Tropical dry forests

“You mean tropical rainforests?”

When I tell people that I study tropical dry forests, odds are that I get that response. Although they’re much better known and better studied than they were a few decades ago, most people are unfamiliar with the idea of tropical dry forests. In their classic work1 on the topic, Pete Murphy and Ariel Lugo observed

Although literature has proliferated concerning the ecology of tropical ecosystems…far less attention has been given to tropical and subtropical dry forest and woodlands.

More than two decades later, Rodolfo Dirzo and coauthors2 could still write

The usual perception that the term tropical forests refers to evergreen tropical rain for moist forests is inaccurate. The tropical forest biome is, in reality, a mosaic of different vegetation entities includes, at mid elevations of the tropics, the patchy and biogeographically restricted tropical cloud forests and, in the lowlands, the rain forest per se and the seasonally dry tropical forests (SDTF). At least part of the biased perception of the term tropical forest stems from the fact taht, by far, tropical rain forests are teh most studied and, indeed, most popularized among the general public. SDTFs, in contrast, have been seriously neglected.

So what are tropical dry forests? The common definitions rely on the Holdridge life zone system which defines tropical and subtropical3 dry forests as forests with a mean annual temperature3 greater than 17°C with 250-2000 mm of rainfall annually which have a ratio of potential evapotranspiration to precipitation that exceeds 1.0 (or, in plain terms, areas where the potential water loss through evaporation exceeds the actual rainfall). An alternate way of defining tropical dry forests takes into account the seasonality of the rainfall – areas that have 4-6 dry months (commonly defined as months with less than 100 mm of rainfall).2 Other definitions have used physiognomy – these forests are sometimes describes as tropical deciduous, semi-deciduous, or semi-evergreen forests; the latter category overlaps with what Holdridge considers moist forest while, at the same time, dry forests on poor soils can be evergreen.

None of this, however, answers the real question: what are tropical dry forests? How would you recognise of if you encountered it? It’s not always easy – Dan Janzen once said of Costa Rican dry forest: if you visit in the rainy season you can go home and tell your friends you went to the rainforest. But come in the dry season and you’re going to encounter something altogether different.

In general, tropical dry forest trees have a shorter stature and more open canopy than rainforests or moist forests. The trees have wider, more open crowns. More light penetrates the forest canopy, so the understorey is more brightly lit. Tree stems are smaller than in the rainforest and fewer trees have buttresses. Many dry forests are dry-season deciduous – some or most of the canopy trees lose their leaves in the dry season. This isn’t unique to dry forests – many moist forests also display some degree of deciduousness – but it tends to be more pronounced in dry forests.

The most difficult line to draw though isn’t between rainforests and dry forests – the hardest line to draw is between dry forests and savannas. The classic definition of the savanna is an area of grassland with some amount of tree cover. Since savannas and dry forests both can have tree cover, how do you distinguish one from the other? The key difference is probably the grass cover. If the understorey is dominated by grass cover, then you have a savanna. If the understorey lacks continuous grass cover, then you’re probably dealing with dry forest. This is, however, just one way to distinguish tropical dry forests from savannas.

  1. Murphy, P.G. and A.E. Lugo. 1986. Ecology of Tropical Dry Forest. Annual Review of Ecology and Systematics 17:67-88.
  2. Dirzo, R., H.S. Young, H.A. Mooney and G. Ceballos. 2011. Introduction. Pp. xi-xiii in R. Dirzo, H.S. Young, H.A. Mooney and G. Ceballos. Seasonally Dry Tropical Forests: Ecology and Conservation. Island Press, Washington.
  3. Holdridge distinguished between tropical and subtropical in a slightly different way than most people do – his definition of ‘subtropical’ only included frost-free areas. He also used biotemperature instead temperature; as he defined it, temperatures below 0°C or above 30°C were changed to 0 when calculating mean biotemperature. His rationale was that photosynthesis stops below 0°C and above 30°C. Modern use of the Holdridge life zone system is sometimes inconsistent in its use of biotemperature.

Published Nov. 26, 2012. Last update, March 23, 2013.

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