Sunday, March 17, 2013

Where have all the dry forests gone?

The environmentally conscientious citizen is well aware of the plight of the world’s tropical rainforests and our moral obligation to protect these biodiverse shelters of the next new cancer drug. But how many know of the troubles facing the tropical dry forests? (Or could even find them on a map?1) Just over 40% of tropical and subtropical forests are ‘dry’ forests where the trees lose their leaves each year, not because of a cold winter, but because of a rainless dry season.

Mopane woodland in Zambia,
a unique type of dry forest.
A study out this month in the Journal of Biogeography finds that dry forest loss in tropical Africa rivals that of deforestation in the rainforest. By comparing satellite photos from 1990-2000 across a grid of 784 locations spanning the continent, the researchers determined that 0.34% of dry forest was lost annually during the nineties compared to 0.16% in the central African rainforest (found by a previous study). This loss amounts to an area the size of Maine over the ten-year period studied and doesn’t include an equal amount of dense forest that is degraded to open forest each year.

These satellite-based estimates of deforestation are lower than estimates based on reports from individual countries and are lower than estimates of forest loss from local hotspots of forest degradation. However they are a relatively accurate summary of continent-wide forest loss. Whether local or broad scale estimates are more useful managing forest resources is a question for a conservation biologist. Regardless, 350 square-miles per year is no small amount for an ecosystem that supports more than half of Africa’s population.

So where have all the trees been going? Fields and firewood are the chief culprits. Everyone needs to eat and this requires land for growing and fuel for cooking.

What is a post about deforestation doing on a blog about biogeography? Two reasons. Images taken from satellites (‘remote sensing’) have enormous potential to inform our understanding of the geography of biological systems, which I hope to highlight this and other posts. Second, there are interesting geographic differences in deforestation rates across Africa. West African dry forests had much lower rates of tree loss than southern African dry forests, but this turns out to be because West Africa lost most of its dry forest prior to 1990 and there wasn’t much left to lose. Instead, in this part of the continent the researchers saw numerous switches between wooded shrub-land and non-wooded land that reflected agricultural practices.

Even though this research just came out this month, it’s actually rather old news. Over ten years have passed since the last satellite photo used in this study was taken. What has happened to forests in Africa since 2000? What story might recent photos tell about the plight of the dry forests?

You can find this article at:

Bodart, C., Brink, A.B., Donnay, F., Lupi, A., Mayaux, P., & Achard, F. (2013). Continental estimates of forest cover and forest cover changes in the dry ecosystems of Africa between 1990 and 2000 Journal of Biogeography. DOI: 10.1111/jbi.12084

1 Here’s a fun Google Earth map of the Holdrege life zones.

Footnote: I spent 2009 living in Malawi, Africa where I observed first-hand the impacts of rural agricultural life on the tropical dry forest ecosystem. The dependence of humans on wood has never been more apparent to me than when I flew across the border from Malawi (90 people / km2) to Zambia (13 people / km2) in a small aircraft. The political border could have practically been delineated by trees. 

Left: Zambia, Right: Malawi