Community-level assessment of freezing tolerance: frost dictates the biome boundary between Albany subtropical thicket and Nama-Karoo in South Africa

AimThe sub-escarpment coastal plains of South Africa provide remarkable opportunities to study the determinants of biome boundaries as numerous biomes are found closely juxtaposed, including the Nama-Karoo semi-desert shrubland and Albany subtropical thicket. The Nama-Karoo shrubland is centred on the semi-arid and frosty high-elevation interior plateau of South Africa, whereas the Albany subtropical thicket inhabits the comparatively warmer sub-escarpment coastal plains. We examined the role of winter frosts in determining the boundaries between these two biomes on the coastal plain. LocationKaboega, Eastern Cape, South Africa. MethodsWe determined the relative freezing tolerance of thicket and Nama-Karoo communities by sampling dominant species from each biome in a small study site (c.50ha) spanning a clear vegetation and minimum temperature boundary. Freezing-induced stress on leaf photosynthesis was measured using chlorophyll fluorescence imaging across a range of subzero treatments. ResultsIn general, largely irrespective of any possible effects of temperature acclimation, freezing exposure significantly reduced photosynthetic efficiency (F-v/F-m) values in thicket species relative to those from the Nama-Karoo shrubland across all treatments. Main conclusionsAs reduced photosynthetic efficiency is generally associated with leaf damage, and species from both these biomes are largely evergreen, we interpreted our results in terms of species-level frost resistance. Therefore, our results support the hypothesis that frost occurrence is a primary driver of the boundary between the subtropical thicket and Nama-Karoo shrubland in South Africa. This has implications for both regional- and landscape-level planning of restoration efforts and predicting boundary shifts under altered climates of the past and future.