Water relations as a factor in Rubus armeniacus (Himalayan blackberry) invasive success in the Pacific Northwest

Invasive blackberries are widespread in regions with Mediterranean-type climates. We sought to determine if one such invasion, that of Rubus armeniacus (Himalayan blackberry) in the Pacific Northwest, is facilitated by adaptation to seasonal changes in water availability. At monthly intervals in spring and summer 2007, we evaluated water stress index scores (WSI; based on leaf water potential), stomatal conductance, and root and shoot hydraulic resistance for co-occurring stands of R. armeniacus and the native R. spectabilis (salmonberry) and R. parviflorus (thimbleberry). We also characterized atmospheric and edaphic conditions. In summer 2006, mean stomatal conductance of R. armeniacus (460 mmol m^-2 s^-1) was significantly higher than R. spectabilis and R. parviflorus (by a factor of two) at low to moderate evaporative demands (VPD 3-27 mb). Also, mean leaf water potential of R. armeniacus was significantly greater than that of R. spectabilis throughout this period. If data from 2007 similarly show lower and less variable hydraulic resistance and WSI scores, and higher stomatal conductance for R. armeniacus vs. R. spectabilis and R. parviflorus, it will suggest that R. armeniacus is more strongly buffered against the region’s large seasonal change in water availability. An ability to maintain a favorable water status may help R. armeniacus maximize carbon assimilation during summer months, and thus facilitate its rapid growth and spread.