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Puerto Rico’s Forest Inventory:
Adapting the USDA Forest Service’s Forest Inventory and Analysis
Program to a Caribbean Island
Puerto Rico’s
forests
History of USDA
Forest Service inventories in Puerto Rico
1980 Survey
1985 Survey update
1990 Re-survey
Objectives for
the current inventory
Methods
Expanding the
inventory scope
Maintaining continuity
with previous inventories
Framework for
inventory intensifications
Increasing sample
size for forests of limited extent
Regional inventory
intensifications for special interest forests
Urban forest
and watershed-scale inventories
Modifying and
applying FIA field data collection methods
Basic tree information
Additional tree
information
Conclusions
Future challenges
Literature cited
Acknowledgements
Based on the
observations of early explorers and historians, the island of Puerto
Rico, (which lies between 18°31’ and 17°55’ N latitude and 65°37’
67°17’ W longitude, and has a total area is 890,000 ha, Fig. 1),
was heavily forested prior to European settlement (Wadsworth 1950)
. The island’s diverse forests, holding over 500 native species,
reflect its equally diverse physiography (Little et al. 1974) .
Marked moisture and elevation gradients result in changes in forest
species composition and structure over relatively short distances.
Figure 1.
The island of Puerto Rico
Pre-Columbian populations cleared
land for shifting agriculture and harvested trees, but their impacts
on the forests were relatively minor (Domínguez Cristóbal 1989)
. European impacts on the forest began in 1505 with the introduction
of goats and pigs by Spanish sailors (Domínguez Cristóbal 1989)
. Forest clearing for timber, fuelwood, charcoal and agriculture
followed colonization in the early seventeenth century and continued
unabated in various forms until the mid-twentieth century. Koenig
(1953) estimated that in 1951 only 6% of Puerto Rico’s land area
was still covered in forest.
Reversion to secondary forest followed
widespread abandonment of agricultural lands during the latter half
of the twentieth century. In 1990, forest covered 32% of the island
(Franco et al. 1997) . These secondary forests contain an additional
117 naturalized exotic species whose ecological impacts are still
unclear (Francis and Liogier 1991) . The present day Puerto Rican
landscape is a fragmented, dynamic mosaic of shifting land uses,
predominately continued agricultural abandonment, reversion to secondary
forest, and urban expansion. USDA Forest Service inventories have
begun to document the land use changes and forest recovery on Puerto
Rico.
The first forest inventory in Puerto
Rico, completed in 1980, had the primary objective of assessing
the timber production potential of the island’s forests (Birdsey
and Weaver 1982) . Nine hundred and seventy eight permanent sampling
points were established island-wide along a square systematic grid
at a 3 km square spacing (900 ha per square) (Birdsey and Weaver
1982) . This sampling intensity was derived from sampling errors
estimated from extensive fieldwork prior to the inventory (Birdsey
and Weaver 1982) . An island-wide estimate of forest area for commercial
and non-commercial areas was made from aerial photographs, although
coverage was incomplete.
Permanent plots were installed in
two of the six forested Holdridge life zones on the island (as described
in Ewel and Whitmore 1973) that were considered to have the potential
for commercial production: the subtropical moist and subtropical
wet forests (Birdsey and Weaver 1982) . Four hundred and thirty
seven inventory points fell within these two commercial forest types
and 133 were found to be forested during field visits (Fig. 2).
The two forest types and four soil categories were used to stratify
the plots. Areas that were excluded from the survey included public
forests, floodplains, urban areas, mangrove forests; areas with
poorer soils, dry subtropical life zones, steep slopes (>60%),
and critical watersheds with high rainfall.
Five years after the first inventory,
a more complete estimate of forest area was made from newer, more
complete aerial photographs of the entire island. Forest types
included in the aerial estimation of forest cover were expanded
to include all present except the montane forests and plantations.
A sub-sample of the 1980 inventory plots installed in commercial
forest was re-visited to update survey results (Birdsey and Weaver
1987) . The data collected during the survey update was expanded
to include erosion factors, hydrology, operating conditions, as
well as tree crown and branch measurements.
