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Edge Effect of a Path in La Selva

Miles Robinson, Carmen Nunez, Jennifer Snyder and Karen Aleman

Soils are the primary nutrient medium for plants and provide not only physical support and nutrient supply, but also influences the amount of water and aeration that reach the roots. Soil exerts substantial influence on the forest community (Kaven et.al.1981). The most important factors involved in soil formation are climate, parent material, relief, hydrology, organisms, time or age, and human impact (Young, 1976). We tested the human impact on soils.

Much is known about the soils at La Selva a tropical forest in Costa Rica. We sought to determine the extent of the edge effect of human impact on the soil that is used as paths and how far the edge effect extends into the forest. The path we tested is alluvial soil deposited by ancient rivers.

The soil composition is important for the ability of plants to germinate seeds and to grow efficiently. We conducted physical tests such as compaction and water absorbing ability to give us an indication of the amount the soil will erode in the constant rain at La Selva. We also tested for two very essential nutrients for plant growth, important for the movement of nutrients in the soil. We expected to see significant differences in the physical qualities of the soil of the path and of the soil 5 meters in from this human impact of a path.

Methods:

We studied a 2 meter wide foot trail with no stepping stones. See the map for location. The measurements were done at locations along the trails which had no steps or concrete covering and extended out from the center of the trail 5 meters along a transect into the forest. We chose this path since it was obviously used and lacked the usual stepping stones at LaSelva. We chose a 5 meter transect because forest canopy had recovered by 5 meters in. We made three transects and recorded data at 0 m, 1m, 2m and 5 meters. Specifically we determined the soil compaction, soil profile (A, B and O soil horizons), depth of the soil, organic cover, moisture content, percolation and did some chemical testing including nitrogen, and phosphate.

We followed the protocols for chemical testing as detailed in the LaMotte Garden Guide Manual for LaMotte Soil Chemistry Test Equipment. Samples were collected and analyzed for nitrogen and phosphates according to the protocol in the test kit. We only analyzed one transect due to a lack of chemical supplies.

Soil compaction was analyzed using a method described in the book Techniques for Ecological Testing. A sharp stick was dropped from a standard height, 1.3 m, straight into the ground. The average amount of penetration over 5 trials was used to indicate soil density. (Sutherland, 1996)

As part of a general habitat survey we performed a qualitative measure of soil moisture. The method of categorization was 1) dry soil, which was crumbly or dry to the touch, 2) moist soil, which was pliable to the touch, and 3) wet

soil, which exudes water when squeezed. Quantitative moisture content was measured by weighing samples of soil before and after drying to determine the percentage of water present. Samples were dried in an oven for four hours at a temperature of 80 degrees Celsius.

We profiled the soil by making a pit with trowels to measure the depth of the O and A levels of the horizon. In addition, we attempted to determine the composition of the B-horizon during our analysis. The amount of organic matter found atop the soil was determined by drying the samples collected within a 30 square centimeter plot. Our group used an Imperial aluminum can as a tool for percolation. The top and bottom of the can were removed. The can was pushed into the ground ½ way and 100 ml of water was poured in. We recorded the amount of time the water required to percolate into the soil.

The map above is a map of the soils type found at La Selva. The arrow at the top points to the sample point on an unpaved trail leading to the river.

Results: The following tables show our physical and chemical test results for 3 transects from the middle of the trail (0 meters) to 5 meters away from the 0 meter mark. Visually, the trail was worn from the 0 to 1 meter mark and had little wear and tear from 1 meter to 5meters.

Transect 1

Test	0m	1m	2m	5m
Percolation time	70s	21s	3s	3s
O horizon depth	Trace	Trace	1cm	1cm
O horizon dry mass 30cm^2	78g	42g	154g	64g
A horizon % water	43%	58%	75%	56%
Soil moisture	Wet	Wet/moist	Moist	Moist
Penetration	3cm	3.6cm	3.8cm	4cm
Canopy cover	75%	85%	95%	95%

Transect 2

Test	0m	1m	2m	5m
Percolation time	90s	8s	8s	5s
O horizon depth	Less than .5cm	.5cm	1cm	.5cm
O horizon dry mass 30cm^2	44g	68g	145g	142g
A horizon % water	46%	38%	33%	45%
Soil moisture	Wet	Wet	Moist	Moist
Penetration	2.2cm	2.8cm	2.8cm	4.3cm
Canopy cover	85%	85%	85%	85%

