Please click here to return to main page

 

What is the canopy effect on rainfall

amount and its chemical composition?

 

Woodrow Wilson National Fellowship Foundation

Environmental Science Institute

Costa Rica

 

Toni Enloe

Ron Hellstern

Charles Lee

Bruce McCandless

INTRODUCTION

 

Since rain is a major component of a tropical rain forest, we became interested in the possible canopy effect on rain.  Precipitation is intercepted, retained, and redistributed by the canopy.  Water ultimately evaporates from the canopy (interception) or drips through (throughfall) or runs down the stems (stemflow) to the forest floor.  The chemical composition of the precipitation may be altered dramatically by the canopy. (Parker, 1995)  In the primary forest water is constantly falling from the sky, percolating through the vegetation, being absorbed by plants, running away into streams and rivers, evaporating, or being evapotranspired back into the atmosphere.  Precipitaion lighter than 5mm per day does not contribute significantly to soil moisture because 3mm of rain is intercepted by the forest canopy and evaporated back into the atmosphere. (Parker, 1995) As fast as much of the water falls, it is being returned through the respiratory activities of trees and other plants where it gathers for a fresh series of storms. 

 

The implications of deforestation are profound.  The remainder of the rainforest is less capable of evapotranspiring because not as much moisture is circulating through the ecosystem as before, therefore desiccating the ecosystem. (Myers,1984)  For example, in a study at La Selva a thirty-one percent reduction in evapotranspiration occurred in a clear-cut during the first six months compared to an adjacent intact primary forest.  Forty-seven percent of the total annual precipitation is returned to the atmosphere by evapotranspiration at La Selva. (Luvall, 1984) We chose to measure the amounts of rain collected at ground level in an open field and compare these amounts to those collected in a primary forest and to analyze some of its chemistry. 

 

HYPOTHESIS

 

We predict that the canopy in the primary forest will affect the amount of  throughfall and its chemistry.

 

 

MATERIALS

 

·        LaMotte water analysis kit

 

APPARATUS

    

 

 

 

 

PROCEDURE

 

Protocol

 

            Four different sample sites were used to collect rainfall:

1.)    Two sites were selected in a close cut grassy area with no canopy cover located approximately 100 meters apart.

2.)    Two sites were selected in a primary forest area with 80% to 100% canopy cover and no understory.  The two sites were located approximately 100 meters apart.

*  Canopy cover was estimated by sighting through a 4 cm circle held directly overhead.

 

One 250 ml beaker was used to collect rainfall samples at each site.  Identical beakers were buried approximately one inch into the ground for stability.  Buckets were used in the canopy area to insure adequate volume for chemical testing.  Samples were collected four intervals over a 24-hour period and total amounts were measured on site using a graduated cylinder.  Interval collection assured that no overflow would occur during the periods of heavy rain.

 

Testing

 

Due to the depletion of testing chemicals we were unable to replicate the tests. Samples from each site had to be combined to have adequate amounts of rain for testing.

           

            Evaporation*

Two petri dishes were massed with approximately 16 mL of water added.  One dish was placed in an open field under a roof overhang 12” above the sample to prevent additional precipitation from entering.  The second dish was placed in the primary forest under some large canopy trees, and protected from precipitation by placing it 12” under some large foliage leaves.  The dishes were allowed to sit for 24 hours and then collected and massed. 

 

pH *

                        Using the LaMotte water analysis kit, we filled the test tube to the 5.0 mL line with the sample water. We then added ten drops of the Range Finding pH Indicator solution. The color that resulted from the mixture indicated the approximate pH value of the sample.

                       

Ammonia*

Using a LaMotte water testing kit, we filled the 5.0 mL test tube to the line with sample water.  We then added  4 drops of Ammonia Nitrogen Reagent #1.  Capped and mixed.  We added 8 drops of Ammonia Nitrogen Reagent #2.  Capped and mixed.  The test tube was into the Ammonia Nitrogen Comparator and the sample matched to a color standard.  Results were recorded as ppm Ammonia Nitrogen.