The island’s forests were re-surveyed
in 1990 using the 1985 survey update’s methodology (Fig. 2). 433
of the permanent plots installed in 1980 were re-visited to confirm
their status, and 167 forested plots were measured (Franco et al.
1997) . Field data collection followed the methodology of 1980,
with the inclusion of nested 3.6-m fixed radius regeneration subplot
(Franco et al. 1997) .
Figure 2. Puerto Rico forest survey
plots installed 1980-1990.
In preparing for the latest forest
inventory of Puerto Rico, we found it necessary to adapt the FIA
program’s standard protocols to produce a comprehensive forest inventory
that meets the requirements of statistical rigor, scientific validity,
and the customer’s information needs. The challenges to adapting
the FIA program’s current methodology to the forests of Puerto Rico
include:
- Expanding inventory scope to include and
adequately sample all forest types, including rare forests of
limited extent.
- Maintaining continuity with previous inventories
to capture long-term tree growth information and 20 years of
land use change, and incorporating forest health monitoring
(FIA Phase 3) pilot study plots.
- Developing a framework that allows for incorporation
of additional inventories at varying intensities that address
specific questions and needs.
- Modify and apply nationally standardized
FIA field data collection methods to a Caribbean island.
A new hexagonal sampling grid was
generated over Puerto Rico that serves as the framework around which
all future inventory intensifications can be built. All forest
types were included in this sampling scheme. The new grid is hexagonal
and has the same dimensions as the FIA grid installed in CONUS,
which is approximately one sampling point every 2400 ha (370 hexagons).
The Caribbean island inventories
have been implemented according the USDA Forest Service’s National
protocols. (For more information on FIA, visit the About Forest Inventory & Analysis
library webpage). FIA is implemented in 3 phases. Phase 1 refers
to classifying each sampling point as being forest or non-forest
using aerial photographs or satellite imagery. Phase 2 is the collection
of data on the ground at points classified as forested. Phase 3
is the installation of forest health monitoring plots at a subset
of the points visited during Phase 2 data collection. (For more
information on Forest Health Monitoring and FIA, visit the Forest Health Monitoring
website).
Figure 3. Puerto Rico forest inventory
sampling grid
(The hexagonal grid and sampling points depicted in Figure 3 are
not the actual ones used in the inventory. The sampling points shown
here are located at the center of each hexagon in an example of the
grid that will be used. However, the actual plot location will
be "wobbled" a random azimuth and distance between 100 and 600
meters from each hexagon center to protect landowner
confidentiality).
As a pilot study, forest health monitoring
(FIA Phase 3) plots were installed at 55 locations on the new systematic
grid across the entire island in 2001. Based on the sampling intensity
of the 1980 and 1990 forest surveys, it was decided that intensifying
the base grid three times to give a sampling point every 800 ha
(1110 hexagons island-wide), then installing a Phase 3 plot in every
sixteenth hexagon, would give an adequate forest health monitoring
network over the island. Crown condition, coarse woody debris,
soil and erosion data were collected along with FIA Phase 2 forest
inventory data.
Where a previous inventory plot falls
within a new hexagon, that plot was used to represent that area
rather than the new hexagon’s center. There were instances of multiple
old plots falling within a single, new hexagon. In those cases,
only one plot was chosen for re-measurement. Forest health monitoring
plots had the highest priority when singling grid hexagons, so all
Phase 3 plots installed in 2001 were included in the final, island-wide
sample.
Every effort was made to re-measure
the 167 forested plots installed in 1980 and 1990 because the previous
forest surveys contain valuable data that give insight into long-term
forest growth and changes in land use. However, when two or more
older inventory plots fell within the same hexagon, the plot closest
to the hexagon’s center was retained and the others excluded from
the sample. At single intensity, only 108 plots from the previous
surveys were maintained, a loss of 35% discarded in favor of new
Phase 3 plots or other old plots. Figure 4 presents the results
of the hexagon singling process that forms the framework of Puerto
Rico’s forest inventory and health monitoring plot network.
