Atlantic Forest

Exercise 9: Edge Effects on Selected Aspects of Forest Microclimate
Module 9: Light and Temperature

Your Questions

1. How far do the edge affects of light intensity, temperature, wind speed, and relative humidity penetrate within the Atlantic Forest?

Background

A continuous vegetation cover can modify the light environment in different ways, depending on the geometry of the plants, the distribution of plant biomass, and the spectral properties and orientation of the leaves. Plant cover also has a significant and complex effect on the temperature regime present in the area. These effects change throughout the day (diurnally) and throughout the year (annually). An area with complex vegetation cover, with many canopy layers, will have a much more constant set of abiotic values in that the temperature, moisture levels, brightness, and wind will all be more consistent both diurnally and annually.

Tropical forests, especially in areas where human pressure is present, are subject to clearing, leaving sharp edges between two community types: edges and interior communities. The presence of a discontinuity in the vegetation cover can produce several effects on the microclimate of the forest. Many recent studies have shown that this effect extends variable distances into the forest, depending on the factor studied and the environment in which it is studied. Edge effects will percolate into complex forests for a shorter distance than less complex or shorter canopied forests.

In this study we will collect data on variation in two abiotic factors, light intensity and temperature with distance from the edge of a forest fragment towards the interior. Light intensity is probably the most important factor controlling plant growth in the understory of a forest ecosystem, since plants rely upon it for photosynthesis. The types and abundance of plants that are present largely determine the type of flora and fauna that can live there.

However, temperature is also vital, particularly in areas with large thermal ranges. In temperate forests and deserts, such as the Black Rock Forest and Biosphere 2, the average temperature is not what determines whether a species can live there. Instead, the most important aspect of temperature are the thermal extremes. For example, Saguaro cacti (Cereus giganteus) cannot live in areas with extreme temperatures below freezing that last for more than two days (or more accurately, around 42 hours). As an interesting side note not related to what we discussing here, Saguaro are routinely found in these areas, but only on south-facing slopes where the sun is able to melt the frost, even though the ambient temperature hovers around freezing.

Wind speed also changes tremendously from the edge of a forest to the interior. Obviously when there are many trees around, wind speed greatly decreases such that a forest interior is much less windy than is a forest edge, which is much less windy than a field without tree cover. High wind speed is an undesirable environmental condition for those forest interior dwelling animal species that are feeble fliers, and therefore do best at dispersing to a precise location when wind speed is low. A strong breeze could be enough to cause these animals to fly off course and end up in potentially lethal environments, such as a lake or a hot, exposed field. Similarly, seeds of those plants that require forest interior habitats to grow will not germinate if they are blown off course into an unfavorable environment.

Relative Humidity (RH) measures how much water is actually suspended in the air at the present temperature, when compared with the maximum amount of water that could be suspended in the air at the current temperature. When the relative humidity is 100%, it means that the maximum amount of water is suspended in the air for that temperature. Whereas, if the RH is much lower then much less water is suspended in the air than the air could possibly contain. Note that warmer air can theoretically hold more water than colder air, so the amount of water suspended is scaled accordingly. As an example, the RH of a rainforest is on average much much higher than the RH of a desert, even though both may be at the same temperature. This obviously has implications for the amount of plant biomass that can be sustained in a given area.

Your Assignment

1. Measure light intensity, temperature variation, wind speed, and/or relative humidity along a transect from the forest edge into its interior and determine the extent of modification of the light and temperature regime in the forest as a function of distance from the forest edge.

Methodology

1. Task A—Establish sample areas
   • Using maps and previous knowledge of the sites, select 2-3 forest areas with well defined edges and of dissimilar canopy structure. Ideally, one of the forests will be more simple than the other.
   • At each site establish a transect of at least 200 m and at a perpendicular angle to the forest border and mark sampling stations
   
   
   
2. Task B—Data collection
   • Determine what intervals you should collect the data along each transect. Should it be clustered at any point along the transect? Should it be regularly spread out? What would be the benefits and drawbacks of each plan, in terms of the scientific method?
   • Determine how many transects you will collect? What would be the benefit of adding more transects?
   • Along each transect, collect data at regular intervals (determined by class) on light intensity, temperature, and relative humidity (using a sling psychrometer - consult the demonstration video)
   
   
   
3. Task C—Data analysis and write up
   • Data entry into computer
   • Analyze the data
   • Compare distances from forest edge
   • Conjecture on how these factors may impact on the plants and animals of the area, based on what you have learned thus far the in course.
   • Summarize results for PowerPoint presentation (written and oral)

Objectives

1. Introduce measurement techniques for abiotic variables
2. Understand what relative humidity is and how it may impact the biotic environment
3. Study edge effects on selected abiotic factors (light intensity and temperature)
4. Understand the implications of simple versus complex forest canopy structure to the relative impacts of edge effects on forests

Key Skills

1. Learning measuring techniques for several abiotic factors
2. Reinforce capabilities in establishing transects
3. Statistical comparisons
4. Learning how to use a sling psychrometer

Timetable

Total time—1 day

1. Morning
   • Establish transects and sampling points
   • Collect data
2. Afternoon
   • Data entry and analysis
   • Write up and discussion of results

Materials Needed

1. Field notebook and writing equipment
2. Material for establishing transects (stakes, hammer, flagging)
3. Tape measures (50 m)
4. Compass and GPS
5. Collection equipment for environmental data — radiation intensity and temperature (wet and dry bulb)
6. Sling psychrometer
7. Anemometer