This project had three objectives. The first objective was to identify variation in discrimination of Δ13C and intrinsic water use efficiency (iWUE) in Ponderosa pine (Pinus ponderosa) tree rings from 1800 to 2012 at two Fire and Fire Surrogate study sites (Arizona, Washington). The sites are both dominated by ponderosa pine but have contrasting climatic conditions (see study area description). Since climatic patterns vary at each site we also conducted an analysis of how discrimination (Δ), intrinsic water use efficiency (iWUE), and basal area index (BAI) a standard measure of annual tree growth, are influenced by climate and disturbance processes that influenced the forests. Our second objective was to compare fuel treatment effects (control, mechanical, mechanical+burn) on tree’s physiology by comparing discrimination (Δ), iWUE, and BAI amongst treatments. Our third objective was to compare discrimination (Δ) and intrinsic water use efficiency (iWUE) pre- and post-treatment as a metric of restoration of forest ecosystem function on the two FFE sites. Our first hypothesis is that Δ13C will increase after treatment and more so in mechanical+burned stands than mechanical and this will reduce intrinsic water use efficiency (iWUE). We also hypothesize that BAI will increase after treatments. Secondly, we hypothesize that post treatment iWUE is lower than during the reference period. Fuel treatments that thin stands are likely to increase carbon isotope discrimination (Δ) and BAI due to increases in water availability and greater stomatal conductance. A higher rate of increase in stomatal conductance (gs) compared to the rate of photosynthesis (A), would result in a decrease of iWUE.