OAK INFLUENCE ON NUTRIENT AVAILABILITY IN PINE FORESTS OF CENTRAL ARIZONA

Existing evidence suggests that Gambel oak (Quercus gambelii Nutt.) improves soil fertility of the ponderosa pine (Pinus ponderosa Dougl. ex Laws) forest of central Arizona. Greenhouse bioassays were conducted to determine if Gambel oak also influences nutrient availability in the 0- to 15-cm mineral soil layer. Barley (Hordeum vulgare L. cu. Gustoe) and pine seedlings were grown to estimate availability of N, P, K, and S in soils from 15 pine stands with basal area of oak varying from 0 to 62% of the total basal area, but equivalent in all other site factors. Without supplemental nutrients, barley yield from soils with 50% oak was three times greater than from those with no oak. With addition of N, P, K, and S, barley yield increased 370% on soils without oak, but only a 50% yield increase occurred on soils from stands with 50% oak. Regression analysis of barley yields for each nutrient revealed that availabilities of N and S were significantly related to amount of oak present. Availabilities of P and K were not related to oak presence. The bioassay with pine seedlings confirmed that oak positively influenced N supply. This test, however, showed no effect on S supply. Contrasting results for barley and pine tests are attributed to greater sensitivity of barley and high variance in the pine test.