Plant invasion into high elevations implies adaptation to high UV-B environments: a multi-species experiment

Environmental factors change with an increase in elevation, such as increased UV-B radiation and decreased water availability, and thus may pose as barriers in upwards range expansion of plant species. Although evidence has been provided for an increasing number of exotic species reaching high elevations, the mechanisms behind still remain to be resolved, specifically, as to what extent processes of plasticity and adaptation are involved in overcoming these environmental barriers. In order to test for the role of genetic differentiation, specifically attributable to changes in water availability and UV-B radiation along elevational gradients in exotic plant species, we conducted a greenhouse experiment with eleven exotic herbaceous plant species sampled along an elevational gradient from 1500 to 3000 m a.s.l. in Drakensberg, South Africa. Seeds were collected across sections over the entire elevational gradient of Sani Pass. The experiment was run in a greenhouse with individuals grown from seeds of different elevational origins and subjected to combinations of three levels of UV-B radiation and two levels of soil moisture. Measures of performance and functional traits were repeatedly assessed as response measures in a species-wise manner. The results revealed a significant impairment of plant performance by increased levels of UV-B radiation as well as by reduced water availability. This emphasizes the existence of cross-resistance mechanisms that alleviate the negative impacts if plants are subjected to both combined stressors at the same time. Additionally, there seems to be a strong dependency of leaf trait responses on the developmental stage of the individual. We found evidence for adaptive processes related to increasing UV-B radiation levels in high montane areas, suggesting that factors fostering evolutionary responses, such as longer residence time and high genetic diversity could promote additional ascent of exotic species into high elevation areas.