Stand Dynamics of Pinyon-Juniper Woodlands After Hazardous Fuels Reduction Treatments in Arizona

Abstract

Pinyon-juniper ecosystems occur extensively across western North America, and at the landscape scale, variation in structure and composition is influenced by topographic position, soils, disturbance history, and local climate. The persistent pinyon-juniper woodland is a common structural form, and though they are known to be infrequent-fire systems, there is increasing interest in implementation of hazardous fuels reduction treatments in woodlands, especially in the wildland-urban interface. Few studies have quantified stand dynamics following fuels reduction treatments in persistent woodlands or compared treatment outcomes to conditions that develop under natural disturbance and successional processes. In 2004, we established a randomized, replicated study in woodlands of northern Arizona, and monitored stand dynamics and understory responses to determine how stand-level changes differed between common fuels reduction approaches. We compared the resulting structure with a conceptual state-and-transition model. Results showed that, over the 11 yr after treatment, juniper tree densities decreased by 8.4% and 0.9% but increased by 14.0% and 27.3% in Control, Burn, Thin, and Thin + Burn treatments, respectively. Pinyon tree densities decreased by 1.1% and 3.3%, increased by 12.2%, and decreased 7.9% in Control, Burn, Thin, and Thin + Burn treatments, respectively. All treatments showed fuel load reductions throughout the 11-yr study period and minimal rebound of tree recruitment toward pretreatment conditions. Prescribed fire alone (Burn) maintained persistent woodland conditions. Thinning treatments substantially reduced small tree densities and, with the addition of prescribed fire, produced losses of large trees. Thinning with prescribed fire (Thin + Burn) tended to produce conditions qualitatively unlike those described by our state-and-transition model. Evaluation of these commonly used fuels treatments against our state-and-transition model suggested that concerns regarding loss of ecological integrity may be warranted.