Diana Getty, Barbara Hawes, William Mills, Melanie Revere, James Whelan

The Significance of Tree Species Diversity in the Tropical Dry Forest
 

Introduction

Less than one percent of the original Tropical Dry Forest exists today in Costa Rica.  The political and economic activities of the past have supported the destruction of the Tropical Dry Forest.  The rich volcanic and alluvial soils provided fertile ground for cattle and crops.  A fast growing grass, Jaragua, was imported from Africa to provide fodder for the cattle.  The thick grasses quickly covered the cleared lands, turning most of the forest into a vast Savannah.  This loss of habitat severely impacted the shelter and food sources for most of the indigenous species. These thick grasses also provide an additional concern because of the potential fire hazard related to such a large accumulation of biomass.  These fires can sweep across the landscape, burning what few trees have managed to begin re-growth.

Restorative efforts have been documented at the Lomas Barbudal area where there is a grassroots effort being conducted to try and create a local concern for the environment.  The Organization of Tropical Studies has funded many studies for restoring the Tropical Dry Forest.  Research is being conducted at the Palo Verde National Park in the Guanacaste Region of Costa Rica. Gerardo Barbosa is a research biologist studying the effects of cattle grazing on the grasses that impede the re-growth of the secondary forests.

Our study is focused on several plots of land that Barbosa has set aside to show the regeneration of the original forests.  Since the grasses pose such a high fire risk, one of the plots has been burned annually to study the hardiness of the tree growth.  A second plot has been isolated, with no human intervention.  Controlled grazing of cattle is in adjacent pastures to reduce grass competition with regenerating forests. A Shannon-Weiner statistical test will be used to analyze the data gathered.  By the end of the study we expect to find a higher rate of diversity in the plots with no systematic burning.  We expect to find the cattle grazed plots to show higher diversity than the isolated plots.
 

Method
Materials
 

Flag Tape (yellow)	Original Logger Tape (m)
Trowel	Scoop Net
Ziploc 1 Gallon and 1 Quart Bags	pH Paper
Knife	Cotton
Clipboard	Alcohol
Paper	2oz. Specimen Bottles
Writing Utensils

 
Procedure

Researchers (R) chose two sites of 20m by 20m sample corrals that had been fallow for nine years.  One site had been burned within the year, but no contact with cattle and the second site had no contact with cattle or fire.  Two parallel plots were chosen at random from the land  adjacent to the corrals.  These sample sites represent areas where cows had been present, but no fire.  R’s randomly chose and cordoned off a 2 by 10 meter site in each of the four areas (See Figures 6 through 9).  After cordoning, the R’s swept each site for arthropods (twenty strokes per sweep).  Soil was collected and noted for texture, color, and pH.  Plant identification and grass and seedling percent cover were recorded.  R’s counted the variety of flora particularly focusing on saplings and trees.  A sample of each tree species was collected for identification at the lab.   Arthropods were sorted and counted per site.
 

Results

Table 1, shows the data gathered on the sample sites.  Site 1(no cattle/no fire) was compared to Site 2 (cattle/no fire) which was sampled adjacent to Site 1.  The data suggests Site 1 was not significantly different from Site 2 when searching for species variety  (H1= -0.062, H2 =-0.77; t = 0.969, df = 25, p< 0.5).  Site 4 (cattle/no fire) was sampled adjacent to Site 3 (fire/no cattle) and the data suggests that there is a significant difference between these sites (H3 = 0, H4 = -0.39; t=4.022, df = 60, p<0.001).  Site 3 compared to Site 1, also shows a significant difference (H3 = 0, H1 = -0.62; t=-5.31, df=10, p<0.001. (See Figures 1-5)
 Grass coverage comparisons were Site 1 to Site 2, with the grass coverage at a 35% difference, Site 4 to Site 3 at a 99% difference, and Site 3 to Site 1 at a 53% difference. (See figures 6-9)

The seedling cover was compared across sites, see Figure 7;  Site 1 to Site 2 there was a 10% difference, Site 4 to Site 3 had 20% difference, and Site 3 to Site 1 a 15% difference.

