Introduction

Palo Verde Biological Station is located in the south-central Guanacaste Province in lowland Costa Rica. The entire site is considered a tropical dry forest receiving between 1000 – 1500 cm of rainfall per year. The average temperature is above 24 C. The tropical dry forest at Palo Verde Biological Station contains both deciduous and evergreen examples of forests in pristine, disturbed and regenerating successional stages.

Tropical forest disturbance has been identified as an important process affecting the regeneration and composition of dry forest communities. Disturbances, natural and human, are the major determinants of change in landscape vegetation patterns (Baack et al, 1997). A variety of factors, such as hydrology, incident light and temperature, can be altered as a result of disturbances. Changes in these abiotic factors can affect the biotic factors of the tropical dry forest. Natural disturbances generate gaps in the canopy of tropical forests. Gaps have been proposed as a mechanism that promotes the maintenance of high diversity in the tropics (Wolfe, 1996). However, the physical environment of disturbed areas can differ significantly from that of adjacent forests resulting in vegetative stratification.

A clear example of the distinct, nonoverlapping layers of vegetation can be observed in an area dominated by a nuclear tree. We described such a site as having one large canopy tree, surrounded by smaller, mid-story trees and herbaceous plants closest to the forest floor. According to Morales (1997), different species of trees appear to be fundamentally adapted to particular heights in tropical forests.

Our question was, "How does the presence of a nuclear tree affect the biodiversity of a given area?" We hypothesized species diversity will be greater under the nuclear tree as opposed to the same area directly outside the canopy of the tree. Our null hypothesis indicates the canopy of a nuclear tree has no effect on the biodiversity.
 
 

Methods

We located nuclear trees in undisturbed and regenerating forest areas. After defining and identifying two nuclear tree sites, our group modified our data collection protocol. We decided to identify two plots (2 m X 2 m) in the ordinal directions. We chose one plot under the canopy of the tree and another outside the tree canopy, for a total of thirty-two, four two square meter sectors. After marking off each plot (Appendix 1), we collected our data. Vegetation was divided into three groups: seedlings (< 0.5 m), saplings (0.5 – 1.5 m) and trees (> 1.5 m). We collected data by species and individual numbers. During data collection, we described the abiotic and biotic conditions of each site (Appendix 2). Measurements were made from 8.00 - 10.00 and 13.30 – 15.00. We collected and analyzed the data from both sites. Our data is reported in Figures 1 - 5 (Appendix 3).
 
 

Results

When we analyzed the total tree specimen data, we found more specimens in the sectors without cattle (Figure 1). We might explain the results because there is less animal induced damage in the regenerating forest. To determine the influence of the canopy, we compared the total number of specimens inside and outside the canopy. Outside the canopy, the total number of species was greater in the area without cattle (Figure 2). When we compared the morphospecies inside and outside the canopy, we found the morphospecies in the east, both inside and outside the canopy, were equal. Both the north and south sectors had more morphospecies inside the nuclear tree canopy. Only in the west side sector was there more vegetation outside the canopy (Figures 3 & 4). When we examined the total number of species in each direction, we found fewer species grew in the northern sectors (Figure 5). Without cattle, the mean number of morphospecies under the canopy was 7.25 and outside the canopy was 7.0. With cattle, the mean number of morphospecies under the canopy was 2.5 and outside was 3.25.
 
 

Discussion

Forest regeneration under nuclear trees is dependent upon several factors. The factors include seed dispersal, seed predation and habitat. Adequate seed dispersal depends on means and opportunity. Seed predation can influence forest regeneration in degraded areas, such as abandoned pastures. Habitat affects seed germination. The presence of a nuclear tree should increase the species richness and diversity under its canopy because of increased seed dispersal, decreased seed predation and a protected habitat. The results of our study support this idea.

According to our data, species richness and abundance under the canopy of a nuclear tree in a disturbed area is greater than beyond the canopy. Species richness was determined by counting the number of species comprising the mid level tree growth. Species abundance could be measured as number of individuals of species X/ total number of individuals in further studies.

One factor that could have effected the results is the presence of cattle. This could be due to an increase in habitat fragmentation when cattle grazing use of the land is expanded. This would effect the seed dispersal as well as seed predation. The high biodiversity under nuclear trees could result because specialized herbivores prevent any one species from becoming too abundant.
 

Acknowledgements

We would like to thank WWNFF and OTS for providing us the opportunity to work with Philippe Hensel in the field oriented learning experience.
 