Figure 4. Puerto Rico forest inventory
sampling grid with pre-existing plots incorporated
Preliminary examination of the single-intensity
sampling point distribution using a land use and vegetation of Puerto
Rico (Helmer et al. 2002) indicated that three ecologically important
forest types would have less than 10 plots. Therefore, we decided
to increase the sample sizes for forest growing on serpentine parent
material, mature dry forest, and cloud forest. The base hexagonal
grid was intensified twelve times to generate hexagons that each
had an area of 200 ha. Additional serpentine forest points were
chosen using a GIS and 1991 LandSat imagery to select hexagon centers
that fell on forest growing on ultramarfic parent material. Additional
mature dry and cloud forest points were selected from LandSat imagery.
Once all possible points for these forest types were identified,
20 sampling points were randomly chosen; 10 as the primary plot
locations and an additional 10 as secondary locations in the event
of inaccessibility or conversion to non-forest since the 1991 satellite
images were taken.
Additional
inventory plots were installed in forest areas of special interest
by intensifying the base sampling grid. One high priority area
for inventory is the northern karst belt. Karst landforms are the
product of chemical dissolution of limestone bedrock. These processes
have formed spectacular landscapes of steep conical hills (“mogotes”),
sinkholes (“sumideros”), and caverns in Puerto Rico. The forests
that grow in the northern karst belt have many unique features.
They provide habitat for over 200 species of birds (16 endemic to
the area), and 34 endangered species (Lugo et al. 2001) . (Download
Puerto Rican
Karst-A Vital Resource). These karst forests also have the
highest tree species diversity on the island with over 100 species/ha
found (Lugo et al. 2001) . The northern karst belt’s biological
and geological importance and uniqueness has been recognized, and
conservation efforts are underway. There is a proposal to transfer
significant portions to the public domain, making a comprehensive
inventory of the forest resources especially timely and relevant
(Lugo et al. 2001) . In an attempt to approach an adequate sampling
of these forests, a triple-intensity hexagonal grid was used to
generate 158 sampling points in the northern karst belt (See the
Puerto Rico Northern Karst Belt Forest
Inventory webpage for details). Previous inventory plots were
incorporated into the sample and the additional hexagons were singled
using the same methodology as was used for the island-wide single
intensity inventory.
Hexagon centers from the twelve-times
intensified sampling grid that fell within the San Juan Bay estuary’s
watershed were selected for an urban forest inventory. (For more
information, see San Juan Bay Estuary Watershed Urban
Forest Inventory webpage). The 250 km2 San Juan
Bay Estuary encompasses San Juan Bay, several large lagoons and
channels, extensive wetlands, mangrove forest, and is home to 622,000
people (Webb 1998, as cited in Villanueva et al. 2000) . The greater
San Juan Metropolitan area holds over 1.3 million people. The watershed
is part of the EPA’s National Estuary Program. We selected 100 plots
as a minimum sample size based on UFORE experiences in continental
US cities (Nowak et al. 2001) . The twelve-times intensified grid
produced 109 sampling points.
A standard FIA subplot cluster, (for
details see: USDA Forest Service
FIA Field data collection procedures for Phase 2 plots. Manual Version
1.6), was installed in areas that met the Caribbean FIA criteria
for forested land (a contiguous area > 0.4 ha, or > 30 m wide
for forested strips, with >10% canopy coverage in trees). Non-forest
plots were located and briefly described without any further data
collection.
For each tree within the plot, the
following data are recorded:
- Azimuth from plot center to the tree
- Distance to tree from plot center
- Species
- Diameter at breast height (1.37 m)
- Height to top of tree
- Height to base of live crown
- Crown width (average width based on two measurements:
N-S and E-W widths)
- Percent of normal live crown that is in leaf
The 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:
- Percentage of epiphytic foliage in the tree
crown
- Uncompacted live crown ratio
- Crown light exposure
- Crown position
- Vigor class
- Crown density
- Crown dieback
- Foliage transparency
Puerto Rico has a forest inventory
and monitoring framework with flexibility, longevity, and a statistically
rigorous design. Expanding the scope of the inventory effort recognizes
the need for information on a broader suite of questions than those
addressed in the past. Maintaining continuity with the previous
work allows the examination of long-term trends in forest growth
and dynamics.
