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Short term monitoring of soil and vegetation recovery following alien plant removal is required to reveal how ecological restoration is progressing. This study examined the recovery of soil physical properties and vegetation following Acacia mearnsii removal at Zvakanaka farm in Limpopo Province, South Africa. Soil and vegetation measurements were conducted in paired cleared, invaded and natural sites on 10 x 10 m plots. Results of the study show significantly (P < 0.001) higher soil moisture content in invaded and natural compared to cleared sites. Soil penetration was significantly (P < 0.001) higher in cleared than invaded and natural sites. Both infiltration rate and hydraulic conductivity showed no significant (P > 0.05) difference between the three sites. Strongly repellent soils were recorded in cleared sites only. Results showed a significant (P < 0.05) increase in measured diversity indices (species richness, Shannon-Wiener, Simpson’s and evenness index) in cleared and natural than in invaded sites. However, most secondary woody invasive alien plants were recorded in cleared sites. The study concludes that A. mearnsii clearing triggers varying changes in soil physical properties. Although native plants are present in cleared sites, recovery may be hampered by the growth of secondary woody invasive alien plants. [ABSTRACT FROM AUTHOR]

Abstract: Riparian ecosystems in South Africa's fynbos biome are heavily invaded by alien woody plants. Although large-scale clearing of these species is underway, the assumption that native vegetation will self-repair after clearing has not been thoroughly tested. Understanding the processes that mediate the recruitment of native species following clearing of invasive species is crucial for optimising restoration techniques. This study aimed to determine native species recovery patterns following implementation of different management interventions. We tested the influence of two clearing treatments (“fell & remove” and “fell & stack burn”) on the outcomes of passive restoration (natural recovery of native riparian species) and active restoration (seed sowing and planting of cuttings) along the Berg River in the Western Cape. Under greenhouse conditions we investigated seed viability and germination pre-treatments of selected native species. There was no recruitment of native species in sites that were not seeded (passive restoration sites), possibly because of the dominance of alien herbaceous species and graminoids or the lack of native species in the soil-stored seed bank. Germination of our targeted native species in the field was low in both “fell & remove” and “fell & stack burn” treatments. However, “fell & stack burn” gave better germination for the species Searsia angustifolia, Leonotis leonurus and Melianthus major. Seedling survival in the field was significantly reduced in summer, with drought stress being the main cause for seedling mortality. Germination rates in the greenhouse were high, an indication that harvested seeds were viable. Most seeds germinated without germination pre-treatments. We conclude that failure of native seeds to germinate under field conditions, secondary invasion of alien herbs and graminoids, the lack of native species in the soil-stored seed bank, and dry summer conditions hamper seedling establishment and recovery on sites cleared of dense stands of alien trees. For active restoration to achieve its goals, effective recruitment and propagation strategies need to be established. [Copyright &y& Elsevier]

Abstract: The impacts of invasive alien plants (IAP) and their subsequent clearing by the Working for Water Programme (WfW) on (a) overstorey (woody plant) vegetation structure, (b) invasion intensity (overstorey aerial cover of woody alien plants) and (c) ground cover, in a temperate to subtropical riparian ecosystem were studied in 1996/7 and again in 2005, in order to provide a longer-term perspective on the effectiveness of WfW clearing. Forty 1000 m2 plots were surveyed and resurveyed, comparing between (a) higher altitude Grassland and lower altitude Savanna, (b) high (>50% invasion intensity) versus low (<50% invasion intensity) alien invasion sites, and (c) WfW cleared versus uncleared sites (the three ‘treatments’). Pre-clearing estimates from cut stumps in 1996/7 indicated high alien invasion intensities of 72±8% in Grassland and 69±11% in Savanna. From 1996/7 to 2005 there was a large decrease in aerial cover of alien trees of >5 m and to a lesser extent 2–5 m in height, and a large increase in alien plants of <2 m. Hence WfW was initially successful, with the original tall Eucalyptus grandis tree layer largely removed. However, total invasion intensity remained unchanged over the first decade (30.4±4.6% in 1996/7, 31.9±3.2% in 2005). From 1996/7 to 2005, grass and herbaceous cover decreased, while bare soil and litter increased, indicating reduced surface stability. This was in response to (a) the major flood event of February 2000, (b) the effects of IAP invasions and (c) WfW clearing. Total ground vegetation cover was negatively related to alien aerial cover in both biome reaches in 1996/7 and 2005. By 2005, there were no longer any differences in the aerial cover of woody alien plants in response to the original 1996/7 invasion intensity or clearing ‘treatments’, and hence progressive homogenization of IAP cover. Aerial cover of woody indigenous plants also responded negatively to increasing alien aerial cover in 1996/7 and 2005 in both Savanna and Grassland. In conclusion, the nature of the IAP problem has changed from dealing largely with relatively few large E. grandis trees in the mid-90s, to the present large suite of invasive species with numerous smaller individuals. This has implications for the time needed for clearing. This is one of few studies to have assessed the initial and longer-term (1995–2005) effectiveness of WfW clearing operations. It shows that improved IAP clearing protocols are needed. More follow-up treatments are recommended to ‘capture’ alien resprouts and new seedlings before they establish and reproduce. Secondly, integrating clearing with restoration of the tall indigenous riparian canopy tree species in heavily invaded sites would help to shade out many alien recruits. [Copyright &y& Elsevier]

