San Juan Bay Estuary Watershed Urban Forest Inventory
The USDA Forest Service’s International Institute of Tropical Forestry and the Puerto Rican Conservation Foundation, with the help of the Urban Forest Research Unit of the USDA Forest Service, Northeastern Research Station, implemented a comprehensive inventory of the San Juan Bay watershed’s forest resources. Our goal is to help managers, decision-makers and researchers quantify urban forest structure and functions. Specifically, the study’s objectives are:
1) Improve management and protection of the remaining forested ecosystems within the San Juan Bay watershed by providing necessary information about the resource a) Quantify watershed area in a forested condition b) Describe watershed’s natural, planted and naturally regenerated forests in terms of: i) Tree cover and density ii) Species composition (native and introduced) iii) Tree health, damage, and potential for pests and disease iv) Leaf area and canopy coverage v) Woody aboveground biomass and carbon
2) Identify urbanized areas in the watershed where increasing tree cover would be feasible and beneficial a) Quantify land uses in the watershed for the following categories: (1) Residential (a) High density, multi-family residences (b) Medium density, single family residences (c) Low density, single family residences (2) Commercial/Industrial (3) Institutional (4) Forest (5) Parks (6) Agricultural (7) Vacant b) Quantify ground cover types for each land use for the following categories: i) Cement, pavement and other impervious surfaces ii) Bare soil and plantable space Trees, shrubs and grass
Combine the inventory data with meteorological and air quality information the ecosystem services provided by the urban forest of the San Juan Bay Estuary in terms of:
a) Urban forest emissions b) Pollution removal by urban trees c) Effect of trees on building energy use d) Air temperature reduction from transpiration
The analyses necessary to complete objectives outlined in items 1-3 will use the urban forest inventory data collected in the field. The analyses required to complete objectives outlined under item 4 involve taking the data collected in the field and processing it in the Urban Forest Effects Model (UFORE) from the USDA Forest Service Urban and Community Forest Ecosystem Unit.
Study areaThe 250 km2 San Juan Bay watershed on the island of Puerto Rico (Fig. 1) encompasses San Juan Bay, several large lagoons and channels, extensive wetlands, mangrove forest (Fig. 2), and is home to 622,000 people (Webb 1998, as cited in Villanueva et al. 2000). The watershed is at the center of the greater San Juan Metropolitan area, which holds over 1.3 million people. The watershed has been chosen for inclusion in the Environmental Protection Agency’s National Estuary Program, and the San Juan Bay Estuary Program. While forest cover island-wide has been increasing since widespread agricultural land abandonment in the 1950’s to reach an estimated 32% in 1990 (Franco et al. 1997, Rudel et al. 2000), urban areas throughout the island have expanded at the expense of the adjacent forest (Thomlinson et al. 1996, Lopez et al. 2001, Ramos-Gonzalez 2001, Helmer 2003). Recent estimates place the forest cover in the San Juan metropolitan area at 26% (Ramos-Gonzalez et al. 2003). An analysis of Landsat TM imagery from the period of 1991 to 1999 done by the American Forests organization showed a 14% loss of tree cover (Anonymous 2002). The San Juan Bay watershed’s political and ecological importance made it an obvious candidate for applying the principles of an urban forest inventory to a major metropolitan area in the tropics.
Figure 1. Puerto Rico and San Juan Bay Estuary watershed
Figure 2. San Juan Bay Estuary features Sampling designThe San Juan metropolitan area is highly dynamic and expanding. We chose to systematically sample the watershed to better follow long-term changes in land use. There is a strong possibility that many of the sampling points which are not presently developed will change strata over the next 5-10 years, the planned urban forest inventory re-visit interval. By using a systematic sampling design, selection of new points and re-stratification will not be necessary, and long-term tree growth data will not be lost. We were also able to fully incorporate the urban forest inventory data into the concurrent island-wide forest inventory being carried out under the USDA Forest Service’s Forest Inventory and Analysis (FIA) program.
The island-wide FIA inventory provided a framework (Fig. 3) for our systematic sampling grid. Implementing the FIA program involves overlaying a hexagonal sampling grid over the area to be inventoried. Each hexagon in the standard FIA grid used on the continental United States has an area of 2400 hectares. A sampling plot is then installed in the center of each hexagon, or located at random distance and azimuth from that center.
Figure 3. FIA hexagonal sampling grid over the island of Puerto Rico, and the San Juan Bay Estuary Studies in North American cities indicated that 200, 0.004ha (1/10 acre) plots (for a total of 8.08 ha or 20 acres sampled) produced standard errors of 10% for estimates of the mean number of trees per ha over the entire urban area. Broken down by land uses in the urban area, standard errors were 5.9% for mean percent tree cover in urban parks, 2.4% for mean percent tree cover in residential areas, and 1.0% for mean percent tree cover in commercial/industrial areas (Nowak et al. 2001). For our study, we chose to decrease within plot variation by increasing the size of sampling plots rather than decrease between plot variation by increasing the number of sampling plots. Data collection for the San Juan watershed urban forest inventory is in a 0.07 ha (1/6 acre) circular plots. This is also the same total area in a single FIA subplot cluster.
We calculated that the San Juan Bay watershed would be covered with 11 standard sized FIA hexagons (Fig. 4). Therefore, we intensified the base grid (decomposed it into smaller hexagons) by a factor of 12 (Fig. 5). Once points that fell onto water were removed, we had 108 sampling points within the watershed boundaries (Figs. 6 and 7). This plot size and number of plots gave a total sampled area of 7.35 ha (18.2 acres).
