The humidity has been rising all morning in the hot rain forests of the rugged Chagres River watershed of Panama. A little after noon, it climbs to a stultifying 90%, and farm and forest laborers working through the oppressive heat of morning now begin to wrap up the day’s projects. Thunder rumbles, and the first huge raindrops splatter on the uppermost leaves of the forest canopy. Water quickly spills into the leaves of branches below, wetting acres of leaf surfaces and trickling along a million leaf stems. Soon the rain is coming down as if buckets of water were being thrown from the sky.
Billions of dollars of world trade are dependent on the quality, quantity, and timing of runoff from the slopes of the Chagres catchment and five smaller watersheds that supply water to the Panama Canal. Each day, according to the Panama Canal Commission’s Web site, about 40 ships transit three sets of locks on the 51-mi. canal. Together, the locks require a volume of water that would cover 10 mi.2 to a depth of 10 ft. The watersheds supplying this flow comprise only about 1,259 mi.2 If this source area were square, it would measure only 35 mi. on each side. Yet sustained groundwater flows from these watersheds are needed to supply the gravity-feed canal system during a four-month dry season when surface flows are diminished. And it is vitally important to keep sediments from diminishing the water storage capacity of the two man-made lakes that supply water to operate the locks.
At midnight of the last day of the 20th century, control of watersheds that maintain one of the world’s most critical shipping lanes transferred from the United States of America to the Republic of Panama. In the previous two decades, the topic of deforestation of the canal watersheds and its potential impacts had received some play in the popular press. The population of the deeply dissected and rugged watershed had risen from about 30,000 people in 1950 to about 140,000 people in 1997, according to a 1998 paper by the US Agency for International Development (USAID). By 1980, rural colonization by subsistence farmers who practiced shifting agriculture had reduced forest cover to less than 30%. In response to this situation, Panama established a system of parklands to protect forest cover in the headwaters and initiated an aggressive program of reforestation.
In an effort to learn more about erosion control in these small tropical watersheds so vital to world trade, the author spoke with people doing research on rain-forest ecology, hillslope dynamics, and sustainable development in Panama and nearby Central American countries of the rainy tropics. What emerged was a collage of fragile and complex ecosystems whose complete understanding will require more independent study by qualified scientists, of poverty and the need for sustainable short- and long-range agricultural production systems, of the financial and educational resources that local governments need to help communities affect social and economic change at a landscape scale, and of the need for more research on the complex interrelationships between human and natural systems in the rainy tropics.
Waiting in the forest canopy to catch water from the afternoons’ treetop drenching are myriad perching and climbing plants whose roots have never touched earth. Despite the almost monsoonal rainfall conditions that give rise to the tropical rain forest, researchers have found that many rain-forest epiphytes possess the same water-conserving adaptations of desert plants. And high in the canopy, the water that these plants capture supports worlds within worlds of miniature ecosystems: amoebas and protozoans normally associated with lakes, blue-green algae that fix nitrogen, and busy insect and animal communities of frogs, birds, lizards, and mammals.
Even the host trees that support these canopy-dwelling plants have developed mechanisms to scavenge water, carbon, and other nutrients from the teeming aerial terrariums they support. These mechanisms, according to Gary Hartshorn, director of the Organization for Tropical Studies, have only become completely understood in the past decade or so-since canopy cranes and walkways opened up this new frontier of biology. When they began to study up-close what they could only glimpse by craning their necks from the ground, says Hartshorn, they learned that there is “incredibly tight” nutrient cycling in the canopies of tropical rain forests.
Darwyn Coxson, a canopy ecologist at the University of Northern British Columbia who studies both temperate- and tropical-zone rain forests, has found that an important source of nutrients in these rain forests are those captured from rainfall by canopy epiphytes. Wind delivers these nutrients in the form of dust from as far away as the Sahara Desert. When the nutrients mix with rain, the result is a dilute “soup” containing essential plant nutrients that washes through the forest canopy.
