Potential of selected Karoo plant species for rehabilitation of old fields

[摘要] ENGLISH ABSTRACT: The passive recovery of old fields in the Karoo is a slow process, hampered by lowand erratic rainfall, poor seed germination due to limited availability of suitable microsites for seedling establishment, competition from existing vegetation, altered soilproperties and the reduction of key soil biotic processes. The objectives of the studywere to investigate the role of seed banks in the recovery of old fields, and to identifypossible plant species and methods of establishing these species with the primaryaim of initiating the process of succession / recovery of old fields in the Little Karoo.The investigation of the seed bank addressed the following issues: the resemblanceof the seed bank to the above-ground vegetation in an old field and the effect ofdisturbance on the seed bank. Furthermore, the role of propaguie migration wasinvestigated to establish possible propaguie movement from undisturbed to disturbedareas was investigated. The study indicated that the perennial seed bank had a 31 %similarity to the above ground vegetation in the old field. The seed bank wasdominated by annual species. In the above-ground vegetation perennial canopycover was higher compared to annual cover. The perennials with the highestdensities in the soil seed bank were disturbance-adapted species with littleimportance for grazing animals except perhaps in the short-term. Disturbance causedannual densities to increase and perennial densities to decrease. The investigation ofpropaguie migration compared adult canopy cover and seedling densities. Theresults show that perennial distribution was patchy and that propaguie migration islow to non-existent. This led to the conclusion that old fields require supplementalseed additions.A total of seven species were assessed for their restoration potential. The speciesused in the investigation were Tripteris sinuata, Ruschia spinose, Drosanthemumspeciosum, Indigofera sessifolia, Pteronia incana, Ehrharfa calycina andChaetobromus dregeanus. Seed viability was examined using one of two techniquesie. tetrazolium or a standard germination technique. The optimal temperature forgermination was determined using the following temperature regimes: 15°C day /1DoC night, 20°C day /1 DoC night and 30°C day / 15°C night. The temperature rangewith the best performance was 20°C day / 1DoC night indicating that species shouldbe sown in autumn or early winter. This timing coincides with the onset of rains in thisregion.The field trial investigated the influence that various mechanical cultivationtechniques (ploughing, disking, tilling and clearing) and soil amendments (seed,seed+aquasorb and seed+straw+branches) have on the establishment of theselected species. Seed germination and seedling survival was monitored. Theinfluence of treatments on water infiltration and soil moisture was investigated. Onlyfour of the seven species germinated (Tripteris sinuata, Ehrharta calycina,Chaetobromus dregeanus and Pteronia incana). As far as species performance wasconcerned, T. sinuata performed best followed by E. calycina and C. dregeanus,while P. incana failed to persist. The cultivation treatments that yielded the bestresults were tilling, disking and ploughing. Emergence success in cleared anduntreated plots was relatively low. As far as seedling emergence was concerned themost appropriate soil amendments were seed+aquasorb, seed andseed+straw+branches. Although soil moisture was higher on-heuweltjies than offheuweltjiesthere was no significant difference in seedling emergence and survivalbetween these localities. Even though soil moisture was higher inseed+straw+branches treatment than in seed+aquasorb and seed treatments,seedling emergence in this treatment were lower than in the two latter treatments.This clearly indicates that soil moisture is not the only factor that influences theestablishment of species.In the trial a mixture of late successional and pioneer species were sown, primarilywith the aim of initiating the process of succession! recovery of old fields. Contrary towhat was expected the late successional species germinated first. This has led to theconclusion that these late successional species have no innate dormancy, furtherproved by the inability of species to germinate after the second season. It could thusbe that these late successional species have a short live span, and that theygerminate when conditions are favourable. It must also be kept in mind that the seedsown were freshly harvested, and it could be that the pioneer species needed anafter-ripening period before they germinated.

[发布机构] Stellenbosch University