This study investigated the methodology and utility of dendrochemistry in the assessment of spatial and temporal concentrations of gaseous elemental mercury. Tree cores from the Pinus species in California and Nevada, U.S.A. were collected from previously sampled areas to test the stability of tree ring concentrations over time. Cores were collected from 2 new locations to assess spatial variability among trees within and between stands located at two elevations in the same watershed. Results indicated that using 2 to 3 cores from ∼10 or more trees provided the best framework for understanding tree ring concentrations within a population of trees located in uncontaminated areas. At the least 2 sides of a tree should be cored to account for radially asymmetric variations associated with growing conditions or injury. An agreement of concentrations and trends measured in trees cored with previous research indicated that tree rings are suitable proxies for historical air mercury concentrations and that mercury concentrations have increased since the Industrial Revolution. Data collected demonstrate that tree rings record regional gradients in GEM concentrations. In addition, temporal consistency may vary within a geographic location due to differing biotic and abiotic factors influencing ring growth since trees are active samplers of atmospheric Hg.