Browsing by Author "Holmes, P M"

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    Open Access
    A decision-making framework for restoring riparian zones degraded by invasive alien plants in South Africa
    (2005) Holmes, P M; Richardson, D M; Esler, K J; Witkowski, E T F; Fourie, S
    Riparian habitats in many parts of South Africa are severely degraded by invasive alien plants, especially trees. These invasions reduce water yields from catchments and affect riverine functioning and biodiversity. Initiatives are under way countrywide to clear alien plants from watercourses and surrounding catchments. Current understanding of key processes that regulate riparian functioning and define options for restoration is rudimentary. We review the impacts of riparian invasions and identify factors limiting the recovery of natural vegetation following alien clearance. We propose a framework of strategic interventions for optimizing restoration success. The framework identifies abiotic and biotic barriers to restoration at the scales of catchments and local reaches. In highly transformed catchments, interventions at the reach scale may fail if important barriers at the catchment scale are not addressed. The extent to which propagule supply and microsite conditions inhibit vegetation recovery is unknown. We also know little of the relative importance of dispersing vegetative propagules, dispersing seeds and soil-stored seed banks in vegetation dynamics, particularly after severe disturbances such as dense invasion by alien plants. The importance of geomorphological and hydrological factors in mediating recovery of riparian vegetation has not been adequately explored for all climatic areas in South Africa. More research is needed to determine the influence of different alien species and clearing treatments on the recovery of riparian vegetation. The literature strongly suggests that in highly alien-transformed catchments, the re-introduction of riparian species is required to promote recovery and suppress re-invasion. However, such interventions are unlikely to be widely implemented unless the cost: benefit ratios are favourable.
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    Open Access
    A comparative study of the seed bank dynamics of two congeneric alien invasive species
    (1989) Holmes, P M; Moll, Eugene J
    The unique vegetation in the lowlands of the fynbos biome is threatened by alien Acacia encroachment. The seed bank dynamics of the two most widespread invaders in the region, Acacia saligma and A. cyclops, was studied to elucidate those factors contributing most to their invasive success. This information was then used to assist in developing optimal control methods. On the basis of information available prior to this study, it was predicted that both species would have large, persistent seed banks in the so.il, and that seed bank processes would provide the - key to invasive success: namely, high seed longevity and heat-stimulated germination. Seed banks were monitored for several years following clearing of the parent stand, using either sites sampled in an earlier study, or sites providing chronosequences of clearing dates. Concurrently, a demographic study of the species' seed banks, including processes from seed rain through to seedling emergence and survival, was done in dense Acacia stands and in fynbos vegetation. Acacia saligma seed banks conformed to predictions, being large and persistent owing to high percentage viability and water-impermeable dormancy. Seed banks accumulate rapidly under dense stands and are "disturbance-coupled" as they have potentially high longevity unless stimulated to germinate by fire. Although A. cyclops seed banks also may be large and long-lived, they display variable percentage viability and dormancy, with the majority of a seed cohort surviving less than a year. Acacia cyclops seed banks do not respond to heat treatment and appear to be "disturbance-uncoupled".
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    Open Access
    An investigation of riparian vegetation recovery following invasive alien tree clearing in the Western Cape
    (2008) Blanchard, Ryan; Holmes, P M; Richardson, D M
    Riparian zones are dynamic, as a result of varying levels of disturbance from natural flooding regimes, and this makes them particularly susceptible habitats to invasion by alien plants. In South Africa, particularly the Fynbos Biome, closed-stand invasions by alien Acacia and Eucalyptus species have been able to develop within riparian areas. Their impacts on water resources and biodiversity have been countered by manual clearing in order to protect the valuable ecosystem services provided by intact riparian zones, as well as the biodiversity of indigenous communities. The Working for Water programme is tasked with the important role of controlling invasive alien plants with an assumption that indigenous vegetation will recover naturally. Current management objectives are to reduce above ground biomass of invasive alien plants by labour intensive means, after which indigenous vegetation is usually left to recover without further intervention. However, it is unclear to what extent natural recovery can be achieved. The main aims of this study were to ascertain the nature of riparian vegetation recovery, as well as determine which clearing treatment was most successful in promoting recovery. This was achieved by focusing on: 1) the recovery of species composition and biodiversity, 2) recovery of vegetation structure (assumed to be a surrogate for ecosystem function) and 3) whether a particular clearing treatment best promoted indigenous riparian vegetation recovery. Reference sites (control), as determined by Prins et aI., (2004), were compared to alien impacted sites in order to analyse variation among vegetation variables. Three initial clearing treatments were identified, namely: Fell Only (trees are felled and slash left on site), Fell & Remove (slash is removed from the riparian zone) and Fell & Bum (the slash is left for six months to a year before it is burnt).
