Abstract:
Rainfall variability is expected to change the soil water regime thereby impacting negatively
on rangeland species composition, productivity and ecosystem services. The aim of this study was to
assess the impact of different intensities of rainfall reduction (RR) on vegetation annual net primary
productivity (ANPP). Twenty 7 × 7 m experimental plots with different intensities of RR structures
consisting of transparent acrylic bands were built on a natural grassland. The interspaces between
acrylic bands varied in size to intercept different intensities of ambient rainfall (0, 15, 30 and 60%) as RR
treatments, with each RR treatment replicated five times in a complete randomised block design. A fixed
1 m2 quadrat was marked at the centre of each plot and the ANPP within the quadrats was determined
by harvesting the quadrant at the end of the growing season. Generally, as compared to the control
(ambient rainfall intensity) the overall grass ANPP (P > 0.05) showed resilience to lower and moderate
intensities of (15 and 30%) RR, but at a severe intensity of RR (60%) the ANPP was significantly reduced.
Compared to the control the percentage contribution of grasses towards the overall ANPP increased
at a lower intensity of RR (15%). In contrast, the percentage contribution of forbs towards the overall
ANPP significantly reduced at lower intensity of RR. Within the grass species, however, those grasses
that decrease when the veld is undergrazed or overgrazed (decreaser grass species) showed resilience
at lower intensity (15 and 30%) of RR, while at a severe intensity of RR the ANPP of decreaser grasses
were significantly reduced (1841 vs. 220 kg DM/ha). Those grasses that increase with undergrazing
or overgrazing (increaser I or increaser II grass species) recorded a higher ANPP at moderate intensity
of RR (30% RR) than at a higher intensity of RR, while the difference between 60% RR and 0% RR in
terms of increaser grasses ANPP were not significant (P > 0.05) (650 kg DM/ha). Up to 88% reduction in
ANPP were recorded for decreaser grass species at severe intensity of RR as compared to the control the
corresponding reduction in ANPP noted for increaser grasses were relatively less (up to 56% reduction
in ANPP at 60% RR vs. 0% RR). Generally, the overall ANPP yield of the semiarid grassland in Gauteng
province showed resilience to a low intensity of RR (15% RR) and moderate intensity of RR (30% RR)
partly due to a shift in the species composition of grasses from decreasers to increasers ecological groups,
as well as due to a decrease and an increased contribution of forb functional groups at a lower and
moderate intensity of RR, respectively.
