“One way of adapting involves changing the mix of species, for example by encouraging drought-tolerant trees if the climate is getting drier,” explains Ian Dawson, the lead author of a major review on climate change and agroforestry genetic resources. However, these compositional adjustments, as they are known, need to be accompanied by other measures which involve maintaining, enhancing and improving the management of trees on farms.

The authors of Climate Change and Tree Genetic Resource Management have identified three possible strategies. The first involves the human translocation of seeds and seedlings to areas where they can survive and thrive under new conditions. This may also involve the transfer of associated microorganisms, such as nitrogen-fixing bacteria and animal pollinators. However, choosing which species will flourish, and which will not, is far from easy. “Translocation isn’t a major problem for annual crops, because if farmers grow a certain crop and it doesn’t succeed, they won’t grow it again the next year,” explains Dawson. It’s a different matter with agroforestry species, which may take 20 years to mature and whose suitability can be difficult to assess over a short period of time.

The second strategy involves encouraging local genetic adaptation to changing environmental conditions. The review found that a number of features enable tree species in natural stands to adapt to change. These include large population size, high levels of genetic diversity and high seed yields. The problem for smallholders is that most trees on their farms have small effective population sizes, thus reducing their ability to adapt. However, measures can be taken to encourage greater on-farm genetic diversity and genetic change to promote adaptation. Farmer-to-farmer exchange networks for seeds and seedlings can help, and so can ‘diversity fairs’. The latter are currently being promoted in the West African Sahel by John Weber, a scientist with the World Agroforestry Centre.

The final strategy involves planting species which tolerate a wide range of environmental conditions, such as Pinus patula and many Eucalyptus species. These are now grown throughout the tropics in environmental conditions which vary greatly. However, the authors of the review warn that these species often have an aggressive disposition and may displace others of value to farmers.

“For the review, we drew on over 100 scientific papers, but more research needs to be done,” explains Dawson. In particular, we need to know more about fruit trees and how to help them adapt. Fruit trees appear to be especially vulnerable, partly because most depend on animal pollinators to yield fruit and these pollinators are themselves sensitive to climate change.
The right tree in the right place

If tree species are going to be successfully translocated – one of the strategies for adaptation identified in the review paper – we need to predict where they will grow well. “The best way to find out whether a particular species will thrive in a particular area is to plant it there and see what happens,” explains Roeland Kindt, an ecologist at the World Agroforestry Centre. “However, there are over 60,000 tree species in the world and in practical terms we simply do not have the resources to do this for more than a handful.”

There is, however, an alternative, currently being explored by the Vegetation and Climate Change in East Africa (VECEA) project. Led by Forest and Landscape Denmark, the project has been developing ‘potential natural vegetation maps’ for East Africa. These are created by combining detailed vegetation maps drawn up by surveyors during colonial times, when there was more natural vegetation, with current knowledge about the flora in seven East African countries. “It wasn’t just a question of finding old maps, but finding old botanists too,” says Kindt. Once the old maps had been digitised, the botanists ‘populated’ them with tree species.