Drought Impacts to Tropical Forest Ecosystems in the U.S. Caribbean
Learn about the impacts of drought on tropical forest ecosystems in the U.S. Caribbean below.
Authors: Shelley Crausbay, Conservation Science Partners; William Gould and Stephen Fain, USDA Caribbean Climate Hub
Click here to download the 2-page fact sheet with graphics, or read below for an extended version of the fact sheet.
Introduction
Forests in the U.S. Caribbean are spectacularly diverse, with more than 500 native tree species in Puerto Rico alone. These forests and trees provide many services to the region's 3.5 million people, including watershed and coastal protection, economic benefits from fruit and wood, cooling in urban environments, and improved water quality, recreation, habitat, and biodiversity protection. Caribbean forests range from coastal mangroves and dry forests, to rainy cloud forests on the mountain peaks. They have been shaped by frequent natural disturbances such as hurricanes, drought, flooding, landslides, and wildfire. Projected increases in temperatures and reduced or greater variability in rainfall may lead to increased drought frequency and intensity. Drought effects will vary by forest type, with greater impacts on forests adapted to perennially wet conditions. Droughts can affect the timing and amount of fruiting and flowering with cascading effects on wildlife. Droughts can affect tree productivity, with loss of leaves and reduced canopy cover affecting nutrient cycling, and the amount of sunlight reaching forest floors. Changing nutrient and environmental conditions can affect the biota and related services delivered by forests. Under a changing climate, droughts that cause tree mortality may affect forest succession, with drier adapted species replacing wetter adapted species. Drought and wildfire can lead to loss of forests and invasion of fire-adapted nonnative shrubs and grasses.
Landscape-Scale Effects
Drought affects forests in the U.S. Caribbean across the region’s distinct precipitation gradient, but may be more substantial in drier forests.
- Individual droughts exhibit spatial variation in severity. In 1992, a drought occurred in Puerto Rico with impacts on consumption and rationing, but with no lasting effects on the wet forests. However, in 1994-95 a severe drought on the island of St. John caused mortality of 12% of trees — much greater than the mortality experienced after Hurricane Hugo in 1989.
- A 2015 drought that impacted the U.S. Caribbean reduced flowering and seed production in the drier Guánica State Forest (Lasky et al., 2016). In contrast, wetter forests have shown increases in reproduction in drought years due to the increase in light availability and water stress
Temporal Impacts
Short-term Impacts
Droughts and other disturbances have historically shaped forests in the U.S. Caribbean, and many forest species can adjust quickly to drought conditions. But, with hotter and longer droughts already occurring and anticipated for the 21st century, drought-tolerance thresholds could be crossed.
- Adaptive Capacity: Many tree and animal species are adapted to drought. Anolis lizards can regulate water loss through their skin during dry periods, and dryland tree species have smaller, thicker leaves and other adaptive traits to reduce water loss.
- Crossing Thresholds: Frogs are also adaptive and can change their physiology to prevent drying, but prolonged droughts can exceed this adaptive capacity. Juvenile Coqui are particularly sensitive to drought conditions (Stewart, 1995).
Long-term Impacts
Increased frequency of hotter droughts, or longer, more extended droughts, can shape Caribbean forests by favoring certain species over others in a particular forest or resulting in the loss of species in particular places.
- Changing Communities: Prolonged drought can change rates of reproduction for many plants and animals. The likelihood of mortality can be increased when drought and other disturbances, such as wildfire or insect outbreaks occur. Over time this can lead to shifts in species composition that persists long after a drought ends.
- Loss of Species: Interactions between extended hot droughts, Amphibian Chytrid fungus (Burrowes et al., 2004), and hurricanes threaten the the Coqui frog across the western Cordillera (Barker and Rios-Franceschi, 2014). Similarly, some bird species dependent on seasonal rains for insects and fruit, as in the Guánica State Forest, a dry forest in southwest Puerto Rico, may be lost locally through extended droughts (Faaborg, 1982; Faaborg et al., 1984). The most vulnerable communities are those associated with the rainy cloud forests of Puerto Rico, which harbor a number of endemic and endangered plant and animal species.
Cross-Sector Impacts
In a warming climate, 21st century droughts may coincide with increased demand for water for people, agriculture, and ecosystem management. Ultimately, increasing drought frequency and severity, coupled with other disturbances and competing demands for water drought - may lead to changes in forest plant and animal communities in the U.S. Caribbean. These changes will affect human communities through altered recreation opportunities, water quality, productivity, economic benefits, and watershed protection.
Solutions & Future Research Directions
Scientists recognize the potentially important role of future droughts in the health of Caribbean forests and are beginning to track the effects in the Luquillo Experimental Forest. A better understanding of how droughts impact tropical forests across the Caribbean is still needed. Key questions include:
- Are dry forests more vulnerable to drought across the broad precipitation gradient in U.S. Caribbean forests?
- Are new, emergent forests that are dominated by alien species more or less vulnerable to drought? These ‘new’ forests are highly fragmented and likely prone to soil moisture losses, or alternatively may function as drought refugia for many animals.
- How do drought impacts to forests affect the natural functions people depend on, like erosion prevention and water quality?
- What plant species are likely to come in after widespread tree mortality following drought?
- We know that soil moisture strongly controls tropical tree species distributions, community composition, and diversity. Could more frequent drought associated with climate change transform U.S. Caribbean forests?
References Cited
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Faaborg, J., 1982, Avian population fluctuations during drought conditions in Puerto Rico: The Wilson Bulletin, v.94, no.1, 20-30. Access here.
Faaborg, J., Arendt, W.J., Kaiser, M.S., 1984, Rainfall correlates of bird population fluctuations in a Puerto Rican dry forest—A nine year study: The Wilson Bulletin, v.96, no.4, 575-593. Access here.
Lasky, J.R., Uriarte, M., Muscarella, R., 2016, Synchrony, compensatory dynamics, and the functional trait basis of phenological diversity in a tropical dry forest tree community: effects of rainfall seasonality: Environmental Research Letters, v.11, no.11. Access here.
Stewart, M.M., 1995, Climate driven population fluctuations in rain forest frogs: Journal of Herpetology, v. 29, no. 3, 437-446. Access here.
Additional References
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Campos-Cerqueira, M., and Aide, T. M., 2017, Lowland extirpation of anuran populations on a tropical mountain: PeerJ, 5, e4059. Access here.
Crausbay, S. D., Ramirez, A. R., Carter, S. L., Cross, M. S., Hall, K. R., Bathke, D. J., … Sanford, T., 2017, Defining ecological drought for the twenty-first century: Bulletin of the American Meteorological Society, v.98, no.12, 2543–2550. Access here.
DRNA, 2016, Informe sobre la Sequía de 2014-2016 en Puerto Rico: División Monitoreo del Plan de Aguas, Departamento De Recursos Naturales Y Ambientales (DRNA), San Juan, Puerto Rico.
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