Vegetation Cover Dynamics And The Environmental Threats To Biodiversity Conservation In Free State National Botanical Garden, South Africa

Abstract

The conservation gardens under South African Biodiversity Institute (SANBI) preserve biodiversity areas with unique and threatened vegetation types including national priority Critical Biodiversity Areas (CBA’s). The mandate of SANBI is to champion the exploration, conservation, appreciation, and enjoyment of South Africa’s exceptionally rich biodiversity for all people. Since the SANBI mandate emphasises sustainable biodiversity conservation, however, there were no studies that have quantitatively appraised the biodiversity conservation value and natural phenomena dynamics over time for different SANBI conservation gardens. This study determined the dynamics of the vegetation cover over a 30-year period (1987-2017) and potential environmental drivers of change in Free State National Botanical Garden (FSNBG). The FSNBG is located in the broader southern African grassland biome that is under huge conservation threats in South Africa. The FSNBG preserved national priority for biodiversity conservation (CBA1) in South Africa: the “Bloemfontein Karroid Shrubland”. The study applied the remote sensing technology and Geographical Information System (GIS) imagery to analyse the spatial vegetation cover changes that occurred over 30-years using 10 year’s intervals. Overall study results show that vegetation cover of the study area significantly increased (in term of woody biomass) over the 30 year period, partly attributed to the combination of factors, which include severe droughts and the rise of CO2 concentrations in the atmosphere. In addition, it was possible that absence of the biophysical disturbance such as fire and browsers in the natural vegetation to suppressed wood species has encourage encroachment of woody cover species. Using Normalized Difference Vegetation Index (NDVI) approach, the vegetation of FNSBG was divided into four categories, namely: Dense vegetation, Moderate vegetation, Sparse vegetation and open soil. Overall, the sparse vegetation cover significantly decreased over time (R2 = - 0.29). Similarly, the moderate vegetation cover increased by 25.1 ha while dense vegetation also increased by 8.6 ha. However, the bare soil cover was found to be relatively consistent over the 30-year period. Since the use of the remote sensing and GIS imagery could not clearly validate the current ecological status of CBA1 vegetation type, the manual field sampling approach was used looking into documenting the typical plant species provided by Mucina and Rutherford (2006); and measuring the vegetation cover. Vegetation cover of CBA1 was relatively high (i.e., 79.6 ± 15.9%); and this was above the minimum threshold of 60% habitat intactness reported to represent the habitat functionality that is normal although overall integrity is reduced. The existence of CBA1 vegetation type was supported by a record of 27 plant species out of 77 species documented as typical in the key descriptions provided by Mucina and Rutherford (2006). Major conservation threats entailed of 27 invasive alien plant species, human settlement encroachment have reduced the ecological buffer zone of FSNBG by 18% (119.07 ha) in the past 18 year; and 10 pests and pathogens that were interspersed in different vegetation patches of the study site. The observed changes suggested a need for regular vegetation monitoring of CBA1 vegetation type to manage the prevailing conservation threats that could have impact to ecological integrity. Animal-plant mutualisms are important for the maintenance of both animal and plant population since both participating partners’ benefit. The phenology of seed dispersal is critical to the successful recruitment of species since dispersed seeds require conducive conditions for germination. Having noted the increasing bush encroachment in FSNBG in the past 30 years, the study also investigated the seed dispersal phenology and the potential influencing factors for the encroaching tree/shrub species of FSNBG focusing on biotic interaction of flesh fruits resource foraged by birds as study model. During fieldwork sampling for the seed deposited by birds in the 17 roosting sites, I collected a total of 22 161 seeds belonging to 14 native and non-native tree/shrub species. It was found that black and red fruits were more preferred by birds since most of the dispersed tree/shrub species had those colours and attracted 22 resident and colonial migratory frugivorous birds. I found that 80% of the bird-ingested and dispersed seeds of many tree/shrub species were collected between March and July. Two species: Ehreta rigida and Searsia lancea, displayed relatively short fruiting peak times between August and December when other fruits were scarce. Whereas the observed two fruiting patterns could reduce the competition for seed dispersal services between two groups of tree/shrub species, the resident and wintering birds are also likely to benefit substance in food resources. In addition, it was found that frugivorous birds adapt to localised foraging patterns and to limited fruit diversity. It was concluded that the highly preferred tree/shrub species are likely drivers of the reported bush encroachment in FSNBG. Frugivorous birds disperse seed away from parent plant to new microsites that have relatively lower competition for the resources, and ingested seeds get scarified during feeding on fruits and ingestion by birds. Therefore, the study also investigated whether bird-dispersed seeds benefit from improved germination after their passage through the bird’s gut and the potential impact of seed density on competition at the microsites. On the germination trail results of bird-ingested seeds versus manual depulped seeds, Ziziphus mucronata displayed the highest germination rate of all trees/shrubs yet frugivorous birds did not improve its seed germination. However, only S. lancea seeds had significantly high seed germination after passage through the avian digestive tract. The test experiments for seed competition during germination at the microsite showed that only Z. mucronata and Olea europaea subsp. africana displayed significant positive performance with increasing seed density. The study concluded that the tree/shrubs species with high germination could account for the observed bush encroachment in FSNBG. It was recommended that adaptive conservation management plan of FSNBG should include monitoring and prioritised bush encroachment control of those species.