Fire history and stand structure of two ponderosa pine-mixed conifer sites: San Francisco Peaks, Arizona, USA

We reconstructed historical fire regimes and contemporary and historical stand structures in two stands of ponderosa pine-mixed conifer forests on the San Francisco Peaks in northern Arizona, USA. Thirty-four fire-scarred specimens recorded 256 fires from the EAST and WEST study sites. Fires were recorded between 1739 and 1903 for the EAST site and between 1548 and 1947 for the WEST site. The mean fire return interval (MFI: ≥25% scarred) for the period 1690-1892 was 10 years with a range of 3-21 years for the EAST site. The WEST site MFI (period 1612-1876) was 9 years with a range of 3-21 years. Seasonal patterns of fire occurrence showed that the majority of fires burned during the summer months. Fire interval (years) means, variances and distributions between the EAST and WEST sites were not statistically different from one another for the common analysis period of 1690-1876. Historically, both the EAST and WEST sites were dominated by ponderosa pine, with scattered individuals of Douglas-fir, limber pine and white fir, with tree densities that ranged from 43 to 60 trees per hectare (TPH). Current forest composition has shifted from fire-tolerant ponderosa pine to less fire-tolerant, more shade-tolerant species, with tree densities ranging from 928 to over 1700 TPH. We suggest that the dramatic structural changes recorded at our study sites occurred since fire regime disruption.