Are local range expansions in southerly populations of Aloidendron dichotomum early indicators of a future range shift?

• Main Author: Grey, Kerry-Anne
• Other Authors: Cunningham, Susan
• Format: Dissertation
• Language: English
• Published: Faculty of Science 2020
• Subjects: biological sciences
• Online Access: http://hdl.handle.net/11427/31142

Aloidendron dichotomum is predicted to undergo a south and south-eastern range shift in response to anthropogenic climate warming. Despite this, its range is suggested to be limited by low temperature extremes at the cool range edge and no such range shift has yet been confirmed. However, eco-physiological knowledge necessary to predict and detect this range shift is lacking. This study aimed to determine whether physiological performance of A. dichotomum differs at a regional and local scale, and if so, whether regional and local temperatures explain physiological differences. Performance metrics and temperature data were collected during summer at 14 microsites in two sites at the cool range edge of A. dichotomum. Microsites were stratified by aspect and elevation. This study confirmed that A. dichotomum is performing CAM photosynthesis at its cool range edge. Further, the results provide evidence for the temperature dependence of A. dichotomum’s physiological performance. Higher performance was associated with warmer nights and the warmer north facing slopes. This is in keeping with our hypothesis that A. dichotomum individuals at the cool range edge would take advantage of higher temperatures more typical of their core range areas. Opposing this, higher carbon gain was found at Gannabos - the cooler of the two study sites. This may be due to the less variable rainfall pattern at Gannabos. Furthermore, performance was higher on the flats where temperatures were also cooler. This may be due to lower water runoff on the flats compared to the slopes. As a result, I suggest that the cool range edge of A. dichotomum is limited both by low temperatures and rainfall variability and that, with warming, constraints on these populations are being removed. This is substantiated by the observed high levels of juvenile recruitment in these populations relative to others, and in time, is likely to lead to range expansion in the region.