Impacts on individual savanna plants growth
Typically, primary production in a South African savanna is shared equably and persistently between two different life forms, trees (C3 photosynthetic pathway) and grasses (C4 photosynthetic pathway) (Scholes and Walker, 1993). The C4 photosynthetic pathway evolved in the Tertiary period when atmospheric CO2 levels were low, which translated into a reduction in photosynthesis and thus reduced growth. Under conditions of elevated CO2 conditions these responses are reversed with trees exhibiting enhanced photosynthesis and growth. However by the middle of this century, CO2 concentrations will exceed the threshold at which C4 plants have a photosynthetic advantage over C3 plants and this will be a critical factor in projecting changes in ecosystem functioning (Bond 2008).
Savanna plants are not predicted to be overly sensitive to increases in temperature as savanna seedlings can germinate at a wide range of high temperatures (Choinski and Touchy, 1991) and although most savanna plants can tolerate maximum temperature extremes, Chidumoya (2001) showed that some savanna trees are sensitive to seasonal highs. The interaction of temperature and rainfall may however, have different impacts on specific plants. The life-history stage, in savanna plants, that is most susceptible to environmental variation is the seedling stage. Savanna seedlings per se are tolerant of drought stress (Hoffman et al., 2004) but their initial success is dependent on frequent, not necessarily high, rainfall for the first 7 weeks for successful establishment and seedling survival (Wilson and Witkowski, 2002). The predicted increase in rainfall variability could therefore change the amount of time available for successful seedling recruitment.
The increased levels of CO2 in the atmosphere have a fertilising impact, particularly on woody plants (Ainsworth and Long, 2005; Scheiter and Higgins, 2009) and can enhance water use efficiency (Hughes 2003) through reducing stomatal conductance, which in turn could increase the soil water balance (Schieter and Higgons, 2009; Fischlin et al., 2007) Yet the increase in temperatures can favour the growth of C4 herbaceous grasses (Epstein 2002; Scheiter and Higgins, 2009).
Currently, there is a disagreement as to the extent CO2 will impact on the tree-grass combination in savannas. The response of trees to more CO2 is also modified by the amount of Nitrogen available as it impacts the tree-CO2 interaction (McMurtie et al., 2008).
Changes in the phenology and physiology of plants and animals
Phenology is the timing of seasonal activities of animals and plants, many of which are sensitive to climate (IPCC 2007). Changes in the phenology of plants and animals will impact conservation areas as it may in future change the species assemblages of these areas. Due to the changing climate a disconnect occurs between the timing of behaviour and the available resources on which the behaviour depends. The most clearly recordable change is the change of bird migration and plant phenology, most of which has been recorded in the northern hemisphere. These long term data sets have revealed that changes in the timing of spring activities (earlier breeding or first singing of birds), earlier arrival of migrant birds, earlier appearance of butterflies amongst others have in general, occurred progressively earlier since the 1960s (Walther 2002). Unfortunately, few such data sets exist in southern Africa, but recent work by the South African Bird Atlas Project has shown that on average, barn swallows (a common species in the K2C area) now leave South Africa 11 days earlier than they did a decade and a half ago, and now arrive back in SA on average 9 days later than they used to. It is not yet clear if this change is due to changes in the South African or European climate (SABAP2)
Larger scale impacts on communities: Range shifts and biomes shifts
Following on from the previous section, the individual impacts of these changes will scale up to have several ecosystem responses; including the range and distribution shifts of species and communities, the composition of and interactions within communities, and the structure and dynamics of ecosystems (Walther 2002). The impacts expected in savannas are perhaps not as dramatic as those elsewhere in the world, yet these areas will still be vulnerable to species range shifts and biomes switches.
Plants and range changes
Rutherford et al. (1999) predicted that several dominant savanna plant species would undergo dramatic range shifts which will change the plant species composition of an area and will impact the assemblages utilising them. Amongst these plant species predicted to shift was the ecologically important plant, Colophospermum mopane. An especially notable trait of C.mopane is its ability to form, large monospecific stands at high densities (Timberlake 1995). Mopane woodlands are known to have a low density of herbivores due to the low grass biomass and the high density of the trees which increases ungulates susceptibility to predation. An expansion of this species will have strong negative impacts on the tourism experience through reduced game viewing opportunities and potentially reduced numbers of game within these patches.
Animals and range changes
Like Rutherford et al. (1999), Erasmus (2002) used bioclimatic modelling to predict the responses of South African animals to a doubling of CO2 levels. The models used bird distribution data to draw its main conclusions. They showed that under climate change most species will show a range alteration characterised by a shift in species from the west of the country to the east, where many species shift away from increasingly arid areas. Another concerning prediction was that most species will experience a range contraction. A concern as smaller populations become more susceptible to stochastic events and population crashes and hence extinction. Relevant to the K2C area, the model looked at the impacts in the Kruger National Park and predicted a 66% loss of bird species occurring in the area. Obviously caution must be maintained when reviewing the impact predicted by models, particularly bioclimatic models. Bioclimatic models do not take into account behavioural adaption, cannot model biotic interactions such as competition and predation and therefore the predictions should not be taken as absolute, however they do provide a good indication as to which species will be impacted (specifically the more climatic dependant species) and do provide a good initial idea of what can be predicted to happen at a course resolution.
