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A Study of Intensity of Light and its Effects on the

Microclimate of Primary and Secondary Forest

La Selva Biological Station, Sarapiqui, Heredia, Costa Rica

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Figure 2-Luminosity    Figure 3-Soil Temp    Figure 4- Air_Temp    Figure 5-Humidity    Table 1

Abstract:

Fragmentation of the forest and the subsequent resulting edge effect causes extreme changes in the tropical rain forest.  We feel one of the greatest immediate effects is a tremendous increase in light intensity which results in various meteorological changes for the forest.  Three areas, which exhibited various stages of forest growth, were selected as study sites.  These included a site with a one-year re-growth, a five plus re-growth, and a primary forest containing trees older than 100 years.  This study shows the correlation between light intensity and forest characteristics.

Introduction:

A Study of Intensity of Light and its Effects on the Microclimate of Primary and Secondary Forest. 

There is constant drama in the game of survival of the fittest, and it is quite apparent in the tropical rainforest.  Not only are the animals involved, the plants are also entrapped in the game.  For plants, a major factor is the presence or absence of available light needed for life processes.

 It is obvious to the naked eye, that light falling on cleared areas is much more intense than light that reaches the forest floor of dense, wooded areas.  What is not so obvious, however, is the quantitative difference light makes in regards to the microclimate of these areas.

Figure 1 – Sun Positioning at Various Times

 Our group studied the amounts of light in three distinctive areas: a recently cleared (but currently re-growing) parcel with approximately five years growth, an older secondary forest approximately ten years old, and a primary forest that is greater than 150 years old. Our experimental design attempted to answer how the intensity of light affects the microclimate and structure of the forest.  We focused on ambient air and soil temperature, relative humidity, and size and density of vegetation.  We hypothesize that lumens of light available will vary with each habitat site, and that the variation can be explained by the vastly different forest floor and overlying canopy coverings.

Methods

Study Site:

We chose to study three distinct areas in the successional plots at La Selva Biological Station that are in close proximity to one another.  These areas are each unique in that they contain different densities of vegetation and are variable in age.  The first site was a recently cleared area of vegetation that has begun to regrow.  This area, which has been growing for approximately five years, contains relatively short grasses, herbaceous shrubbery, and small trees.  The second study site was an older secondary forest that has been regenerating for approximately ten years.  The secondary forest is one that is very dense with not only grasses and shrubs, but also a nearly impenetrable tree layer thick with vines, causing extreme competition for light.  The largest tree in the area, a Cecropia, was located near the 50 meter sampling site and had a circumference of 45 centimeters.  The third was an old growth primary forest having a minimum of 150 years of growth.  This tremendously diversified forest site is one with a distinct canopy containing very large trees, and a relatively thin understory.  Large trees were located at each of the sampling locations within the primary forest area.  A tree with a circumference of 3.5 meters was located near the 20-meter sampling location and another with a circumference of 8.5 meters was located at the 50-meter sampling site.  The three distinct sites were chosen in an area where all could be explored in close proximity to one another and so that consistencies in brightness of light could be found.

Data Collection:

We collected microclimate data on July 26, 2001 using a continuous transect that crossed each of the study areas.  We collected data from measurements located at –10, 0, 10, 20, and 50-meter increments in a direct east to west compass direction (90 to 180 degrees).  The sampling location at –10 meters was within the recently cleared area of vegetation.  Sampling locations at 0 meters were located at the edges of both the primary and secondary forest plots.  The other points were located up to 50 meters within the primary and secondary forests along the transect line.  At each transect point, we gathered data on intensity of light, air and soil temperature, relative humidity, and other related weather indices.  All measurements were taken in replicate and in a consistent fashion at ground level using an “all in one” tool called a Kestrel 300 “weather machine.”   Also, using a clinometer, the slope of the study area measured 1.5 percent.  Three team members went out at dawn, around noon and dusk to collect climatic data at the study sites.  The climatic data was collected for information that would be compared with light intensity measurements.  On the day we collected data the skies were continually overcast. Additionally, all samples were collected in replicate using the four cardinal points of a compass as reference and within 0.5-meters of the central sampling point.

Results   

LUX (lumens per square meter):

As anticipated, the lumens of sunlight per square meter were lower in the morning, higher around noon, and lowest prior to dusk in the late afternoon (Figure 2).  This is likely are result of the angle of the solar rays reaching the earth at each of these times (Reading, 1995).  At 1300 hours, the 50-meter sampling location of the primary forest had higher lumens per meter readings due to its close proximity to a nearby light gap in the canopy made by falling trees.  The gap was located approximately 10 meters to the south of the sampling site.  In the late afternoon, this same area of primary forest had lower lumens per square meter because of the low angle of the sun and a scattering and absorbing of the solar rays by the overhead multi-tiered closed forest canopy.

Soil Temperature:

The highest soil temperature recorded during the sampling was obtained from the open field.  The height of the vegetation at this sampling location was mostly less than 10 centimeters.  This resulted in little scattering of light and permitted the direct rays of the sun to heat up the soil.  As we entered into either the primary or secondary forests areas, the soil temperature readings went down between two to four °F with the biggest decrease occurring in the primary forest (Figure 3).  Lower temperature readings in the forested regions likely resulted from the overhead canopy cover and the microclimate formed by the vegetation located near ground surface of these areas.

