Copyrighted Material Chapter 1 Forests of Panama and Costa Rica Tree Diversity in the Tropics are far fewer botanists, more species, and less of Central America money available for environmental concerns than there are in North America. Indeed, there Southern Central America excels in tree spe­ remain immense gaps in our knowledge of cies richness. On a world map, Costa Rica and trees in Central America. We have carried out Panama, along with western Colombia, Ecua­ inventories for 20 years throughout Panama, dor, and Peru, have the highest plant diversity yet we are still unable to identify every tree we (Barthlott et al. 1996). Understanding why be­ see. In fact, in more remote areas that are dif­ gins with understanding the topographic and ficult to visit, it is typical for tropical botanists climatic diversity of the region. The Andes in to leave as unidentified 25% of the species en­ South America and the central cordillera of countered. There is nowhere in North America Central America are a long, near­continuous where an experienced botanist would have spine of mountains, and the variation in eleva­ anywhere near this difficulty. tion from lowland to mountain peaks creates Those North American botanists have variation in habitat. From the warm lowlands to excellent field guides (e.g., Elias 1980; Petrides the cool and windswept mountain tops, com­ 1972). Comparable guides for trees anywhere munities differ, whether of trees, birds, or but­ in the tropics are scarce, though the number terflies. Particularly important in Central Amer­ has been greatly expanding in the past decade ica is the contrast between the Pacific and (e.g., Zamora et al. 2000, 2004). Our hope here Caribbean flanks of the mountains, which we is to follow this trend and help other biologists will return to shortly. expand their knowledge about the tropics. This But there is more to high diversity in Central guide covers 493 tree species, including 438 America than a mountain range. Colorado with color photos, and we offer descriptions of and Switzerland have mountains, but nowhere key features for identification and show where near the number of tree species. Indeed, all of they are known in Panama and Costa Rica. North America has about 1000 tree species, We also offer brief notes about the tree species compared to 2300 in much smaller Panama. we do not cover, where they are known, and We don’t know the total number of tree species which of the illustrated species they resemble. in all of South America, but it is somewhere around 20,000. Scientists have debated this Geography, Climate, and Forest contrast in species richness since von Hum­ boldt traveled the Amazon 200 years ago, and Panama and Costa Rica are well inside the there are nearly as many theories as there are tropics, at 8–10°N latitude, where an atmo­ theoreticians. One obvious possibility is glacia­ spheric conveyor belt delivers storm systems tion: during the past 200,000 years, while North out of the east—off the Caribbean—during the America and Europe were under ice sheets, northern summer. During the winter, from De­ Central America maintained a moist, warm cember through April, this belt shifts south­ climate and dense vegetation. There is plenty ward and the region has its annual dry season. of evidence that northern regions suffered The mountain axis of Central America is extinction during the glacial epoch of the past high enough to intercept storms off the Carib­ few million years. bean, and thus the wettest areas of Panama High species richness is one reason bota­ and Costa Rica are on the Caribbean flanks nists find it difficult to learn, describe, and doc­ of the mountains. There, annual precipita­ ument plant richness of Latin America. There tion is greater than 3000 mm, with many sites Copyrighted Material 12 Chapter 1 getting 4000 or 5000 mm. The Pacific slope of (2) Wet forest. The lowland forests of the Ca­ the isthmus is generally drier, getting less than ribbean slope are wet forests of the Holdridge 3000 mm, and as low as 1500 or even 1000 mm system. Here, the classic image of rainforest at the driest locations. Though generally drier holds: trees are tall, the canopy is dense and than the Caribbean slope, the Pacific slope is continuous except for small gaps created by variable and has both wet and dry areas (Figs. fallen trees, and epiphytes are conspicuous. 