A view from the air--Where have the forests gone? (Sierpe River Area, SW Costa Rica)
Tropical cyclones have as their primary source of energy the release of latent heat. The initial stages of a hurricane initiate with the warming and subsequent evaporation of a large body of water already at its peak thermal capacity. When the oceanic waters off of western Africa at the latitude between Senegal and Guinea reach at least 270 C (810F), convective waves of energy in the form of cumulus clouds begin building and developing along warm oceanic currents (N. Equatorial) as the convection migrates westward (www.nhc.coaa.gov). If conditions are favorable for the intensification of a tropical disturbance, a cyclonic circulation develops and the central air pressure begins to fall at the core of the storm due to an increase in air movement from the outside. Unstable air begins to rise at this point with the development of a low-pressure core and defined isobars surrounding the increased convection. This stage of the storm is the tropical depression characterized by winds up to 50 km/h (31 mph) and intense precipitation in the release of latent heat energy. The next phase of the storm’s life greatly depends on the release of further latent heat into the atmosphere (the engine of the mature hurricane) and a more drastic drop in central pressure and isobar development. With the tropical storm phase, there are now several closed circular isobars and producing inflow and outflow gusts between 52 km/h (32 mph) and 117 km/h (73mph). However, the most pronounced feature of the tropical storm is the intensification of its internal circulation into a flattened disk of cumulus convection with a central depression initiating the development of the eye.
Once wind speeds within the eye-wall exceed 119km/h (74 mph), the storm is an official hurricane with the same basic structure as a tropical storm but with a far lower core pressure and wind speeds that can exceed 155 mph and storm surges greater than 18 ft. The mature hurricane can best be compared to a voracious combustion engine with its eye wall churning explosive amounts of moisture into pure potential and kinetic energy. The mechanics of the hurricane are based on the circulation of air around a central warm-core through a combination of the Coriolis effect and friction acting on the pressure gradient force of air moving into the storm (Miller, 1967). As moisture-rich air moves into the eye-wall because of a pressure gradient between inner and outer air parcels, the opposition of the frictional force and perpendicular drag of the Coriolis force act to veer incoming winds generating a profound level of circulation at the core. It is this circulation of air currents that not only gives the storm its disk-like appearance but also the incredibly dynamic pattern of heating and cooling that allows such systems to mediate the earth’s climate. From their inception to landfall, the hurricane is just one more way the earth is able to deal with excess solar radiation across latitudes by releasing the latent heat of evaporation and converting potential energy into a kinetic form through unparallel rain, wind, and tidal action. As the air rises 14,000 meters or more into the central core cooling at the rate of 540 cal per gram of water, the vast percentage of the potential energy goes into the formation of a massive canopy of cirrus clouds in the outflow layer high above the water surface. It is estimated that the hurricane exports an incredible amount of potential energy in the adiabatic cooling of air along the eye wall instead of simply converting all the available energy into a kinetic form (an estimated 3% of all available energy) (Miller, 1967). However, once the hurricane’s source of heat/energy is removed upon landfall, like all thunderstorms, the process of updraft and downdraft that keep the storm’s dynamics in equilibrium becomes one of cooling only. In the case of specific landfall studies that follow, the vast majority of the devastation caused during a hurricane strike is primarily the result of potential energy being converted to a kinetic form without any recycling of latent heat. This causes an effective “blow out” of the storm’s organized structure and the result of inundating rains (kinetic energy) that result in the devastation of such dissipating storms as Hurricane Floyd and Tropical Storm Alison. A very different scenario arises, however, in the event of an island or peninsula landfall such as Hugo over the forests of Puerto Rico, or Andrew over Florida. In the studies that follow, the internal dynamics of the storm in each case remained largely intact to the extent that the inflow/outflow systems that power the storm’s internal engine resulted in the greatest changes of coastal ecosystems.
Luquillo Experimental Forest –
What we primarily noticed in the passing of such strong tropical disturbances over forested islands such as the Luquillo Experimental Forest (LEF) in Puerto Rico is a sharp decline in above-ground biomass and tree density due from wind and flooding (Walker et al. 1991). Populations of many organisms declined immediately following Hurricane Hugo in the LEF either because of direct negative impacts to their populations or because individuals migrated out of hurricane-damaged areas. Many species such as a variety of insect and a wide-ranging species of terrestrial bat, Artibeus jamaicensis, declined during the five year period of re-growth and re-concentration of nutrients only to increase to greater than pre-hurricane levels immediately thereafter (Zimmerman et al. 1996). The beauty of these particular symbioses between communities of species is in their response to the passing of hurricanes and a communal effort in rebuilding the forest in accordance with equilibria through nature. Whereas many organisms respond negatively to the increase forest debris left in the wake of the hurricane, some organisms respond different to changes in resource availability and habitat composition through increase litter. Populations of Atya shrimp and adult coqui frogs similarly increased during the five-year period of rebuilding, only to decrease to pre-hurricane levels once a more ordered forest infrastructure had been established (Zimmerman et al. 1996). Increases in the river shrimp were probably due to changes in resource levels (detritus) and increased habitat complexity caused by coarse woody debris in streams whereas frogs appeared to be related to the increased number of retreat sites caused by forest floor debris and a decline in an abundance of predators.
