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The age structure of aspen in Rocky Mountain National Park was characterized to determine if the number of aspen established in recent decades was substantially lower than would be expected from long-term trends. At lower elevations on the eastside of the Park, aspen density averaged 1250 trees/km2, compared with 550 trees/km2 at higher elevations and on the westside. Most aspen in the Park were relatively young, with trees less than 80 years old accounting for 90% of aspen trees and 50% of aspen basal area. The number of trees in each decadal age class increased exponentially from 1855 through 1965, with no decade showing significantly higher or lower aspen numbers than expected from the long-term trend. The number...
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Populus?Salix forests are a valued riparian vegetation type in western North America. These pioneer, obligate phreatophytes have declined on some rivers, raising conservation concerns and stimulating restoration plantings, but have increased on others. Understanding patterns and causes of forest change is essential for formulating conservation, restoration and management plans. Our goal was to assess spatio-temporal patterns of vegetation change on the Upper San Pedro River in semiarid Arizona, USA, one of the few undammed rivers in the region. Over 100 years ago, intense floods initiated channel incision and substantially altered hydrogeomorphology. Pioneer trees began to establish in the widening post-entrenchment...
In 1998, Mexican gray wolves (Canis lupus baileyi) were introduced into the Blue Range Wolf Recovery Area (BRWRA) that spans adjacent portions of Arizona andNewMexico. In 2009 we selected three mixedconifer sites on the Apache National Forest, within the BRWRA of east-central Arizona, to characterize long-term age structure of aspen (Populus tremuloides) and to check for the possible occurrence of a tri-trophic cascade involving Mexican wolves, Rocky Mountain elk (Cervus elaphus nelsoni), and aspen. These mixed-conifer sites included (a) a refugium site, (b) an old-growth site, and (c) a site thinned in 1991?1992. The refugium site was inaccessible to elk and cattle whereas the old-growth and thinned sites were...
The fire history of Piñon–Juniper (Pinus edulis–Juniperus osteosperma) woodlands in much of the southwestern United States is poorly understood, and as a result, fire management decisions are being made without a rigorous ecological underpinning. We investigated the historic fire regimes in Piñon–Juniper woodlands on the Mesa Verde cuesta utilizing stand and age structures. All Piñon trees in eight stands were aged and stand age was extrapolated to the surrounding landscapes using digital imagery, creating a time-since-fire map of the 1995 landscape. Six sampled stands were over 400 years, while two were between 200 and 300 years. Stand-replacing fire with a rotation of 400 years or longer characterized...
Encroachment of singleleaf pinyon (Pinus monophylla) into adjacent low sagebrush (Artemisia arbuscula) and basin big sagebrush (Artemisia tridentata ssp. tridentata) communities may be enhanced by the efficient use of limited water resources by tree seedlings. Seedlings and sagebrush nurse plants were monitored over two growing seasons to determine water-use patterns. Predawn xylem water potential of low sagebrush declined rapidly, reaching ?3.5 to ?5.5 MPa by late summer. Big sagebrush values dropped to ?2.0 to ?3.0 MPa during summer drought. The drop in sagebrush xylem water potential was related to the decline in soil water potential (r=0.68 and 0.82). The change in pinyon predawn xylem water potential was moderate,...
Piñons and junipers, that dominate many semi-arid landscapes in the western United States, have invaded some sagebrush and grassland areas and possibly increased in density since EuroAmerican settlement. Exclusion of fire by livestock grazing and intentional suppression is thought to have been a cause of these changes. National assessments suggest that many woodlands have missed one or more low-severity surface fires and are thus in poor condition, requiring restoration. We undertook a systematic review of seven questions about fire history, fire severity, and the role of fire in these woodlands to evaluate the scientific basis for the national assessment. First, unless piñons and junipers record fire by means of...
Pi�ons and junipers, that dominate many semi-arid landscapes in the western United States, have invaded some sagebrush and grassland areas and possibly increased in density since EuroAmerican settlement. Exclusion of fire by livestock grazing and intentional suppression is thought to have been a cause of these changes. National assessments suggest that many woodlands have missed one or more low-severity surface fires and are thus in poor condition, requiring restoration. We undertook a systematic review of seven questions about fire history, fire severity, and the role of fire in these woodlands to evaluate the scientific basis for the national assessment. First, unless pi�ons and junipers record fire by means of...
The Colorado River Basin has been, and continues to be, the focus of a wide diversity of research efforts to learn more about the effects of natural and human-induced disturbances on the processes and functioning of the basin's upland watersheds. These watersheds are situated at the headwaters of streams and rivers that supply much of the water to downstream users in the western United States. Responses of streamflow to vegetation manipulations have been, and are, one of the research foci in this water-deficient part of the country. The watershed-scale research, led by the U.S. Forest Service and its cooperators, has spanned nearly a century and included an array of vegetation types along a wide range of precipitation...
We used data from 142 stands in Colorado and Wyoming, USA, to test the expectations of a model of growth dominance and stand development. Growth dominance relates the distribution of growth rates of individual trees within a stand to tree sizes. Stands with large trees that account for a greater share of stand growth than of stand mass exhibit strong growth dominance. Stands with large trees that contribute less to stand growth than to stand mass show reverse growth dominance. The four-phase model predicts that forests move from a period of little dominance (Phase 1), with trees accounting for similar contributions to stand growth and stand mass. Phase 2 is a period of strong growth dominance, where larger trees...
map background search result map search result map A century of riparian forest expansion following extreme disturbance: Spatio-temporal change in Populus/Salix/Tamarix forests along the Upper San Pedro River, Arizona, USA A century of riparian forest expansion following extreme disturbance: Spatio-temporal change in Populus/Salix/Tamarix forests along the Upper San Pedro River, Arizona, USA