Evolution, Diversity, and Habitats of Poikilohydrous Vascular Plants

Abstract

Water stress is a common environmental constraint in terrestrial ecosystems. Among the various strategies to cope with temporal lack of water, poikilohydry is an important adaptive trait under specific habitat conditions. Desiccation tolerant vascular plants are particularly common among ferns, fern allies, and angiosperms that colonize forest canopies and mainly tropical rock outcrops (e.g., granitic/gneissic inselbergs, ferricretes) where environmental conditions (e.g., high temperatures, lack of water, and soil) are harsh.

Within vascular plants poikilohydry has evolved more than a dozen times independently. Within angiosperms desiccation tolerance is absent in the basalmost angiosperm clades and has evolved in rather advanced lineages. Most species rich are ferns and fern allies (e.g., Hymenophyllaceae, Polypodiaceae, Selaginellaceae) with angiosperms (monocotyledons: Boryaceae, Cyperaceae, Poaceae, Velloziaceae; dicotyledons: e.g., Gesneriaceae, Linderniaceae, Myrothamnaceae) playing only a minor role. It can be speculated that a number of Bromeliaceae could be poikilohydrous too. The total number of desiccation tolerant vascular plant species could reach c. 1,300 (c. 1,000 ferns/fern allies, c. 300 angiosperms).

In particular, on tropical rock outcrops, desiccation-tolerant vascular plants may become dominant. Particularly characteristic are mat-forming monocotyledons that cover steep rocky slopes whereas other desiccation tolerant vascular plants occur in crevices, shallow depressions, and temporally water-filled rock pools. Prominent mat-formers on inselbergs in South America, Africa, and Madagascar are treelet-like Cyperaceae and Velloziaceae that can attain an age of several hundred years. Their stems mainly consist of adventitious roots that possess a velamen radicum that might contribute to the rapid uptake of water.

Desiccation tolerant vascular plants are most richly represented in constantly wet to seasonally dry (length of the dry season 3–8 months) tropical regions with their species numbers decreasing under desert-like climatic conditions. Centres of diversity for resurrection plants are certain tropical regions with East Africa, Madagascar, and southeastern Brazil being most speciose. In the temperate zone parts of Australia and North America are likewise rich in poikilohydric species.

Increasing human pressure (e.g., fire, quarrying) on the natural habitats of poikilohydric vascular plants is a serious threat to their long-term survival and measures for their ex situ conservation should be taken into account.