Analyses of Intrinsic Water-Use Efficiency Indicate Performance Differences of Ponderosa Pine and Douglas-fir in Response to CO2 Enrichment

Aim: Increasing intrinsic water-use efficiency (iWUE) in trees is a global-scale response to increasing levels of atmospheric carbon dioxide (Ca). Our primary goal was to determine whether significant differences in either iWUE or radial growth exist between locally co-occurring ponderosa pine (Pinus ponderosa var. ponderosa; PIPO) and Douglas-fir (Pseudotsuga menziesii var. glauca; PSME) trees growing in minimally disturbed environments and exposed to gradually increasing levels of Ca. Methods: We developed 14 tree-ring chronologies, calculated basal area increment (BAI) and examined stable carbon isotope ratios (d13C) contained in the tree cores to determine: (1) the consistency of climate/BAI responses and iWUE between the two species and the magnitude of climate change for the primary drivers of BAI; (2) changes in iWUE and BAI values during ad 1850–present; and (3) the extent of pan-regional consistency in iWUE, BAI and climatic responses.Results: PIPO and PSME experienced exponentially increasing iWUE rates during ad 1850–present suggesting either increased net photosynthesis or decreased stomatal conductance, or both. There was pan-regional consistency, with similar temporal trends in iWUE and climate/growth relationships between species. Both species experienced above-average BAI in the latter half of the 20th century despite no favourable changes in climate. We found substantive differences between species as PIPO has experienced greater increases in iWUE and BAI relative to PSME. The trends in BAI and iWUE were significantly correlated for PIPO, but not for PSME.Main conclusions: Differential radial growth and iWUE responses exist among co-occurring species and are most pronounced during the period of highest Ca. As PIPO and PSME forests have extensive geographical ranges and are critical to North American forestry, any substantive change in forest dynamics related to water use and growth may have ecological and economic implications