Literature Cited

Aldrich-Wolfe, Laura. 1996. Variation in leaf morphology of Spathiphyllum fulvorium (Araceae): response to differences in the light environment. OTS: Tropical Biology: An Ecological Approach 96-3. pp. 190-193.

Baack, Eric, Lori Pommerenke. 1997. Effects of a small scale disturbance on forest floor vegetation in a dry tropical forest. OTS: 97-3. pp. 93-97.

Copeland, Sandi, Jim Hamilton, Jim Peters, Anne Gearheart, Laura Lundquist. 1997. Treefall densities within forest fragments of various sizes. OTS: 97-1. pp. 251-253.

Jarzowski, Christine and Lucia Lohmann. 1997. Lianas at Palo Verde dry forest. OTS: 97-3. pp. 87-92.

Morales, Manuel, Greg Pryor, Gregor Schuurman, Joe O’Brien, Sandi Copeland, Jim Hamilton, Kara O’Keefe. 1997. Testing for tropical forest stratification. OTS: 97-1. pp. 103-108.

Welch, Allison, Shannon McCauley, Mark Laska, Matt Brady, Brian Rehill, Nina Skinner. 1996. 3 Sheets to the Wind: barriers to forest regeneration in cattle pastures. OTS: 96-1. pp. 26-31.
 

 
Group Work

APPENDIX 2

Cow Path Plot: Nuclear Tree CPP1

CPP1 is located 203 paces from the road. The CPP1 is located on the North side of a barbwire fence. The fence marks off the cow path that is used during the dry season. The East and West plots are divided in half by the barbwire fence. The characteristics for both plots vary depending on their location to the fence. The inside of the cow path, which contains ½ East plot, ½ West plot, and the South plot, has little to no foliage. The soil is dark and cracked. On the other side of the fence, ½ East plot, ½ West plot, and the North plot, shows an obvious sign of a cleared path parallel with the fence. There is a large termite nest on the West side of CPP1. The West and North plots show herbivore damage on the leaves. There is a shared canopy on the East side of the tree. There are lots of seedlings on the North side of CPP1 that show a promise of growing if not damaged. The soil on the non-cow path is brown/black and slightly cracked. The temperature at 8:20 a.m. was 82½ degrees.

Cow Path Plot: Nuclear Tree CPP2

CPP2 is located 217 paces from the road. The CPP2 is located in the middle of the cow path. There is little foliage on the ground. There is an abundance of seedlings on the North side of CPP2 outside of the foliage. North side of CPP2 in plot A contains broken twigs. There is an artificial nesting environment on the South side of the tree. The soil is easily seen all around the tree. There are major vines all around CPP2. Under the vines there is little to no foliage or seedlings. On the West side of the tree there is a major vine that covers most of the ground. The soil is dark and cracked. Where there is a direct cow path there is nothing growing. On the South side there is weedy flowers outside of the canopy along with standing water. The West side in plot B has more light and more seedlings. There is several tin cans discarded within the West plot A. The temperature at 9 a.m. was 83 ½ degrees.

Field Plot: Nuclear Tree FP1

FP1 is located 85 paces from the road. The foliage on the ground is sparse and very decomposed. There are some cracks in the soil surface. The soil is dry and dark in color. There are leaf minors present in the foliage. The measured temperature at 1:20 p.m. was 90 degrees. There is a slight breeze flowing through the canopy. There is a dual canopy cover on the West side. The FP1 has a circumference of 4 meters. The bark contains small spikes. There is no noticeable moss or other parasites on the tree. Although there is shade under FP1 there is light away from the canopy. There are large roots from other threes that invade the area.

Field Plot: Nuclear Tree FP2

FP2 is located 26 paces from the road. The foliage on the ground is very decomposed with very little even present. There is an abundance of small sticks on the ground around the base of the tree. The soil is cracked and black in color. There appears to be no foliage disturbed on the tree. At 2:15 p.m. the temperature was 92 degrees. There was a slight breeze. The canopy was not shared by another tree. Moss is present on the East side of the tree only. Spikes are present on the tree bark. The tree is 3 m<t<4m. There is a large root system present. FP2 has minimum sunlight on the North and East sides. On the West side there is a fallen tree in the marked plot. West and South sides contain a lot of foliage. The FP2 is near the road.

Fig. 1

Fig. 2

Fig. 3

Fig 4.

Fig. 5

Fig. 6

Fig. 7

Fig. 8