Forest change on a steep mountain gradient after extended fire exclusion:: San Francisco Peaks, Arizona, USA

1. More than a century of forest management, including fire exclusion, livestock grazing and tree harvesting, may have affected forest structure and composition in south-western USA. Dendroecological techniques were used to reconstruct an 1876 baseline against which modern conditions could be compared. We assessed the magnitude of changes on the San Francisco Peaks in five distinct forest types: ponderosa, mixed conifer, aspen, spruce-fir and bristlecone. 2. We established a systematic grid of 135 plots, each 0.1 ha in size, over a 1117-m altitudinal band. 3. In the contemporary forest, density was greatest in spruce-fir and least in bristlecone whereas basal area was greatest in spruce-fir and lowest in ponderosa. In 1876, all forest types had significantly lower densities and basal areas. 4. The period since 1876 was associated with increased forest density, a shift in species composition as a result of invasion of shade-tolerant conifers, and a trend for mesic species to migrate to lower altitudes. Changes were least evident in the highest altitude forests. Climate and human-caused and natural biotic disturbance factors probably all played a role in forest change, but we argue that the most prominent factor was probably exclusion of the thinning effect of fire, especially on fire-susceptible mesic species. 5. Synthesis and applications. Mesic species have encroached to lower altitudes and forest density has increased since 1876. These changes have created to conditions opposite to those suitable for warmer, drier future climates that will display increased fire risk, setting the stage for sudden and severe change. Management is complex because of heavy fuel loading, administrative constraints and high public visibility. However, 'sky island' landscapes such as the Peaks represent protected ecosystems of great importance in arid regions. Testing of wildland fire use and other management interventions to restore composition and fuel structures more resilient to warmer climate should proceed.