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1 January 2012 Predicting the Occurrence of Downy Brome (Bromus tectorum) in Central Oregon
Sara C. P. Lovtang, Gregg M. Riegel
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Where the nonnative annual grass downy brome proliferates, it has changed ecosystem processes, such as nutrient, energy, and water cycles; successional pathways; and fire regimes. The objective of this study was to develop a model that predicts the presence of downy brome in Central Oregon and to test whether high presence correlates with greater cover. Understory data from the U.S. Department of Agriculture (USDA) Forest Service's Current Vegetation Survey (CVS) database for the Deschutes National Forest, the Ochoco National Forest, and the Crooked River National Grassland were compiled, and the presence of downy brome was determined for 1,092 systematically located plots. Logistic regression techniques were used to develop models for predicting downy brome populations. For the landscape including the eastside of the Cascade Mountains to the northwestern edge of the Great Basin, the following were selected as the best predictors of downy brome: low average March precipitation, warm minimum May temperature, few total trees per acre, many western junipers per acre, and a short distance to nearest road. The concordance index  =  0.92. Using the equation from logistic regression, a probability for downy brome infestation was calculated for each CVS plot. The plots were assigned to a plant association group (PAG), and the average probability was calculated for the PAGs in which the CVS plots were located. This method could be duplicated in other areas where vegetation inventories take place.

Nomenclature: Downy brome, Bromus tectorum L.; western juniper, Juniperus occidentalis Hook. var. occidentalis.

Management Implications: Currently, there is interest in how downy brome might change fire return intervals throughout the West and whether prescribed fire will help or cause further problems by favoring downy brome infestations. Of special interest is the question of how to manage dry western rangelands and forests in the face of climate change with the recent push by the U.S. Federal Government to recognize climate change and manage forests for future climates. Land managers must consider how large-scale disturbances may reset plant association groups as climate changes. Areas with healthy native plant populations can keep downy brome populations minimized; however, if the native plants are wiped out by a large-scale disturbance and the climate is slightly warmer and drier than it had been historically, downy brome could easily spread and then alter fire regimes that maintain the downy brome population. Keeping downy brome out of an area is far easier than trying to eradicate it once it becomes established.

This article presents a statistical model that land managers can use to predict the presence of downy brome in Central Oregon. For example, land managers in Central Oregon who know the values of average March precipitation, minimum May temperature, total trees per acre, western junipers per acre, and distance to the nearest road can calculate the probability of downy brome presence. For land managers in the Deschutes National Forest, Ochoco National Forest, and Crooked River National Grassland, a map has been generated showing the probability of finding downy brome. Land managers in other areas can use the methods in this article to create a statistical model, using standard inventories, such as Current Vegetation Survey or Forest Inventory and Analysis. The probability of downy brome presence can be used to make informed decisions about fuels treatments and other management actions.

Weed Science Society of America
Sara C. P. Lovtang and Gregg M. Riegel "Predicting the Occurrence of Downy Brome (Bromus tectorum) in Central Oregon," Invasive Plant Science and Management 5(1), 83-91, (1 January 2012).
Received: 19 April 2011; Accepted: 1 December 2011; Published: 1 January 2012

Central Oregon
Current Vegetation Survey
downy brome
logistic model
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