Tropical Dry Forests - Part One: The Dry Season

Figure 1. View from the Mirador (Spanish word for “lookout”). Evergreen trees stand out amongst the deciduous trees that are leafless towards the end of the dry season.

Welcome to the tropical dry forests of Guanacaste, Costa Rica! You can see in the photo above (Fig. 1) where this ecosystem gets its name, at this time of year it is VERY dry. For approximately half the year, tropical dry forests receive very little rain. Streams dry up, trees lose their leaves, and water becomes scarce and as precious as gold. Even for field biologists like us, access to water has been an issue. With wild animals of the tropical dry forests becoming desperate for water, some dig up our water line and create holes in the pipes to gain access to precious and delicious water. Good for them, bad for us. For many days in a row our friends who work in the park as forest fire fighters and park rangers were coming by to help us repair the water line. We would celebrate with beers after our water was fixed and thank them profusely…only to wake up the next morning to no water. Again. Now they fill up the tank behind our house so that we have water for dishes and showers and we fill up our water bottles with potable water at their station up the road. Perhaps once the rainy season hits, we will have more luck in keeping our water line free of holes!

In Santa Rosa, evidence of the value of water can be seen along the now almost completely dry Quebrada Cuajiniquíl (one of the main streams that runs through Santa Rosa). During the wet season, the quebrada is full and flowing but now is reduced to muddy puddles. These puddles are an oasis for thirsty and hot wildlife, and it’s a hotspot for birding and wildlife viewing (Fig. 2 & 3).

Figure 2. Baird’s Tapir, an endangered species that was once widespread in Costa Rica, is now restricted to protected areas like Santa Rosa. This one is cooling off in one of the few spots of standing water left in the area.

Figure 3. A Russet-naped Wood-rail searches for food in what's left of a pond.

While migratory birds can be seen passing through the tropical dry forests of Guanacaste, many choose to avoid this route as spring migration coincides with the driest and harshest time of year in the tropical dry forests here. Despite that, some migratory birds can still be seen, such as wood-warblers, tanagers, and thrushes (Fig. 4 ).

Figure 4. Two male Scarlet Tanagers hanging out near the same pond the Russet-naped Wood-rail was at.

How do wildlife cope with such an extreme dry season? Some migrate to nearby rain and cloud forests. Some species of moths will migrate to moister forests, live out several generations, then return to the dry forest and continue their life cycles until the drought and heat become again unbearable. Some animals seek refuge in the evergreen patches of mature forest that remain up to 5 degrees cooler than surrounding forests (Fig. 5). These patches are fragments of primary tropical dry forest that once would have covered much of the landscape. These forests have been cleared by humans for hundreds of years to make way for settlement, cattle ranching, logging, and agriculture. Today, one of the biggest threats to these forests are human-caused forest fires, usually from carelessness or revenge by angry poachers. Some days we can smell smoke from nearby forest fires. Other days, we drive by burned areas that had been forest or field a day before (Fig. 6).

Figure 5. Canopy cover photos from two different forests during the dry season. Top = young regenerating forests; Bottom = mature evergreen forest. The difference in the amount of sunlight that reaches the ground results in a difference in temperature in each forest.

Figure 6. This particular spot is purposely burned within the park to create a fire break. This fire break is meant to protect the other half of the park in case of runaway wildfire

This extreme seasonality of tropical dry forests is one of my favourite things about this amazing and unique ecosystem. Once the rainy season hits in (usually) mid-May, the forests will change drastically. I can’t wait to share that transformation with you! Prepare yourself for more green, more insects, and more wildlife (especially herps!).