Lab 6 - Global Patterns in Green Up and Green Down

100 points - This activity has been adapted from an activity developed through the GLOBE project at SRI International.

Directions

1. Read through this handout, then print out the four worksheets. 
2. Look carefully at the color visualizations of January and July vegetation vigor, and Global land cover. 
3. Cut out and assemble the flip books by cutting out each month; then, assemble the months in order 
4. Fill in the worksheets.
5. Answer the questions at the bottom of this sheet and turn in only the answers to these questions.

Key Concepts

• The Normalized Difference Vegetation Index (NDVI) measures vegetation vigor.  NDVI is calculated from satellite data, thus allowing global coverage.
• Vegetation growth and decline follows the seasonal cycle with an annual “green up” in the spring and summer and a “green down” (or senescence) in the fall and winter. The cycle of greenup and greendown occurs within ecosystems at all scales.  Large-scale regional land covers show characteristic patterns that identify them (e.g., rain forests nearly uniformly high, deserts nearly uniformly low, tundra has a rapid rise that lasts only a short time, etc.).

Figures

• Vegetation Vigor in January and July
• Difference between January and July & Earth vegetation
• Student worksheet (4 pages)

Background

Everyone experiences local changes in vegetation that occur in fall and spring.  This activity is designed to connect your local experiences of seasonal change in vegetation with global patterns of change.  These global patterns in vegetation growth and decline or “plant waves of green up and green down” follow the annual climate cycle.  As a result, what occurs in the Northern and Southern Hemispheres is roughly reversed.  Just as summer in the Northern Hemisphere occurs during winter in the Southern Hemisphere, so Green-Up occurs in the North while Green-Down occurs in the South. 

This activity focuses on visualizations of vegetation vigor that were collected by satellites.  We have global coverage because the data was collected by satellite. The measure of vegetation vigor is made in terms of a calculated quantity called Normalized Difference Vegetation Index (discussed in lecture). NDVI provides a measure of how much sunlight is being used by plants for photosynthesis.  These values are unitless, since NDVI is calculated as a ratio of absorbed light in the red wavelengths and reflected light in the near infrared wavelengths. While the measurement does not provide a precise measure of vegetation and is subject to several issues that compromise its consistency, it is very useful. 

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What to Do and How to Do It

 There are three visualizations used in this activity.  The initial two illustrate the seasonal extremes of vegetation vigor by showing vegetation vigor during January and July (Figure 1).  The other one helps to explain the observed seasonal change by showing the type of vegetation and land cover present.

Figure 1. Global vegetation vigor (NDVI) in January (top) and July (bottom).  Darker shades of green represent a higher vegetation vigor.

Seasonal Extremes in January and July 

• The visualizations of vegetation vigor are drawn using shades of green (dark values are higher).
• In the vegetation vigor visualization, darker shades of green are used to represent greater amounts of growing vegetation and lighter shades used for less actively growing vegetation.

Primary patterns shown in the January and July visualizations of vegetation vigor.

• In January most of the high values are found in the Southern Hemisphere particularly in Central America, South America, Africa, and South Asia. 
• Now look at the vegetation vigor in July.  In July, much of the mid-latitudes of the Northern Hemisphere show substantial vigor, especially in the eastern part of North America and in Europe and Asia.  Further, there remains substantial vegetation vigor in the Southern Hemisphere (in South America, Africa, and South Asia) a similar pattern, but in reduced amounts as in January.
• From the visualizations, it can be seen that July has much higher values overall because there is substantial vigor in both hemispheres.
• You can quantify how much higher July is than January using the global means. These means show approximately a fifty percent rise from 0.16 in January to 0.29 in July.
• But why is the Northern Hemisphere's summer so much more productive globally than the Southern Hemisphere's summer?  One important part of the answer is that the Northern Hemisphere experiences a sharper seasonal change than the Southern, since most of the Earth’s land is located in the North and land is more easily heated and cooled than are oceans.  However, another part of the answer lies in their distinctive landcovers. For example, the rain forests of the Southern Hemispheres stay green all year.

