Vegetation –environment relationships and ecological species groups of an Arizona Pinus ponderosa landscape, USA

Pinus ponderosa forests occupy numerous topographic and soil complexes across vast areas of the south-western United States, yet few data exist on species distributions and vegetation –environment relationships for these environmentally diverse landscapes. We measured topography, soils, and vegetation on 66, 0.05-ha plots within a 110,000-ha P. ponderosa landscape in northern Arizona, USA, to discern vegetation –envi-ronment relationships on this landscape. We analyzed associations of environmental variables with plant communities and with single-species distributions, and we classified ecological species groups (co-occurring plant species exhibiting similar environmental affinities). Gradients in community composition paralleled gradients in soil texture, available water, organic C, total N, and geographic precipitation patterns. Soil parent material, affected by the presence or absence of volcanic activity, is a primary factor constraining vegetation patterns on this landscape. Using discriminant analysis, we built a model that correctly classified the most important of four grasses (Bouteloua gracilis, Muhlenbergia montana, Sporobolus interruptus, or Festuca arizonica) on 70 –80 % of plots based on five environmental variables related to soil moisture and resource levels. We also classified 52 of the 271 detected plant species into 18 ecological species groups. Species groups ranged from Phacelia and Bahia groups occupying xeric, volcanic cinder soils low in organi