A 12-hour windstorm flattened conifers at Devils Postpile, and bigger trees fell first

A windstorm on November 30 to December 1, 2011 knocked down large areas of conifers across the southern Sierra Nevada, including Devils Postpile National Monument. The source calls it the most extensive wind event on record for California's Sierra Nevada, with possible long-lasting effects on forest composition and function. A resource brief from the Park Service and a field note in the Western Journal of Applied Forestry describe what the monument's forest lost.

An unusual storm

An extreme north-to-south pressure gradient over Nevada and the Sierra drove the winds, which were unusual in three ways: long duration, an atypical direction and high intensity. Extreme winds lasted for 12 hours, 6 pm to 6 am, strongest from 10 pm to 4 am. An anemometer on Mammoth Mountain recorded winds above 145 km/hour, mostly from the north-northeast.

Aerial views of the northwest part of the monument, where the worst blowdown occurred, in 2010 (before) and 2012 (after). The green line is the monument's northern boundary, the red lines are roads and the yellow dotted lines are trails. Credit: National Agricultural Imagery Program of the USDA.

How the study worked

In May 2012 scientists characterized the windthrown trees in a Devils Postpile mixed-conifer forest, mostly red fir, white fir and lodgepole pine. They wanted to relate tree size to wind damage, to see what separates uprooting from snapping, and to compare species' vulnerability by weighing the forest's composition before and after.

Fire effects plots gave the before picture. Eight were installed in 1992 after the Rainbow Falls fire and re-measured in 2002, and two unburned ones were added in 2005, with data on 390 live trees. The plots doubled as photo points for repeat pictures. In May 2012 the team also set up 13 transects, each 100 meters long, and recorded density, fall angle, type of damage, and species and abundance of the new windthrown trees.

An unburned fire effects plot photographed in 2005 and again six months after the blowdown. NPS / Sequoia & Kings Canyon National Parks Fire Effects Program.

What they found

Standing live trees were far more likely than dead ones to uproot than to snap. The findings agree with other studies that link tree size to storm damage. Because windstorms mainly hit larger trees, their effect on regeneration and function is likely different from other disturbances such as fire.

Why it matters: uprooting leaves mounds of exposed soil that can help seedlings take hold, while snapping leaves dead snags that matter to wildlife such as cavity-nesting birds. Precipitation, stem rot, whether a tree is alive or dead, slope and stem size all play a part in which happens. The study describes the forest at time zero after the storm, and the authors say it will be important to keep monitoring how the event affects forest structure and diversity.

Sources

• National Park Service, "Windstorm Effects on Conifer Forests of Devils Postpile National Monument" (nps.gov article, a resource brief based on K. Hilimire, J. C. B. Nesmith, A. C. Caprio and R. Milne, "Field Note: Attributes of Windthrown Trees in a Sierra Nevada Mixed-conifer Forest," Western Journal of Applied Forestry, 2013); the page's photograph of a field crew is left out here; rewritten in hubnx's own words.