In terms of water dependent species, the warmer temperatures will result in water temperature increases which will shift species assemblages and algal assemblages which may impact the variety of mammals and birds living off these ecosystems.
Ecosystem consequences: biome switches
In terms of large scale ecosystem responses in the savanna conservation areas, the most clear-cut ecosystem response will be the increase of tree cover in the savannas. The general opinion is that the enhanced C02 and increased water use efficiency will over-ride the benefits C4 grasses obtained from increased temperatures resulting in a shift towards greater tree dominance in savannas. Two alternative mechanisms have been proposed to explain the causes of tree dominance.
The first hypothesis proposed is that rising CO2 concentrations have a 'fertilization' effect on trees (C3) as opposed to grasses, resulting in increased yield and growth rates in trees. The increased tree and grass water use efficiency may allow deeper percolation of water so that both tree seedling establishment and the growth established trees may be enhanced (Polley et al., 1997; Scheiter and Higgins, 2009). The second mechanism proposed by Bond and Midgley (2000) suggests woody plant cover increase will occur most rapidly in fire prone systems, such as savannas and grasslands, as the elevated CO2 will tend to favour regrowth of juvenile trees trapped (sometimes for decades) in the `topkill' zone, thus allowing them to escape more readily from periodic fires. It should be added that in addition to increased background levels of CO2, land management practices through altered fire and grazing regimes, have a very important influence on woody plant increases.
The increases in woody cover will be exacerbated by potential increases in rainfall. The predicted 150mm rainfall increase in the lowveld will drive rainfall over the 650mm level which marks a separation between stable and unstable savannas (Sankaran et al., 2005). In stable savannas, the maximum woody cover is constrained by the amount of water available. However, when the rainfall increases above 650mm savannas become unstable systems where the mean annual precipitation is sufficient to cause woody canopy closure. Range management becomes of particular importance in these instances as the management of fire and herbivory is required to maintain the co-existence of trees and grasses and prevent a biomes switch from savanna to closed woodland (Sankaran et al., 2005). Such management implications may incur additional expenses.
The combination of such interactions certainly could cause a significant biomes switch. Early models in South Africa predicted that the savanna biome is predicted to encroach onto the grassland biome along almost all grassland borders, reducing its extent considerably (Rutherford et al., 1999). This change, in conjunction with the human impacts, result in virtual confinement of the grassland ecosystem to high altitude refugia in the Drakensberg range (Rutherford et al., 1999), making the conservation areas in the escarpment of national importance. The savanna biome is itself expected to retreat from the northern and eastern border of the country, again suggesting the opening of opportunities for incursion of more arid and/or heat-adapted elements from across South African borders (Rutherford et al., 1999). Yet Rutherford's model did not fully get to grips with the loss of savanna habitat through woodland closure to the exclusion of grasses. These predictions are raised in a model by Higgins and Schieter (2009) which predict that in Africa parts of savannas will be replaced by deciduous woodlands; 45.3% of savannas to deciduous woodlands and 34.6% of grasslands to savannas.
Tourism impacts
A study looking at the heat-stress at popular tourist destinations around South Africa, under current climate conditions, ranks the heat-stress levels felt by a person in the eastern lowveld as "unpleasant" (Becker 2000). The increased temperature stress experienced through climate change may create a negative perception by tourists resulting in a shift in the visitation season by tourists. This will directly impact the tourism industry and may result in an increase in the associated costs (increased air-conditioning) of keeping people cool.
Summary: Impacts on the conservation community
The conservation sectors encompass several different aims with similar end points. The end point common to this sector is the conservation of/non-development of natural land. The reasons behind this are numerous and hence climate change will impact the sector in different ways. Conservation agencies, particularly state run conservation areas generally face the mandate to maintain biodiversity and through several acts are obliged to meet several biodiversity targets. It is in this sector that the heaviest impacts lie as the projected changes carry a heavy responsibility of conserving the countries biodiversity in spite of such impacts. The implications are far-reaching as changes in plants and animal ranges and biome shifts as well as the alteration of ecosystem functioning means that these organisations have the duty to use the climate change predictions and plan ahead so as to continue to meet its biodiversity responsibility and need to ensure that areas of future diversity hotspots are incorporated as conservation areas at this stage to ensure continued biodiversity conservation. This involves expanding or changing the configuration of protected areas, often at great expense.
Several other aims in this sector are for economical gain, the most common being hunting and eco-tourism. These sectors will be affected by changes in the biodiversity of the area and will also be impacted by factors that will negatively impact the tourist experience, such as increased bush, altered animal and plant assemblages and an increase in discomfort levels caused through increased temperatures. Management actions certainly will be able to mitigate against some of these impacts but will no-doubt result in an increased cost to this sector.
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