Responses to chronic N fertilization of ectomycorrhizal piñon but not arbuscular mycorrhizal juniper in a piñon-juniper woodland

Responses of mature trees to chronic N additions are poorly understood in ecosystems with high seasonal and spatial variability. To determine the effects of increased N deposition on mature conifers, we fertilized a piñon-juniper woodland in New Mexico at a rate equivalent to the urban interface. Fertilization (10 g m −2 y −1) reduced numbers of mycorrhizae and increased leaf production in the ectomycorrhizal (EM) piñon but not in arbuscular mycorrhizal (AM) juniper. Based on N fractionation between EM fungal sporocarps and piñon, EM in piñon utilized 20% of the net primary production in control plots. No sporocarps were produced in fertilized plots. N uptake by piñon could be accounted for by fertilization without mycorrhizae. Leaf N and size increased with fertilization in both species, and positively correlated with leaf δ 13C. Leaf N:P increased in piñon but not juniper. Piñon mortality commenced in the N-fertilized plots in 2001, a year before the widespread die-off in western conifers, and continued through 2003. No mortality was observed in control plots or in junipers. The coupling of N enrichment and mycorrhizal decline could affect piñon production and mortality in semi-arid woodlands in the western US.