P.M. Holmes

Diversity, composition and guild structure relationships between soil-stored seed banks and mature vegetation in alien plant-invaded South African fynbos shrublands

We investigated vegetation-seed bank relationships at three fynbos sites on the Cape Peninsula, South Africa, and the impacts to these sites of invasion by the alien tree Acacia saligna. Soil-stored seed banks in uninvaded fynbos were of a similar density to those previously measured in fynbos (ca. 1100–1500 seeds m-2) and were dominated by mostly short-lived species. Lack of similarity between mature vegetation and seed banks, suggests that seed banks are poor predictors of mature vegetation composition and structure in fynbos. This lack of correspondence was attributed to the ephemerals (present only in the soil seed bank) and the dominance of serotinous (aerial seed bank) and sprouting (soil seed bank low to absent) species, in mature vegetation. Long-lived seeders were among the 10 most abundant species in the seed banks at all sites and at two sites shrub species contributed more to seed bank richness than any other growth form. Soil-stored seed banks, therefore, boost species richness and diversity both in early post-fire and later seral stages.There was a decline in fynbos species richness, diversity and abundance both in the standing vegetation and seed banks with increasing duration of invasion by the alien tree, Acacia saligna. However, the rate of decline was higher for the vegetation than the seed banks, suggesting that many fynbos species have long-term persistent seed banks. At two sites, there was no obvious shift in community composition associated with Acacia invasion: invaded sites were depauperate versions of the uninvaded site. However, at a third site, the vegetation composition shifted towards a community dominated by bird-dispersed thicket species and its seed bank shifted towards a community dominated by wind-dispersed perennials. Community composition of the soil seed banks under dense, recent Acacia was very similar to that of the corresponding uninvaded fynbos at all sites, indicating that there is good potential to return to species-rich fynbos vegetation after removal of the alien Acacia. Most seed bank species persisted in the soil seed bank of the long-invaded fynbos at low frequency and density, indicating high seed longevity in many species. We suggest that either a thick Acacia litter layer or a deep (>5 cm) burial moderated the fire and ambient temperature effects, preventing these seeds from germinating after fire and thus preventing loss from the seed bank.

Effects of dry heat on seed germination in selected indigenous and alien legume species in South Africa

The effect of dry heat on the percentage germination, germination rate and lethal temperatures of selected legume species was investigated by exposing seeds to different intensities and durations of heat. Two alien invasive legumes (Acacia saligna and A. cyclops) and two indigenous legume species: Podalyria calyptrata, which is a fynbos species, and Virgilia oroboides, a forest precursor, were tested. Results were compared to predictions based on published accounts of the species’ ecology. Acacia saligna and P. calyptrata fitted predictions: high percentage germination, fast germination rates and a tolerance to high temperatures; with A. saligna outperforming P. calyptrata in the latter two responses. Both A. cyclops and V. oroboides did not conform to predictions: dry heat had little positive effect on germination, and in A. cyclops it had a negative effect.

Patterns of seed persistence in South African fynbos

In fire-prone communities such as fynbos, many species rely on regeneration from seed banks in the soil. Persistent seed banks are particularly important for species with life spans shorter than the average fire cycle, in order to counter local extinction. Persistent seed banks also give potential for restoring ecosystems following disturbances such as alien plant invasion. This study investigated the seed persistence patterns of 25 perennial species, representing several growth forms and life histories, during a three-year burial. Long-term persistence (i.e., seed bank half-life exceeding two years) was found in the hard-seeded Fabaceae and Pelargonium, and the nut-fruited Proteaceae. In this group, germinability was low and dormancy increased further following burial, resulting in a highly viable, dormant seed bank after three-years burial. A second group with potentially long-term persistent seeds includes four taxa (Pseudopentameris, Passerina, Elegia and Restio) that either have low germinability or develop secondary dormancy following burial. Dormancy in the latter group was partially countered by exposure to smoke-seed primer. Of the small-seeded species, only two Erica species with high initial dormancy had long-term persistent seed banks. The other species mostly displayed high initial germinability and short-term persistent seed banks (i.e., seed bank half-life less than two years). This group included taxa with short to medium life-spans (Syncarpha, Roella) that were expected to have long-term persistent seeds in order to buffer against local extinction following average to long fire-return intervals. We hypothesize that light may play a role in overcoming secondary dormancy in those species, and could have resulted in an underestimate for seed persistence in this study. Alternatively, those short to medium life-span species persist via inter-fire recruitment in gaps or long-distance dispersal (of the smallest seed). No correlations were found between seed persistence and seed mass or variance in seed dimensions. Nor was a correlation found between seed persistence and phenol concentration. In fynbos, seed burial of larger seeds by ants and rodents are major processes that operate in conjunction with passive burial of small seeds. Selection for persistence can be expected to operate across all seed sizes and shapes in fire-prone communities.

