Are rare species less shade tolerant than common species in fire-prone environments? A test with seven Amorpha (Fabaceae) species

Increases in tree density resulting from fire suppression have contributed considerably to the loss of savanna and grassland habitats in North America. Inability to tolerate shade is likely an important cause of species loss in areas that have not maintained historical burning regimes. We conducted an experiment to test whether differences in shade tolerance can explain rarity within the genus Amorpha. Four common and three rare species of Amorpha were grown in a greenhouse for 100 days in direct sun or under 90% shade. Overall, shading significantly reduced growth and survival and affected allocation among species, but these differences did not differ consistently between common and rare species. Ability to tolerate shade was best explained by phenotypic plasticity, with greater shade survival being exhibited by species with the largest changes in leaf area ratio. Furthermore, this study demonstrated that all three of the rare species can be readily cultivated under greenhouse conditions.