Figure 4. Standard (2400 ha) FIA hexagonal grid over the San Juan Bay Estuary.
Figure 5. Twelve-time decomposition of the standard size FIA hexagonal sampling grid over the San Juan Bay Estuary.
Figure 6. Sampling points added at 12-x hexagonal grid centers.
Figure 7. San Juan Bay Estuary urban forest inventory sampling points. Click here to see interactive map
Preliminary classification of the points using a 1991 vegetation map produced from Landsat TM imagery (see Helmer et al. 2002) indicated that there were potentially 21 points with forest, 60 urban/barren points, and 28 points on agricultural land (primarily pasture). We found this distribution of points to be acceptable, exceeding the minimum recommendation of 10 points per strata in urban areas Nowak et al. (2001).
The final San Juan watershed inventory sampling design has the following characteristics:
Field data collection
Two field crews visited the plots from July to November of 2001. Potentially forested plots were assigned to the FIA-trained crew. If the plot was forested according to FIA definitions, the crew installed a sampling plot of standard FIA configuration (a cluster of 4 subplots. For details see: USDA Forest Service FIA Field data collection procedures for Phase 2 plots. Manual Version 1.6). If the plots did not meet the minimum FIA requirements for forest, they were assigned to the urban forest inventory crew and field data was collected according to Urban Forest Effect (UFORE) model; urban forest inventory procedures (For details see Nowak et al. 2001). These protocols required some modification for Puerto Rico. Of the UFORE field data collection options, we chose the following for the San Juan urban forest inventory:
Plot-level information
At the level of each plot, the following data was collected:
Ground cover information
The crew noted what proportion of the plot ground area is covered by the following materials:
Building information
UFORE procedures for noting building information were modified and re-defined to better fit the structures, architecture, and building materials that predominate in San Juan. Plots with multiple buildings, walls made from multiple materials, and partial walls are common in high-density residential areas of San Juan. The following information was collected to describe the buildings that were within each plot:
· Total length that the building wall intersects the ground in the plot
Tree, shrub, and vertical vine information
For each tree within the plot, the following data are recorded:
The following data are recorded for the shrub area:
For vines that climb up vertical flat structures (e.g., walls), we record:
Additional tree informationThe prevalence of epiphytes and vines in tree canopies required we add this variable to the data collection. In addition, a full FIA Phase 3 Crown Condition Assessment is done on all trees within the urban inventory plots. The additional information collected from each tree included the following:
Expectations for the San Juan Bay watershed urban forest inventory
The goal of every forest inventory is to provide information that will guide future management of the resource being inventoried. Our hope is that the information produced by this inventory will help guide city planners and municipal decision-makers toward policies that encourage conservation of the estuary’s remaining natural forest, improve management of trees in urbanized areas, and take into account the ecosystem services provided by trees in this heavily urbanized watershed.
This effort represents the first intensified inventory to be derived from the island-wide FIA inventory and monitoring framework installed on the island in 2001. The inventory and monitoring methods that will be applied in this study could potentially be applied over the entire San Juan Metropolitan area or any other city or urbanized watershed in Puerto Rico. Further, these methods could be used in applied to other Caribbean cities with only minor modification to better-fit local conditions.
Literature Cited
Anonymous. 2002. Urban ecosystem analysis: San Juan, Puerto Rico Metropolitan area. Report by American Forests, Washington, DC. Franco, P. A., P. L. Weaver, and S. Eggen-McIntosh. 1997. Forest resources of Puerto Rico, 1990. Southern Resource Bulletin SRS-22, USDA Forest Service Southern Research Station, Asheville, North Carolina. Helmer, E. H. 2003. Forest conservation and land development in Puerto Rico. Manuscript submitted for publication in Landscape Ecology. 15 pp. Helmer, E. H., O. Ramos, T. del Mar-López, M. Quiñones, and W. Diaz. 2002. Mapping the forest type and land cover of Puerto Rico, a component of the Caribbean Biodiversity Hotspot. Caribbean Journal of Science 38:165-183. Lopez, T. d. M., T. M. Aide, and J. R. Thomlinson. 2001. Urban expansion and the loss of prime agricultural lands in Puerto Rico. Ambio 30:49-54. Nowak, D. J., D. E. Crane, and J. C. Steven. 2001. The Urban Forests Effects (UFORE) model: Field data collection procedures. USDA Forest Service, Northeastern Research Station, Syracuse, NY. Ramos-Gonzalez, O. M. 2001. Assessing vegetation and land cover changes in northeastern Puerto Rico: 1978-1995. Caribbean Journal of Science 37:95-106. Ramos-Gonzalez, O. M., C. D. Rodríguez-Pedraza, and A. E. Lugo. 2003. Forests and vegetation of the San Juan Metropolitan area. Report to the Dept. of Transportation and Public Works. USDA Forest Service International Institute of Tropical Forestry, Rio Piedras, PR. Rudel, T. K., M. Perez-Lugo, and H. Zichal. 2000. When fields revert to forest: development and spontaneous reforestation in post-war Puerto Rico. Professional Geographer 52:386-397. Thomlinson, J. R., M. Serrano, I., T. d. M. López, T. M. Aide, and J. K. Zimmerman. 1996. Land-use dynamics in a post-agricultural Puerto Rican landscape (1936-1988). Biotropica 28:525-536. Villanueva, E., L. J. Rivera-Herrera, S. Rivera-Colón, M. Tacher-Roffe, C. Guerrero Pérez, and C. Ortiz-Gómez. 2000. Comprehensive Conservation and Management Plan for the San Juan Bay Estuary. U.S. Army Corps of Engineers, San Juan, Puerto Rico.
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