- Fire feedback (local, established): An increase in herbaceous vegetation increases fire frequency which then maintains low tree densities.
- Flooding/High levels of soil saturation (local, established): Constant flooded conditions and high levels of soil saturation ensure the persistence of herbaceous vegetation adapted to wet conditions. Flooded conditions prevents trees from invading the wetlands which cannot invaded and the structure and function of the wetland remains herbaceous and saturated.
- Fire suppression (local, established): The lack of fire in the system provides seedlings the opportunity to invade into the wetlands and mature into forest which then excludes fire itself.
- Drought conditions (local, established): Lack of constant conditions which favours the hygrophilous vegetation, the intermediate to long duration of flooding, creates conditions which enable forest species to invade wetlands. These conditions persist with the establishment of swamp forest species which further reduce the water table as trees utilize more water than herbaceous vegetation.
Herbaceous wetlands to forest
Important shocks (eg droughts, floods) that contribute to the regime shift include:
- Droughts (regional, well established): The hydrological regime (the depth and duration of flooded conditions) is the major driving force in plant community development in wetlands. Any disruption to the hydrological regime disrupts the functioning of the wetlands and the patterns of species composition. In drought periods forest species establish in the wetlands and outcompete the hydrophytes which are drought intolerant.
The main external direct drivers that contribute to the shift include:
- Plantation (local, well established):The large scale plantation forestry in the landscape has led to the drying of the landscape (which affects the hydrology of the wetlands) and therefore reduces the levels of soil saturation. Simultaneously, plantation forests are fire suppression areas to avoid tree loss. These factors, together with the disturbance of converting wetlands into plantation forest and clear felling, have allowed forest species such as the fern Staenoclina tenuifolia and Macaranga capensis to invade the wetland areas. Over time, the combination of fire suppression, disturbance and drying encourages the establishment of woody seedlings, turning wetlands into swamp forests/woodlands.
- Atmospheric carbon dioxide (Global, speculative):Concentrations of carbon dioxide in the atmosphere have varied greatly throughout the earth’s history and the appearance and spread of C4 grassland biomes is commonly linked to conditions of low carbon dioxide concentration (Bond et al., 2003b; Bond and Midgley, 2012). According to Bradley et al. (2010), the rising atmospheric carbon dioxide concentration has a direct fertilization effect on C3 plants by increasing resource availability that will favour shrubs and trees over grasses and sedges. According to Bond and Midgley (2012), the high prevalence of the conversion of open savannahs to woodlands is consistent with experimental and simulation studies of carbon dioxide effects.
The main external indirect drivers that contribute to the shift include:
- Global warming (global, well established): Global warming not only maintains high concentrations of atmospheric CO2; it also increases the likelihood and intensity of droughts in certain areas of the globe. Climate change predictions forecast further amplifications in seasonal variations (high run-off through intensified flooding and high evaporation rates due to droughts) and inter-annual variations (Hannah et al., 2002), thus putting wetlands under even greater stress.
- Demand for paper (regional, speculative): As the population size increases and subsequently, the number of people needing paper has increased. From school to university to office environments, their efficient functioning relies on the sufficient supply of paper. This then puts pressure on forestry companies to grow trees that grow faster. The shorter the harvest cycle, the greater the forest’s use of water. Trees use up more water in their early growth years. Higher transpiration rates negatively affect wetland functioning.
Slow internal system changes that contribute to the regime shift include:
- Fern and Tree density:Fern and tree density change slowly but it is the best indicator of the regime shift. Adie et al. (2011) and Murdoch (1942) showed that the presence of Pteridium aquilinum (bracken fern) aided in the transition of grassland to forest in periods of long fire suppression by providing establishment opportunities for resprouting early-successional forest species. Once a swamp forest is formed, fire becomes infrequent due to swamp forest species having low flammability, providing further establishment opportunities for tree seedlings.
- Change of the fire frequency: Fire is influenced by climate, rain variability and direct management actions. In this landscape rain variability and management actions have had the greatest influence on the fire frequency. Being in a plantation limits the time frame in which managers can burn the wetlands and with recent high variations in rain, this has been significantly shortened.
- Saturation levels in the wetlands:The hydrological regime is the major driving force in plant community development in wetlands. This determines the species which can persist in the wetlands as some species have varying tolerances of saturation. High levels of saturation favour herbaceous wetland species. If saturation levels are lowered beyond a certain point, it allows for non herbaceous, often non-wetland species to invade and change community structure. In this example, swamp forest fern and tree species have been favoured by the reduced saturation levels which then reinforce the low levels of water in the wetlands due to their increased transpiration levels.
Summary of Drivers
|#||Driver (Name)||Type (Direct, Indirect, Internal, Shock)||Scale (local, regional, global)||Uncertainty (speculative, proposed, well-established)|
|3||Atmospheric carbon dioxide||Direct||Global||Speculative|
|4||Global warming||Indirect||Global||Well established|
|5||Demand for paper||Indirect||Regional||Speculative|
|6||Fern and Tree density||Internal||Local||Well established|
|7||Change of the fire frequency||Internal||Local||Well established|
|8||Saturation levels in the wetlands||Internal||Local||Well established|
Summary of Ecosystem Service impacts on different User Groups
||References (if available)|
|Feed, Fuel and Fibre Crops|
|Wild Food & Products|
|Air Quality Regulation|
|Soil Erosion Regulation|
|Pest & Disease Regulation|
|Protection against Natural Hazards|
|Cognitive & Educational|
|Spiritual & Inspirational|