A Polar Approach for Defining a Spatially-Explicit Phenological Year and Quantifying the Degree and Date of Seasonality

Thursday, December 15, 2016 - 11:05
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Polar analysis of the annual distribution of NDVI allows for temporally seamless determination of vegetation seasonality, irrespective of the arbitrary human calendar. Evergreen vegetation appears as a perfect circle in such a polar graph presentation. Circular statistics can be used to calculate the mean resultant vector of the annual NDVI distribution over a number of years. The magnitude of this mean NDVI resultant vector describes how far "off-center" the center of mass of the NDVI distribution is, and therefore characterizes the degree of seasonality of the actual mixture of existing vegetation within each MODIS cell. The angle of the mean NDVI resultant vector indicates the Day-of-Year of the center of mass of the entire multi-year NDVI distribution (regardless of its form), and the anti-vector of mean vector NDVI divides the annual cycle into a beginning and ending at the antipode of greenness center-of-mass, creating a unique, vegetationally defined "phenological year" in every MODIS cell. Once the strength and mean date of seasonality are quantitatively defined, national maps can be drawn showing the nature of dominant vegetation seasonality. The polar phenology analysis approach is general and transferrable, and can also be applied to phenologies other than vegetation. Then avian seasonalities, for example, are directly commensurate with vegetation seasonalities, and shifts in the degree of synchrony or asynchrony between the two can be quantified and mapped.

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