Where Have All the Grasslands Gone?

Fire and Vegetation Change in Northern New Mexico (page 2 of 5)

Author: Craig D. Allen. Adapted from: Allen, C.D. 1998. Where have all the grasslands gone? Quivera Coalition Newsletter, Spring/Summer.

Tree and Shrub Invasion of Open Grasslands

Consider the decline of montane grasslands in the Jemez Mountains [Adobe Acrobat map], which are found on the upper, south-facing slopes of nearly all of the larger summits and ridge crests. These are the most productive grasslands in New Mexico, and their deep, prairie-type soils indicate that grasslands have persisted for thousands of years on these sites. Yet today a tidal wave of young ponderosa pine, Douglas-fir, and aspen are observed to be invading these grasslands. By coring and dating hundreds of trees I found that the tree invasion began in the 1920s. Vegetation mapping from a time sequence of aerial photographs confirms the timing of the tree invasion and reveals the extensiveness of the tree encroachment. Between 1935 and 1981, tree invasion reduced the area of open montane grasslands by 55% across the 250,000 acre mapped area that covers the southeastern Jemez Mountains. Several small montane grasslands present in 1935 have disappeared, while the larger grasslands have become fragmented.

Similar tree and shrub invasions are also observed in many other open vegetation types throughout northern New Mexico, including blue spruce encroachment on moist meadows, Engelmann spruce invasion of subalpine parks in the Pecos Wilderness, and the spread of juniper and sagebrush and snakeweed into valley grasslands. These patterns of woody invasion into formerly grassy environments are tied to changes in land use history, primarily livestock grazing and fire suppression.

Follow these links to:
Tales That Trees Tell
Fires in the Forests: Then and Now
Restoring the Balance