The geographic distribution of pubescence in the sea daisy, Borrichia aborescens, on Bahamian Islands

Borrichia aborescens (L.) DC. is a salt-tolerant perennial herb that is common on small islands in the central Bahamas. Two morphotypes are present: one with densely pubescent leaves and one with glabrous leaves. I conducted surveys in three archipelagos to document the geographical distribution of pubescence in this species and to infer the underlying mechanisms. B. aborescens was also grown from seed in a greenhouse. The pubescent form of B. aborescens was relatively more abundant on small islands than on nearby large ‘mainland’ islands. In two of the three archipelagoes, pubescence increased with distance on small islands. The pubescent form was relatively more abundant on small islands exposed to the open ocean compared to small islands that were protected by mainland islands or reefs on all sides. On a large mainland island, the pubescent form decreased in relative abundance inland from the coast. B. aborescens cultivated in a greenhouse revealed the effect of a genetic factor on the expression of pubescence. The observed patterns of variation are consistent with a physiological explanation for the adaptive benefit of pubescence. Trichomes may prevent physical blockage of the stomata by accumulation of salt in areas near breaking waves and salt spray.