New Zealand's only mangrove species, Avicennia marina (Forsk.) Vierh. subsp. australasica (Walp) Everett (Avicenniaceae), has a clearly defined southern natural limit around latitude 38° S. Historically, this has been attributed to the lethal effects of winter frosts (Chapman & Ronaldson, 1958) and, more recently to poor species dispersal and lack of suitable habitat south of the present species boundary ( de Lange & de Lange, 1994). A series of investigations of frost tolerance, leaf gas exchange, stomatal conductance, chlorophyll a fluorescence, water relations and stable carbon isotope analyses of New Zealand Avicennia were undertaken in the laboratory and field in order to characterise frost tolerance limits and photosynthetic performance under normal and stressed conditions. By so doing, the relative importance of winter frosts and low air temperatures in defining the range of Avicennia in New Zealand could then be determined. It was found that photosynthetic performance in Avicennia was increasingly depressed near its southern limits. Photosynthetic responses were consistent with those typical for cold-sensitive species after exposure to low temperatures. Large decreases in photosynthetic production, photoinhibition and chlorophyll loss were evident following exposure to night temperatures lower than 4°C, suggesting that critical damage occurred even at temperatures well above freezing. Frosts probably still play an important, additional and episodic role in defining the range of mangrove in New Zealand. The physiological stress effects of low temperatures were compounded by sub-zero temperatures and productivity in frostprone areas was significantly reduced by physical damage to leaves, branches and reproductive tissue. Freeze-injury of this form also reduced the likelihood of successful seedling establishment and long-term survival of saplings. These findings indicate that latitude 38° S acts as a physiological barrier for mangrove in New Zealand. Avicennia might persist further south than its present natural range if dispersal to suitable microclimates occurred. However, the probability of this occurring naturally is small, given the species poor dispersal abilities, and the reduced likelihood of survival in the face of damage and physiological stresses brought about by cold winter temperatures.