Exercise 4: Vegetation Vital Signs
Each section below describes five different ways to map certain land surface measurements using satellite remote sensing data. Look carefully at all of the images and read their explanations. Each image is a map of land surface “vital sign” produced using data from NASA’s Terra satellite. After you are familiar with these maps and what they mean, you can use the ICE tool to analyze them (details at the bottom of this page).
Layer Upon Layer of Leaves: Leaf Area Index
One of the key “vital signs” of vegetation is the total green leaf area for a given ground area. Knowing the total leaf area in a plant canopy helps scientists determine how much water will be stored and released by an ecosystem, how many dead leaves (leaf litter) might be on the ground, and how much photosynthesis the vegetation is capable of. It also helps scientists understand the flow of energy among the various layers of vegetation, the atmosphere, and the ground; that flow of energy influences Earth’s climate climate.
In these satellite images, values of leaf area range from 0 to 5 square meters of leaf area per square meter of land surface, in shades of tan to light green to dark green. How can there be five square meters of leaf area in a one square meter area? Because the vegetation is in layers. A leaf area index value of five means that there are five equivalent leaf layers in that area. A value less than one means that if you took all the leaves and laid them flat on the ground, there wouldn’t be enough to cover a square meter.
Questions to consider:
- Which has a greater leaf area, a rainforest or a grassland?
- What can leaf area tell us about the vegetation in an ecosystem that NDVI cannot?
- How is leaf area related to NDVI?
Driving the Photosynthetic Reaction: Absorbed Sunlight
To make food through photosynthesis, vegetation’s major needs are water, carbon dioxide, and sunlight. Not all the wavelengths of sunlight are useful to plants for photosynthesis. And depending on environmental factors such as water availability, vegetation may not be able to absorb all of the useful sunlight. The useful wavelengths of light are called “photosynthetically active radiation,” and scientists have to know how much of that radiation plants in different locations on Earth are absorbing before they can say how much photosynthesis is going on there.
Satellite images show absorbed sunlight in values ranging from 0 to 1, in shades of green to yellow to red. A value of 0 means no photosythetically useful light is being absorbed, while a value of 1 means vegetation is absorbing all the photosythetically active light available. In addition to driving the chemical reactions that occur inside plants during photosynthesis, absorbed radiation influences Earth’s total energy budget (incoming minus outgoing radiation) and climate.
Question to consider:
- Rates of photosynthesis in vegetation can be slowed down by lack of water, cold temperatures, and lack of sunlight. Which of these factors do you think most influences vegetation in the rainforests of Panama? Why do you think so?
Taking the Tropics’ Temperature, Day and Night
Anyone who has stepped into a deep forest or walked across a black top parking lot on a hot summer day knows that the presence or absence of vegetation can have a big impact on temperature. Temperature can influence vegetation, too, for example, by increasing vegetation’s water needs when temperature rises and decreasing it when temperature drops. Cold winter temperatures in the Northern Hemisphere are one reason why vegetation doesn’t grow there all year round.
Unlike conventional observations of surface temperature that are actually measurements of air temperature collected by thermometers 2 meters (6.6 feet) above the ground, these satellite observations are the actual thermal radiation emitted from the surface itself, whether that surface is bare ground, lakes, treetops, or rooftops. Land surface temperature influences the rate of evaporation and photosynthesis.
Land surface temperatures can also reveal where the natural vegetation has been modified by humans. Areas of natural vegetation tend to show a bigger difference between day time and nighttime temperatures than urban areas do. This smoothing out of day-night temperature differences is a “spectral signature” of urban areas.
Questions to consider:
- Do you see any relationship between either day or night time temperature and the net photosynthesis (productivity)?
- Do you see any relationship between land surface temperature and land cover type?
- Why would urban areas not cool off as much at night as an area with natural vegetation? (*Hint: Imagine walking barefoot across a blacktop asphalt parking lot on a hot summer day.)
Taking it All In: An Ecosystem’s Net Productivity
Photosynthesis is the process by which green plants use light, water, and carbon dioxide to make plant structures (carbohydrates). All the previously discussed measurements—including greenness (NDVI), leaf area, absorbed sunlight, and land surface temperature all play a part in estimating photosynthesis from satellite data. The greener, thicker, and healthier an area’s vegetation appears on a satellite image, the more photosynthesis takes place there. The more photosynthesis that takes place, the more productive the ecosystem is.
Net productivity is the total amount of carbon that winds up in leaves, roots, stems and trunks of vegetation. Knowing the productivity for each vegetation type or ecosystem makes computer models of Earth’s climate and carbon cycle much more accurate. With increasing concerns that human-produced carbon dioxide is heating up Earth, knowing where vegetation might be helping store atmospheric carbon is very important. Knowing how much carbon is being drawn down into vegetation also provides a good estimate of crop and timber yields.
Questions to consider:
- Compare Panama’s plant productivity for different times of the year. Do you see any differences? When do you observe the greatest productivity? When do you observe the least productivity?
- Why do you think plant productivity in Panama changes over the course of the year?
Putting Plants in Their Place: Mapping Land Cover Types
By measuring the way the surface reflects, absorbs, and emits radiant energy in visible and infrared wavelengths, scientists can produce the spectral signatures for Earth’s varied landscapes. Knowing a region’s spectral signature allows scientists to construct maps of the type of vegetation growing there. Such land cover maps help scientists and policy makers monitor Earth’s environment and better manage our world’s natural resources.
This image shows the various land cover types across Panama in different colors. Each land cover type responds differently to changes in climate, both on a seasonal scale and over a span of decades.
Questions to consider:
- What land cover types do you observe across Panama? Which is the most widespread? Which is the least widespread?
- Compare Panama’s leaf area index map to the land cover map. Which land cover types show the greatest leaf area? Which show the least leaf area?
- Compare Panama’s plant productivity map to the land cover map. Which land cover types show the greatest productivity? Which show the least productivity?
Now pass your cursor over the labels below in the righthand margin. Notice how the scene displays each of the vegetation vital signs discussed above. This interactive image allows for easy comparison of each.
Now, using the Image Composite Editor, you can compare any of the land surface measurements during different times of the year. Using the Transect Tool, produce graphs of each of the measurements during the “wet” season and the “dry” season. Make a note of the difference that you observe for each of these parameters for the two times of year for each of the various land cover types in Panama.
- Use the pop-down menus above to select the measurements you wish to compare. A new window will launch.
- In the ICE window, click the thumbnail that you wish to display in the large window.
- Click the "Probe" button and pass your cursor over the large window to get the unit values for each pixel.
- To get an analysis of how values change from one part of an image to another, click “Plot transect” and draw a line across an interesting part of the image. You’ll get a graph that plots the unit values of each pixel on the line segment you draw, for both of the scenes. The starting point of the line you draw will appear at the left end of the graph, the end point will be at right.
- To learn more about what you can do with ICE, read the User’s Guide.