The soil samples were relatively the same in regards to pH, the pH ranged from 7.0 to 8.0.  There was no significant difference in soil composition; they were all dark brown. .

The sorted arthropods showed more abundance in population and variety at both the no burn, no cattle site (34 individuals of 9 different groups) and the no burn, cattle grazing site ( 31 individuals of 8 different groups) as opposed to the burn site (14 individuals of 4 different groups).
 

Discussion

One of the biggest stumbling blocks in restoring the Tropical Dry Forest has been the two major problems posed by the Jaragua grass.  The first problem is its ability to out-compete indigenous tree species and the second; the large accumulation of combustible biomass.  The three management systems have different effects on both of these factors.  Burning removes the grass, but damages or kills the fire sensitive regenerating trees. Cattle grazing also removes the grass, but does not do the extensive damage that fire does to trees.  The cattle grazing on regenerating trees has been minimized by limiting grazing to few cattle per hecter, and grazing on an area for only four months of the year.  The isolation technique allows the regenerating trees to shade the grass.

The results of our work showed that the fire treatment was the least effective for controlling grass and regenerating forest.  This was as we hypothesized.  We found no significant difference between the grazed sites and the isolated site.  There may have been some differences between these two sites during the earlier years for this study.  It appears according to our results that after nine years, tree diversity is about the same for each site.

The research currently being conducted by Biologist Gerardo Barbosa has found a different conclusion.  His research project, begun in 1991, has included 230 hectares of the Palo Verde National Park.  He has implemented the method of introducing cattle into the park to control the Jaragua grass.  He was responsible for setting aside the other to study the burning method and the isolation method.  So far, his results are showing a significant difference in the area with no fire and cattle grazing.  He has found that when fires are introduced, very few species of tress are hardy enough to withstand the stress.  These trees were injured and died, leaving more space for the fast growing grass.  He has also found in cases where fire was only introduced every few years that the amount of biomass built up caused the same type of tree destruction.  In the plots left in isolation, there has been a greater variety of tree species re-growth than in the burned plots.  The problem stems from the fact that it is a very long process in which there is still an incredible amount of biomass that remains for the first fire that comes along.  Barbosa’s plots that have had cattle grazing have shown a significant decrease in biomass and a significant increase in healthy tree species diversity.

This project has many impacts, not the least of which is the restoration of a natural environment.  Barbosa’s approach also offers an immediate impact on the socioeconomic condition of the area by providing an area for cattle grazing.  The forest is once again able to provide habitats for indigenous species.  At the beginning of this project only 10 mating pairs of jabiru were located in this region.  Over the nine year course of the study there are now an estimated 45 mating pairs producing 3 to 4 chicks in a season.

Neither Barbosa’s presentation to our group, nor the two OTS studies reported in 1992 and 1998, clearly stated the purpose of the project.  Did they want to regenerate forest approaching the old growth forest, or did they just want to grow trees instead of grass? Was fire control the purpose?  Certainly an effect, and possible a goal, has been the acceptance of the local population for the park because of the use of cattle.  Cattle ranching has a long history in the area.  Cattle are one of the main economic activities of the region.  By including cattle in the park’s land management there is something familiar and acceptable in the park.  Another benefit of this project is that it is a model for local ranchers to use.  Several are using this plan as they accept funds through the Carbon Sequestration Program to reestablish dry tropical forest in the region.

We offer two suggestions for the future development of this project.  First, we would like to see a clear statement as to the goals of the project, and what mechanisms will be used to contain and end the use of cattle in the park.  Second, It would be helpful if comparative studies of old growth forest are carried out to measure the extent of regeneration.
 

References

Barbosa, Gerardo.  Personal Communication.

Edgar, Blake.  1989.  Seeds of Change in the Dry Forest.  Pacific Discovery.  3:23-37.