Light and desiccation responses of some Hymenophyllaceae (filmy ferns) from Trinidad, Venezuela and New Zealand: poikilohydry in a light-limited but low evaporation ecological niche.

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Hymenophyllaceae (filmy ferns) are typically plants of shady, constantly moist habitats. They attain greatest species diversity and biomass in humid tropical montane forests and temperate hyperoceanic climates. This paper presents ecophysiological data bearing on their worldwide ecological niche space and its limits. Chlorophyll fluorescence was used to monitor recovery in desiccation experiments, and for measurements of 95 % saturating irradiance [photosynthetic photon flux density (PPFD(95 %))] of photosynthetic electron flow and other parameters, in the New Zealand Hymenophyllum sanguinolentum, and three species each of Hymenophyllum and Trichomanes from forests in Trinidad and Venezuela. Hymenophyllum sanguinolentum was comparable in desiccation tolerance and light responses with the European species. The more common species in the two tropical forests showed PPFD(95 %) >100 µmol m(-2) s(-1), and withstood moderate desiccation (-40 MPa) for several days. The four most shade-adapted species had PPFD(95 %) ≤51 µmol m(-2) s(-1), and were sensitive to even mild and brief desiccation (-22 MPa for 3 d). Light and desiccation responses of filmy ferns can be seen as an integrated package. At low light and windspeed in humid forests, net radiation and saturation deficit are low, and diffusion resistance high. Water loss is slow and can be supported by modest conduction from the sub-stratum. With higher irradiance, selection pressure for desiccation tolerance increases progressively. With low light and high humidity, the filmy fern pattern of adaptation is probably optimal, and the vascular plant leaf with mesophyll and stomata offers no advantage in light capture, water economy or CO(2) uptake. Trade-offs between light adaptation and desiccation tolerance, and between stem conduction and water absorption through the leaf surface, underlie adaptive radiation and niche differentiation of species within the family. Hymenophyllaceae are a rare example of an evolutionary shift of adaptive strategy from typical vascular plant adaptation to the poikilohydry most typical of bryophytes.