Abstract
Pteronia paniculata is an indigenous, unpalatable shrub that invades mismanaged Karooveld, resulting in degraded rangelands with low species diversity and grazing potential. We conducted a series of trials in the Succulent Karoo Randteveld near Barrydale to determine if the uniform defoliation of P. paniculata dominated vegetation at two heights (0.05 m and 0.20 m above ground level) using a brush-cutter and, in one trial, application of a second cut will improve the plant species composition, productivity and grazing capacity of the veld. Brush-cutting treatments and the uncut control all resulted in a change in species composition towards greater species diversity and more palatable species and an average increase of 540 kg ha–1 (28%) in above-ground biomass over four years. It appears that there was a pervasive improvement in species composition associated with a general decline in the cover and abundance of P. paniculata over the time-scale of the present study that was not influenced by the defoliation treatments, except for the 1996-cut treatment where the cover of P. paniculata increased. The absence of propagules of palatable species in the soil seed bank and competition from P. paniculata (a long-lived, perennial shrub) are assumed to be among the main reasons for the lack of response of the vegetation community to the defoliation treatments. Brush-cutting (in the absence of reseeding), aimed at reducing the dominance of unpalatable karoo shrubs, was more costly but not significantly better than long-term resting in improving veld composition or forage production.
Introduction
Mismanagement, for example by prolonged overgrazing by domestic livestock, is widely recognised as a direct agent of rangeland degradation in semi-arid rangeland (Milton and Dean 1995, Khresat et al. 1998, Hoffman and Ashwell 2001, IFAD 2007, Syngenta Foundation for Sustainable Agriculture 2007) and more mesic grassland (Frank et al. 1998). Holechek (2002) and Ralphs (2002) found in studies conducted in deserts, grasslands and woodlands in the USA and Mexico that poor grazing management, especially high grazing intensity, resulted in an increase of unpalatable species and a decrease in palatable species.
Degradation of the arid Succulent Karoo due to overgrazing can result in an increase of unpalatable species such as Pteronia paniculata Thunb. (Roux and Vorster 1983, Milton and Dean 1994, Wiegand and Milton 1996, Burger 2001). Rehabilitation of overgrazed arid or semiarid shrublands cannot be achieved by the withdrawal of livestock alone (Westoby et al. 1989, Milton and Dean 1994, Stokes 1994,
Wiegand et al. 1995, Wiegand and Milton 1996). According to Wiegand et al. (1995) and Wiegand and Milton (1996) the rehabilitation of overgrazed rangelands dominated by unpalatable shrubs is hindered by demographic inertia and lag-effects: once established, the unpalatable species dominate for a long time because of long life-spans, and they therefore do not necessarily facilitate succession to better alternative states (Milton and Dean 1994). The change in species composition of vegetation can possibly be explained by the ‘state-and-transition’ model, which suggests that unpredictable and predictable impacts such as a variable climate (natural events) and overgrazing (human activities) can change the vegetation from one state to another (Briske et al. 2008), and the movement from one state to another is not necessarily reversed by succession (Milton and Hoffman 1994).
In arid environments degradation may be rapid, but the recovery, if any, is slow because plant growth is limited by soil moisture availability owing to low, erratic rainfall and a high evaporative demand. Loss of plant cover further reduces rainfall efficiency (Milton et al. 1994, Wiegand et al. 1995). Seedling recruitment will normally take place only once competition from the established (long-lived) plants is removed and further interventions will be required as seedlings do not recruit on bare, undisturbed soils in other Karooveld types (Milton 1995, Visser et al. 2007). The removal of competition from long-lived unpalatable plants is normally due to prolonged drought, age and/or physical disturbance that results in their death. Midgley and Moll (1993) and Milton (1995) state that competition in Karoo shrublands is primarily for soil moisture, a resource that is depleted rapidly by established Karoo shrubs, such as by P. paniculata. Pteronia paniculata (Asteraceae) occurs over vast areas in the Succulent Karoo, especially in the higher-lying areas.
It is an evergreen indigenous shrub that is unpalatable and poisonous to livestock (Shearing and van Heerden 1994, Esler et al. 2006). Pteronia paniculata increases in areas subjected to sustained mismanagement (Milton and Dean 1994, Burger 2001). Cultivation of natural veld or mismanagement of herbivores leads not only to degradation of the vegetation but also of the physical and chemical attributes of the soil (Stokes 1994). Plants that colonise degraded arid environments tend to be unpalatable, long-lived, hardy perennials and/or unpalatable plants whose abundance may be maintained by prolific seed production even if the plants do not have a strong competitive effect on, or response to, neighbouring plants (Stokes 1994). Vegetation composition and cover influence the amount of palatable herbage available to grazing/browsing livestock. Changes in plant species composition towards unpalatable shrubs and loss of cover thus reduce the carrying capacity of rangelands and the productivity and economic potential of the rangeland to render various services, especially forage production for livestock (Milton et al. 1995). The objectives of this paper are to test the hypothesis that the uniform defoliation of P. paniculata dominated vegetation using a brush-cutter would result in: (1) a reduction in the cover of P. paniculata, (2) improve plant species
composition towards a higher proportion of more palatable and forage-productive species and less undesirable species, and (3) promote the establishment and productivity of palatable species. The purpose of the trial was to find an effective and financially viable method of restoring vegetation and carrying capacity of degraded vegetation in Succulent Karoo Randteveld, based on the results of the treatments on measurable parameters (1–3).