Sunday, 4 February 2018

(oranges versus thicket) OR (oranges and thicket)?

by Alastair Potts

Much pristine thicket vegetation has been cleared for agriculture. This includes for the cultivation of oranges — which is major land-use option in the Sundays and Gamtoos drainage basins due to the availability of irrigated water. (There are other land uses, including melons, but that is a blog for another day).

The standard approach for obtaining land for orange farming in the Eastern Cape has been to clear large tracts of thicket.

However, would there be any benefit to farmers to maintain tracts of thicket — for example, in linear strips of 10-20 m in width? Would these strips have any conservation value? 

A provide a list of points below of possible benefits that thicket vegetation may have for orange groves if these occur side-by-side (the configuration still needs some thought, but there are lots of options).
  • Wind damage. Citrus trees and oranges can sustain significant damaged from high winds. Thicket vegetation is a dense matrix that grows between 2 - 3 m in height. It is an excellent wind buffer — anyone who experienced the thicket matrix on a windy day can attest to this. The ratio of thicket to grove size and the shape of groves could be used to drastically reduce the wind speeds in the grove.
  • Pest protection. Globally, citrus is either directly threatened by a range of insects or other invertebrates, or the bacteria they carry. For example, the invasion of the exotic Asian citrus psyllid (an insect) has led to the spread of  bacteria responsible for "Citrus greening". Such invasions can occur in systems do not have a functioning food-chain. By maintaining thicket vegetation, one maintains a food-web plus the predators that may feed on citrus pests. By keeping thicket vegetation, farmers may be able to reduce the susceptibility to such threats. This is the basis for concepts such as organic farming. (Note that there may be local potential pests in the thicket vegetation — but it would need to be investigated whether these represent a major threat to citrus; this probably involves asking the citrus growers)
  • Improved water availability. Intact thicket plays an important role in absorption and slow release of water, especially in the more arid forms of thicket (e.g. van Luijk et al. 2013). This may provide orange groves with more water. Alternatively, the evapotranspiration from thicket may lead to localised cooling thus ameliorating high temperatures.  
  • Improved pollination (?). The level of self-pollination appears to vary across different cultivars of citrus (e.g. this report). Where cross-pollination (pollination needs to travel from on plant to another) may increase yields, then maintaining thicket vegetation and all the associated pollinators could very well increase the pollination rate. 
There may be other points to consider on the citrus side of the equation (please share these!), but what about on the value of retaining tracts of thicket for conservation? 
  • Biodiversity. Thicket vegetation occurs as a solid type and in a mosaic with other vegetation types (e.g. grassland, fynbos, Nama-karoo). In the mosaic form, distinct thicket clumps are nested within a matrix of another vegetation type. What is crucial to understand is that these thicket clumps can maintain their biodiversity without any management — in contrast, fynbos or grassland patches need fire (a management headache!) to maintain their species. The patch size to maintain species is surprising small. In his PhD on bontveld (a vegetation type with thicket clumps in the mosaic), Justin Watson demonstrated that a surprisingly small area is required to maintain a high species diversity: 10 ✕ 10 m (with over 200 sqm consistently having high diversity; 2002, pg 129).  
  • Ecological functioning. Thicket operates as isolated patches in many landscapes (the "mosaics" mentioned above). Thus, irrespective of the system, thicket should be able to maintain its ecological functioning — this may also include when thicket grows in a mosaic with orange groves. (Although some thought of the role of pesticides may play needs to be considered).
Thus, there may be a a rare economic and conservation opportunity here where conservation goals with economic growth are achieved by maintaining thicket in a landscape with cultivation. To achieve this, we need to investigate the the size, shape, and co-benefits for both conservation and agriculture.

 If I may end with an example of economic utilisation and biodiversity conservation that coexist in harmony. In a Costa Rican National Park, containing some of highest biodiversity estimates on the planet, sits a power station. Dan Janzen, one of the world's leading scientists, has demonstrated that —with the right mitigation measures in place — there has been no loss in biodiversity or ecosystem functioning. Forests and thicket operate on similar principles: maybe one day say the same about thicket and oranges...