The systematic sampling grid in place
allows researchers to better assess land use change in a highly
dynamic landscape. Different questions regarding Puerto Rico’s
forests will require different sampling intensities. Composing
and decomposing the hexagonal grid to zoom in and zoom out in scale
according to the demands of the data being collected is and will
continue to be a vital part of the sampling framework’s design.
This will be important to adequately
describe Puerto Rico’s forests and meet the minimum requirements
of FIA. Extensive intensifications such as those in the San Juan
area and the northern karst belt will be needed throughout the island
to keep sampling errors at an acceptably low level. Also, without
adequate intensification, too many of the previous survey’s plots
will be lost in the singling process, resulting in the loss of valuable
long-term growth data.
Our goal has been to develop a forest inventory and
monitoring network with longevity and flexibility to meet the foreseeable
and unforeseen demands for information about Puerto Rico’s forests.
We envision greater interest in applying Puerto Rico’s inventory
data to criteria and indicators of sustainable forest management.
Because Puerto Rico’s rate of urbanization continues to increase,
the future inventory will have to expand to better incorporate urban
and suburban development into formerly forested land. Finally,
we hope this inventory framework could eventually be expanded in
scope into a true biodiversity inventory of the island.
Birdsey, R. A., and P. L.
Weaver. 1982. The forest resources of Puerto Rico. Resource Bulletin
SO-85, USDA Forest Service Southern Forest Experiment Station, New
Orleans, Louisiana.
Birdsey, R. A., and P. L. Weaver.
1987. Forest area trends in Puerto Rico. SO-331, USDA Forest Service,
Southern Forest Experimental Station, New Orleans, LA.
Domínguez Cristóbal, C. M. 1989.
La situacíon forestal pre-hispanica de Puerto Rico. Acta Cientíca
3:63-66.
Ewel, J. J., and J. L. Whitmore.
1973. The ecological life zones of Puerto Rico and the US Virgin
Islands. ITF-18, USDA Forest Service Institute of Tropical Forestry,
Rio Piedras, Puerto Rico.
Francis, J. K., and H. A. Liogier.
1991. Naturalized exotic tree species in Puerto Rico. General Technical
Report SO-82, USDA Forest Service, Southern Forest Experiment Station,
New Orlean, LA.
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., 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.
Koenig, N. 1953. Land for forests
and tree crops. Chapter VI. Pages 103118 in A comprehensie
agricultural program for Puerto Rico. USDA, Washington, D.C.
Little, E. L., R. O. Woodbury, and
F. H. Wadsworth. 1974. Trees of Puerto Rico and the Virgin Islands.
Agriculture Handbook No. 449, USDA Forest Service, Washington, D.C.
Lugo, A. E., L. Castro-Miranda, A.
Vale, T. del Mar-López, E. Hernández-Prieto, A. García-Martinó,
A. R. Puente-Rolón, A. G. Tossas, D. A. McFarlane, T. Miller, A.
Rodríguez, J. Lundberg, J. R. Thomlinson, J. Colón, J. H. Schellekens,
O. Ramos, and E. H. Helmer. 2001. Puerto Rican Karst - A Vital Resource.
General Technical Report WO-65, USDA Forest Service, Washington,
D.C.
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.
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.
Wadsworth, F. H. 1950. Notes on the
climax forests of Puerto Rico and their destruction and conservation
prior to 1900. Caribbean Forester 11:38-47.
I would like to thank Drs. Eileen
Helmer (USDA-FS IITF) and Ariel Lugo (IITF) for their collaboration
on this project; Joe McCollum (USDA-FS SRS-FIA) for generating the
hexagon compositions and decompositions for Puerto Rico; Vince Few,
Angie Rowe, and Sarah Combs (USDA-FS SRS-FIA) for their technical
expertise in field data collection; Olga Ramos, Wilmarie Díaz, Maya
Quiñones (IITF) GIS lab for their invaluable assistance; Carlos
Rodríguez (IITF) for technical support with GPS; Esther Rojas of
the Fundacíon Puertorriqueňa de Conservacíon, and Luis Ortíz,
Orlando Díaz, Humfredo Marcano, Humberto Rodríguez, Omar Monsegur
and Ivan Vicéns for their tireless work collecting data in the field,
and Consuelo Brandeis (FPC) for data entry.
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