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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 pinon-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 ectomy-corrhizal (EM) pinon but not in arbuscular mycorrhizal (AM) juniper. Based on N fractionation between EM fungal sporocarps and pH-ion, EM in pinon utilized 20% of the net primary production in control plots. No sporocarps were produced in fertilized plots. N uptake by pinon could be accounted for by fertilization without mycorrhizae. Leaf N and size increased with fertilization in both species, and positively correlated with leaf delta(13)C. Leaf N:P increased in pinon but not juniper. Pinon 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 pinon production and mortality in semi-arid woodlands in the western US. (C) 2010 Elsevier Ltd. All rights reserved.