1. heavily grazed cattle pasture
2. 3 year dormant cattle pasture
3. a secondary forest
4. a primary forest

Amy Biasucci, Raylene Gerber, Mary Giambruno, Dale Mast, Mike Novemsky, and Rachelle Rapp-Dickerson

Introduction

Since Palo Verde became a wildlife refuge and a national park, management has attempted to return cattle pasture back to tropical dry forest. Three hundred years ago, the natural forest was cleared for cattle pasture. Thirty years ago (Stewart), African Blue-Stem Grass (Jaragua) was introduced to the area because it provided hardy food for the animals. The problem was if the grass was not eaten by the cattle, it could grow to be 3 meters high. It was so dominant that native grass and woody plants could not compete, and in the dry season, the grass would die. There would be a fire danger where the successional plants around these grassy areas would be harmed. Five years ago, cattle were reintroduced in several pilot plots in the lagoon areas. Our group wanted to test the soils of a cattle and a non-cattle pasture areas, a secondary forest, and a primary forest. We knew that this area had a bedrock of limestone, ( Dudley), so we wanted to see how the above three areas would compare with a primary forest. In 1995, (Angelis), an OTS group studied soil samples comparing a grazed area with a secondary forest. They only probed the soil at a 5 centimeters depth using a grid pattern of 10 meters. They suggested that soil composition, structure, and erosion, be studied. We hypothesize that when active pastureland is left to recover, its soil properties will return over time to those of a primary forest.

Methods and Materials

Our group decided to take 4 soil samples in the 4 corners of a 5 meters square at each of the 4 different sight areas on Sunday, August 1, 1999. Using a Global Positioning System, we found that the active pasture (ap) was located at N10 degrees, 21.20’, W85 degrees, 17.72’. The inactive pasture (ip) was located at N10 d, 21.32’, W 85 d, 18.12’. The secondary forest (sf) was located at N10 d, 21.22’, W85 d, 18.07’. The primary forest (pf) was located at N10 d, 22.09’, W85 d, 23.07’. We used a Thommen altimeter and found the altitude of ap, ip, sf, and pf to be 90 meters, 102 meters, 108 meters, and 110 meters respectively. We found the altitude reading at sea level to be 60 meters. We then corrected each reading to be 30 meters, 42 meters, 48 meters, and 50 meters. We dug 1 hole at each corner using a shovel, pick ax, and trowel. Raylene collected a 34 cc sample of humus and a sample of A horizon soil at 5 cm below humus using a film canister. These samples were used in determining pH. Amy collected a 34 cc sample of soil 5 cm into the A horizon. These samples were used in determining soil density and moisture content. Michael measured the depths of the humus layer and A horizon using a tape measure. Rachelle recorded the data. We recorded the plant types, amount of sun or shade, and general weather at each site. We mixed distilled water (68 cc) with soil (34 cc) and let the samples sit for 12 hours. We found the pH by using pHydrion paper. Michael massed the density samples using an Ohaus analytic scale (+/- 1g). Mary baked these samples in a gas oven at 400 degrees F for 3 hours and 50 minutes. Michael measured the dry mass of the samples. We examined particle size and identified characteristics such as clay, silt, sand, and pebbles. Each person examined the same soil samples and we came to a consensus about what predominant soil type was. We repeated this method for all soil samples.

Results
 

	Plant 1	Plant 2	Plant 3	Mean	Standard Deviation
Agrarian
drained bract fluid	4.5	5.5	6.0	5.33	0.763762616
refilled bract	5.5	4.5	5	5.00	0.5
drained bract covered	6	5	5	5.33	0.577350269
Roadside
drained bract fluid	8	5.5	6	6.50	1.322875656
refilled bract	5	6	6	5.67	0.577350269
drained bract covered	6	5	5.5	5.50	0.5
Forested
drained bract fluid	8	9	7.5	8.17	0.763762616
refilled bract	8	8	7.5	7.83	0.288675135
drained bract covered	7.5	8	7.5	7.67	0.288675135
Garden
drained bract fluid	8.5	5.5	5	6.33	1.892969449
refilled bract	8	5.5	5	6.17	1.607275127
drained bract covered	8	6	5	6.33	1.527525232

 

Discussion

From looking at soil textures we found there is a progression in soil characteristics from (ap) to (pf). The (ap) contained more sand and rock than the others, possibly due to erosion and deposition, which would remove the finer particles from the top soil. In the (ip) where grasses start to take over, we still observed sand but a larger proportion of clay was present. This was probably due to a decrease in weathering. In the progression of (ip) to (sf) we noticed that clay was still a major component, while there was evidence of the production of silt. This could be explained because of the presence of grasses and shrubs that hold the finer silt particles in place. When moving from (sf) to (pf) the soil basically became clay. This could be due to the presence of larger and more numerous trees that protect the forest floor and retard the growth of grasses. When looking at the density of the A horizon, we noticed a decrease in the density of the soil when moving from pasture to primary forest. This trend was expected because in pastures with high erosion, rocks and other high-density materials would remain. In the forest the smaller, less dense particles stay in place.

The increased percent of moisture as we moved from (ip) to (pf) is as expected. This is due to more run-off and erosion in a pasture. In the forest, there is more humus to catch the rain, along with more cover to prevent the rain from reaching the ground. Also, forces such as wind and sunlight increase evaporation in a pasture, while they are not as strong of factors in the forest.

The pH of the humus and the A horizon remained constant at an acid pH for all four locations. We would expect a basic pH in the region due to the presence of limestone found in the Guanacaste province (Janzen 1983). The acidic pH could be due to the fact that we were only 5 centimeters below the humus in the A horizon. The limestone could be found in a lower horizon that we did not test. As tree littler decomposes, organic and inorganic materials are formed that can reduce the soil pH (Brady 1996). Therefore, the deeper you progress in the soil horizons, the more basic they become. Interpreting our data we found a pattern in the difference of the pH between the humus layer and the A horizon. When a t-test was applied, we found that this data was within the 5% error, therefore, our data supports this assumption. There seems to be trends from our data that would show a pattern in recovery from pasture to primary forest.

Classroom Applications

The exact procedure for this experiment could be followed in all classrooms. The use of technologies such as probes, would increase the quality of data. This would lead to a better understanding of soil profiles and the physical properties of the various horizons. It would also demonstrate the importance of land management and conservation strategies in different regions. A real life experience such as this would give students a better understanding of the impact of weathering, erosion, and deposition.

Bibliography

Angelis, l., Corbell,N., Netti, B. "Soil Analysis at Grazed and Ungrazed Sights at PUNP." OTS Tropical Conservation 1995-11, p. 40,41

Dudley,M., Boyule,B., Blanco, B. Cruz, J., Grose, S., Kershner, R., and Moody, N. "A Characterization and Com[parison of the Floristic Composition of limestone rock outcroppings at Palo Verde National Park." Tropical Plant Systems. June-August, 1994 p. 94.

Janzen, Daniel H. Costa Rican Natural History. University of Chicago Press, Chicago, 1983 p. 63, 64

Stewart , I. "Effect of Cattle Grazing on Understanding Vegetation at Palo Verde Wildlife Refuge."

Dartmouth Studies in Tropical Ecology, 1990 p. 7,8,and 9.

Unpublished paper: Wesch, R. ,Gonzales, E., Fisher, R., Anthony, S.R. "Tree Effects on Soils in a Tropical Dry Forest." Department of Soil and Crop Sciences, Texas A. and M. University. Palo Verde Research Station, Spring 1999