Physiological controls over the northern range limit of kudzu (Pueraria montana var. lobata): Implications for predicting its future spread

Background/Question/Methods: Climate change is expected to exacerbate biotic invasions by promoting poleward migration of invasive species.  To forestall climate change-induced invasions, accurate predictions of the factors controlling range limits of invasive species are required. In this study, the physiological factors determining the northern range limit of the severe invasive vine kudzu were investigated.  Kudzu is currently invasive in the eastern United States, following its introduction in the late 1800s from Asia.  Its current northern range limit correlates with the -20°C minimum winter temperature isocline, leading to the hypothesis that its northernmost distribution is controlled by extreme winter temperatures; however, the physiological mechanism underlying such a thermal control is unknown. Using kudzu collected from a recently identified stand in southern Ontario, Canada, we determined the cold tolerance limits for overwintering kudzu tissues and low temperature effects on photosynthesis and growth during the growing season.

Results/Conclusions: The most tolerant tissues were aboveground stems and belowground stem nodes, which survived to -26.8°C and -17.0°C, respectively, when acclimated to subzero temperatures.  Roots and leaves were frost sensitive, dying at -6°C and -2°C, respectively.  Stem nodes are the regenerative organs of kudzu, and thus must be killed to prevent stand survival, but belowground nodes in stands at the northern range limit do not see temperatures below about -5°C, due to soil insulation.  Thus, winter cold does not appear to be the main control over kudzu distribution, because killing temperatures do not occur in the soil at its current range limit.   There was no evidence for irreversible photoinhibition or photosynthetic collapse at chilling (0°C to 10°C) temperatures until leaves froze.  Moreover, leaf gas exchange and chlorophyll fluorescence show that leaves rebound rapidly from chilling exposure. While the elongation rate of kudzu vines also rebounds rapidly from chilling, elongation stops below 15°C.  We propose that in cooler northern climates, an inability to produce new tissues below 15°C may be a severe limitation on kudzu performance, and may be a key control over its northern range limit.  If so, changes in early growing season temperatures may best predict kudzu’s response to climate change.