Throughout the American West, pine forests are dying off in record numbers. Take a drive through the Northern Rockies and you're likely to see entire hillsides covered by the bare, gray remains of dead trees. Once lush forests are now like ghost towns, empty and rotting.
Often, when we talk about this kind of unbalanced destruction in nature, we point the finger of blame toward an invasive species, a plant or animal which has been introduced to a new ecosystem by man. Without natural predators or other checks, invasive species can grow out of control, out-competing native species and driving them towards extinction.
In the case of the pines, however, the culprit is not a foreign invader, but a native resident. The tiny mountain pine beetle (Dendroctonus ponderosae)
has played an important role in the western ecosystem for eons, killing off diseased and overcrowded trees, and keeping the forests healthy. Recently, however, these tiny tree surgeons have begun spreading to higher elevations and are destroying healthy trees, sometimes killing entire forests in a single summer.
So what's driven this sudden change in appetite? Scientists are still debating the exact cause of the current blight, but climate change is believed to play a key role. Hot, dry summers and mild winters have enabled the beetles to survive higher in the mountains where there are new species of trees to attack. Whitebark pines, which play a major role in the Greater Yellowstone Ecosystem are particularly vulnerable and seemed to have become a preferred victim of the beetles.
At just five millimeters long, or about the size of a single grain of rice, the mountain pine beetle is not a particularly obvious foe. In the summertime, they fly through the forest, seeking out suitable trees. Once they have located one, they use their jaws to chew through the bark and lay their eggs inside. The tree is not entirely defenseless, however, and in response to the beetles' attack, it excretes sticky sap that can trap the insects as they bore inward.
To compensate, the beetles have developed a symbiotic relationship with the bluestain fungus (Grosmannia clavigera
). The spores of this fungus cling to the head of the beetles and are dispersed into the wood as the beetles chew through. Here the spores begin to grow, creating thread-like masses that clog the tree's phloem, inhibiting its ability to excrete sap. This allows more beetles to enter, but it also prevents the tree from moving water and nutrients through its tissues, effectively starving it. The tree dies quickly, often succumbing in a single season.
So what can we do to help stave off the beetle blight? Unfortunately, once a tree is infected there is almost nothing that can be done to save it. Hope lies in prevention. If you see evidence of infected trees in your area — popcorn-shaped pitch tubes on the trunk, yellow or red foliage on an evergreen pine, or tunnel-like galleries in exposed wood — please alert your local Forest Service representative. Infected trees can be carefully destroyed, preventing the spread of beetles to healthy neighbors. Check out Colorado State University's mountain pine beetle fact sheet
for more information.
Also, remember that even small disturbances in our planet's climate can have big impacts. Do what you can to lower your carbon footprint, and support research into the causes and prevention of climate change.
Photos: WBUR/Flickr, Dru!/Flickr