Conventional wisdom says that cutting down tropical forests causes devastating floods, dries up streams and rivers, makes it rain less, and clogs up dams and waterways. To solve those problems you need to plant trees, but god forbid you plant a eucalypt because that will dry everything up. Then you will be in really big trouble. It’s simple.

Sorry friends. The world is a bit more complicated, and sound bites won’t solve your problems. Logging and deforestation can make small floods worse, but probably don’t affect big floods much. In most cases removing forests will not dry up streams or rivers, not even during the dry season. Still, in some cases it may, particularly if the land use that replaces the forests compacts the soils and keeps them from holding water. There is no solid evidence that chopping down trees reduces rainfall, but we should still be concerned because various studies suggest it might. Replacing forests with crops or pasture usually increases soil erosion and sedimentation, but not always. In any case building roads and houses may be the real culprits when it comes to sedimentation. Planting trees can be part of the problem, not the solution, even if you don’t plant eucalypts.

That is what Sampurno Bruijnzeel tells us in ’Hydrological Functions of Tropical Forests, Not Seeing the Soil for the Trees?’- a recent review published in Agriculture, Ecosystems, and Environment. He should know. He may well be the world’s leading expert on how deforestation, reforestation, and logging affect water in the tropics. He first reviewed the topic almost two decades ago and has focused on it ever since.

Bruijnzeel chose the title for his latest review to emphasize that often the vegetation itself is not the most important factor. The key is what happens to the soil. Well-managed agriculture can sometimes be as good for watersheds as forests, while treating soils poorly can do more harm than most experts realize.

Bruijnzeel is practical enough to realize that practitioners need general guidelines they can follow. Yet he also knows governments and NGOs have wasted a lot of money on watershed projects that had clear ideas but fuzzy thinking. So many of his conclusions have caveats and qualifications and over the years he has changed his mind about several of them. This review doesn’t claim to be the last word. Nor will the next. Still it is the best we’ve got, and can help make good decisions.