In accordance with the fragile ecosystem of Caribbean forests, the rapid turnover of leaf litter from pioneer species provides a high rang of diverse habitat structures and high quality detritus for consumers and producers. So in the wake of a powerful storm or hurricane where we see the instant leveling and restructuring of this biosphere, the trees standing are the ones directly responsible in the regeneration of the forest canopy and rapidly alleviate the disorder that exists in the understory immediately thereafter. And, despite widespread habitat alteration by the hurricane, preferred habitats were quickly regenerated in response to a slight shift in the equilibrium of the ecosystem (i.e. influx of producers (simple celled organisms) and lack of consumers due to the temporary disarray) (Boose et al. 1994). It is this flexibility and adaptability in the mountainous rainforests of the Caribbean that we as scientists have come to admire in light of the destructive forces of nature surrounding them. And it is also such a system of similar checks and balances on the forest that we come to expect throughout all of nature as an almost adaptive quality to the conditions that regularly come to pass. So in keeping with an almost Darwinian view on nature, the forests of the Caribbean have come to evolve this quality of producers over consumers to take over in the event of such disasters in quickly rebuilding the forest infrastructure back to an original level of homeostasis. However, it is through a recent trend in human activity that these forests have come under fire in the deforestation of viable communities able to adapt to natural levels of destruction. Through research of the LEF and human density patterns throughout the Caribbean (5 of the 25 most densely populated countries occur in the Caribbean basin) we have confounded that the economic changes and subsequent increases in secondary forests in Puerto Rico provide the opportunity to predict increases in forest cover in other areas of the Caribbean. “Prior history of human disturbance can lead to changed in tree species compositions that predispose these secondary forests to high levels of hurricane damage.” (Zimmerman et al. 1996) So in essence what we are seeing today is a far more difficult outlook in the successful habits of such adaptive ecosystems due to human interference through extraction, pollution, and reclamation (Boose et al. 1994). As mentioned earlier, the importance of a natural occurring soil biomass in regenerating the levels of producer productivity in these mountain forests are being harmed through recent deforestation. In this scenario, soil vital to the ecosystem’s survival is being lost through runoff because there no longer exists the vegetation and root structure to hold the nutrient-rich mass together. This is similar problem we are finding throughout the deforested regions in South America where areas of human activity have not responded at all to any form of nutrients due to a lack of original content washed downstream. These examples of human disturbance emphasize the diverse ways in which humans affect natural ecosystems.
Coral Reefs/Coastal Ecosystems –
Hurricanes play a role across the Caribbean that is similar to fires on terrestrial systems, “releasing resources (i.e. space) and preventing monopolization by a small subset of species.” (Hughes, Connell, 1999) Understanding the impacts of such multiple stressors as hurricanes requires a long terms approach. For the most part, multiple stressors on coral reefs and elsewhere can act simultaneously across long periods (i.e. chronic pollution and overfishing) or at different times (hurricanes). Such notable examples through my research have been the ecological effects of both Hurricane Allen (1980) on the reefs of Jamaica and that of Andrew (1992) on coastal ecology of Florida. What we see here, as a common connection between the effects on forest and underwater communities, is the relative shift in equilibrium of plant and animal organisms that results in response to a hurricane’s passing. In comparison, both storms affected the Caribbean ecosystem in the break up and release of stored nutrients in the biosphere and resulted in a shift in biotic productivity. With hurricane Allen, damage was greatest at shallow sites where the hurricane hit shallow-water branching species, most notably the elkhorn and staghorn corals (Acropora palmata and A. cervicornis). Also, the nutrients released in the destruction of such coral beds as A. palmata flooded other communities down shore causing extensive damage to communities such as Zoanthus lying outside the storm’s reach. After the hurricane’s pass, large amounts of nutrients from destroyed forest above-shore came washing down as runoff, depositing nutrient-rich compounds such as nitrates and phosphates causing wide-spread algal booms in response to a temporary depression of herbaceous sea-urchin Diadema killed off through the sediment scouring by Allen’s sustained winds and storm surge. In the following years and in accordance to the same redevelopment taking place in the free space allotted in Hurricane Hugo in ’89, new coral communities, namely Porites, settled in large numbers onto free space generated in the shallows that had previously been over-run by dominant communities. (T.P. Hughes et. al., 1994) The selectivity of disturbance is a key issue. If hurricanes or other disturbances affected all species equally, they would have no direct impact on community structure; overall abundance would decline, but there would be no direct change in the abundance of each species. Primarily in the case of hurricane destruction, differential mortality rates for each community is the norm (Hughes, Connell, 1999). Differential recruitment after a hurricane will also result in changes in community composition where some species recover faster and, in effect, change the overall composition of the ecosystem as a whole. In such studies, it appears that the immediate, destructive effects of hurricanes are selective. Consequently, their effects depend on the length of time since the last disturbance. “Where hurricanes are rare, they selectively remove the competitive dominants (usually fast-growing branching or tabular species), which have had sufficient time to recover. Where disturbances are frequent, these vulnerable species may be rare, but they will still suffer higher mortality than resistant species. Consequently, disturbances can increase or decrease diversity and evenness depending on circumstances.” (Hughes, Connell, 1999)