As it flows over the plant materials of the canopy, the soup becomes enriched by decaying plant material and dissolved animal droppings. It is absorbed by leaf surfaces, sought after by the roots of perching plants, and scavenged by adventitious roots that host trees send out to tap the organic material and moisture of plant mats that perch in the crotches of branches and on outstretched limbs.
In the lower canopy, light is a limiting resource for many plants, Coxson points out. Yet some plants can thrive by capturing carbon and other nutrients delivered by stem flow from above. It can take many centuries for the entire group of organisms in this nutrient cycling system to mature, he says. At top performance, they may be capturing up to half of the available nutrients required by the forest.
Coxson and others have been researching how the local nutrient pool is affected after rain-forest trees are removed by natural disturbances (such as wind storms) or through human means (such as burning or harvesting). They have found a threshold effect concerning canopy microclimate. If the canopy opening is small, new members of the forest community can be inoculated with the epiphytic plants that capture nutrients in rainfall. But if the canopy opening exceeds the temperature threshold, the canopy becomes too dry to support even the bromeliads, which are adapted to capture and conserve rain that falls on the bright, hot, upper-forest canopy.
These findings suggest that selected harvest of tropical rain forests should be based on decisions about what species composition, age classes, and spacing need to be left after harvest in order to ensure continuing capability of the canopy to cycle nutrients at maximum productivity, explains Coxson.
This is a shift from traditional selective harvest in which “take trees” are identified. Coxson points out that if 10% of the forest is harvested and the harvest trees carry 50% of the canopy epiphytes, their loss will cause the surrounding forest system to lose access to an essential nutrient pool that might not recover for centuries. If the forest cover is converted to other landscape types, this nutrient source will be cut off.
This is the familiar tale of slash and burn, or shifting agriculture in the tropics, in which a patch of forest is cleared and farmed for two or three years until the available nutrients are depleted. Then the farmer moves on, leaving the depleted clearing to lie fallow for 20 years or so. Researchers agree that such loose rotation systems worked well at low-population densities, but not where populations have boomed and people cannot afford to abandon the land.
When a canopy ecologist creeps out on an outstretched branch and sits on a mat of aerial roots in which the detritus of canopy-dwelling plants has collected, it is similar to sitting on a giant sponge, Coxson describes. Many canopy epiphytes, such as mosses and lichen, can hold up to 10 times their dry weight in water. This thirsty, perching plant community has a tremendous capacity to attenuate runoff from the intense convectional rainstorms that gather every afternoon with clockwork predictability in the rainy tropics.
In these intense afternoon storms, rainfall energy on steep slopes cleared for farming has the potential to detach very high quantities of sediments. Lowlands that lie in the path of northeasterly winds moving across the narrow Isthmus of Panama from the Caribbean can receive more than 11 ft. of precipitation each year. Much of the rainfall is condensed into an eight-month period and occurs in intense afternoon convective storms of a few hours’ duration. This is a characteristic pattern of rainfall in the Inter-Tropical Convergence Zone in which a low-pressure system facilitates convective rising, which together with orographic lifting produces intense afternoon thunderstorms.
Field-scale studies of paired watersheds in southern Honduras conducted by researchers from Texas A&M University from 1993-98 recorded an average soil loss of 92 tons/ha/yr. on steep land that was farmed using the traditional slash-and-burn methods. Tom Thurow, a hydrologist who participated in the study, notes that rainfall intensity in the tropics is three times higher than the highest rainfall energy recorded for any county in the US. It is not uncommon, he says, for slopes in excess of 55% to be farmed in these rainy tropics. In Honduras, farmers sometimes cultivate fields so steep that they tie ropes around their waist to prevent them from falling while cultivating their maize or sorghum crops.
