Variation in gas exchange and water use efficiency patterns among populations of western redcedar

Western redcedar (Thuja plicata Donn) populations were planted out on a reforestation site and measured, during the summer, to define their gas exchange processes in relation to evaporative demand under non-limiting light and edaphic conditions. These populations came from various locations across a longitudinal transect representing various biogeoclimatic subzones, differing in annual and summer precipitation, in British Columbia, Canada. The study was designed to determine whether there was population variation in gas exchange patterns and if this variation was adaptive to environmental conditions of the source geographic region (i.e., descriptors of population seed-origin). Both instantaneous gas exchange and carbon isotope composition were measured. All western redcedar populations had a decrease in foliage conductance (g(wv)) and net photosynthesis (P-n) as vapor pressure deficit (VPD) increased. There was evidence of genetic variation in both g(wv) and P-n responses to VPD, with the two Vancouver Island populations having greater g(wv) and P-n levels than most other populations across the full range of VPD conditions. For all populations, intrinsic water use efficiency (WUEI) was fairly stable across the full range of VPD conditions. There was a relationship between foliage carbon isotope discrimination (Delta) and both the ratio of internal to ambient CO2 concentration (C-I/C-a) and WUEI measured during the summer growing season. There was a stronger relationship between g(wv) and Delta than between P-n and Delta. There was population variation in Delta with trees from two submaritime locations having the lowest Delta levels and the highest WUEI across all field site VPD conditions. In contrast, the two Vancouver Island populations as well as trees from a very dry interior location had the lowest seasonal WUEI and the highest Delta levels. Measured gas exchange response and long-term water use efficiency (WUE) patterns did not consistently correspond to environmental conditions of the source geographic region.