Puerto Rico Northern Karst Belt Forest Inventory

Introduction

Additional inventory plots are being installed in forest areas of special interest on the island of Puerto Rico by intensifying the USDA Forest Service’s island-wide Forest Inventory and Analysis (FIA) sampling grid.  One such area is the northwest portion of the island where the forests grow on limestone bedrock weathered by the tropical climate into karst landforms (Monroe 1976) .  These processes have formed spectacular landscapes of steep conical hills (“mogotes”), sinkholes (“sumideros”), and caverns in Puerto Rico.  The largest area of karst landscape (142,544 ha) is found in the northwestern part of the island and is referred to as the northern karst belt (Lugo et al. 2001) (Fig. 1).

Figure 1. Puerto Rico’s Northern Karst Belt

Generations of agricultural activity left the northern karst belt largely deforested.  Farming of both subsistence and cash crops took place in the flatter depressions between the hills where soils were deeper, while steeper hillsides were used for grazing and fuelwood cutting (Rivera and Aide 1998, Lugo et al. 2001) .  Only small, isolated patches of heavily degraded forest remained in less accessible or less productive as late as the 1950’s.  Subsequent changes in the Puerto Rican economy and migration to urban areas resulted in the abandonment of much agricultural land across the entire island (Rudel et al. 2000) .  This trend was also true for the karst region, and forest has returned to many of the abandoned fields and pastures (Rivera and Aide 1998) .  This newly reforested region now has some of the largest contiguous blocks of forest remaining on the island.  However, these forests are now coming under pressure from the needs of the more urbanized population.  Karst forest is now being cleared for urban expansion, such as highway construction and housing developments (Rivera and Aide 1998, Lugo et al. 2001) . 

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 and the Puerto Rican Karst fact sheet from the USFWS).  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.

In a collaborative effort of the USDA Forest Service’s International Institute of Tropical Forestry, the NGO Citizens for the Karst, and the Puerto Rican Conservation Foundation, an intensified forest inventory is being completed by intensifying the island-wide FIA base sampling grid   (Brandeis 2003) (or view on-line summary at Puerto Rico’s Forest Inventory: Adapting the USDA Forest Service’s Forest Inventory and Analysis Program to a Caribbean Island).

Methods

Sampling design

The northern karst belt forest inventory is a regional intensification of the island-wide Forest Inventory and Analysis (FIA) sampling grid installed over Puerto Rico.  (For more information on FIA, visit the About Forest Inventory & Analysis library webpage and the Forest Health Monitoring website).  First, the standard FIA hexagonal grid was decomposed three times (triple-intensity sampling) and points generated at the centers of each new hexagonal cell.  Then, the study area was defined using a geological map that shows the locations of the island’s limestone formations.  In a GIS, points from the triple-intensity grid that fell on limestone formations were chosen as locations for field sampling points (Fig. 2).  Only points that fell on karst geology within the subtropical moist, wet and rain forest life zones were chosen for this inventory (See Ewel and Whitmore 1973 for life zone definitions) .  Points that fell on karst geology within the dry subtropical forest life zone in the southern western area of the island were excluded.

Figure 2.  Northern karst belt inventory and monitoring sampling grid

This sampling scheme produced 158 sampling points within the northern karst belt.  Of these points, 55 were FIA sampling points.  FIA sampling points are being visited separately as part of the island-wide inventory.  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.

Inventory Plot Design

A standard FIA subplot cluster, (for details see USDA Forest Service FIA Field Data Collection Procedures for Phase 2 Plots, 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.

Tree Information

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

Expected outcomes from the inventory

We hope to complete the following objectives with this forest inventory:

1)      Quantify forest cover in the northern karst belt

2)      Describe northern karst forest structure in terms of:

a)      Tree cover and density

b)      Species composition (exotic and native)

c)      Tree health, damage, and potential for pests and disease

d)      Leaf area and canopy coverage

e)      Woody aboveground biomass and carbon

We hope to improve our understanding of the forests in Puerto Rico’s northern karst belt through the completion of these technical objectives.  With this improved understanding, better appreciation for the ecological, social and economic value of these forests, by the general public and policy-makers can be achieved.  With this accomplished the chances for increasing protection and conservation of forests in the northern karst belt will improve.

Acknowledgements

I would like to acknowledge the valuable contributions of this project’s many collaborators: Dr. Ariel Lugo (IITF), Abel Vale (CdelK) and Terry Hoffman (USDA-FS NE) for their work on agreements and funding; Joseph McCollum (SRS-FIA) for the sampling grid; Dr. Eileen Helmer (IITF) for plot location selection; Olga Ramos (IITF), Wilmarie Díaz (IITF) and Maya Quiñones (IITF) for GIS mapping; Carlos Rodríguez (IITF) for GPS support; Esther Rojas (FPC) for project administration; Ivan Vícens (field work) of the IITF; Orlando Diaz (CdelK), Luis Ortíz (CdelK), Humfredo Marcano (FPC), Humberto Rodríguez (FPC), Omar Monsegur (FPC), Ivan Vícens (IITF) for field work and botanical expertise, Dr. Armando Rodríguez (Univ. Interamericana) for use of the Mata de Platano Field Station; and Consuelo Brandeis (FPC) for data entry.

Literature Cited

Brandeis, T. J. 2003. Puerto Rico's Forest Inventory: Adapting the Forest Inventory and Analysis Program to a Caribbean Island. Journal of Forestry 101:8-13.

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.

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.

Monroe, W. H. 1976. The karst landforms of Puerto Rico. United States Government Printing Office, Washington, D.C.

Rivera, L. W., and T. M. Aide. 1998. Forest recovery in the karst region of Puerto Rico. Forest Ecology and Management 108:63-75.

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.