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Land Cover Visualization 

Many of your questions about why individual areas are high or low can be answered by looking at the land cover visualization below. For example, compare the rain forests of South America (the broadleaf evergreen class that is blue-green) and the Saharan desert in Africa (a desert and scrubland class that is tan).  The rain forests show high values (bright green) year round while the desert shows low values (pale yellow) year round. Overall, the difference can be explained by the type of ecosystem that occurs there. However, many ecosystems are not constant across the year but change greatly. 

Figure 2. Landcover Type

Check your Comprehension

Answers are at the bottom of this page.

• What does the color dark green indicate?

• What does the color light green indicate?

• What patterns do you notice between landcover as it is classified in Figure 2 and the regions that are dark green in both January and July (Figure 1)? 

• What about the regions that are light green in both January and July?

• Which hemisphere has a stronger vegetation vigor in January? Which hemisphere has a stronger vegetation vigor in July?

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Worksheets

1. Print out the four worksheets. You will need Adobe Acrobat to read the worksheets. Adobe Acrobat is a FREE software program. You can download a copy by going to the Adobe download website.

1. Worksheet #1
2. Worksheet #2
3. Worksheet #3
4. Worksheet #4

2. Cut out the pages of the flipbook and assemble it  in order. 

3. Use the vegetation vigor flipbook to look at change over time in specific areas of the Earth.  For example, focus on the Sahara desert (look it up in Goodes World Atlas if you are not sure where this is).  Does it have a high or low vegetation vigor value in January?  How about in February?  March?  If you were to plot the Sahara's vegetation vigor on a graph by month, what would the line look like?

4. Use the flipbook to help characterize the seasonal cycles for each land cover type on the worksheets.

5.The worksheet has questions for each land cover type.  These questions along with the definitions of the land cover types should help to classify the lines on the graph with the land cover type that produced it.

Questions to turn in

Answer in complete sentences for full credit.

1. (15 pts) What is land cover #1 (solid triangles)?  What evidence did you use to come to this conclusion?

2. (15 pts) What is land cover #2 (open squares)?  What evidence did you use to come to this conclusion?

3. (15 pts) What is land cover #3 (open circles)?  What evidence did you use to come to this conclusion?

4. (15 pts) What is land cover #4 (open triangles)?  What evidence did you use to come to this conclusion?

5. (15 pts) What is land cover #5 (solid circles)?  What evidence did you use to come to this conclusion?

6. (25 pts) In two or more paragraphs, discuss the patterns of green up and senescence around your home. What is the limiting factor in your local ecosystem? How do these patterns of green up relate to the seasons and the amount of available insolation? 

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Answers to Check your Comprehension

1. What does the color dark green indicate?
   
   Dark green indicates a high vegetation vigor.  In other words, the plants in this region are using the majority of the sunlight available.

2. What does the color light green indicate?
   
   Light green indicates a low vegetation vigor.  In other words, the plants in this region are not growing very much, indicative of senescence.

3. What patterns do you notice between landcover and the regions that are dark green in both January and July (Figure 1)?  What about the regions that are light green in both January and July?
   
   The dark green areas in both January and July correspond to 'Broadleaf evergreen' or tropical rainforest.  The light green areas are 'Shrubs, desert and bare ground'. This makes sense because evergreen leaves never fall off – in other words they grow all year.  The desert is very dry, which is not hospitable to growing plants. Note, however, that the deserts do green up slightly in January.

4. Which hemisphere has a stronger vegetation vigor in January? Which hemisphere has a stronger vegetation vigor in July?
   
   As with hemispheric seasonal patterns, vegetation patterns are reversed between the hemispheres. Further the much stronger seasonal change in the Northern Hemisphere results in a much more pronounced rise and fall to its vegetation.

See the Solution and Grading Rubric for this Lab Assignment.

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