A City’s Dynamic Mangrove Forest

Inventory and monitoring of the urban forest in the San Juan Bay Estuary watershed, Puerto Rico

The southern part of the San Juan Bay Estuary, a hilly area with soils derived from volcanic soils, is less densely developed and has higher tree cover than other parts of the watershed. Photo by Tom Brandeis.
The southern part of the San Juan Bay Estuary, a hilly area with soils derived from volcanic soils, is less densely developed and has higher tree cover than other parts of the watershed. Photo by Tom Brandeis.

A recent report by U.S. Forest Service researchers on the forests of the San Juan Bay Estuary watershed provides details about a highly dynamic urban forest that provides important benefits for its residents. Tom Brandeis, supervisory research forester with the Forest Service Southern Research Station (SRS) Forest Inventory and Analysis unit, worked with other Forest Service and university researchers to complete the first decade of a study started in 2001 of the watershed of Puerto Rico’s San Juan Bay Estuary.

Lying along the northeast coast of Puerto Rico, the 53,500-acre San Juan Bay Estuary watershed is the heart of the expanding and densely populated San Juan metropolitan area. The 2010 U.S. Census estimated the population of the area at 2,478,905, with a population density of 3,215 people a square kilometer (about half a square mile), and in some places over 8,300 people a square kilometer.

Forests once covered most of the estuary’s watershed, with mangroves circling the bays and lagoons and a diverse mix of species in the coastal plains and further inland. Human activities and development greatly reduced forest cover over the last century, with losses to mangrove forests ranging from 28 to 67 percent. Reduction in forest cover in such a dense urban area has far-ranging consequences, from the loss of biodiversity to increases in air and water pollution, to higher temperatures (the urban “heat island” effect) for people to endure.

In addition to providing data on the forests in the estuary’s watershed—including forest structure, tree and shrub species present, insects and diseases—the report estimates the value of the ecosystem services provided by the forests, including carbon sequestration, which reduces the carbon dioxide in the atmosphere that pushes climate change. Findings include:

  • Half the trees sampled in 2001 were gone by 2011, either removed or dead. In general, however, new growth exceeded mortality, with an average increase of 40 trees per acre. Natural regeneration in upland and mangrove forests accounted for most of the new trees added.
  • Species richness (the number of different species present) increased from 75 tree and shrub species in 2001 to 86 in 2011.
  • Red, black, and white mangrove made up the most common tree species. The occurrence of tree pests, diseases, and natural or human-caused damage was relatively low.
  • Mangrove and subtropical moist secondary forest covered 12 percent of the area. The estimated 10.1 million total of all trees stored 352,450 tons of carbon and sequestered carbon at a rate of 31,288 tons a year.
  • The estimated value of carbon storage by trees in the San Juan Bay Estuary watershed was $8.1 million, with an annual carbon sequestration value of $718,113 in 2011.

In 2011, shade from trees helped save approximately 19,000 megawatts of energy for cooling buildings in commercial and residential areas of the San Juan Bay Estuary, equating to 1986 metric tons of avoided carbon emissions due to building energy effects.

The 2001 and 2011 urban forest inventories were integrated with island-wide inventories of Puerto Rico carried out at the same times. The primary goal of these and other FIA surveys is to develop and maintain the information needed to support sound forest policies and programs.

“Urban forest benefits can be lost in the future due to development and other activities,” said Brandeis. “But benefits can also be increased by tree establishment and protection programs. We found that 16.8 percent of the estuary could be planted. Proper planning and management can sustain or enhance existing urban forest to increase the environmental and social benefits from trees in the San Juan Bay estuary watershed.”

For more information, email Tom Brandeis at tbrandeis@fs.fed.us

Additional authors for the report are Francisco Escobedo, University of Florida; Christina Staudhammer, University of Alabama; David Nowak, Forest Service Northern Research Station; and and Wayne Zipperer, research forester with the SRS Integrating Human and Natural Systems unit.

Access the full text of the report.

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