Past, Recent, and Twenty-first Century Vegetation Change in the Arid Southwest

Duration: February 1999 - September 2003

In the past, distributions of plant and animal species have shifted dramatically in response to changes in climate. Several atmospheric circulation models predict that over the next 50 to 200 years, climates may change as radically as they did near the close of the ice ages, about 10,000 years ago. If these projected changes come about, they will affect the distribution of most organisms, and may occur much more rapidly than similar changes in the past. Managers of natural areas will need as much information as possible to help forestall undesirable effects of these potentially rapid and severe environmental changes.
Understanding the responses of plants and animals to past climatic changes can help us predict responses to future changes. This project will reconstruct vegetation changes and other environmental shifts in the southwestern U.S. during past periods of climatic change. The primary tool will be the analysis of plant assemblages from fossil packrat middens. In addition, we will examine the climate requirements of individual modern plant species in order to reconstruct past climates and generate future plant distributions from projected future climates.

Fossil packrat (or woodrat) middens often contain abundant fossils of leaves, seeds, fruits, twigs, bones, shells, and other materials. These items comprise a physical record of species that lived within a packrat’s range of the midden. The debris within the midden also contains records of past atmospheric conditions through ratios of stable isotopes of oxygen, carbon, and deuterium. Radiocarbon dating has identified some middens as being over 50,000 years old. Analysis of packrat middens thus provides a powerful tool for reconstructing biotic communities and environmental conditions at specific locations, far into the past.

Approach

Our project will use information derived from packrat middens as well as other sources (fossil pollen, historical photographs, permanent vegetation plots, tree-ring studies) to identify and understand past responses to climatic change. We will work at three spatial scales:

First, intensive data on past vegetation change and climatic zones will be generated from several specific sites within National Parks units along the Colorado River (red triangles). 

The site-specific data will be compiled with other existing data into a larger-scale analysis of the Colorado River profile extending from the Gulf of California to western Colorado (blue triangles). 

Past and future projections will be extended throughout the entire Colorado River drainage basin and Mojave Desert using a geographic information system (GIS) and available regional paleoecological data.

Vegetation changes over the last few hundred years will be studied by integrating data from paleoecological, historical, and ecological studies. Extreme changes are usually evident from numerous sources.
For example, the expansion of juniper across many southwestern rangelands over the last 150 years has been detected in studies of packrat middens, fossil pollen, historical photographs, permanent vegetation plots, and tree-ring studies.

Compilation of these different sources of retrospective data allows for a more complete understanding of past changes.

Application of Results:

The results of this project will contribute to a more complete understanding of how future vegetation changes are likely to compare to past changes in both rate and severity. This information will help land managers prepare to cope with possible radical changes to the landscape.

Products:

Collections, databases, maps and publications will include the following:

More complete series of fossil packrat middens, including the warmer Holocene intervals, from several important, previously studied, fossil sites.
Complete series of fossil packrat middens from important areas which have not yet been studied.
A database of Colorado Plateau fossil packrat midden information detailing past plant distributions.
A database of modern plant distributions of the Colorado Plateau analyzing their climatic correlations.
Modern and past climates of the Colorado Plateau projected onto geographic grids.
Analyses of vegetation responses to past periods of rapid climate change.
Comparisons of past rates of vegetation change to those that are presently occurring and those projected for the future.

Collaborators:

Samantha Arundel

	Primary Contact:  Kenneth L. Cole, USGS Southwest Biological Science Center (SBSC), Colorado Plateau Field Station