Introducing the Subtropical Thicket Biome
The Subtropical Thicket (also known as Albany Thicket) is a specialised type of dwarf forest covering approximately 2.2% of South Africa. It is in a biodiversity hotspot with more than 25% of the plant species being endemic. It is considered unique in terms of its origin and largely emerged during the Eocene age (56–33 million years ago). It is characterized by the absence of a continuous grass cover (and is evolutionarily older than grasslands) with succulents forming an important part of its biodiversity. The Subtropical Thicket is located at the convergence of eight of the nine biomes in South Africa. The complexity of the geographical area and associated plant species has some of the highest landscape-level heterogeneity in the world and supports a high diversity and density of species.
The Subtropical Thicket extends from the Breede River Mouth in the West, to the Kei River mouth in the East, and inland to Graaf Reinet and Cradock. The vegetation tends to be thorny and is densely intertwined by creepers and lianas. The Subtropical Thicket vegetation changes with rainfall and soil type to form five very different broad vegetation types that again change in a gradient from west to east resulting in 44 different subdivisions. The rainfall in summer and winter is very similar, decreasing towards the interior. Temperatures are generally mild with no frost in winter and not very high temperatures in summer. There are five major types of Subtropical Thicket:
- Dune thicket occurs along the coast where the rainfall is between 350–950mm per annum on sandy soils. Milkwood (Sideroxylon inerme) is common here.
- Mesic thicket is found further from the coast towards the more mountainous areas with a rainfall of 500–800 mm per annum. Soils tend to be loamy and wild olive (Olea africana) trees are common. Although tree Euphorbias are present, they are not very common.
- Valley thicket connects different thicket types as it follows rivers and drainage lines in the valleys between the mountains. The clay soils support a very typical subtropical thicket, with a closed canopy and prominent tree Euphorbias.
- Arid thicket is at the dry end of the rainfall gradient of thicket with an average rainfall of 100–300mm per annum. Temperatures in this vegetation are more extreme dropping to almost zero in winter with very hot summers of over 30°C. There are many smaller succulents present with Spekboom and Noors prominent in the vegetation.
- Thicket mosaics represent fragmented vegetation areas with thicket clumps interspersed with the adjacent vegetation types such as succulent karoo in the north and forests in the south.
Thicket Under Threat
The Subtropical Thicket Biome is highly variable and the vegetation structure is complex and often site specific. The exact historical distribution of thicket patches is not always clear (for instance, historical paintings of early Bathurst show open areas which now contain diverse thicket). It is clear that intact and recovering thicket is continuously being removed, and commonly altered to a far less productive state. For instance, where the original intact vegetation has been over utilised or removed (due to injudicious management e.g., of grazing, or during clearing for various developments) the landscape can enter a vicious cycle of removal of topsoil and the formation of gullies, with resultant loss of ecosystem function, disruption of the water cycle and more severe floods from increased runoff and reduced infiltration. Soils in the Subtropical Thicket commonly have very good agricultural potential which is a product of a functional thicket ecosystem. However, without intact thicket, nutrients are rapidly depleted and soil micro-organisms are lost. This results in a sterile environment which has detrimental consequences to land users. Land users need to carefully weigh up short term financial benefits to long term ecosystem collapse. Many other challenges beset thicket habitats, not least of which is invasion by alien plants. Although many aspects are still to be studied thoroughly, there are multiple knock-on effects such as loss of pollinators which is deleterious for adjacent ecosystems and farming activities. Collection by local communities of non-timber forest products such as edible, medicinal and ornamental plants is impacted, as are several other important cultural values. These include the sense of place, recreational values such as ecotourism, and spiritual values associated with ihlati (the forest).
A compelling flagship story of how species loss can be affected by the interconnections in our ecosystem follows, that of the pipefish, a charismatic relative of the Knysna Seahorse. Although primarily the role of regulators, thicket loss is increasingly becoming a more general concern to many farmers and to society in general. The Rhodes Restoration Research Group (RRRG) plans to collaboratively involve stakeholders in discussing and finding more satisfactory and sustainable pathways.
Estuaries of the Subtropical Thicket: Home to the Estuarine Pipefish but for how long?
The Estuarine Pipefish (Syngnathus watermeyeri) is both a narrow endemic and is critically endangered. The pipefish is only found in five estuaries in the world, namely the Bushmans, Kariega, Kasouga, West & East Kleinemonde estuaries of South Africa. It was previously thought to be extinct but was rediscovered in 2006. The catchments of the estuaries are situated in the unique Subtropical Thicket Biome at the heart of the Albany Centre of Plant Endemism. Frequent and prolonged droughts, in combination with illegal clearing of thicket vegetation and extensive impoundments have led to negative effects on most of the downstream estuaries along the eastern sea board of South Africa. Pipefish are sensitive to high levels of sedimentation, nutrients and pollution and hypersaline conditions.
Vegetation within the key catchments has been transformed due to changes in land use, most notably being commercial agriculture. This is threatening the remaining patches of intact thicket and also has negative impacts on the water quality in the estuaries and marine protected area. One of the major drivers contributing to the state of the estuaries in which the pipefish is found is from excessive nutrient and sediment loading from the catchments. By mapping, monitoring and evaluating thicket condition one can start to understand the effects that land cover change is having on overall catchment conditions. Sustainable catchment management can aid in the maintenance of these sensitive ecosystems which are at risk of disappearing due to anthropogenic processes. With the disappearance of functioning ecosystems one also loses endemic species such as the estuarine pipefish. It is imperative to balance economic developments with environmental conservation to avoid a species extinction.
Forest loss and monitoring
Forest degradation is a global phenomenon and is unfortunately also evident in the Subtropical Thicket Biome. Methods have been developed by the international scientific community to detect changes and trends in forest and woody vegetation degradation at a regional or county level (e.g. Global Forest Watch, Dynamic World, and Resource Watch). Trends in canopy cover and above ground biomass are easily monitored in high risk areas of the Subtropical Thicket Biome. The data can be evaluated in near real time and passed back to higher management for decision-making, adaptive measures and enforcement.
