Response of arid shrubland to nitrogen and potassium additions over 9 years at Tierberg Karoo LTER, South Africa

The vegetation of the arid Succulent Karoo region of South Africa is dominated by slow-growing shrubs and succulents <0.5 m tall. However, soil mounds over termitaria support distinctive fast-growing, but short-lived plants. The differences in composition could be attributed to increased organic matter, nitrogen (N) and potassium (K) enrichment, or physical disturbance of the soil by termites and their predators. We tested the hypothesis that N enrichment alone may lead to changes in plant community composition through applications of N, K or N+K at a rate of 10g m^-2 twice yearly over a period of 9 years. Variables measured were rainfall, live and dead cover, species composition, species richness, and seedling survival. Live and dead vegetation cover increased significantly in N-enriched plots over 9 years. Inter-annual variation in live cover within control and K plots was positively correlated with 6-month lagged precipitation, however, rainfall did not account for live cover increases within N or N+K treatments. Nitrogen addition generally increased growth and flowering but differentially reduced survival during drought. The net result was a slow change in species composition that favoured short-lived, fast-growing plant species. The probable mechanism for these changes is that abundant N allows weak competitiors for nitrogen to outcompete stronger competitors by rapid depletion of water and reduction of access to light. Differences in vegetation composition on and off termitaria can, in part, be attributed to N enrichment. This finding has relevance for restoration enclosures intensively used by livestock in the Karoo.