Insights and Opportunities Offered by a Rapid Ecosystem Service Assessment in Promoting a Conservation Agenda in an Urban Biodiversity Hotspot

Regional and global scale ecosystem service assessments have demonstrated the socioeconomic value of protecting biodiversity and have been integrated into associated policy. Local government decision makers are still unsure of the applicability, return on investment, and usefulness of these assessments in aiding their decision making. Cape Town, a developing city in a globally recognized biodiversity hotspot, has numerous competing land uses. City managers, with a tightly constrained budget, requested an exploratory study on the links between ecosystem services and biodiversity conservation within this municipal area. We set out to develop and test a simple and rapid ecosystem service assessment method aimed at determining the contribution natural vegetation remnants make to ecosystem service provision. We took selected services, identified in conjunction with city managers, and assessed these in two ways. First we used an area weighted approach to attribute services to vegetation types and assessed how these had changed through time and into the future given development needs. Second, we did a regulatory and cultural service remnant distance analysis to better understand proximity effects and linkages. Provisioning services were found to have been most severely affected through vegetation transformation. Regulatory services have been similarly affected, and these losses are more significant because regulatory services can only function in situ and cannot be outsourced in the way provisioning services can. The most significant losses were in coastal zone protection and flood mitigation services, both of which will be placed under even greater pressure given the predicted changes in climatic regimes. The role of remnant vegetation in regulating and cultural services was shown to be a significant additional consideration in making the case for conservation in the city. Our rapid assessment approach does not allow for nuanced and individual understanding of the trade-offs presented by individual remnant patches, but is particularly strong in quickly identifying issues, key focus areas, and opportunities provided by this research direction, and thereby serving to facilitate and drive constructive engagement between ecosystem service experts and city planners.

Can Cape Town's unique biodiversity be saved? Balancing conservation imperatives and development needs.

Cape Town is an urban hotspot within the Cape Floristic Region global biodiversity hotspot. This city of 2,460 km2 encompasses four local centers of fynbos plant endemism, 19 national terrestrial vegetation types (six endemic to the city), wetland and coastal ecosystems, and 190 endemic plant species. Biodiversity in the lowlands is under threat of extinction as a result of habitat loss to agriculture, urban development, mining, and degradation by invasive alien plants. Cape Town’s population is 3.7 million, increasing by an estimated 55,000 people/yr, which puts pressure on biodiversity remnants for development. South Africa is a signatory to international instruments to reduce biodiversity loss and has a good legislative and policy framework to conserve biodiversity, yet implementation actions are slow, with limited national and provincial support to conserve Cape Town’s unique and irreplaceable biodiversity. The lack-of-action problem is two-fold: national government is slow to implement the policies developed to realize the international instruments it has signed, with conservation initiatives inadequately funded; and local governments are not yet recognized as important implementation partners. A further problem is created by conflicting policies such as the national housing policy that contributes to urban sprawl and loss of critical biodiversity areas. The City’s Biodiversity Management Branch, with partners, is making some headway at implementation, but stronger political commitment is needed at all levels of government. Our objective is to improve the status and management of biodiversity in existing conservation areas through the statutory proclamation process and management effectiveness monitoring, respectively, and to secure priority areas of the BioNet, Cape Town’s systematic biodiversity plan. The most important tools for the latter are incorporating the BioNet plan into City spatial plans; communication, education, and public awareness; negotiating appropriate management of public land; and conservation stewardship on private land. The timeframe to save Cape Town’s biodiversity is short, and it is unlikely that full success will be achieved without national or international funding and political will at all levels of government.

Effects of different clearing treatments on the seed-bank dynamics of an invasive Australian shrub, Acacia cyclops, in the Southwestern Cape, South Africa

Abstract Acacia cyclops is an invasive shrub of natural vegetation in the fynbos biome of South Africa. Its large soil-stored seed banks make eradication difficult. In this study, a dense, mature stand of A. cyclops was cleared using four commonly applied treatments. Few seeds above 30-mm depth in the soil profile survived burning treatments, and percentage viability was reduced at depths up to 40 mm. Seed germinability was not enhanced by fire. Seedling emergence and mortality, monitored over 13 months post-clearing, were much higher following non-burning and ‘fell and burn’ treatments than ‘burn standing’ and ‘fell, pile and burn’ treatments. Nevertheless, the pattern of seedling emergence and survival was similar in all treatments, with survival low until October, 9 months post-clearing. Although all treatments resulted in significant reductions in seed density 1 year post-clearing, burning treatments were more effective than the non-burning treatment. The ‘burn standing’ treatment killed more seeds than the ‘fell and burn’ treatment, and may be recommended where labour costs are prohibitive. Since all treatments induced a rapid seed-bank decline in A. cyclops , it is recommended that the choice of treatment be dictated by other factors, such as possible effects on the recovery of indigenous plant species.

Decay rates in buried alien Acacia seed populations of different density.

