Historical and modern disturbance regimes, stand structures, and landscape dynamics in pinon-juniper vegetation of the western United States.

Authors: ROMME, WILLIAM - Colorado State University; ALLEN, CRAIG - Us Geological Survey (USGS); BAILEY, JOHN - Oregon State University; BAKER, WILLIAM - University Of Wyoming; Bestelmeyer, Brandon; BROWN, PETER - Colorado State University; EISENHART, KAREN - Edinboro University Of Pennsylvania; FLOYD, M. LISA - Prescott College; HUFFMAN, DAVID - Northern Arizona University; JACOBS, BRIAN - National Park Service; MILLER, RICHARD - Oregon State University; MULDAVIN, ESTEBAN - University Of New Mexico; SWETNAM, THOMAS - University Of Arizona; TAUSCH, ROBIN - Us Forest Service (FS); WEISBERG, PETER - University Of Nevada

Submitted to: Rangeland Ecology and Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/4/2009
Publication Date: 5/1/2009
Citation: Romme, W.H., Allen, C.D., Bailey, J.D., Baker, W.L., Bestelmeyer, B.T., Brown, P.M., Eisenhart, K.S., Floyd, M., Huffman, D.W., Jacobs, B.F., Miller, R.F., Muldavin, E.H., Swetnam, T.W., Tausch, R.J., Weisberg, P.J. 2009. Historical and modern disturbance regimes, stand structures, and landscape dynamics in pinon-juniper vegetation of the western United States. Rangeland Ecology and Management. 62:203-222.

Interpretive Summary: This paper presents a summary of what we currently know – and don’t know – about historical and modern stand and landscape structure and dynamics, in three major and fundamentally different kinds of piñon-juniper vegetation in the western U.S.: persistent woodlands, savannas, and wooded shrublands. It is the product of a workshop that brought together experts from across the geographical range of piñon-juniper vegetation. The intent of this synthesis is to provide a source of information for managers and policy-makers, and to stimulate researchers to address the most important unanswered questions.

Technical Abstract: Piñon–juniper is a major vegetation type in western North America. Effective management of these ecosystems has been hindered by inadequate understanding of 1) the variability in ecosystem structure and ecological processes that exists among the diverse combinations of piñons, junipers, and associated shrubs, herbs, and soil organisms; 2) the prehistoric and historic disturbance regimes; and 3) the mechanisms driving changes in vegetation structure and composition during the past 150 yr. This article summarizes what we know (and don't know) about three fundamentally different kinds of piñon–juniper vegetation. Persistent woodlands are found where local soils, climate, and disturbance regimes are favorable for piñon, juniper, or a mix of both; fires have always been infrequent in these woodlands. Piñon–juniper savannas are found where local soils and climate are suitable for both trees and grasses; it is logical that low-severity fires may have maintained low tree densities before disruption of fire regimes following Euro-American settlement, but information is insufficient to support any confident statements about historical disturbance regimes in these savannas. Wooded shrublands are found where local soils and climate support a shrub community, but trees can increase during moist climatic conditions and periods without disturbance and decrease during droughts and following disturbance. Dramatic increases in tree density have occurred in portions of all three types of piñon–juniper vegetation, although equally dramatic mortality events have also occurred in some areas. The potential mechanisms driving increases in tree density—such as recovery from past disturbance, natural range expansion, livestock grazing, fire exclusion, climatic variability, and CO2 fertilization—generally have not received enough empirical or experimental investigation to predict which is most important in any given location. The intent of this synthesis is 1) to provide a source of information for managers and policy makers; and 2) to stimulate researchers to address the most important unanswered questions. La vegetación de Piñon-junípero es un tipo de vegetación muy importante en el Oeste de Norte América. El manejo efectivo de estos ecosistemas se ha obstaculizado por el inadecuado entendimiento de 1) la variabilidad en la estructura del ecosistema y los procesos ecológicos que existen entre las diversas combinaciones de pinos, juníperos, arbustos, hierbas, y los organismos asociados del suelo; 2) regímenes prehistóricos e históricos del disturbio, y 3) mecanismos que conducen cambios en estructura y composición de la vegetación durante los últimos 150 años. Este publicación hace un resumen de lo que sabemos (y no sabemos) acerca de tres clases fundamentales de vegetación del piñon–junipero. Arbolados persistentes se encuentran donde los suelos locales, el clima, y los regímenes del disturbio son favorables para el piñón, el junípero, o una mezcla de ambos; los fuegos han sido siempre infrecuentes en estas áreas. Las sabanas del Piñon–junipero se encuentran donde los suelos locales y el clima son apropiados tanto para árboles y gramíneas; es lógico que los fuegos de baja-intensidad pueden haber mantenido densidades bajas de árboles antes de que se interrumpió los regímenes del fuego después del establecimiento Euro-Americano, pero existe poca información para apoyar cualquier afirmación afirmativa sobre los regímenes históricos del disturbio en estas sabanas. Los matorrales se localizan donde los suelos locales y el clima apoyan a comunidad de arbustos, pero los árboles pueden aumentar durante condiciones climáticas y períodos húmedos sin disturbio, y disminuyen durante sequías y después del disturbio. Los aumentos dramáticos en la densidad de los árboles han ocurrido en las porciones de los tres tipos de vegetación del piñon–junipero, aunque eventos