We develop new methods to study and monitor changes in the distribution of vegetation in North America. The Normalized Difference Vegetation Index (NDVI), which is a level 3 data product from channel 1 and 2 of NOAA/AVHRR Pathfinder, is used for the study. Frequency analysis for uniformly divided NDVI ranges is used to study interannual trends in vegetation distribution. The study is done for each region specified by vegetation type and some geological considerations, and covers the entire area of North America. The period of the study covers 1981 to 2000. Correlation coefficients are found for different vegetation greenness levels and Southern Oscillation Indices (SOIs). Correlations between vegetation distribution and climatic phenomena such as ENSO occurrences are suggested. Long-term increase in vegetation greenness on decadal time scales for some regions in North America is suggested. The work is of significance to interannual variability of natural vegetation and its connection to known physical phenomena such as ENSO and droughts.