Salomón Aguilar

The Status of the Panama Canal Watershed and Its Biodiversity at the Beginning of the 21st Century

P is a small Central American country, but it operates a big canal and the world keeps an eye on developments there. Problems with the canal or ecological disasters in its watershed would attract a lot of attention. As Theodore Roosevelt planned, the Panama Canal is a major shipping corridor, transporting 37 ships a day and providing substantial income to the Panamanian government. Yet as Roosevelt could not have recognized—despite his interest in conservation—the canal sits in the center of one of the world’s most biologically diverse areas (Myers et al. 2000): Southern Central America has more forest bird species than any other region in the world, except Amazonia and the northern and central Andes, each of which is vastly larger than southern Central America (Stotz et al. 1996); and Panama has as many plant species per 10,000 km2 as any region in the world, more than Amazonia or the Malay Peninsula (Barthlott et al. 1996). Roosevelt may have suspected, though, that forests are crucial for protecting the water supply of the Panama Canal and for maintaining the plant and animal communities. Fortunately, the year 2000 still found extensive forests around the canal, protected largely thanks to military and shipping interests, but it also found the watershed adjacent to a large and expanding capital city. Maintaining the ecosystem integrity of the canal will pose a major challenge for conservation in the 21st century. Is urban and economic development compatible with a hydrologically functioning canal and conservation of an extremely diverse flora and fauna?

AN ECOSYSTEM REPORT ON THE PANAMA CANAL: MONITORING THE STATUS OF THE FOREST COMMUNITIES AND THE WATERSHED

In 1996, the Smithsonian Tropical Research Institute and the Republic of Panamas Environmental Authority, with support fromthe United States Agency for International Development, undertook a comprehensive program to monitor the ecosystem of the Panama Canal watershed. The goals were to establish baselineindicators for the integrity of forest communities and rivers. Based on satellite image classification and ground surveys, the2790 km2 watershed had 1570 km2 of forest in 1997, 1080 km2 of which was in national parks and nature monuments. Most of the 490 km2 of forest not currently in protected areas lies along the west bank of the Canal, and its managementstatus after the year 2000 turnover of the Canal from the U.S. to Panama remains uncertain. In forest plots designed to monitorforest diversity and change, a total of 963 woody plant specieswere identified and mapped. We estimate there are a total of 850–1000 woody species in forests of the Canal corridor. Forestsof the wetter upper reaches of the watershed are distinct in species composition from the Canal corridor, and have considerably higher diversity and many unknown species. Theseremote areas are extensively forested, poorly explored, and harbor an estimated 1400–2200 woody species. Vertebrate monitoring programs were also initiated, focusing on species threatened by hunting and forest fragmentation. Large mammals are heavily hunted in most forests of Canal corridor, and therewas clear evidence that mammal density is greatly reduced in hunted areas and that this affects seed predation and dispersal. The human population of the watershed was 113 000 in 1990, and grew by nearly 4% per year from 1980 to 1990. Much of this growth was in a small region of the watershed on the outskirts of Panama City, but even rural areas, including villages near and within national parks, grew by 2% per year. There is no sewage treatment in the watershed, and many towns have no trashcollection, thus streams near large towns are heavily polluted. Analyses of sediment loads in rivers throughout the watershed did not indicate that erosion has been increasing as a result ofdeforestation, rather, erosion seems to be driven largely by total rainfall and heavy rainfall events that cause landslides.Still, models suggest that large-scale deforestation would increase landslide frequency, and failure to detect increases inerosion could be due to the gradual deforestation rate and the short time period over which data are available. A study of runoff showed deforestation increased the amount of water fromrainfall that passed directly into streams. As a result, dry season flow was reduced in a deforested catchment relative to aforested one. Currently, the Panama Canal watershed has extensive forest areasand streams relatively unaffected by humans. But impacts of hunting and pollution near towns are clear, and the burgeoningpopulation will exacerbate these impacts in the next few decades.Changes in policies regarding forest protection and pollution control are necessary.

Use of native tree species by an Hispanic community in Panama.