 

Phosphate*

Using a LaMotte water testing kit, we filled the test tube to the 5mL line with the sample water.  Using the 1.0 mL pipet we added 1.0 mL of the VM Phosphate Reagent to the test sample.  Capped and mixed.  We waited 5 minutes. Using the pipet to we added 3 drops of the Reducing Reagent to the mixture and inverted to mix the contents. The test tube was inserted in the VM Phosphate Comparator and matched  to a color standard.

 

Nitrate*

Using a LaMotte water testing kit, we filled the test tube to the 2.5 mL mark with the sample water.  Mixed Acid was added until the tube was filled to the 5.0 mL mark.  Then capped and mixed.  We waited for two minutes.  Using the .1g spoon one level measure of Nitrate Reducing Reagent was added to the mixture in the test tube.  The test tube was inverted 50-60 times in one minute.  Wait 10 minutes.

 

Potassium* 

                        Using the LaMotte water testing kit, we filled the test tube to line five with

the water sample.  To this we added one Potassium Indicator Tablet.  The test tube was capped and shaken until the tablet dissolved.  The Potassium Test Solution was added two drops at a time until the color changed from purple.  Using a conversion chart, the number of drops were converted to a concentration level of Potassium

 

 

Results

 

            The following data were collected for rainfall amounts during a 24-hour period at the open field and forest sites. 

 

Time

Field Volume

Forest Volume

Difference

Standard Deviation

hrs:min

(mL)

(mL)

(%)

 

16:30

14

5.5

-60.71

6.01

22:30

33.5

12

-64.18

15.20

7:00

73

40

-45.21

23.33

10:30

106

96.5

-8.96

6.72

 

 

 

 

 

            The following data were collected for the indicated chemical components. 

 

Site

Ammonia

Nitrates

Potassium

Phosphate

pH

Units

ppm

ppm

 

ppm

 

Field

0.0

0.0

slight

0

5.5

Forest

> 5

1.0

low

0

8.0

 

 

The following graph indicates the percent difference between the amount of sample collected and the amount of sample collected in the forest during the same time interval.  Generally, the percent difference decreased as the rain kept falling.  This may have been due to saturation of the canopy cover.

 

 

Evaporation results were inconclusive. The forest sample had been tampered with and time constraints prevented us from repeating the comparison.  Some interesting results were obtained in the open field.  During the 24-hour period 226.5 mL of precipitation fell.  11% of the water in the petri dish was lost due to evaporation.

 

Discussion

 

We can only speculate about the results of the chemical tests.  We may infer the pH values are due to alkaline ammonium compounds that result from decay of plant matter. 

Tree canopy does affect rainfall amounts that can be collected at ground level.  Seventy-two mL more of rain was collected in the open area. The pH of the collected samples varied greatly.  The pH in the primary forest area was more basic than in the open area.  Since compounds of ammonia are alkaline and concentrations of these compounds was higher in the primary forest, there may be a relationship.  It appears that even during intense periods of rainfall, the intermittent dry spells maintain conditions to promote some evaporation.

 

Apparently elimination of large stands of tropical primary forest could reduce the rate of evapotranspiration.  This could have a dramatic influence on local weather and regional climate patterns influencing the tropics.  On a larger scale this possible change could contribute to global climate change.

 

Classroom Application

           

Appropriate Subjects:  General Science, Earth Science, Geography, Mathematics, Chemistry

Grade Levels:  5-12

 

This investigation requires an open field and canopy microclimate.  It also requires a minimum of 24hrs of data collection and extensive classroom time for testing.  The testing kits should be appropriate for the grade level taught.  Both LaMotte and Hach make appropriate kits which can be purchased through science supply companies.  Basic water test kits are adequate for this study.

 

Bibliography

 

Myers, Norman. The Primary Source. 1984. pp 280-281.

 

Parker, Geoffrey G. Structure and Microclimates of Forest Canopies. 1995. pp 121-123.

 

Sanford, Robert L. et. al. “ Climate, Geomorphology, and Aquatic Systems,” La Selva. 1994. pp 23-24.

 

Please click here to return to main page