Tree mortality following the severe 2002 drought at the ponderosa pine/pinyon-juniper ecotone in northern Arizona: Biotic and abiotic influences

Tree-ring and meteorological data show that the 2002 drought, which resulted in extensive tree and shrub mortality, was the most severe drought year in over 400 years in the Southwest U.S. We conducted a study in 2004 within the ponderosa pine/pinyon-juniper ecotone of northern Arizona at sites with soils derived from three parent materials (SPM; sedimentary, flow basalt, cinder) to assess the extent of tree dieback and mortality that occurred during this drought. Our objectives were to determine if tree condition varied among sites of different SPMs that differed in water-holding capacity, and among woody plant species. We categorized trees into three groups: alive with low-dieback (<25% of canopy volume), alive with high-dieback (>25%), and recently dead. SPM influenced the percentage of trees with high-dieback only for one-seed juniper (cinder 51%, sedimentary 3.3%, flow basalt 7.0%). Across all SPMs, high-dieback was greater for juniper and pinyon (19.8% and 10.9%) than for ponderosa (3.4%). Mortality did not differ significantly among SPMs for any species. Mortality over all SPMs was greater for ponderosa and pinyon pines (17%, n=948 and 15%, n=1120) compared to juniper (0.5%; n=407). Logistic regression of mortality on a suite of physical (elevation, slope, aspect) and biotic (dbh, basal area of the target species, BA of other woody species) factors explained 9.8-60.0% of variation in tree mortality for ponderosa and 14.7-26.8% for pinyon, depending on SPM. In general, abiotic factors were a more important source of variation in mortality at the drier cinder SPM than other SPMs.