Adaptive strategies of African pteridophytes to extreme environments

Abstract

SynopsisWater deficiency is the key factor limiting the occurrence of pteridophytes in seasonally dry tropical areas and shaping their adaptive strategies in respect of habitat preferences, life-forms, phenological patterns, and reproductive biology. In Zambia, which is situated in the savanna woodland zone, a total of 146 pteridophyte species occur mainly in special habitats: extrazonal evergreen forest patches and initial successional stages of lithoseres and hydroseres. Life-forms with perennating buds well protected against desiccation (hemicryptophytes and geophytes) dominate, while those with more exposed buds (epiphytes, chamaephytes and phanerophytes) are much less numerous and restricted mainly to the higher-rainfall areas. Selaginella tenerrima represents the life form of a therophyte, unknown in any other pteridophyte genus. Three major seasonal patterns of growth and dormancy may be distinguished: the evergreen type (ca. 40% of species), the poikilohydrous type (ca. 20%) and the deciduous (‘summer-green’) type (ca. 40%). No less than 20% of species are able to survive recurrent bush-fires, and some of them possess the features of advanced pyrophytes. In the driest parts of the savanna zone in Africa, e.g. in the Lake Chad Basin of northeastern Nigeria, seasonal pools are remarkably rich in water ferns, especially of the genus Marsilea which has undergone an intensive adaptive radiation in this environment.