Mature Ponderosa Pine Nutrient Use and Allocation Responses to Air Pollution

Current-year needles from mature ponderosa pine (Pinus ponderosa Dougl. ex. Laws.) were sampled at four sites across the air pollution gradient in the San Bernardino Mountains in southern California. The sites, in order of decreasing air pollution exposure, included: Sky Forest (SF), Conference Center (CC), Camp Angelus (CA) and Heart Bar (HB). Needle nutrients measured were: nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), manganese (Mn), zinc (Zn), iron (Fe) and aluminum (Al). Concentrations of N in foliage (12.5 g/kg) were significantly higher at SF, the site most exposed to air pollution. This level is similar to concentrations for containergrown seedlings fertilized with nutrient solution. Mature tree needle concentrations of K, Mg , Fe and Al were higher at sites more exposed to air pollution. Mature trees with two annual whorls of needles (more severely injured) had significantly higher current year needle concentrations of P, K, Zn, and Fe at SF than trees with three annual whorls of needles (less severely injured). However, significant differences were not found in foliar nutrient concentrations between trees with different levels of needle retention (injury) at CC, which is also a high pollution site. At Barton Flats (BF), a low to moderate air pollution site in the San Bernardino Mountains, N, K and P were measured in current-year needles and litter fall. The resorption efficiencies (the percent reduction in nutrient concentration upon senescence of leaves or needles) for mature trees at BF were: 45 percent for N, 60 percent for P and 80 percent for K. N resorption proficiency (the concentration a nutrient is reduced to before needle abscission) was high with N reduced to 4.4 g-N/ kg (considerably less than the defined threshold of 7g-N/kg). However, P resorption proficiency was very poor with P reduced to 0.59 g-P/kg (higher than the defined threshold of 0.4 g-P/kg). A complete understanding of the changes in tree nutrition associated with air pollution will require more research.