In a study on Hurricane Andrew’s effects on coastal ecology, there exists the opportunity to study a variety of ways in which hurricane’s impact communities of coastal organisms. Hurricane Andrew was a compact, fast moving and intense hurricane that impacted south Florida on August 24, 1992 ranking as the third strongest hurricane to impact the U.S this century. The response of coastlines and shallow marine environments on both the east and west coasts of Florida reflect both the high intensity and the short duration of wind and current attack that resulted from Andrew's passage. During the early morning of August 24, 1992, the eye of Hurricane Andrew made landfall on the east coast of south Florida. By morning, the storm had passed westward into and across the Everglades and exited the west coast of south Florida along the mangrove shoreline of Everglades National Park. The major effects of the storm were changes in the near shore water quality, localized by intense bottom scouring and beach overwash. The stirring of sediments were found, like with Alllen, to increase dissolved phosphate levels causing plankton blooms and eventually insufficient oxygen levels to support the ecosystem. In hard bottom communities, sponges, corals, and sea whisps were sheared from their substrate and deposited along extensive debris tracks (Pimm, Davis, 1994). For a brief period of time, the marine community of organisms was greatly affected by a sudden decline in habitable space. The juvenile spiny lobster normally found under sponges and coals in Biscayne Bay disappeared. On some reefs, the storm scoured the tops of massive Acropora brain coral, washing away boulders of 200-year-old coral communities and breaking off other branching varieties. As sad as the marked devastation to the coral communities appeared, vertebrate communities throughout Florida saw a significant improvement in nesting sites from predators much like the coqui frogs of the LFE in Puerto Rico due to in increase in debris providing new terrestrial habitats. R.W. Snow of the South Florida research center in the Everglades counted more manatees than on any previous census as well as multitude of new sea turtle and Florida crocodile nesting sites due to the overwash of coastal debris. However, in the spite of little damage done to the vertebrate communities of animals with the exception of the occasional smashed nest, the extent of the destruction to Florida’s valuable mangrove forests was extensive and may take decades to fully recover. Here, where the hurricane's eye left the west coast to move across the gulf, more than 85% of the mangrove trees had blown over. On-ground inspections showed that many of the standing trees had cracked trunks, an observation in line with the type of destruction seen through the LEF in Hugo.
The Possibility of Recovery –
We may think of hurricanes as once-in-a-lifetime events but for ecosystems they are to be expected continuously, if erratically, within an individual tree or coral head's lifetime. The Caribbean as a whole averages more than four hurricanes per year and in some communities, such as Puerto Rico, fifteen hurricanes have crossed the island in less than 300 years. The paths of many hurricanes, including Donna (1960), Betsy (1965), and Andrew (1992), have traversed the three South Florida parks this century. Hurricanes have been a central feature in evolution of subtropical Caribbean ecosystems due to the fact that they are widespread and unavoidable in the long term. Due to the storm's rapid transit, damage from Andrew to the coral reefs and marsh ecosystems was less than that caused by other Caribbean storms and the marine and freshwater teams viewed the hurricane's effects as consistent with the usual patterns ecosystem growth and disturbance. Although harder hit initially, the forest ecosystems may be as resilient as the marshes and marine environments. Only 16% of wind-thrown trees and 29% of broken trees died within two years after hurricane Gilbert. That hurricane hit the Yucatan Peninsula with greater ferocity than Andrew, clocking winds of 300 km/hr. By extension, many of trees seriously injured in 1992 should survive. Indeed, within three weeks of Andrew, many of the tropical hardwoods were already re-leafing ((Pimm, Davis, 1994). So, in essence, what we see through the above case studies is a similar language in ecological adaptation and growth found throughout the Caribbean. Whether on the mountaintop forests of Puerto Rico, the coral reef communities of the Caribbean basin, or in the mangrove swamps and shallows along the Floridian coastline, the effects from natural disasters remain one of sustained balance through shifts in biotic infrastructure. Each community of the Caribbean plays a vital role in the support of one another in sustaining a level of homeostasis. Here we have a rare gift to behold, not only in the study of one of the most eclectic groups of organisms on the planet, but also a real-world experiment on the methods of change and recovery through evolution.
