Decline in canopy gas exchange with increasing tree height, atmospheric evaporative demand, and seasonal drought in co-occurring inland Pacific Northwest conifer species

Interspecific variation in stomatal conductance (G<inf>S</inf>) and transpiration (E<inf>L</inf>) has been documented in stands of co-occurring species, and this variation has been observed to differ with tree size and canopy height increase. In this study, we present data that examine fluctuations in canopy gas exchange across co-occurring species and varying canopy heights for three montane forest chronosequences located in an inland Pacific Northwest mixed-conifer forest. With the exception of Douglas-fir (Pseudotsuga menziesii var. glauca (Beissn.) Franco), we observed consistent declines in canopy E<inf>L</inf> and G<inf>S</inf> with increasing height for the majority of species examined in our 2-year study. Along with declines in canopy G<inf>S</inf>, we observed decreases in leaf-specific hydraulic conductance (K<inf>L</inf>) across species as canopy height increased. Seasonally, we observed declines in canopy GS during warmer and dryer summer months of both years. These decreases in G<inf>S</inf> were significant (up to 50%) and suggest that carbon assimilation in trees was limited during dryer months due to a combination of high evaporative demand and reduced soil H<inf>2</inf>O availability. Such reductions in G<inf>S</inf> during periods of increased plant water stress suggest that forest productivity in the inland Pacific Northwest may be impacted negatively if future climate predictions of increasing growing-season water stress are realized.