This approach leverages the USA-NPN's Spring Indices, models of spring onset developed using a long-term data set of early-season shrubs. Leaf out dates generally correspond to the earliest spring activity, while bloom corresponds to mid-spring activity. Here we use the Spring Index raster layers based on the PRISM dataset, which provides a 30 year period of record at a high spatial resolution (2.5 km; learn more). To determine the day of year of leaf out or bloom for each unit, an average of all the pixels that fall in the unit polygon is taken, weighted by the percentage of the pixel that falls within the polygon.
