Tree canopy types constrain plant distributions in Ponderosa Pine- Gambel Oak Forests, Northern Arizona

–Trees in many forests affect the soils and plants below their canopies. In current high-density southwestern ponderosa pine (Pinus ponderosa) forests, managers have opportunities to enhance multiple ecosystem values by manipulating tree density, distribution, and canopy cover through tree thinning. I performed a study in northern Arizona ponderosa pine-Gambel oak (Quercus gambelii) forests to measure the influences of tree canopy types on understory plant communities and soil properties. On ten 2.5-acre (1-ha) sites, I sampled five 43-ft (4-m) plots below each of the following five canopy types: openings; single ponderosa pine; and Gambel oak single stems, dispersed clumps, and thickets. Soil properties, species richness, plant cover, and the distribution of cooland warm-season grasses were canopy-type specific. Openings contained the most species/plot, three to eight times greater plant cover than any tree canopy type, and warm-season grasses (for example, purple threeawn [Aristida purpurea]) that were infrequent below trees. In contrast, aspen pea (Lathyrus laetivirens) and Fendler’s meadow-rue (Thalictrum fendleri) were most frequent below Gambel oak canopies. There were no species that were most frequent below ponderosa pine. Results suggest that canopy openings need to be reestablished and maintained on this landscape if understories are to be productive, diverse, and contain species dependent on these microsites.