U.S. Geological Survey

Piñon-juniper woodlands are one of the most extensive vegetation types in New Mexico, including large portions of Bandelier National Monument and the Pajarito Plateau in the southeastern Jemez Mountains. The woodland soils on local mesas formed to a large degree under different vegetation during cooler, moister conditions of the late Pleistocene; in other words, they are over 10,000 years old, and many are over 100,000 years old (McFadden et al. 1996). Changes in climate and vegetation in the early Holocene (9,000–6,000 years ago) led to at least localized episodes of soil erosion on adjoining uplands (Reneau and McDonald 1996, Reneau et al. 1996). During this time, the dominant climatic and associated vegetation patterns of the modern southwestern United States developed, including grasslands, piñon-juniper woodlands and ponderosa pine savannas and forests (Allen et al. 1998). On the basis of local fire history information (Allen 2002), the young ages of most piñon-juniper trees here (Julius 1999, unpublished data), and soils data (McFadden 1996), we believe that many upland mesa areas now occupied by dense piñon-juniper woodlands were formerly more open, with fewer trees and well-developed herbaceous understories that: 1) protected the soil from excessive erosion during intense summer thunderstorm events, and 2) provided a largely continuous fuel matrix, which allowed surface fires to spread and maintain these vegetation types (Figure 1). In contrast, rocky canyon walls have probably changed relatively little through the centuries, as grazing and fire suppression had fewer effects on such sites. Native American effects on local woodlands are thought to have been insignificant or highly localized until the late 12t