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.
Highlights
Riparian habitats provide many ecosystem services, including riverbank stabilisation, nutrient cycling, flood attenuation, regulation of streamflows and stream temperatures, groundwater recharge and water purification (Richardson et al, 2007)
To achieve our aims we addressed the following questions: (1) Which clearing method is most effective for promoting natural recovery of native species? (2) How effective is active restoration for restoring indigenous vegetation following two treatments for removing stands of the invasive tree Eucalyptus camaldulensis: fell & remove and fell & stack burn? (3) Were seeds of introduced native species viable and which germination pre-treatment is appropriate for each of them?
The recorded high frequencies of alien herbs and graminoids during spring translated into significantly (P b 0.001) higher cover of these two growth forms in fell & remove (F&R) sites compared to invaded sites (IS) and natural sites (NS: Table 2)
Summary
Riparian habitats provide many ecosystem services, including riverbank stabilisation, nutrient cycling, flood attenuation, regulation of streamflows and stream temperatures, groundwater recharge and water purification (Richardson et al, 2007). Alien species diversity and abundance have increased in riparian systems worldwide (Hood and Naiman, 2000; Richardson et al, 2007). Most rivers in South Africa's fynbos biome are lined by dense stands of Australian Acacia and Eucalyptus species (Forsyth et al, 2004; Richardson and Van Wilgen, 2004; Meek et al, 2010, 2013). These invasions have displaced native species (Richardson et al, 1997; Richardson and Van Wilgen, 2004) and have caused significant changes to both above- and below-. Alien tree invasions have substantially reduced streamflow (Dye and Poulter, 1995)
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