Variability of historical forest structure and fire across ponderosa pine landscapes of the Coconino Plateau and south rim of Grand Canyon National Park, Arizona, USA

We undertook reconstructions of historical ponderosa pine forest structure and fire regimes across an entire landscape to expand understanding of spatial variability in forest structure and dynamics. The study area includes the ponderosa pine forests of the Coconino Plateau, Arizona, USA. Using General Land Office survey data and newly developed methods, we examined surveyor descriptions of overstory and understory composition and structure across 60,998 ha. For 41,214 ha of forests, we reconstructed density, basal area, diameter-class distributions, and fire severity and used GIS to analyse their relationships with topography. Ponderosa pine forests were continuous in only 34 % of the 60,998 ha landscape. In 40 %, they were mixed with pinon–juniper, 24 % was pure pinon–juniper and 2 % were grass or shrubs. In ponderosa pine forests, the focus of this study, mean tree density was 144.2 trees ha−1; 18.8 % of the landscape had low tree density ( 200 trees ha−1). Small trees ( 50 % of all trees on 47 % of the landscape and large trees (>40 cm) composed >50 % of all trees on 21 % of the landscape. Low-severity fire likely structured the forest on 59 % of the landscape while 39 % was structured by mixed-severity fire, likely including small, patchy crown fires. Forest parameters displayed wide variability across environmental gradients. Broader-scale reconstructions revealed a much more spatially complex forest that was structured by wide range of fire severities. Variability in ponderosa pine forests was often under-reported in past studies, but is important in perpetuating biological diversity.