The notion that Aspen leaves change colors every year seems simple, but it can become complicated when observing an individual tree and expecting to see it change within a time frame. Using a reflectance spectrometer we measured the amount of a leaf reflectance in an Aspen tree in Cheney Plaza, in the visible and infrared region of the spectrum.
We expected to see changes in leaf colors over a period of about a month, starting on September 19 and running through October 18, 2013. We used the measured red (645 nm) and infrared (735-810 nm) reflectance and calculated Normalized Difference Vegetation Index (NDVI). NDVI is computed by taking the near infrared value minus the visible wavelength value, then dividing this value by the near infrared plus the visible light value.
NDVI shows the difference in leaf reflectance in these regions of the spectrum. Healthy leaves have high NDVI values while stressed and dying leaves have low NDVI values. We expected to see a gradual decline in NDVI values which would indicate an increase in the amount of reflectance observed in the visible spectrum and a decrease in the infrared spectrum. Based on a trend line we fitted to the NDVI values we observed a drop from about 0.62 to about 0.5 which indicates a small change in reflectance (Figure 1).
|Figure 1. NDVI values for a young aspen tree in |
Chaney Plaza at the University of Wyoming in Laramie, Wyoming 2013.
We did not see major changes in this trees’ leaf color and the NDVI values we obtained shows a small decline. If we had continued this experiment for more than a few weeks or a month, then we would have noticed further decline in NDVI values. This study is a good example of how you can use remote sensing to study different aspects of phenology.