1.1 and 1.2). The central axis of mountains Trees do not shed their leaves during the weak between the Caribbean and Pacific is nearly dry season. Much less conspicuous is what you everywhere wet, often with clouds and fog. can decipher only after learning tree species: To the tree species of Central America, the wet forests have many more species than do variation in rainfall is crucial. On the wettest dry forests. There are substantial blocks of wet Caribbean slopes, there is enough moisture forest throughout Central America, including even during the December–April dry season extensive and unbroken stretches in San Blas, to keep the soil damp, and forests are wet and much of western Panama, and the national green all year. In contrast, much of the Pacific parks of Costa Rica’s Caribbean slope as well slope has hard, dry soil by April, and many as the Osa Peninsula (Fig. 1.1). species are deciduous then. By sitting on the (3) Moist forest. For the forests between cusp of this gradient from dry forest to wet wet and dry, we use Holdridge’s phrase “moist forest, Panama and Costa Rica capture a great forest.” This grows where more than 1500 but diversity of tree species, because the ability to less than 3000 mm of rain falls, but where tolerate a dry season is a crucial adaptation. the dry season is pronounced and lasts 3–4 In the 1950s and 1960s, a pioneering ecolo­ months. Trees are tall; indeed, the biggest trees gist named Leslie Holdridge worked for years in the area are characteristic of moist forest in Costa Rica and Panama (Holdridge 1967). (e.g., the kapok [Ceiba pentandra] and the Holdridge noticed how the variable climate of espavé [Anacardium excelsum]). On the other the region has a conspicuous impact on the hand, there are deciduous species during the forests and developed a system for classifying dry season. All the easy­to­see forest near the vegetation to describe it. The Holdridge system Panama Canal is moist forest (Fig. 1.2), as is a has been widely used by tropical ecologists large block in Darien National Park in eastern since, especially in Central America. There are Panama and much of the Pacific slope of Costa a total of 17 categories in the Holdridge system, Rica (Fig. 1.1). but a good basic understanding of forest com­ The remaining climatic zones in our abbrevi­ munities in Central America can be gained by ated version of Holdridge are montane: forests abstracting five broad units: dry, moist, wet, above 800 m elevation. There is an important lower montane, and upper montane. division at about 1500 m, so we consider two (1) Dry forest. In Panama and Costa Rica, categories. dry forest is found in two regions where rainfall (4) Lower montane forest. From about 800 to is less than 1500 mm annually, one in central 1500 m elevation, forests resemble lowland wet Panama west of Panama City, the other in forests, with tall trees and dense canopy, and northwestern Costa Rica (Fig. 1.1). True dry for­ wet and lower montane zones share many spe­ est in the tropics is short, seldom more than 20 cies. Tropical cloud forests fall in this range, m tall, and many trees lose their leaves during and are typical on the tops of lower mountains, the dry season, quite different from the classic where clouds often sit. There are high densities image of tropical rainforest. Fine­leaved trees of epiphytes—orchids, bromeliads, ferns, and of the legume family are common. In the driest mosses—in cloud forests. areas of Central America, there are even co­ (5) Upper montane forest. Above 1500 and lumnar cacti and the mesquite tree (Prosopis especially 2000 m elevation, forests are quite jubata), both common sights in North Ameri­ different, and are sometimes classified as tem­ can deserts. But it is not a desert in Panama perate, like forests of North America. Indeed, and Costa Rica: trees would occupy the entire oak (genus Quercus) and alder (Alnus) are con­ region, given the chance. We will return to spicuous above 2000 m, along with many other human impacts later. temperate groups familiar in North America Copyrighted Material Forests of Panama and Costa Rica 13 Santa Rosa NP Braulio Carrillo NP Caribbean Guanacaste NP San Jose La Amistad Intl Park San Blas Bocas del Toro Nusagandi El Cope Cerro Boquete Corcovado NP Punta Chiriqui Osa Peninsula Santa Fe Darien Herrera Sarigua NP Los Santos Azuero Pacific Ocean Peninsula Figure 1.1. Place names and forest zones in Panama and Costa Rica. The dry zone is indicated by yellow, the moist zone by light green, and the wet zone by dark green. A light green topographic line demarcates the lower montane zone, at 800 m elevation, and a black line shows the upper montane zone, at 1500 m. Caribbean Santa Rita Ridge Upper Chagres River Fort Sherman Colon Fort San Lorenzo Chagres R. Soberania NP Pipeline Rd Cerro Jefe Barro Colorado I. Madden Dam Cerro Azul Gamboa Sendero El Charco Summit Garden Panama Canal Parque Metropolitano Panama City El Valle Punta Chame Altos de Campana NP Pacific Ocean Figure 1.2.