Do freeze events create a demographic bottleneck for Colophospermum mopane?

Abstract

Frost disturbance is often mentioned in southern African savanna literature, but it is seldom discussed or investigated further. However, it can represent an above-ground disturbance as effective as fire or browsing, depending on the resistance capacity of the effected plants. A severely freeze-damaged stand of Colophospermum mopane along a slope in the Venetia Limpopo Nature Reserve provided an opportunity to investigate the nature of freeze-damage impacts on C. mopane. Is this disturbance a possible demographic limitation of C. mopane preventing its southwards spread? Freeze-damage of individual trees was assessed according to tree height and landscape position — with lower elevations representing the most severe freeze zones and higher elevations representing the least severe. Lower elevation trees were relatively small (2.24 m) and coppicing, whilst higher elevation trees were taller (3.65 m) with no coppice present. No freeze-damage was observed on tree canopies above 4 m in height. Trees < 4 m in height that had experienced 100% freeze-damage, failed to regrow to their original heights of the previous season. This is a possible driver of the pre-freeze height differences seen across the slope; with trees at low elevations having to recover from freeze events and subsequent topkill more frequently, resulting in a net decrease in tree height for that growing season. It appears that C. mopane has limited resistance to freeze events, and this may be linked to the absence of this species at colder latitudes in the Southern Hemisphere.