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Saving the Clanwilliam Cedar

17 Jul 2020 - 16:30

Despite much research and management effort, the iconic Clanwilliam cedar (once Widdringtonia cedarbergensis now sadly W. wallichii) continues the decline it has been in for over the last century or more.  Prof Jeremy Midgley, PhD student Joseph White, honours student Marianne Stevens and colleagues have come up with some new ideas on saving the cedar. They concentrated on lack of recruitment as the main problem, rather than death of adults. Firstly, they showed that the large wingless nuts of the cedar are scatterhoarded (buried for later consumption) by a local small mammal species but that this dispersal process is not working in many areas because adult plants are too scarce. Trail cameras and uv fluorescent threads attached to seeds, showed that the cape spiny mouse (Acomys subspinosus) was the burier of seeds. Also, they excavated seedlings and their germination depth of about 2-3 cm below the soil surface confirmed that only buried seeds developed into seedlings. Another sure sign of seeds having been buried by a scatterhoarder is that they occur in groups of two or more from the same spot, as in the figure below. Trail cameras showed rodents putting a couple of seeds in their mouth and then rushing off to bury seeds. Burial of seeds was mainly found in dense cedar stands, such as the old cedar plantations and seedlings were most commonly found after fires. For various reasons cedar stands are very sparse presently; historical cutting down of trees no doubt is an important factor in explaining the sparseness. Scatterhoarders mainly bury seeds when seeds are in sudden excess; such as in the old cedar plantations where trees are close together and thus seed-fall will be intense. Seeds released from sparsely spread trees do not stimulate burial by rodents and are either eaten by granivores or incinerated in fires. Secondly, they showed that germination is fire stimulated (heated or smoked seeds germinate best). Again, this part of the regeneration process is not working because most fires take place in summer before seeds are mature, which occurs in the late autumn. Two management options are to prevent or fight summer fires in cedar areas and to plant seeds or seedlings close together to create dense patches, to get the scatterhoarding process going again. (Understanding recruitment limitations in a critically endangered species: The story of the iconic Cape cedar (Widdringtonia wallichii); Global Ecology and Conservation 23 (2020) e01062).