beetles. However, no matter how cold the winter is, no beetles will be wiped out if they are not there to begin with. So, demographic release, which makes possible but does not cause an outbreak, could cause an abundance of beetles across a much larger area where, no matter what natural suppression may occur, they will then become more abundant over time.
As noted, the trees themselves matter. We can safely assume that generally changes in overall climate will mean that plant communities adapted to a given region might lose that adaptive edge and be subject to a number of problems which can then be exploited by a potentially spreading parasite. These changes in viability of plant communities are not all climate change related. Forest management, disturbance, and regional demographics (as forests age, they tend to change what they do) are also factors in this complex set of ecological relationships.
The bottom line. This study confirms the effects of warming, especially the increase of winter low temperatures, on the potential for D. ponderosae to spread rapidly locally and regionally. The study also calls into question the simplistic model that this is all that happens to explain the widespread epidemic of this beetle. Other factors, including other aspects of global warming, also contribute to the epidemics. In addition, and importantly, the study demonstrates a high degree of variability in the outcome of ecological and climate change.
This epidemic is probably the largest observed kill-off of forests caused by a parasite. So far it is much more severe in its effects than forest fires, but over the long to medium term, we will probably see increased frequency and severity of forest fires because of the abundance of fuel provided by the die-off.
Soucre:
Weed, A. S., Bentz, B. J., Ayres, M. P., & Holmes, T. P. (2015). Geographically variable response of Dendroctonus ponderosae to winter warming in the western United States. Landscape Ecology. doi:10.1007/s10980–015–0170-z
Text for the image at the top of the post, from the USDA:
The Mountain Pine Beetle is at epidemic levels throughout the western United States, including here in the Rocky Mountain Region … Forests affected here include several in Colorado, Wyoming, South Dakota and Nebraska. In northern Colorado and southeastern Wyoming, Mountain Pine Beetles have impacted more than 4 million acres since the first signs of outbreak in 1996. The majority of outbreaks have occurred in three forests: Arapaho-Roosevelt, White River and Medicine Bow/Routt.