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Lilacs (Syringa vulgaris) near old cabins in Arizona. Researchers with the USGS-led USA-National Phenology Network use climate change indicators called the Spring Indices to determine when spring arrives in different parts of the USA. From these indices, researchers can build scientifically backed models and maps based on nationwide field observations collected of when enough heat has accumulated to initiate leafing and blooming in lilacs and honeysuckles, two common and temperature-sensitive flowering plants.