A front brought fairly heavy cloud cover to the Sierras on one afternoon of my recent trip to Yosemite National Park, preventing any warm afternoon light from reaching El Capitan, Clouds Rest, Half Dome, or the Cathedral Group. But if I found the absence of light somewhat disappointing, I found the sight of a growing number of brown trees in the valley below Tunnel View far more disturbing.
Stressed by four years of drought, a growing number of the now-brown pines are dying, almost certain victims of pine bark beetles. Indigenous to the western United States, bark beetles usually serve a useful purpose in the life cycle of a forest, hastening the demise of unhealthy trees and creating favorable conditions for new growth.
But climate change has upset the natural order in the West. Whereas freezing winter temperatures once helped keep beetle numbers in check, record-setting warm temperatures have allowed many more beetles to overwinter, resulting in a bumper crop of hungry beetles that have attacked not just stressed and older trees but young and healthy ones as well.
To make matters worse, the warmer temperatures have resulted in an earlier beginning of spring and a later onset of winter, allowing beetles not only to reproduce multiple times during the calendar year (thus increasing their numbers even faster) but also to extend their ranges. As a result of climate change, pine bark beetles have even migrated to trees above 10,000 feet, such as the pines in the Ancient Bristlecone Pine Forest in California’s White Mountains.
The warmer temperatures have also altered the mix of winter precipitation in the Sierras and Cascades, resulting in a greatly reduced snow pack throughout the far west. In California, the snow pack in the Sierras was only at 5% of capacity at the end of the winter season. After a normal winter, a slowly melting snow pack can sustain a forest for months. But at the beginning of spring, there was little snow in the mountains to irrigate the already parched pine forests of the Sierras (further stressing the trees) or to fill reservoirs the state depends on for its drinking water.
Right now, the brown trees scattered below El Capitan seem insignificant below the looming peaks of the Sierras. But climate models for the immediate future do not offer much hope for a change in weather patterns. Warmer temperatures, reduced snow fall, and severe drought conditions may persist for years. Maybe longer. And if they do, significantly more pines in Yosemite Valley—and elsewhere in the park and the Sierras at large—will turn brown.