Rock outcrops as interglacial refugia for plants: ecological evidence from the forest-steppe transition of Patagonia

Abstract

In the forest-steppe transition of Patagonia from Argentina, the intense physical variability promotes the occurrence of contrasting vegetation units, among which is the saxicolous meadow developed on rock outcrops. In temperate and tropical regions, these geoforms of highly variable climate and topography, and poor soil development play an ecological and evolutionary role disproportionate to their reduced area. In Aguas Frías (38°46´S, 70°54´W), the range, diversity, density, and adaptation of vascular plant species growing in the saxicolous meadow were compared to those of the surrounding steppe, forest and hygrophilous meadow. We hypothesize that rock outcrops exhibit a compositionally and functionally divergent, cold-adapted flora of a previously more widespread distribution, allowing them to be categorised as refugia for species whose range retreated during the current interglacial. In comparison to the other units, the saxicolous meadow exhibited a small area, low plant cover, high total, native and rare species richness, high density and diversity of species, high richness of unique families and species, and an exclusive presence of (rock) ferns. It also presented a high richness of endemic species with more extreme latitudinal and altitudinal distribution, particularly > 46°S and > 3000m a.s.l. The similarity of species between the saxicolous meadow and the other units was low. Chamaephytes and nanophanerophytes were dominants, hemicryptophytes and geophytes were infrequent, and mesophanerophytes and therophytes were absent. Equivalent results in term of area of rock outcrops, and origin, strategy, composition, diversity and range of species were found in different regions. The biological and physical attributes would allow these outcrops to be characterised as reduced habitats of high diversity which could be acting as refugia for cold-adapted plants. Conservation actions must prevent and mitigate the effects of fire, biological invasion, and cattle grazing that represent the current threats to this valuable ecosystem.