The fact that the eastern United States is green and supports extensive forests and vegetation is direct everyday observation and geographical knowledge that does not require citation.
The claim states that the eastern US remains green despite prevailing westerlies. The retrieved papers discuss broad climatic variability, drought, drylands, phenology, and wildfire, but none specifically address or test the geographic truism of eastern vegetation greenness relative to prevailing winds in a manner requiring empirical citation. The claim is a matter of common geographical observation.