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    Riparian vegetation recovery after invasive alien tree clearance in the Fynbos Biome
    (2008) Blanchard, R; Holmes, P M
    The Working for Water programme is tasked with the important role of controlling invasive alien plants with an assumption that indigenous vegetation will recover naturally. This study assessed vegetation composition and structure following alien clearance in closed-stand invasion of riparian areas and a minimum of two years' passive recovery. Three initial clearing treatments — Fell Only, Fell & Remove and Fell & Burn — were compared to uninvaded Reference conditions. The aim was to ascertain the nature of vegetation recovery, as well as to determine which clearing treatment was most successful in promoting recovery. A Detrended Correspondence Analysis revealed that the Fell & Remove treatment most closely approached the Reference condition while Fell Only and Fell & Burn plots had altered composition and structure. All clearing treatments had significantly lower vegetation cover than the Reference and species composition was altered by invasion and clearance. Important growth forms, such as small (3–10 m) trees were suppressed by felled slash and burning. Although burning was the best method to reduce woody alien species, secondary invasion by alien herbaceous species occurred where natural riparian vegetation did not re-establish. The Fell & Remove treatment is recommended as the best to use in promoting indigenous vegetation recovery, and together with continued alien follow-up control, is able to minimize alien re-invasion of riparian ecosystems. Managers are advised to consider active restoration measures in areas where recovery is likely to be protracted.
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    Open Access
    The effects of past management practices for invasive alien plant control on subsequent recovery of fynbos on the Cape Peninsula, South Africa
    (2004) Parker-Allie, F; Richardson, D M; Holmes, P M
    Integrated control aimed at reducing impacts of alien woody plant invasions on biodiversity is underway in many parts of the Cape Floristic Region. However, the kinds of control measures applied may themselves affect the recovery of the natural vegetation. In view of this, we investigated the effects of past management practices, viz: ‘bulldozing and aerial-spraying’, ‘aerialspraying’, ‘boom-spraying’ and ‘foliar-spraying’ aimed at clearing invasive alien woody plants on subsequent fynbos recovery in the Table Mountain National Park. Changes in soil (pH and depth) and vegetation (species cover, richness, diversity, evenness) properties, the total cover of species representing different dispersal guild, regenerative mode, and life form categories were compared between the differently treated and control plots at three different sites in the reserve. Only partial recovery of fynbos was observed in the ‘boom-sprayed’ and one of the ‘foliar-sprayed’ plots where measured species richness was significantly lower than that in control plots. However, marked changes in community structure were observed following ‘boom-spraying’ and ‘foliar-spraying’ at one site where a significant reduction in long-lived obligate reseeding species and an increase in the graminoid component was measured, though high fuel loads with possible post-fire erosion may also have aggravated the effects of foliar-spraying. Growth form structure changed the least following ‘aerial-spraying’ and ‘foliar-spraying’. Multivariate analysis of plant functional types indicated a greater similarity between the control plots than the differently treated plots, implying a treatment effect on subsequent fynbos recovery. However, past land use and residual effects of the alien woody invaders may also have contributed to the differential recovery in the treated plots. It is concluded that re-introduction of species in the under-represented guilds may speed up fynbos recovery. The recommendation is a comprehensive seed mix containing the major guilds, and an initial seed mix of fast-growing indigenous species to stabilise the soil.