The Caribbean today seems as sustainable as ever to the constantly shifting world of human and natural activity around, however, the simulated effects of a global warming on the climate may prove its one downfall. Theoretical models of hurricane intensity predict that the maximum potential intensity (MPI) of hurricanes will increase in a warmer climate (Knutson et al. 1998). A recent study was done of typhoons in the Northwest Pacific region in the event of a warmer climate compared to storms spawned under present conditions which concluded a marked increase in wind speed (3 to 7 meters per second) and decrease in pressure (7 to 20 millibars lower) in the simulated systems (Knutson et al. 1998). There also exists conclusive evidence that an increase in global temperature along with melted ice caps would greatly alter the climatic control of the oceanic currents and the paths of all tropical systems. Under such a scenario, hurricanes may take direct aim at more populated centers such as Washington D.C. or even New York City where not only more people and property are susceptible, but also the natural systems themselves because of a lack of experience in dealing with the regular destruction brought through tropical weather. Under such a scenario, the ecosystem would not have learned the evolutionary success of the Caribbean in adapting to sustained tropical disturbances and thus may be altered or destroyed for good. We as a community of learners and teachers have the chance to reverse the trend by educating the world around us to the seriousness of the present situation. We not only have the Caribbean to fear and fight for, but all of humanity in the event of such a shift where the weather begins to work against the very fabric of nature that it has sustained throughout recorded history.
Works Cited -
Boose, Emery R., Foster, David R., Fluet, Marcheterre. Hurricane Impacts to Tropical and Temperate Forest Landscapes. Ecological Monographs. Vol. 64, No. 4 (Nov., 1994), 369-400.
Knutson, Thomas R., Tuleya, Robert E., Kurihara, Yoshio. Simulated Increase of Hurricane Intensities in a CO2 Warmed Climate. Science, New Series. Vol. 279, No. 5353 (Feb. 13, 1998), 1018-1020.
Miller, Banner I. Characteristics of Hurricanes. Science, New Series. Vol. 157, No. 3795 (Sep. 22, 1967), 1389-1399.
The National Hurricane Center Tropical Prediction Center. NOAA. May 7, 2004. Pimm, Stuart L., Davis, Gary E. Hurricane Andrew. Bioscience. Vol. 44, Issue 4 (Apr., 1994). Terence P. Hughes. Catastrophes, Phase Shifts, and Large-Scale Degradation of a Caribbean Coral Reef. Science, New Series. Vol. 265, No. 5178 (Sep. 9, 1994), 1547-1551. T. P. Hughes, J. H. Connell. Multiple Stressors on Coral Reefs: A Long-Term Perspective. Limnology and Oceanography. Vol. 44, No. 3, Part 2 (May, 1999), 932-940. Zimmerman, Jess K., Willig, Michael R., Walker, Lawrence R., Silver, Whendee L. Introduction: Disturbance and Caribbean Ecosystems. Biotropica. Vol. 28, No. 4, Part A (Dec., 1996), 414-423.
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Pimm, Stuart L., Davis, Gary E. Hurricane Andrew. Bioscience. Vol. 44, Issue 4 (Apr., 1994).
Terence P. Hughes. Catastrophes, Phase Shifts, and Large-Scale Degradation of a Caribbean Coral Reef. Science, New Series. Vol. 265, No. 5178 (Sep. 9, 1994), 1547-1551.
T. P. Hughes, J. H. Connell. Multiple Stressors on Coral Reefs: A Long-Term Perspective. Limnology and Oceanography. Vol. 44, No. 3, Part 2 (May, 1999), 932-940.
Zimmerman, Jess K., Willig, Michael R., Walker, Lawrence R., Silver, Whendee L. Introduction: Disturbance and Caribbean Ecosystems. Biotropica. Vol. 28, No. 4, Part A (Dec., 1996), 414-423.
We also have a GUIDE for depositing articles, images, data, etc in your research folders.
Article complete. Click HERE to return to the Pre-Course Presentation Outline and Paper Posting Menu. Or, you can return to the course syllabus
Listen to a "Voice Navigation" Intro! (Quicktime or MP3)
It is 7:20:43 AM on Saturday, April 21, 2018. Last Update: Wednesday, May 7, 2014