When rain forest is cleared, rainfall interception is reduced, resulting in much more precipitation reaching the soil and much greater erosive energy being applied to directly to it. The resulting potential for exponentially increased surface erosion, particularly on steep slopes that have been cleared for agriculture, is well known.But Thurow points out a more subtle result of rain-forest clearing that potentially can lead to even more remarkable soil loss through mass wastage: a change in the stability of soils on slopes when they become saturated by water that infiltrates from the surface. In many soils, he continues, the percolation rate of water moving downward through the soil profile slows at depth. This can be the case particularly with some volcanic soils in which subsoil layers that restrict percolation have developed.
In a tropical cropping system where mulches are used, Thurow explains, mulches serve multiple functions: to dissipate rain-splash energy, to return decomposed organics to the soil, to control erosion, to minimize runoff, and to enhance infiltration of rainfall. However, the field studies showed that mulches do not protect against mass wasting events. In a steep, tropical rain-forest soil that no longer possesses the live roots of many plants that tie the soil together at many depths in the soil profile, the accumulated weight of water within the soil can trigger shearing.
Thurow and his colleagues found that such mass wastage accounted for more than 90% of soil loss measured in farmed areas where tree cover had been removed and the topsoil remained saturated during prolonged rainy periods. This finding and results of field-scale assessments of soil and water conservation practices on these steep lands led Thurow and his colleagues to favor hybrid terrace systems to prevent soil loss and landslides in the multilayered plantations and homestead gardens common in cropped, steep lands of Latin America.
Such systems combine mulches, young trees, vetiver grass hedges, and rock walls. They help tie soils onto the hillsides and provide gradient breaks where sediments can be deposited and nutrients retained, Thurow says. The root systems of vetiver grass grow downward like a curtain, helping to tie the soil together with minimum competition with the roots of adjacent crops. The barrier formed by the vetiver traps eroded soil from upslope. Over time, deposition behind the vetiver grass creates level areas that can be used for cultivation. The roots of alley-cropped trees add more structural support to soils. Crop residues and leaf fall provide dissipation of rainfall energy, erosion protection, and nutrients.
These soil and water conservation practices have a terrible return on investment as measured at the plot level by increases in production, Thurow is quick to point out. This can be especially true in tropical countries where populations are booming, alluvial agricultural lands have long since been spoken for, and people are desperate for subsistence crops. But, he adds, the benefits of such practices need to be weighed against the long-term societal benefits of healthy forests: productive soils, clean water, and clean air.
Decisions about development and land use need to be made in larger temporal and spatial scales using a wholly different kind of green accounting to make the link between environment and economics. The impacts of Hurricane Mitch exposed these interconnections, Thurow says. According to the Lanic weather Web site, on October 26-27, 1998, Hurricane Mitch brought wind speeds of up to 180 mph and heavy rains that caused extensive floods and landslides. For the Central American commercial corridor, which depends 90% on road transport,the impacts were economically devastating.
Other economic reverberations can occur downstream because of accelerated erosion brought on by conversion of rain forests on steep slopes to agriculture. Deposition might require dredging. Nutrient loading and eutrophication might require costly fixes to water-quality problems. So a short-term gain in one area of the watershed could have tremendous societal costs in another. And a decision to hold upland systems in place can have cascading impacts throughout whole watersheds, notes Thurow, with positive effects on downstream economies.
Hydrologist Bob Stallard of the US Geological Survey is conducting studies of natural and developed watersheds in both Puerto Rico and the Panama Canal. Both regions are excellent metaphors for future development in the tropics, he says. The studies aim to characterize processes that control the distribution and transport of nutrients through watersheds. Using paired watersheds, Stallard is looking at the impacts of development on landslides and on biogeochemical budgets of entire landscapes.
