Puerto Rico’s 21,658-ha San Juan Bay Estuary (SJBE) watershed is a complex, dynamic landscape of urban development, subtropical secondary forest, coastal lagoons, wetlands and mangrove forests. Urban Forest Effects (UFORE/iTree) and U.S. Forest Service’s Forest Inventory and Analysis (FIA) data collection and models were adapted to characterize the SJBE watershed’s forest and estimate ecosystem services in 2001 and 2011. We systematically sampled the watershed with a sampling point every 200 ha to follow long-term land use changes in this highly dynamic landscape; twelve times more spatially intense than standard FIA sampling concurrently done island-wide. Two plot designs were used; single circular plots in urban and agricultural lands and FIA subplot clusters in forest, but even so dense secondary forest patches within the urban matrix required reduced plot sizes. We reinterpreted building and structure measurement guidelines to account for San Juan’s different building materials and designs. Compiling the ancillary data needed for accurate data collection and ecosystem services quantification was challenging. Conflicting nomenclature and newly introduced ornamental species made urban species list creation problematic and accessing detailed weather and air quality data was more difficult than in the continental U.S. cities.
Results/Conclusions
The urban forest inventories provided valuable information about SJBE watershed’s urban forest and lessons on how to better inventory forests in subtropical urban areas. In 2011, there were 2,548 ha of mangrove and subtropical moist secondary forests covering 11.8% of the watershed. 10.1 million trees stored 319,737 metric tons of carbon (C) and sequestered C at a rate of 28,384 metric tons/year. Approximately 19,000 megawatts/year of energy required for cooling building were avoided due to tree shading and climate effects in residential and commercial areas, which equated to 1,986 metric tons of avoided C emissions. We identified 86 tree and shrub species, with mangrove species being most common. Naturalized Spathodea campanulata and native Calophyllum antillanum were predominant in moist forest patches and developed land uses.
Plans to continue within the new urban FIA initiative framework and expand sampling to heavily-developed adjacent municipalities rely on the lessons learned from these studies, like using forest and non-forest plot layouts and adding nested microplots to the urban plots for sampling saplings. Challenges remain, however, and stricter plot and tree tracking are needed due to fast tree growth rates and rapidly changing land uses. We also need to review iTree and FIA models and coefficients for applicability to subtropical environments.