Volunteers will be surveying trees for signs of ash die-back disease this weekend.
The research is taking place on Clapham Common and is being led by tree expert Clive Mayhew.
Mr Mayhew will be giving advice on how to spot the signs of disease in a number of tree species, as well as help with tree identification.
The deadly disease of ash trees is caused by a fungus known as Chalara fraxinea.
Trees with the fungus suffer from leaf loss and crown dieback, which can lead to the death of the tree.
It is estimated there are about 40 European Ash trees on the common, with all expected to be in good health, except for one.
Participants will tour all of the European Ash trees, with the opportunity to learn about other tree pests and diseases affecting London's trees.
Anyone who wishes to take part is invited to meet at the bandstand in the centre of the common at 10.30am on Saturday.
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