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A plant's Heliophily or Heliophyte Index emphasizes the degree to which a species has fidelity to or requires light for its growth, reproduction, and success. Thus the Heliophyte Index emphasizes the species biology of the rated plants – their physiological need for medium to strong light to physiologically grow and reproduce, or conversely their physiological inability to succeed in strong shade. While not a direct measure of fidelity to grassland community types, it addresses fundamentally the ecological requirements of the species, and indirectly assesses their association with Grassland habitats in the 25 state Southeastern United States region.

The scale is 1-9, so that all ratings are single digit. Essentially, a 9 rating means the species is a "sun obligate", and a rating of 1 means the species is a "shade obligate". By obligate, we mean that the species requires those conditions and will not survive, or at least successfully reproduce, under contrary conditions. A pine savanna or prairie grass with a 9 rating may be able to persist in the dense shade of a pine plantation for some years, but will not be able to flower and fruit, and will eventually simply deplete its resources and die. A fern or cove forest forb with a 1 rating may persist after the clear-cutting of the canopy, but will be damaged by unfiltered sunlight, or at least be competitively eliminated by more aggressive plants phsiologically adapted to higher light environments. A 5 rating would be assigned a species that has a broad ability to grow and reproduce in both sunny and shady environments.

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