13 October 2012
The fall colors in America are peaking over 10 days later now than 30 years ago. How can you determine such a subjective thing as when fall colors are peaking? Well, that’s just as interesting as the results!
Take a look at the graph below courtesy of Climate Central.
The NDVI is the Normalized Difference Vegetative Index and in this case it was used to graph the times when the leaves were changing colors the fastest over North America. This is a good proxy for peak colors and the results speak for themselves: Fall colors are peaking well over a week later than in 1980-1985. The USGS has a good explanation of the NDVI:
The Normalized Difference Vegetation Index (NDVI) has been in use for many years to measure and monitor plant growth (vigor), vegetation cover, and biomass production from multispectral satellite data.
NDVI = (Channel 2 – Channel 1) / (Channel 2 + Channel 1)
(Channel 1; 0.58-0.68 micrometers) and near infrared (Channel 2; 0.725-1.10 micrometers):The principle behind NDVI is that Channel 1 is in the red-light region of the electromagnetic spectrum where chlorophyll causes considerable absorption of incoming sunlight, whereas Channel 2 is in the near-infrared region of the spectrum where a plant’s spongy mesophyll leaf structure creates considerable reflectance (Tucker 1979, Jackson et al.1983, Tucker et al. 1991). As a result, vigorously growing healthy vegetation has low red-light reflectance and high near-infrared reflectance, and hence, high NDVI values. This relatively simply algorithm produces output values in the range of -1.0 to 1.0. Increasing positive NDVI values, shown in increasing shades of green on the images, indicate increasing amounts of green vegetation. NDVI values near zero and decreasing negative values indicate non-vegetated features such as barren surfaces (rock and soil) and water, snow, ice, and clouds.
The graph above is based on the image below made from the raw data in the paper in Global Change Biology: