Where Have All the Grasslands Gone?

Fire and Vegetation Change in Northern New Mexico (page 1 of 5)

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

Introduction

Passing through the majestic landscapes of northern New Mexico today one finds valleys thick with sagebrush, dense woodlands of piñon and juniper amidst eroding foothills, closed forests crowded with ponderosa pine and fir cloaking plateaus and mountain slopes, and a variety of grasslands and meadows densely fringed with young trees. Most people think of these wildlands as "natural", pretty much the ways things have always been. Yet old-timers and even older documents tell of pine forests and woodlands with grassy understories open enough to drive a wagon through, while ecologists and environmental historians compile ever-increasing evidence of major vegetation changes over the past century. A repeated theme of these ecological histories is the decline in herbaceous vegetation (grasses and non-woody flowering plants) while forests thicken and brush invades. The details of species and timing vary a bit between studies, but everywhere woody plants (trees and shrubs) are increasingly dominant. Why has this been happening?

Views from Acoma Pueblo to Enchanted Mesa, west of Albuquerque, NM, taken by William Henry Jackson in 1899 and H.E. Malde in 1977.  Note expansion of junipers into surrounding grassland. Source: C. Allen, J. Betancourt, and T. Swetnam. 1997. Southwestern U.S. LUHNA pilot project. USGS Biological Resources Division. http://biology.usgs.gov/luhna/chap9.html.

The main reason for this increase in woody plants in northern New Mexico is human-caused changes in the role of fire in our landscapes. Vegetation patterns are determined by many factors, with climate, topography, and soils often considered paramount. It is less well recognized that disturbances like fires and floods may be equally influential. Particularly here in the Southwest, fire is a key process that determines the ecological structure and function of most ecosystems. In general, fire favors grassy vegetation over woody plants. Trees and shrubs can be killed or severely damaged by fire, while perennial grasses not only survive and quickly recover (the same way they regrow from basal growth points after being grazed), their growth may even be enhanced by fire. A variety of scientific studies show that fires have been frequent and widespread in the lands of Rio Arriba since before the Oñate entrada, shaping the vegetation in ways that favored herbaceous life-forms. About 100 years ago the occurrence of fire greatly diminished across this region, resulting in major vegetation changes that included declines in grassy vegetation and increases in woody plants—a trend that continues today.

Follow these links to:
Tree and Shrub Invasion of Open Grasslands
Tales That Trees Tell
Fires in the Forests: Then and Now
Restoring the Balance