Abstract: Recovery of indigenous species subsequent to the clearing of invasive alien plants (IAPs) is crucial for ecosystem recovery to occur. However, cleared sites are often just left in the hope that revegetation will occur naturally. In riparian areas of Kruger National Park (KNP), the Working for Water (WfW) Programme has cleared IAPs on a regular basis, but little post-clearance monitoring has taken place. Thus investigating short-term effects of IAPs and IAP clearing on plant community diversity and vegetation recovery provided an ideal opportunity to assess feasible targets of natural ecosystem recovery in similar areas. Vegetation was sampled from twelve transects along the Sabie River in and adjacent to the KNP, before (March/April 2006) and after (March 2007) the annual clearing of IAPs by WfW. Rarefied species richness, alpha diversity and evenness of distribution of species all declined with increasing density of IAPs (P <0.05). There was a mean reduction in IAP density of 80% (S.E±6%) (P =0.002) through the clearing by WfW. After clearing of IAPs, indigenous vegetation densities increased, with herbaceous growth forms showing the largest increase in transects that were previously heavily invaded. Thus, in this system, which is relatively undisturbed by human activities, initial recovery of indigenous vegetation can occur without further restorative interventions. This process is more than likely aided by the continuous clearing of IAPs by WfW as this acts to deplete alien seed banks and maintain IAPs at acceptable and manageable levels. [Copyright &y& Elsevier]

Abstract: Solanum mauritianum Scopoli (Solanaceae; ‘bugweed’) is a major weed of natural vegetation and plantations in the eastern higher rainfall regions of South Africa. This study assessed the aerial cover and density of S. mauritianum, prior to and after Working for Water (WfW) clearing on the Sabie River, in both higher altitude grassland and lower altitude savanna reaches, using 40, 50×20 m (modified Whittaker) plots. Clearing of woody plants involves both cutting the stem and then applying herbicide to the cut stump. Despite clearing, S. mauritianum aerial cover rapidly recovered in the initial 1996/7 study period, and overall densities remained similar in 1996/7 (873 plants/ha) and 2005 (838 plants/ha). The study then focussed on the regeneration capacity of S. mauritianum within these sites. Although seed bank densities in 1996/7 were highly variable, mean densities between the 8 treatment combinations (biome, invasion intensity, cleared vs. uncleared) were high throughout (47–554 seeds/m2), showing that the invasion potential remained high despite clearing. Seed densities were higher in savanna than grassland (P =0.0001), and under high- compared to low invasion intensity (P =0.0046). Particularly noteworthy was the lack of a significant clearing effect, or any significant interactions. High proportions of cut S. mauritianum resprouted after clearing (40–80%). Cutting lower on the stem increased mortality substantially, with cutting lower than 18 cm resulting in 100% mortality, while cutting above 50 cm resulted in 100% resprouting recovery. Variation in basal stem (stump) diameter of cut plants had no effect on survival. Therefore the effectiveness of WfW clearing of S. mauritianum can be significantly improved. Seed production occurs on plants of ≥1.50 m tall, and increases linearly with increasing plant height, reaching in the order of 100000–200000 seeds/plant per annum for plants of 3.3 m in height. Seed persistence increased with depth of burial and was higher under canopy shade versus ‘open’ in the field. Seed half life ranged 11–16 months. Hence seedling emergence from the dense relatively long-lived seed bank is a considerable challenge. More thorough and frequent follow-up treatments will be essential to maintain the improved situation after clearing. Exploring a tighter integration of clearing efforts with (a) the restoration of the tall indigenous riparian canopy tree species at some sites, (b) a more strategic application of follow-up clearing to target newly emerging seedlings after major disturbances such as severe floods, and (c) a more strategic application of biological control, should be pursued. [Copyright &y& Elsevier]