Other definitions

Last year, Jeremy Bruno posted a description of dry forest at The Voltage Gate. (He has since moved to ScienceBlogs.) It's a pretty good overview - covers the main points, though I'm a tad uncertain about the assertion that erosion (post-deforestation) is higher than in rainforests.

So what is tropical dry forest?

So what is tropical dry forest? In trying to compile a list of recent dry forest-related literature that question comes to the fore. It's a very familiar question - I spent a lot of time wondering about that early in my grad school career when I thought that I should get a handle on all the literature related to tropical dry forest. (What can I say, I was ambitious naive.) Part of the problem comes from relying on Holdridge's definition - if you define dry forest in relation to precipitation and potential evapotranspiration (which basically boils down to precipitation and temperature), you end up including riparian forests and some forests dependent on ground water - forests that which resemble wetter forests in terms of physiognomy and species composition. When it boils down to it, regardless of rainfall, they don't function as dry forests. In addition, there's the issue of savannas - many savannas fall within the dry forest life zone, but if you look at them as "former" or "potential dry forest" you ignore the fact that they have been grassland for thousands and years. (Other grasslands though, are recent human creations.) Then there are areas that function as dry forest, but get too much rainfall.

In selecting literature I am trying to use broad criteria. I'm including deciduous and semi-deciduous forest even if they seem to be a bit on the wet side. I'm also including savannas. In a case like this, it's probably more useful to use a broad definition.

Introducing tropical dry forests - part I

Although most people are familiar with tropical rainforests, fewer people are familiar with tropical dry forests. Despite this, they are an important tropical biome - according to Leslie Holdridge's definition of dry forests, they may once have covered 42% of the land mass in the tropics. But most people, when you say "tropical dry forest" reply by asking "tropical rainforest?"

So what are dry forests? While Holdridge's system was complicated, in the simplest terms they are forests were potential evapotranspiration exceeded precipitation. In other words, averaged over the year, the amount of rain that falls is less than the total amount of water that could be lost through transpiration and evapouration, if it were available. On average, the vegetation is water stressed. Classic dry forests are dominated by dry-season deciduous trees (trees that drop their leaves in the dry season), and they have five or more months with less than 100 mm of rainfall. There are exceptions - dry forests in Hawai'i don't have that pronounced a dry season, but they are still dry overall. Holdridge's system was a first approximation - there are areas that meet his definition for dry forest which appear and behave like wetter forests, and they there are forests in higher rainfall areas that resemble dry forests in their species composition and ecological function because they grow on steep slopes with thin soils, or because they are subject to desiccating winds.