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[MEXICO CITY] Traditional coffee farms are hotspots for native trees and could be vital reservoirs for forest regeneration, a study has found.

Small-scale farmers usually grow their coffee under tree canopies. Their product —shade-grown coffee — is already promoted as ecological because their farms harbour native birds, bats and other creatures.

But new research conducted in southeast Mexico has revealed that the surrounding trees themselves are unexpectedly genetically diverse — more so than clusters of the same trees in neighbouring forest. The farms may therefore be important corridors of genetic diversity as forests become increasingly fragmented.

Shalene Jha and Christopher Dick of the University of Michigan, United States, studied genetic samples taken from Miconia affinis trees growing in a network of coffee farms and forest fragments.

Typical of coffee farms in Chaipas state, the three farms in the study were clear-cut and burned in the late 1930s and immediately replanted with coffee bushes and canopy trees. Since then Miconia has been allowed to invade because it protects against soil erosion.

Jha found that the Miconia trees in the coffee plantations came from a wider variety of parent trees than those in clusters in nearby forest.

This could be explained by seed dispersal: in forests, seeds are spread by small, forest-dwelling birds, whereas on farms they are spread by larger, wider-ranging birds.

"If seeds are not dispersed, they will remain clumped together under the mother tree, and this will make them easy targets for predators," said Jha. "Without seed dispersal, gene flow will be limited, and this can result in future plant inbreeding."

A concern in agricultural areas is that increasingly fragmented landscapes isolate native plant populations, eventually leading to lower genetic diversity. But this study shows that shade coffee farms, by being hospitable to birds, support widespread dispersal of native trees, in effect connecting patches of surrounding forest.

The research has implications for the recent trend of rustic coffee farms moving from shade-grown to sun-intensive operations where farmers cut down canopy trees and level out the fields so it is easier to get machines in, said Jha.

"It's more essential than ever to pay attention to the ecological benefits shade coffee farms provide."

The study was published in Current Biology last month (23 December).

References

Current Biology doi 10.1016/j.cub.2008.11.017 (2008)

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