Long-term vegetation response to treatments of prescribed fire and chaining in Great Basin pinyon-juniper woodlands

Abstract

Treatments of prescribed fire and chaining have been widely applied across the Great Basin since the mid 20th century in an effort to reduce pinyon and juniper cover and stimulate understory growth. Treatment efforts often result in altered vegetation structure and composition relative to untreated areas. A few studies have examined short-term effects of chaining and prescribed fire treatments, but this study examined long-term changes in vegetation cover, tree survivorship, and tree establishment multiple decades after historical treatments. This study revisited sites of woodland treatments that were originally sampled over 30 years ago. An emphasis was placed on tree recovery because it is an easily interpretable measure of treatment success. Changes in vegetation composition were evaluated by collecting vegetation cover, soil depth, and soil texture data at plots within four chaining sites treated in 1958, 1962, 1968, and 1969 and originally sampled in 1971. The same data were collected at five prescribed burn sites treated in 1975 and originally sampled in 1976. In 2008, tree cover at chaining treatment sites was much greater than at burned sites, where it was nearly absent. Absolute increases in tree cover at chaining sites were comparable to absolute increases in tree cover occurring in corresponding control areas. Relative to original tree cover in 1971, however, increases in tree cover at chaining treatments were greater than relative tree cover increases at controls. At all sites, cover of herbaceous species decreased and cover of woody species increased. Relationships between site overstory and understory cover values were fit with a negative exponential curve, and the parameters were interpreted in the context of understory response to treatment. Finer-textured, shallower soils were the most predictive environmental conditions for rapid tree recovery following chaining. Tree dominance has recovered at a much faster rate following chaining treatments than prescribed fire. Tree infilling in untreated areas is also occurring at a rapid rate, similar to tree recovery at chaining sites. Forty years after chaining, sites are again treedominated with limited understory vegetation. Burned sites, however, continue to be dominated by shrub and herbaceous species 33 years after treatment. Analysis of post-treatment tree age structures helps to clarify species-specific processes of tree survivorship and establishment. Tree age data were collected at every chaining and prescribed fire plot by sampling increment cores and stem cross sections. Tree-ring data indicated that all chaining sites experienced greater juniper survival than pinyon survival. Chaining sites with higher overall tree survival following treatment also experienced the greatest amount of new tree establishment. During the interval between treatment and 2008 sampling, approximately four more trees ha-1 y -1 established following chaining than following fire. Post-treatment establishment was dominated by juniper at prescribed fire sites. At chaining sites, post-treatment establishment was dominated by juniper for the first 15 years; however, by 15 to 40 years following chaining treatment, pinyon establishment greatly exceeded juniper establishment. The greatest density of newly established trees occurred in control (untreated) plots, indicating that both prescribed fire and chaining treatments decreased tree establishment. However, eventual relative dominance of pinyon is a probable long-term effect of chaining treatment. Prescribed fires applied at relatively infrequent intervals may favor extended dominance of juniper over pinyon. Although prescribed fires more closely met resource management goals of the mid 20th century, managers may find the practice too risky when considering current environmental conditions and management concerns. Prescribed burning was more successful than chaining for removing trees, maintaining understory communities, and causing reduced post-treatment tree establishment. However, cheatgrass (Bromus tectorum) was present at prescribed fire sites and not chaining sites. The great increases of tree cover in the untreated areas of this study indicate fuel loads are much larger than when the prescribed fires were conducted in 1975. The risk of an escaped fire becoming a conflagration has increased since then, as has the risk of cheatgrass invasion.