I recently learned about the ForWarn Tool, a satellite-based forest disturbance monitoring system for the contiguous United States. This tool represents the combined efforts of USDA, NASA, USGS, and DOE and generates new potential disturbance maps every 8 days, even throughout the winter. This is a very user friendly tool and perfect for classroom use!
Using this tool you can observe things like: declining hemlock forests in Western NC, forest fires and impacts to greenness, vegetation recovery after a tornado, etc. Images can be downloaded and corresponding NDVI graphs are also available.
Here is one example of something the tool can be used for - leaf out in the spring. It was a cool spring, right? Especially compared to spring 2012. Using ForWarn, I can actually see (below) that, in the NC mountains, leaf out is less than expected - most likely due to the cooler spring temperatures we had in April and May. By comparing current greenness with the "normal" greenness that would be expected for healthy, undisturbed vegetation growing at this location during this time, ForWarn shows that in Western NC, the amount of greenness between April 15 and May 8th, 2013 is less than expected (as indicated by the red, orange, yellow colors).
To acquire this image, I selected the Theme: MODIS Cumulative NDVI Products from the top tool bar, then from the left tool bar I selected "Forest Disturbance Detection maps" and then I selected the current image under "Change from All Year Baseline" by clicking on the light bulb to make it yellow (turning layer on). To see the legend, export the image, or graph NDVI, scroll to the far right.