Regional temperature and precipitation trends in the Drakensberg alpine and montane zones: implications for endemic plant species

Abstract

Mountains are complex environments owing to their varying topography and geographic range, and as a result are havens for a variety of eco-systems and biodiversity. Mountain systems around the globe are potentially transforming due to increasing pressure from human-driven climate change. The possible effects of these pressures and overall consequences of the changes are difficult to predict due to the complexity of mountain habitats. Previous studies have recorded increasing temperatures in mountain systems as one of the consequences of climatic change. As a consequence of this warming trend, plant species that typically grow in lower altitudes may migrate to higher altitudes as those habitats become suitable. There are many different effects this may have on the local eco-system, such as the possibility of the migrating species outcompeting the local species or even hybridization occurring, resulting in a new species. Regardless, the movement of species from low to high elevations will have a direct effect on plant community dynamics in the area. South Africa is experiencing warming temperatures and has experienced a reported increase in mean annual temperature by 0.96 ºC over the last five decades. This research aims to understand the implications of inter-annual temperature and precipitation trends in the alpine, upper and lower montane thermal zones in the Drakensberg on two endemic plant genera, Rhodohypoxis (Hypoxidaceae) and Glumicalyx (Scrophulariaceae). A thermal zone refers to a temperature gradient at a specific altitude in a mountain system. Temperature and precipitation data from 1994 to 2014 were collected from four weather stations: Sani Pass (2874 m), Shaleburn (1614 m) and Giants Castle (1759 m) in South Africa, and Mokhotlong (2209 m) in Lesotho. These sites represent three thermal zones; Sani Pass is in the alpine zone, Mokhotlong is in the upper montane zone, and Shaleburn and Giants Castle are in the lower montane zone. The objectives of this research are to analyse and compare the temperature and precipitation trends, inter-annual variability, and annual number of frost days at each data collection site from 1994 to 2014, as well as infer the potential impact these changes may have on the local endemic plant genera. Results show a more pronounced increase in temperature in the lower thermal zones and a larger decrease in precipitation in higher thermal zones. The lower montane zone experienced the highest increase in temperature of up to 0.6 ºC over two decades. The alpine zone showed the largest decrease in precipitation of on average 27.5 mm of rainfall

per annum over 20 years, while the lower montane zone displayed the largest inter-annual variability in both temperature and precipitation variables. The upper montane zone had a larger decrease in frost days over the 20 year period relative to the lower montane zone. Interestingly this work showed an increased warming pattern in the lower thermal zones relative to upper zones, which contrasts with work in other mountain ecosystems. This warming may create larger intermediate regions which could encourage the movement of endemic flora into neighbouring thermal zones. Movement between thermal zones may increase hybridization within plant genera which could change the structure of the plant communities and possibly result in altered floral populations.

Keywords: Climatic change, Drakensberg, endemic flora, thermal zones, temperature and precipitation trends, inter-annual variability