Impact of a severe frost event in 2014 on woody vegetation within the Nama-Karoo and semi-arid savanna biomes of South Africa

• Near-lack of trees on Nama-Karoo plains is possibly influenced by frost.
• Most (>90%) canopy volume at lower elevations lost to a severe frost event.
• Savanna woody species differ in susceptibility to frost.
• Nama-Karoo biome boundaries with savanna and grassland are affected by frost.

The plains of the Nama-Karoo biome in southern Africa are characterised by a mixture of dwarf shrubs and grasses with near absence of shrubs and trees that are conspicuous in the adjacent savanna biome and on hills. We investigated the impact of a severe frost event in 2014 on the abundance and composition of woody vegetation in relation to local topography at three widely separated locations. Long-term weather records of 44, 53, and 83 years confirmed that the 2014 event was the 16th, 6th, and 12th percentile of cold years across three locations. An event of similar severity had not occurred for >7 years at the Nama-Karoo locations but had at the savanna location. Woody vegetation lost 60–100% of canopy volume at the base of a slope but no loss had occurred at elevations of 30–100 m above this. Regrowth volume after one season was linearly related to volume lost to frost although most plants had not recovered pre-frost volume. All dominant woody species experienced substantial topkill. Microphyllous species appeared particularly sensitive to frost. Results suggest frost contributes to maintaining the treeless character of the Nama-Karoo biome and to containing bush encroachment along drainage lines within the savanna biome.