Human activities have greatly increased the size and frequency of mass-wastage phenomena in these regions, but much research is needed to correlate these findings with their implications at regional and global scales. Stallard notes that landslides in humid mountainous regions may dominate mass wasting on hillslopes. Long periods of quiescence may end when heavy rains or earthquakes destabilize the soil and trigger a landslide. Stallard’s colleague, Richard Condit, a population biologist at the Smithsonian Tropical Research Institute (STRI) in Panama City, reports that sedimentation peaks recorded in 1981, 1983, 1986, and 1987 correlate well with days of extra-heavy runoff. These are days or storms in which roughly 8 in. of precipitation fell. This finding has given rise to a new term: landslide weather.
In another study by STRI, reduction of 16 years of daily sediment data from the Panama Canal shows a decrease in erosion rates in the years following deforestation. Stallard posits that the lack of mechanized agriculture and the quick growth of secondary plants may contribute to this decrease. But, he adds, the main factor in the decrease in erosion during this period is probably the lower number of landslide-producing storms.
One of the questions addressed by hydrology and erosion monitoring in Panama Canal watersheds is whether deforestation will reduce the water-storage capacity of the watersheds that supply the canal. Measurements at three weirs showed that 14% of rainfall falling on forested watersheds resulted in runoff. Runoff from deforested watersheds jumped to 26%. This is important, Condit says, because if two times more runoff enters the streams of deforested watersheds during the rainy season, there might not be enough base flow to operate the canal during the dry season. The greater volumes of runoff during the rainy season cannot be stored for later use and are spilled over the dams. This is critical, for the limiting factor for canal operation is water supply during the dry season.But complicating this hydrologic picture is the fact that an abrupt 10% reduction in rainfall began to be measured in the central Isthmus of Panama in 1965, explains Condit. This downward trend in rainfall lasted for 30 years, until the mid-1990s. There were concerns that this shift was the result of deforestation. Yet, he adds, climatic data show this prolonged shift occurred everywhere in the tropics, including large areas in which no deforestation had occurred, such as in Puerto Rico, where there has been forest regrowth since the 1940s.
Doug Schaefer of the Institute for Tropical Ecosystem Studies at the University of Puerto Rico, thinks El NiÃ±o may be the most grave issue for future operation of the Panama Canal. During the 1997-98 El NiÃ±o, he recalls, Panama experienced a drought in its normal dry season (January-March) that reduced water inflow to an upper reservoir (Lake Alajuela) that supplies water to the canal during the dry season. This resulted in some restrictions to canal use.
Condit notes that there is concern about organic sediments decreasing the water-storage capacity of Lake Alajuela. Aquatic plants generate enormous quantities of decayed organic material that constantly must be removed. However, Stallard points out that less than one half of the lake’s dredging costs are associated with channel maintenance.
To answer questions about nutrient loading of the two lakes that supply water for canal operations, Stallard’s project is sampling nitrate, ammonia, and phosphate throughout the watershed. Both lakes are poised on the edge of becoming at least partially eutrophic, because of phosphate loading, he says. Erosion is a slow, natural source of phosphate. Rainfall, however, is a source of nitrate and ammonia, both of which are in great supply. Thus, the two lakes seem to be phosphate-limited. The largest sources of additional nutrients appear to be entering the canal below Lake Alajuela as runoff from growing urban areas and farms, pastures, and feedlots, Stallard says.
Speaking over the phone from his desk at the Institute for Tropical Ecosystem Studies, Schaefer laughs that a handful of tropical researchers worldwide is busy answering questions with more questions. More independent research is needed to fully understand the complex interrelationships between climate, hillslope dynamics, sediment and nutrient budgets, and human uses of the tropical rain forest. Further, the USAID and others point out that for effective agroforestry practices to be disseminated in developing tropical countries, a transfer of authority and resources to local governments needs to be accomplished. Municipalities need to be partners in the development of goals for sustainable uses of local natural resources. They need financial resources to involve and instruct communities in sustainable agroforestry practices. They need education in how to formulate policies and implement programs that support long-range economic and land-use decisions at the landscape scale.These are the challenges that EC professionals will face in the new century as greater populations seek greater productivity from the rain forests of the developing tropics.