Exploring Pigeon Valley: The Thorny Elm (Chaetacme aristata)

This is the 85th article in an ongoing series that highlights the riches of Pigeon Valley, the urban nature reserve in the heart of Glenwood. The focus of this article is on the Thorny Elm, Chaetacme aristata.

This tree is almost everywhere in Pigeon Valley, yet it is so seldom that I ever see it in anyone’s garden. I know why – it grows unevenly, with much growth of young coppicing shoots, and generates a great deal of leaf and branch litter. Yet, if you look at it, it is such an interesting tree, so variable in shape, with many gaps and crevices. Seedlings of the tree pop up constantly in my garden; the sapling has very serrated leaves, rather like holly. With growing age, the margins of the leaf become entire (smooth).

The fruit is very firm and the nut hard; only Thick-billed Weavers seem able to crack it with their powerful jaws, while Purple-crested Turacos eat the fruit. Apparently Shaka’s impis would use this is an emergency source of food if necessary, according to Jonathan Foley, the DUT horticulturist. I have heard that along the coast Green Mambas will lurk in the tree to catch Thick-billed Weavers.

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Recently I watched a Large-spotted Genet walking in Pigeon Valley; it went to a Thorny Elm and then disappeared inside it. What is remarkable about the tree is the large numbers of hollows it generates; these provide ample opportunity for beetles, reptiles and small mammals to roost, even to breed, thus ensuring that it is a major contributor to biodiversity.

Recent research has shown that these hollows accumulate droppings and leaf litter; at some point rain will wash this to the ground, providing additional nutrients to the tree. It is a reminder of the complex web of interconnections between different species in nature.

Crispin Hemson chairs the Friends of Pigeon Valley, a group that undertakes clearing of alien plants, keeps records of bird and mammal sightings and alerts management to any problems.

The Friends have a monthly walk at 7.30am on the second Saturday of each month. Email: friendsofpigeonvalley1@gmail.com.

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