We investigated the use of plants collected in the wild by a small farming community in Central Panama to document the importance of noncultivated plants by tropical, nonforest-dwelling, nonindigenous people. We visited the community to observe what wood was used to build houses and interviewed local people about medicinal and edible plants collected in the wild state. The community reported use of 119 noncultivated plant species, including 108 tree species, three shrubs, two herbs, four lianas, and two vines. The majority (71) of the species were used for building homes. Other products built with wood collected in the wild were diverse kinds of tools, containers, cages, and fences. The second most important use of wild plants, in terms of number of species, was firewood, for which 40 species were mentioned by the community. Other uses included fruit for human consumption (20 species). Most of the species (82 of 119) were collected in secondary forests near the community, whereas another large group (47 species) were collected in mature forest. Fewer species were harvested in shrubby regrowth or from isolated trees in farm land. Nearly all the species (111 of 119) were native to the area, and never cultivated locally, but 15 species were considered especially valuable, and were often protected when found as juveniles. Only six of the species are commonly used in reforestation programs in Panama. We conclude that even hispanic communities in tropical Latin America, living outside the forest, with no Amerindian inhabitants, make frequent use of the great diversity of trees native to the region.

Tree Species Composition and Beta Diversity in the Upper Río Chagres Basin, Panama

Tree species composition at two sites in the upper Rio Chagres basin of central Panama was evaluated using rapid inventory methods. At each site, two 40×40 m quadrats were demarcated, and each was thoroughly searched for tree and shrub species. The 40×40 m quadrats had a mean of 155 species each, and the four pooled had 285 species; 29 other species were noted along trails near the survey plots. These inventories were compared to 81 others within the Panama Canal Watershed, and forest composition and diversity was evaluated relative to mean dry season duration. The upper Rio Chagres sites have high rainfall and are rich in tree species relative to most of the area; the only area with higher diversity is the Santa Rita Ridge, along the Caribbean coast, which is even wetter. Many tree species are restricted to these wet areas of central Panama, not occurring in drier areas of the Pacific slope or central Panama.

Demographic trends and climate over 35 years in the Barro Colorado 50 ha plot

BackgroundThe first three censuses of the 50-ha plot at Barro Colorado Island spanned an unusually harsh dry season during the 1983 El Niño. By the early 1990s, we had documented increases in tree mortality, tree growth, and large population fluctuations of many species during the 1982–1985 census interval. At the time, we asserted that increasing drought frequency would greatly affect the forest. With the benefit of five more censuses at Barro Colorado from 1995–2015, we can now put the 1980 conditions in a longer perspective and test the hypothesis that increasing droughtiness has continued to change the forest.MethodsA 50-ha forest plot on Barro Colorado Island was censused eight times, in 1982 and every five years since 1985. All free-standing woody stems were measured, mapped, and identified in each census.Results1) The period 1982–1992 included several extreme dry seasons, not just 1983, but since then there have been few such droughts.2) Dbh growth declined from a peak in the early 1980s to its lowest in the early 1990s. From 1995–2015 it increased slightly, but not returning to the initial peak. Nearly every species and all dbh categories followed the same pattern.3) The elevated stand-wide mortality rate of large trees during the 1982–1985 drought has not returned, and most individual species showed the same pattern of elevated mortality in the 1980s followed by low and fairly stable mortality after 1990.4) Sapling mortality declined after 1985, but rose again in the late-90s, so the 1980s drought period no longer looks unusual. Mortality of individual species’ saplings fluctuated erratically, including cases where mortality during the drought was lower than after.5) Population sizes of individual species fluctuated in all possible directions. Some species declined precipitously during the drought, then recovered, but others did not recover. Other species increased in abundance during the drought.ConclusionsDroughts of the 1980s elevated tree growth and mortality at Barro Colorado, but since 1990, demographic rates have remained lower, paralleling a moderate climate with few severe droughts after 1990. Moisture-demanding species suffered during the drought, but many have since recovered. We do not know how often such drought periods recur. Moreover, many species’ abundances fluctuated over 35 years with no known cause.