A comparison of grass production and utilisation in sodic and crest patches on a semi-arid granitic savanna catena in the southern Kruger National Park, South Africa
Date
2010-03-24T07:16:49Z
Authors
Alard, Glynn Frank
Journal Title
Journal ISSN
Volume Title
Publisher
Abstract
Grass biomass productivity was measured in sodic and crest patches in the semi-arid
(581.8 mm per annum) savanna of the southern Kruger National Park, straddling an
herbivory exclosure located on the northern bank of the Sabie River. Sodic and crest
patches are ubiquitous landscape elements within the undulating granitic terrain of the
Lowveld region of north-eastern South Africa. Sodic patches consistently occupy
footslope positions on the catena and are characterised by a sparse cover of woody
shrubs, lower herbaceous standing crop and a higher concentration of large mammalian
herbivores relative to adjacent crest patches. These characteristics imply that sodic
patches possibly make a substantial contribution to the landscape in terms of high
quantity or quality forage production. The long-term use of standing crop indices for
grassland productivity assessment in the region are from areas subject to natural
herbivory, and has created an impression of low grass biomass productivity on sodic
patches. The crest and sodic patches were compared for grass biomass productivity,
forage quality and utilisation, and were sampled in the growing season (November to
May) from 2002 to 2005. Sequential monthly clippings were conducted in permanently
located 1 m2 plots inside the exclosure to determine monthly productivity and outside the
exclosure to determine utilisation. Clippings of grass standing biomass were
simultaneously collected from 1 m2 once-off plots inside the exclosure to determine the
effects of defoliation on productivity. Sodic productivity was 8.74 ± 2.37 g dry matter m-2
month-1 where mammalian herbivory was excluded and did not differ significantly from
crest productivity both inside or outside the exclosure (p>0.05), but was significantly
higher than sodic forage yields of 0.15 ± 2.82 g m-2 month-1 in the presence of sustained
mammalian herbivory (p<0.05). This indicates that sodic forage is more heavily utilised
than crest forage. Productivity was significantly higher in plots clipped once-off than in
sequentially clipped plots (p<0.05). The patch utilisation differential was investigated by
analysing dried productivity clippings for total nitrogen, sodium, carbon and phosphorus.
Significantly higher (p<0.05) foliar nitrogen (>2% N) and sodium (>0.5% Na) in sodic
forage both account for higher sodic utilisation, though relationships are not linear.
Mixed and single species grass foliar samples from the study area were analysed for
iii
stable natural abundance nitrogen and carbon isotopes to investigate nitrogen availability
and cycling, and water availability, respectively. Relatively higher δ15N ranges in sodic
samples concur with higher sodic soil nitrogen availability and infer relatively higher
rates of nitrogen cycling. Relatively higher δ13C ranges in crest samples concur with
lower crest water availability and that crest grass species are more water use efficient.
Compared to the sodic patch, crest patches are relatively water and nitrogen limited
environments resulting in less fertile grass forage which is less utilised.