Red Is Dead - The Full Extent Of Western Conifer Forest Destruction Comes Into Focus

[hatrack]

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In western North America, true deciduous forests are strictly riparian and therefore limited. The vast majority of upland forests and woodlands are dominated by conifers, all but one of which are evergreen. However, one highly important deciduous species—quaking aspen (Populus tremuloides), which turns brilliant yellow—forms groves that range in size from a few trees to vast forested expanses, typically on mid to high elevation mountain slopes with decent groundwater supply through the summer. The greens and yellows provide a poor man’s version of fall color compared to an eastern hemlock-hardwood forest perhaps, but when combined with often colorful rock exposures, blue skies and rivers, and montane topography, the sight usually has its own magnificence. Red foliage however, is one thing you’re not supposed to see a lot of in western North America, any time of year.

In many places in the west–particularly through the central and northern Rocky Mountains and British Columbia–there is now a lot of red foliage, but unfortunately, it’s not just in autumn, and is occurring on non-deciduous species. The proximate causes of this are (1) Dendroctonus spp. (literally, “tree killer”), the most destructive of several destructive bark beetle genera, and (2) tree physiological stress. God’s “inordinate fondness for beetles”*, combined with certain human activities, is now causing some serious problems indeed. Here I’ll try to give background on the issues related to bark beetle outbreaks, working from proximate to ultimate causes, and focusing on the one beetle species currently doing by far the most damage, the mountain pine beetle (MPB), Dendroctonus ponderosae. The MPB is attacking a set of highly important pine species (Pinus spp.) over a very large area of western North America, especially lodgepole pine (Pinus contorta), ponderosa pine (Pinus ponderosa) and whitebark pine (Pinus albicaulis), but also some of the other five needle pines (esp. limber pine). Other species of beetles in the genera Dendroctonus and Ips are also doing some serious damage to these and other species, albeit at smaller spatial scales and/or less intensively, and represent so many variations on the theme illustrated by the MPB.

Theological considerations aside, there are many hundreds of species of bark beetles worldwide, and the Curculionidae, or weevil family, of which bark beetles are a member, contains the largest number of species of any animal family. However, only a very small percentage of “aggressive” bark beetle species has the potential to cause extensive tree death during population outbreaks (“irruptions”). These species kill by overwhelming, with coordinated aggression and sheer numbers, a tree’s defenses, followed by a complete destruction of the tree’s ability to transport the products of photosynthesis (e.g. sucrose, amino acids, hormones, etc.) through its transport tissue, the phloem or inner bark. This is usually accompanied by an impaired ability to transport water to the leaves via the outer xylem (the sapwood), courtesy of blue stain fungi (e.g. Grosmannia or Ceratocystis spp.) carried and introduced by the beetles. As ecological systems go it’s very well studied, and contains the interesting wrinkles, non-linearities and feedbacks so typical of ecological systems, even relatively simple ones like this. Plant physiology and demography, insect population dynamics, insect-fungal symbioses, land management practices, and climate dynamics all play important roles in the overall outbreak process. Simplistic explanations of the causes of these dynamics are, as usual, to be avoided.

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Much of western North America has elevated tree densities, relative to pre-settlement times, either for all trees, the largest tree classes thereof, or both. This is primarily due to active fire reduction/suppression policies over the last century or more by federal and state land managers, and/or timber harvesting practices. The resulting increased competition, without any increased climate stresses, would by itself increase tree physiological stress and affect beetle outbreak dynamics. The addition of warmer and/or dryer conditions simply magnifies this problem. Similarly, increased climatic stress unaccompanied by increased competition would also favor the beetles. Because natural fire regimes varied widely historically, and are complicated in many places by similar variability in logging practices and intensities, the effect of fire reductions on bark beetle outbreaks varies considerably and involves several issues of spatial and temporal scale variability. This makes the topic both interesting and difficult, and requires good information on past land management practices and forest stand dynamics. That topic however is fodder for another post.

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http://www.realclimate.org/index.php/archives/2010/10/seeing-red/

[mike_c]

1. this is an excellent article, with some good references attached....

Thanks for posting it. Rec'd.

[eleny]

2. It's a sad sight

[reggie the dog]

3. i was in northern colorado this summer

and the trees were dead for over 100 miles, hell basically from fort collins to steamboat springs 90% of the trees were dea