Reaction to Competition
Secondary succession following fire or other severe disturbance in pinyon-juniper woodlands appears to follow the general successional model shown in figure 1 (3). However, the first herbaceous species to become established after a fire are often those that were present in the stand before disturbance (19). The shrub stage, often consisting of sagebrush, a common associate in the woodlands, becomes prominent after about 12 years (11). Junipers, which appear to have a wider ecological amplitude than pinyons because of their greater drought resistance, are usually the first trees to regenerate (10,12,75). They rapidly increase in density after 45 years, and dominate the site at 70 years. Thereafter, pinyons tend to succeed junipers at rates determined by available seed sources until the shrub understory is essentially eliminated. If disturbances are less severe, as when cabling, chaining, or bulldozing is used to remove tree cover for range improvement, many small surviving pinyons and junipers and newly established seedlings, reforest the site in about 2 to 3 decades (55,56,64). Under some conditions, however, natural regeneration can take much longer (60).
Figure 1- Possible series and pathways of secondary succession
following disturbance in pinyon-juniper woodlands (3).
Considerable evidence has accumulated to show that the woodlands, especially those dominated by singleleaf pinyon, are invading areas below their historic elevational limits (3,12,17,41). Furthermore, tree density appears to be increasing in some stands that existed before the invasion period. Pinyon-juniper woodland expansion since the time of settlement has been attributed to several factors, including possible climatic changes, control of fire, increased populations of seed-dispersing birds and mammals, and reduced competition from grasses resulting from overgrazing by livestock or the allelopathic influence of juniper foliage and litter (20,39).
- Burns, Russell M., and Barbara H. Honkala, technical coordinators. 1990. Silvics of North America: 1. Conifers; 2. Hardwoods. Agriculture Handbook 654 (Supersedes Agriculture Handbook 271,Silvics of Forest Trees of the United States, 1965). U.S. Department of Agriculture, Forest Service, Washington, DC. vol.2, 877 pp. http://www.na.fs.fed.us/spfo/pubs/silvics_manual/table_of_contents.htm
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