Celastrus orbiculatus Thunb. is a pest species that has become a weed in Michigan. It occurs in many of the same habitats as the native species Vitis riparia Michaux. However, C. orbiculatus frequently develops into extensive monospecific infestations, while V. riparia does not. Root pressures in Vitis have been implicated as vital to the recovery of xylem function in wide vessels following winter freezes. For both of these lianas (woody vines) we investigated water conductance and root pressure as possible explanatory factors for their differential spread. According to our hypothesis, C. orbiculatus should produce greater or more frequent root pressures than V. riparia. However, the reverse proved true. This indicates that root pressure is not a prerequisite for weedy proliferation of the exotic temperate liana C. orbiculatus. Additionally, the seasonal trends for specific conductivity indicate that each species responds differently to environmental constraints. Vitis riparia establishes conductivity early in the growing season, before the leaves emerge, using root pressure to reverse embolism, but loses conductivity early in the autumn with the first freeze. Celastrus orbiculatus is slow to establish conductivity, depending on new wood production, but leafs out sooner than V. riparia and maintains conductivity far into the autumn beyond the first freeze. These results indicate that root pressures are not responsible for the invasive success of C. orbiculatus, and suggest that other parameters must be key to its prolific invasion.

Key words: Celastrus orbiculatus, hydraulic conductivity, liana, root pressure, Vitis riparia, water transport