They can be left to grow in their native, natural habitat, which is known as conservation in situ, or “in their original place”.
If trees are valuable because they produce timber, fruit or other commodities, farmers may transplant them to nearby agroforestry plots, a technique known as conservation circa situm, meaning “close to their place of origin”.
Or, they may be conserved in seed or gene banks, known as conservation ex situ, or “outside their original habitat”.
The three types of conservation are interrelated, and smallholders play a role in each of them, but scientists actually know very little about the extent and the limitations of those connections and how effective they are, said Manuel Guariguata, a principal scientist with CIFOR.
“We know that agroforests can be very diverse, but we don’t really know the dynamics of the links between farms and natural ecosystems,” he said.
“For example, we don’t know exactly what happens with timber and other species in the wild when smallholders domesticate them by planting them outside their natural habitat.”
If a species is to survive, its population must be large enough to allow for genetic diversity, but more research is needed to determine how well small agroforestry farms contribute to conservation, the paper said.
“Conventional wisdom is that if you plant trees in plantation or agroforestry systems, it will contribute to conservation of natural stands of trees,” said Ian Dawson of the World Agroforestry Center, the lead author of the paper.
People are expected to harvest trees from plantations or agroforestry plots, leaving the natural forest intact, but there is little research to support that assumption, he said, adding that “it may be that people take it for granted and don’t think they need to study the link.”
“We need a better understanding of the relationship between agroforestry practices in one location and conservation in an adjacent natural forest or woodland,” Dawson said.
Farmers who combine agriculture and forestry can contribute to conservation in several ways, but it is important to look at how the different approaches work together to ensure the greatest benefit with the fewest unintended consequences, Guariguata said.
Agroforestry plots may serve as “stepping stones”, allowing bees, birds and animals to disperse pollen or seeds as they move through them into natural forest areas – a process that can result in tree reproduction in both land types.
In places where certain species in natural forests cannot adapt to the pace of climate change, conservation on agroforestry plots may offer the only chance for survival, Dawson said.
Scientists need more information about how different species reproduce and about population density – the number of trees of a certain species in a particular area that are necessary for sustainability, he said.
Tropical forests are home to a great diversity of species. When small farmers find trees that are particularly valuable for timber, fuel, fruit, nuts, medicine or some other purpose, they may transplant them or sow the seeds in their own agroforestry plots. As a result, the farmers’ plots tend to have a wide variety of species, too.
“At first glance, it looks positive, but whether it is sufficient for conservation depends on the species and the number of trees,” Dawson said.
If farmers opt for exotic species, they may crowd out native species, counteracting conservation goals, he said. And if there are too few trees of a particular species, they may be too far apart to be pollinated and reproduce.
If just a few individual trees pollinate each other, the result can be inbreeding, which leads to weaker trees that are less likely to survive.
When a species is seriously threatened in the wild, scientists may opt for ex situconservation, sometimes far from the original habitat, to ensure that it is not lost forever.
That may mean storing the seed in a seed bank, under carefully controlled conditions, or it could mean planting trees at a site set aside for “field gene bank.”
When a tree species is seriously threatened in the wild, scientists may opt to store seeds far from their original habitat to ensure they are not lost forever. Picture credit: World Agroforestry Centre.
There are disadvantages to ex situ conservation, however. Tree seeds do not survive long-term storage as well as food crop seeds do, Dawson said, and when they do, it takes much longer for them to grow to maturity.
Ex situ conservation of trees therefore tends to be expensive, and could shift funds away from more cost-effective conservation strategies, he said. A middle ground could be found by planting threatened trees in botanical gardens, which could promote education and awareness raising while conserving the species, the authors of the paper said.
Despite these discoveries, many questions remain about how agroforestry can contribute to conservation.
Scientists must learn more about plantations or policies such as certification really help conserve species in the wild – research that will involve studying both ecology and genetics, Guariguata said.
Because small farmers are constantly domesticating wild species that are useful to them, researchers can learn a lot from studying how farmers manage both the wild and domesticated trees and how the wild and domesticated populations interact, he said.
One crucial question is how a warming climate will affect tropical forests, Dawson said.
“A key concern is whether climate change will affect pollinator populations and what that could mean for tree species,” he said.
“That’s especially important in farm landscapes, where conservation also depends on how valuable a species is to farmers.”