Morphology, water relations, and the invasive success of Rubus armeniacus in the Pacific Northwest

Background/Question/Methods

Members of the Rubus fruticosus complex (wild blackberries) are invasive at the global scale, and are most abundant in regions with mild winters but dry summers.  The objective of this study was to compare morphology, water relations, and growth rate between the invasive Rubus armeniacus and the common, but noninvasive, Pacific Northwest shrubs Rubus ursinus, Rubus parviflorus, Rubus spectabilis, and Rosa nutkana.  We propagated plants from cuttings, and subsequently grew them in the greenhouse under high and low irrigation regimes for approximately 100 days. 

Results/Conclusions

Under both water availability levels, Rubus armeniacus had more massive root and shoot systems, with wider but less dense canes than several of the native species.  Rubus armeniacus canes also contained more water relative to their biomass (mean = 69%) than Rubus parviflorus, Rubus spectabilis, and Rosa nutkana.  Under high water availability, Rubus armeniacus had higher mean stomatal conductance (644±55 mmol m^-2 s^-1) and relative growth rate (0.016±0.001 η η^-1 day^-1) than Rubus ursinus, Rubus parviflorus, and Rubus spectabilis.  Under low water availability, Rubus armeniacus had a less negative mean pre-dawn leaf water potential (-0.29±0.02 MPa) than Rubus ursinus, Rubus spectabilis, and Rosa nutkana.  The larger root system of Rubus armeniacus may enable it to access water resources that other shrubs cannot, and its larger shoot system may allow it to store water over daily timescales.  Ultimately, advantages in morphology and the ensuing advantages in water relations may help Rubus armeniacus maintain rapid growth in seasonally oscillating water regimes and overwhelm competing plants.