Transect 3

Test	0m	1m	2m	5m
Percolation time	Over 5 min	10s	50s	10s
O horizon depth	Less than .5cm	3cm	2cm	2cm
O horizon dry mass 30cm^2	50g	182g	142g	102g
A horizon % water	58%	52%	41%	36%
Soil moisture	Wet	Wet	Wet	Moist
Penetration	4cm	4.5cm	4.1cm	4cm
Canopy cover	75%	90%	90%	90%

Chemical Analysis of Transect 1

	0	1m	2m	5m
Nitrogen	Trace	Trace	Trace	Trace
Phosphorus	Low	Trace	Low	Low

Discussion:

Our results show a definite impact of the path, literally human impact, on the soil of the forest. Visually it is obvious from the pictures that there was no vegetation directly on the path. This left an almost continuous break in the canopy cover which was two to three meters wide. The canopy cover here allowed more sunlight and rain to hit the surface of the path, alternately allowing the soil to be baked and drenched. During the rain the path was indeed covered in puddles that extended over the whole path up to 6cm deep. There was no standing or visible flowing water in the forest proper.

While our penatrometer was crudely made and did detect subtle differences in the compaction of the soil. The apparent minor differences we noted may actually be very significant. The forest soil was “spongy” to walk on due to the leaf litter cover the roots, sticks, and maybe the actual density of the soil. The amount of difference we observed in the density did show generally more compaction in the path compared to the forest. This point needs to be investigated with a more sensitive instrument.

The one significant trend in our data was the leaf litter cover increased dramatically at the edge of the path, 1 meter, and increased into the forest. Leaf litter is hard to measure and collect because there are often large sticks, branches and crushed leaves too small to collect. These odd bits of litter did cause some dispersant points on our graph, but the general trend is that the edge of the path the leaf litter dramatically increases.

The percolation test showed dramatic difference from the path to the forest. The path was saturated with water and the test should be repeated in dry conditions. The reduction of canopy contributed to this situation. The leaf litter on the path was also flatten or embedded in the soil matrix. In the forest, the canopy reduced the amount of water hitting the litter as demonstrated in a companion project (The Canopy Effect of Chemical and Physical Properties of Rainfall). The water that did hit the litter had surfaces to run off on before hitting the soil. The soil in the forest was thus protected from direct rain and it had a sponge effect to allow the water to slowly penetrate the soil preventing saturation.

There was significant changes in the soil of the path in the areas of density, percolation and leaf litter cover. These differences impacted the soil because of the saturation of the soil on the path, the standing puddles and the lack of vegetation. These effects seemed to only extend a meter from the path. There was significant changes in the soil because of direct walking and continued use as a path.

Application to the Classroom:

This research project has a lot of applications to the typical classroom situation. All campuses have paths across the grass or off the sidewalks where students tend to walk. These sites can be the test sites. The experimental equipment can easily be fashioned from available materials. The penetrometer can be made from a weighted sharpened stick or a sharp metal rod. Weighing containers can be made from aluminum cans. The percolation apparatus was easily made from an aluminum can with the top and bottom removed. A graduated cylinder, a balance and a chemical test kit if that part is done, are all the lab equipment that needs to be supplied by the school.

Visually the impact of human walking on a path on the grass or in the forest is obvious. This lab exercise gives the visual differences some numbers. Measuring the run off from the path or the change in soil composition over time compared to the matrix that the path is inserted in can also be added. The flora and fauna on and off the path can be checked to see the impact on the living systems in the matrix. Students can begin to see the impact they have on the land around them. An extension that might be tried is to fence off a part of the path and see how long, if ever, it takes the land to recover.

References:

Brown, Zar, and vonEnde. 1997. Field & Laboratory Methods for General Ecology. WCB McGraw Hill, Boston

Kaven, et.al, 1981. The Biology of Plants. Cambridge University Press, Chicago

Mc Dade, Bawa, Hespenheide and Hartshor, 1994. La Selva: Ecology and Natural History of a Neo Tropical Rainforest. University of Chicago Press, Chicago

Young, A. 1997. Tropical Soils and Soil Survey. Cambridge University Press, Cambridge

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