Click on a thumbnail to see the enlarged image
Figure 2 -  Luminosity	Figure 3 – Soil Temperature	Table 1	Figure 4 – Air Temperature	Figure 5 – Relative Humidity

Air Temperature:
As expected, the highest ambient air temperature was encountered during the 1300 hours sampling event in the open field due to the direct rays of the sun heating the air above the soil surface in conjunction with solar heat radiating off the soil surface.  Conversely, lower air temperature readings were encountered in both the primary and secondary forests around 1300 hours (Figure 4).  This is likely due to foliage scattering and diffusing the light rays as they penetrated to the ground level.  We also infer that these dispersed solar rays did not reach the ground cover or reached it as reflected sunlight from the nearby vegetation.  Early morning and late afternoon readings did not show this trend.  At 730 hours, higher readings were detected at the primary forest edge due to the sun shining across the open parcel into the edge of the forest.  Based on observations of the surrounding vegetation, most of the variations in temperature readings can be accounted for by the angle of the sun’s rays hitting the forest at that particular time of day.

Relative Humidity:
Relative humidity readings are close to the saturation point (>80%) at the La Selva Biological Station because the maritime air of the region is always close to the saturation point (Reading, 1995).  The highest humidity readings in the morning were found in the secondary forest sampling area.  This is possibly due to the retention of the morning dew in the microclimate formed by the canopy and the dense forest groundcover vegetation (Figure 5).  During the 1300 hours sampling regimen, readings in all sampling locations were lower possibly due to evaporation caused by the intense heat of the afternoon sun.  A short downpour occurred, however, while we were collecting data from the open parcel area.  This rain continued while we collected samples from the primary forest sampling locations.  Despite the rain, the relative humidity readings were slightly lower in the primary forest area than in the open field area (with the exception of the sample located near 50 meters).  We suspect relative humidity readings remained lower notwithstanding the rain due to the forest canopy cover as we observed very little rain reaching the ground level.  In the afternoon, the relative humidity readings were higher in the secondary forest possibly due to the heating up of this area, producing evaporation that is retained by the dense lower vegetation.   No explanation can be determined for the lower (85%) percent relative humidity reading measured at the 50-meter sampling site of the secondary forest.

Discussion:

The data shown in Table 1 indicate that the greatest variation between the primary and secondary forests involves the intensity of light on the forest floor with more light available in the secondary forest.  It is important to note, however, that our information will not appear similar to data collected on a sunny day for the same time of year.

 Although some variations were detected in relative humidity, soil and ambient temperature, and wind speed data, these factors are less between the forest types than are the luminosity data in the areas of study. 

 We believe that our hypothesis is substantiated in that the lumens of light available did vary with each habitat site, and that the variation can be explained by the vastly different forest floor and overlying canopy coverings.  Areas of recently cleared forest were observed to contain a diverse community of low-lying vegetation in intense competition for space, nutrients, and sunlight.  These pioneer trees, shrubs, herbs, and climbers differed from the climax species they eventually will be replaced by in that they are more light demanding and less shade tolerant.  The fact that they are very rapid in growth can be inferred from observations of the zero to five-year growth parcel of land.  Plants in this parcel appear to immediately spring from the soil when light becomes available and cover the bare forest floor, protecting it from wind and rain erosion.  Within the primary forest, light is subdued.  Vegetation is composed mostly of huge soaring trees.  The upper levels are difficult to see from the ground.  They form a huge umbrella overhead that not only provides shade but also provides shelter from the wind and rain.  During the 1300 hours sampling event it began to lightly rain.  Once we entered the primary forest covering we no longer felt the falling raindrops.  Additionally, although measurements of the wind showed stagnant air at the forest floor, we could hear the wind blowing in the canopy overhead.  Also, several of the trees in the sampling area were measured at more than five meters in circumference.  The bases of these trees contained huge buttresses pointing out in every direction to stabilize the trees.  Below these huge fortresses were the mid-level sized trees which had the effect of shutting off even more light.  The floor of the primary forest was relatively clear and only a few shrubs existed here and there.  The soil temperature was several degrees cooler, also.  Its surface protected from the intense tropical sunlight outside. 

Some additional considerations in this particular study would be to investigate particular types of vegetation found at each site.  Additional measurements of humidity and soil temperature might be investigated in later studies to account for differences in our study.  Our test also was carried out on a cloudy, sometimes rainy day.  A bright sunshiny day may enhance the effects noted.  It also should be noted that our study only took into consideration visible light rays.  However, there are other components of the visible light (i.e. ultraviolet light) that should be considered in further studies.

The human effects of clearing the rain forest is immense.  Not only does cutting the forest cause huge erosional forces on the exposed land, it has direct effects upon the edges of the remaining forest.  Increased light reaches the forest floor creating areas of disturbances.  Plant and animal life responds to the change and a complete new microenvironment is formed.

Acknowledgments

Very instrumental to the completion of this study was the total dedication and ambition of the members of this team.  Only with the hard work of all involved parties, could we have done our best.

 “Muchos Gracias!” to our wonderful and knowledgeable group leaders.  Tom, Jim, Melanie and John thank you all for your time and trouble to help us stay on track and do our best. 

We are very thankful for the assistance of our technical support person, Chris for his assistance in keeping our machine in working condition, and help with all of our many “crises.”

 Thanks to La Selva Biological Station and The Organization for Tropical Studies for giving us the opportunity to take in all the wonders of this beautiful environment.  Our stay in your facility has been a memorable one. 

Finally, thanks to all our group members for their continued support, humor and genuine concern during our research!

Dorothy, Rafael, Curtis, and Julia

July 28, 2001