Not as straightforward as it seems: the role of agroforestry in tree conservation
Deforestation and climate change are making the development of appropriate conservation methods for trees increasingly important, but how can species and genetic diversity best be maintained on farms, in the wild and in gene banks?
“Loss of tree species and indeed genetic diversity is a particular concern for rural communities in the tropics who have traditionally depended on a wide range of trees for fruits, timber, fodder and other products for their livelihoods,” says Ian Dawson, Associate Fellow at the World Agroforestry Centre and co-author of a new study published in the journal Biodiversity and Conservation.
The study critically reviews the role that smallholder agroforestry plays in tree conservation and points to interventions and research required. The authors question the common belief that tree planting in agroforestry systems has a positive impact on the conservation of wild tree stands, and they suggest that seed collections may also have limited utility in supporting conservation.
The study looks at the relationship between agroforestry and tree conservation in three settings: in farmland where forest was once found; in the forest itself; and in seed collections and other ‘ex situ’ locations such as field trials and live gene banks.
Are trees on farms important reservoirs of local biodiversity?
Dawson and colleagues relate that while dozens and sometimes hundreds of tree species can be found in tropical agroforestry landscapes, this species diversity is unlikely to be sustained in the long-term, as farm management systems change and as exotic trees become more dominant. For example, older cocoa agroforestry farms in Brazil show less diversity over time with exotic species increasing in abundance.
“Maintaining genetic diversity within species on farmland is also extremely important, especially for trees which generate products for farmers,” explains Dawson. “If the trees suffer from inbreeding and produce poorer quality products, farmers are less likely to retain them.”
The genetic diversity found in agricultural landscapes depends on how long and how intensively trees have been managed. If management intensity is low, there can be little difference between farm and neighbouring natural forest populations, but when trees have been managed for millennia – such as some fruit trees have – genetic bottlenecks can be observed in farmland.
Dawson also outlines how genetic diversity alone is also not enough to sustain species. “Inter-connectivity in the landscape is also crucial, which depends on tree density.” If tree density is low, regeneration can be more difficult due to limited and irregular pollination. For example, although a total of 297 different tree species were identified in a survey of Central Kenyan farms, more than 40 per cent occurred at densities of less than 0.1 mature trees per hectare. Where low densities occur, it may be better to focus efforts on conserving these trees in natural forests or in ex situ collections.
Does agroforestry help to maintain wild tree stands?
It is generally believed that planting trees in smallholdings around native forests helps to protect wild stands by providing an alternative resource for harvesting, but Dawson warns that this link should not be taken for granted and there is little direct evidence for it: “When trees are actively planted around forest, this may result in less attention being paid to the sustainable management of natural stands.”
Cultivation can also stimulate the development of markets and infrastructure that inadvertently capture wild resources as well as the harvesting of planted stands, while profitable planting can result in forest and woodland being further cleared for tree cultivation.
Trees in agroforests are often viewed as providing a corridor for pollinators and other animals between native forest fragments. This role is important, but the authors of the study warn that planting trees to fulfil this role may sometimes be counterproductive if the germplasm used has a different genetic composition. Cross pollination may then lead to genetic dilution and the loss of unique wild diversity. For example, wild coffee that still exists in small fragments of Ethiopian montane forest is threatened by hybridisation with coffee cultivars in surrounding farmland.
One solution to this issue could be to select the best performing local wild material for farm planting, an approach that is being taken with Prunus africana and Allanblackia spp., which are being domesticated on African smallholdings surrounding forests where they are naturally found.
Does agroforestry support germplasm presence in ex situ locations?
Currently, there are more than 5,800 woody perennial species conserved as seed ex situ in tree seed centres, by scientific institutions and by commercial suppliers. While this may appear to safeguard against losses in tree species and genetic diversity, Dawson believes this is not necessarily the case.
“With a few notable exceptions, most collections have no particular commitment to conservation and are threatened by changing research priorities and shifts in commercial imperatives,” Dawson stresses. According to one survey of tree seed providers, only 19 per cent of species are held by 5 or more suppliers and 49 per cent only have a single supplier. Furthermore, the natural origin of only half of the species covered by the survey was known, reducing their value as conservation collections.
Even when seed are stored securely, to remain viable, seed samples need to be periodically regenerated, a costly undertaking and often difficult with trees that have long generation intervals, large growth forms or specific regeneration requirements.
On farm planting of trees in exotic locales is helpful for conservation when new landraces have developed over centuries of cultivation, such as with coffee, cocoa and mango, but genuine cases of landraces with useful traits not found within native stands are probably rare for the majority of little-domesticated tree species.
The way forward
What is clear is that the role of agroforestry in conservation requires further research. More needs to be known about the densities and configurations of trees in farm landscapes to support conservation, and further information is required of the circumstances in which tree cultivation can support the conservation of neighbouring natural tree stands. The ‘stability’ of ex situ seed collections also requires assessment.
“One of the most promising conservation advances through tree cultivation is in the domestication of indigenous trees in participatory approaches involving both farmers and scientists,” says Dawson. “This has enormous potential to reduce genetic losses and provide more productive and sustainable farming options that involve trees.”
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Dawson IK, Guariguata MR, Loo J, Weber JC, Lengkeek A, Bush D, Cornelius J, Guarino L, Kindt R, Orwa C, Russell J, Jamnadass R. (2013). What is the relevance of smallholders’ agroforestry systems for conserving tropical tree species and genetic diversity in circa situm, in situ and ex situ settings? A review Biodiversity and Conservation 22 (2): pp 301-324