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The Effects of Different Edge Types on Abiotic
Characteristics
of Primary and Secondary Tropical Dry Forests
By Jennifer Huntsberger, Lisa Lockwood, Ken Perry, Jon Yoder
ABSTRACT
The purpose of this experiment was to determine the effects of different edge types on the abiotic characteristics of primary and secondary forests. To test this we located two types of edges: pasture adjacent to secondary forest, and secondary forest transition to primary (old growth) forest. The following tests were performed: relative humidity, air temperature, soil temperature, light intensity and pH. Our results showed no significant changes for soil temperature and pH at either transition. However, within the first fifty meters beyond the transition, the data indicated that relative humidity, light intensity and air temperature are more intrusive into the secondary forest from the pasture than the secondary into the primary forest.
INTRODUCTION
Fragmentation of tropical dry forests produces a series of biogeographical islands. Two primary effects of this are an alteration of the microclimate within the surrounding island remnant and the isolation of remnant islands from each other. Therefore, in a fragmented landscape there are changes in the physical environment as well as biological changes.
The
“edge effect” refers to those physical and biological changes that occur
between two different ecosystems or habitats. The forest border next to cleared
pasture, for example, represents a boundary between two ecosystems that differ
in air and soil temperature, light intensity. Along these edges there may be
abiotic and biotic changes as you move from one ecosystem to another. In
addition, different ecosystems may be influenced to varying degrees by
different edge types. Living organisms such as plants and animals may be
adversely impacted by these influences. For example, altered temperature
regimes can destabilize predator-prey and parasitic interactions. (Saunders, et
al., 1991)
Biodiversity
may be lower than expected in forest islands due to the edge effect. This
reduces the amount of habitat associated with both secondary and primary
forests. The consequences of this may impact management decisions, especially
knowing that larger remnants are less adversely affected than smaller remnants.
(Saunders,
et al., 1991)
The
Guanacaste region of Costa Rica, and in particular the Palo Verde area, has
undergone extensive modification of habitat by humans since Conquest times. The
purpose was to raise cattle in the lowlands. Prior to exploitation, the
lowlands were forested. (Janzen, D., 1983)
Our
field experiment will examine the edge effect between pastureland and secondary
forest as compared to the edge effect between secondary forest and primary
forest. The following abiotic
characteristics will be used as a means of comparison: air and soil
temperature; relative humidity; light intensity; and soil pH. We would expect the edge effect to be
greater between pasture land and secondary forest due to the reduced canopy
cover in a secondary forest and the previous disturbance to that area.
MATERIALS AND METHODS
The secondary forest/pastureland edge site was approximately half a mile from the OTS dormitories and the OTS headquarters. The pastureland was being used for cattle grazing while the secondary dry forest was characterized by numerous Acacia trees and ……… The secondary forest/primary forest transition edge site was established by traveling approximately three miles in the opposite direction, taking the first right in the road from the dormitories. The secondary forest was characterized by a thicker shrub layer and trees in the 10 meter range while the primary (old growth) forest was characterized by a thinner shrub layer and trees in the 30 meter range.
It should be noted that our attempt to use GPS to identify the exact locations were unsuccessful due to canopy cover. In addition, the terms primary forest and old growth forests were both used and need to be defined. A primary forest is one that has been untouched and unaltered by humans while an old growth forest has been altered at some point in the past but has now taken on the characteristics of a primary forest. OTS staff provided the directions to the primary forest. However, the areas that had these characteristics were small remnant islands among secondary forests that may themselves been altered by early settlement. So we have included both terms, primary and old growth forest to denote our second study site.
The
following materials are needed to perform this field experiment: armored
thermometer, meter tape, light meter, sling psychrometer, trowel, LaMotte soil
testing kit
The
following method was used:
1.
Select
sites at secondary forest edge adjacent to pasture
2.
Select
sites at primary forest edge adjacent to secondary forest
3.
Set
two transects at each site from 100 m into edge to 200 m into the adjacent
forest
4.
Take
measurements at set intervals of: relative humidity, air temperature, soil
temperature, light intensity and soil pH.
·
Humidity:
use sling psychrometer
·
Air
temperature: use dry bulb on sling psychrometer
·
Light
intensity: Digital Light Meter Model 401025
·
Soil
pH: bring marked soil sample back to lab and use La Motte soil kit for pH
RESULTS
Point 1 = 100m into disturbed area (pasture or secondary forest)
Point
2 = habitat edge (pasture/secondary or secondary/primary)
Point
3 = 50m into secondary forest or primary forest
Point
4 = 100m into secondary forest or primary forest
Point
5 = 200m into secondary forest or primary forest
DISCUSSION
Our
cursory data suggest some support for our hypothesis. There appears to be more intrusive impact from the edge of pasture
into secondary forest than from the edge of secondary forest into primary (old
growth) forest. Relative humidity from
the edge of pasture to 50 m into secondary forest increased by approximately 6%
while there was no appreciable difference between the edge of secondary forest
into primary forest (old growth). In
general, a similar trend held true for other abiotic factors as well. Light intensity and air temperature showed
differences between the edge up to a 50 m point in the pasture to secondary
forest edge, while there was less significant difference in these factors
between the edge of secondary forest and primary forest. Soil temperature and pH showed less
significant changes in all tests, possibly due to inadequate instrumentation
and procedure.
If
we were to conduct this experiment again we would change the following:
·
Use
soil thermometer
·
Use
more sophisticated soil test kit
·
Find
edge between cattle pasture and primary forest
·
Find
larger areas of primary forest
·
Increase
number of trials/line transects
From
what our abbreviated study shows we believe that management policies should
consider planting a 50m buffer zone between disturbed land and primary forest.
Because of the predominance of the agro-ecosystem worldwide (Janzen, 1983),
studies on the edge effect of fragmentation warrant additional studies.
SCHOOL APPLICATION
The edge effect concept and field activity is one that can easily be applied to a variety of areas where forested lands are adjacent to cleared lands. With human encroachment on forested lands increasing, it is important to understand the effects of forest fragmentation at the ecosystem level. As a unit, this concept has a variety of curricular approaches in chemical, biological, earth science and life science classrooms. Extensions of the unit could involve the student in more specific studies associated biotic/abiotic interactions and biodiversity.
ACKNOWLEDGEMENTS
We
would like to acknowledge those who pointed us in helpful directions: Nicole,
Maurice, Ulysses, David and Evan.
REFERENCES CITED
Janzen,
Daniel. 1983. Costa Rica Natural History. University of Chicago Press.
Saunders
et al. 1991. Biological Consequences of Ecosystem Fragmentation: A Review.
Conservation Biology. Vol. 5, No. 1.