Decay rates in seed populations of Acacia cyclops and A. saligna of known viability and dormancy were determined after one and two years of burial at different densities. Decay rates differed significantly between the two species, but there was no clear effect of density. Decay rates were high, despite an initially high level of dormancy: an average of 97% and 45% of A. cyclops and A. saligna seeds, respectively, were lost in the first year. Decay rates were significantly lower in the second than in the first year and it is predicted that decay rates would continue to decline in seeds surviving more than two years. Acacia seed populations in the soil do not have a continuous and constant death risk and consequently do not fit the log-linear (i.e. Deevey Type II) survivorship curve reported in the literature for soil-stored seeds. As a small proportion of the Acacia seed population may have great longevity, clearing operations should incorporate a programme of periodic follow-up control.

Implications of alien Acacia seed bank viability and germination for clearing.

Percentage viability of A. saligna and A. cyclops seed banks was determined for stands cleared of mature plants for different time intervals. Seedling emergence following recent clearing was monitored for a year at one site of each species. Percentage viability of A. saligna seed banks was uniformly high and was not affected by age since clearing. Percentage dormancy in A. saligna was high since few seedlings emerged in the field (1% of viable seed bank), but after fire, 70% of the remaining viable seed bank germinated. Percentage viability of A. cyclops seed banks differed significantly among sites, with the proportion of viable seeds in the soil increasing significantly with age since clearing at one site. Percentage germination and viability of fresh A. cyclops seeds also differed significantly among sites. Although seedling emergence in A. cyclops (9% of viable seed bank) exceeded that in A. saligna, pre-emergence mortality was implicated as the more important process in A. cyclops seed bank decline. In order to markedly reduce seed banks, it is recommended that A. saligna stands should be burnt and A. cyclops stands cleared in any way which removes the vegetation cover.

Effects of shade on seedling growth, morphology and leaf photosynthesis in six subtropical thicket species from the eastern Cape, South Africa

Abstract We investigated the plasticity of seedlings to shade in six shrub species common in subtopical thicket, in order to assess their microhabitat preferences for regeneration. Cassine peragua and Sideroxylon inerme , which also occur in adjacent forests, were the most shade tolerant and would be unlikely to require canopy gaps for recruitment: their relative reduction in growth in dense shade was slight, as was their increase in leaf weight ratio (LWR); leaf inclination was adjusted to near horizontal in dense shade and maximum net photosynthetic rate ( A max ) remained fairly low in sun-acclimated plants. Rhus glauca , which also occurs on forest margins, demonstrated the highest growth rates, especially in the open, and together with its high potential A max is well-equipped as a pioneer, and would benefit from canopy gaps for establishment. Cassine aethiopica , which is also common in coastal forests, was slower growing than Rhus , but less shade tolerant than C. peragua and Sideroxylon and would probably benefit from canopy gaps for establishment. Pappea capensis and Schotia afra , which also occur in open savanna vegetation, demonstrated a growth and morphological pattern indicative of species adapted to periodic drought, in having a high ratio of woody conductive tissue to leaf tissue with accompanying slow growth. Their relatively poor performance in deep shade suggests that canopy gaps would benefit their recruitment. We concluded that canopy gaps may be essential for recruitment of the full species complement in sub-tropical thickets.

A reference framework for the restoration of riparian vegetation in the Western Cape, South Africa, degraded by invasive Australian Acacias

Riparian vegetation, which normally differs structurally and compositionally from surrounding vegetation, has been degraded in many parts of the fynbos biome by several species of invasive Australian Acacia . Systematic clearing of dense stands of these alien trees was initiated in 1995, and information is urgently needed to guide the restoration of riparian habitats. A problem is that degradation of these communities is so advanced and widespread that in many cases managers do not know what species to use in restoration, or what kinds of target communities to aim for. This studys aim was to provide baseline information on riparian plant community structure and composition from non-transformed habitats. Species and environmental data were recorded from 76 sites located along the headwater systems of six rivers in the southwestern part of the Western Cape province. Analysis of the data, applying multivariate classification (TWINSPAN) and ordination (Detrended Correspondence Analysis) techniques, identified four prospective plant communities: 1) a Nivenia corymbosa - Brachylaena neriifolia Community; 2) a Leucadendron salicifolium - Berzelia lanuginosa Community; 3) a Cliffortia ruscifolia - Metrosideros angustifolia Community; and 4) a Kiggelaria africana - Brabejum stellatifolium Community. These formed a continuum with only the Leucadendron and Kiggelaria communities separating in ordination space. Soil pH differed between the latter two communities, reflecting different geology. It was found that many riparian specialist species are relatively widespread. For the study area, it is concluded that where information on the historical composition of riparian communities is lacking, target communities for restoration can be defined from pristine communities with similar geology, and secondly, altitude. In all cases the target community will comprise a large proportion of widespread, predominantly resprouting, riparian species.