Flora - Morphology, Distribution, Functional Ecology of Plants
Dew as a key factor for the distribution pattern of the lichen species Teloschistes lacunosus in the Tabernas Desert (Spain)
Introduction
Lichens are poikilohydric and are able to survive in arid regions because they can tolerate desiccation and because of their ability to use not only rain, but also fog, dew or atmospheric water vapor as a source of water. Large biomasses of lichens have been reported in deserts and coastal arid regions. Rundel (1978) and Kappen (1988) reviewed the lichen vegetation in the coastal fog deserts of the world. Rich lichen communities in fog zones of the Atacama Desert (Follmann and Redon, 1972), in a Chilean fog oasis (Redon and Lange, 1983), in the coastal regions of Baja California (Nash et al., 1979; Rundel et al., 1972) and in the Namib Desert (Lange et al., 1990, Lange et al., 2006) were also reported. Kappen et al. (1980) demonstrated a relationship between lichen density, species richness, the exposure of the habitat and dewfall in the Negev Desert. Typically, lichens from these desert regions are able to hydrate during the night from fog, dew or water vapor uptake so that a peak of positive net photosynthesis is possible immediately after sunrise (Kappen, 1988; Lange and Tenhunen, 1982; Lange et al., 1970, Lange et al., 1975, Lange et al., 1990, Lange et al., 2006; Nash et al., 1979).
The southeast of Spain is the most arid region in Europe, and has a characteristic lichen vegetation that is particularly prevalent on the gypsum badlands of the Tabernas Desert (Almeria province) (Gutiérrez and Casares, 1994). In these fragile ecosystems lichens have an important role in stabilizing soil and protecting it from erosion (Canton et al., 2003) and one of the characteristic and plentiful macrolichens is Teloschistes lacunosus (Rupr.) Sav, a species also found in other Mediterranean arid zones. Its distribution pattern is not homogenous and, contrary to other characteristic lichen species in the same communities that are able to grow in any exposure, such as Diploschistes diacapsis (Ach.) Lumbsch (Pintado et al., 2004), T. lacunosus is mainly present on north-facing slopes and on pediments, and no thalli occur on south exposures. It is possible that these habitat preferences are related to the sources of water that can be used by this species and to the time that it can remain active. Laboratory experiments have demonstrated that, in contrast to other fruticose green algal lichens, T. lacunosus is hardly able to reactivate its metabolic activity by water vapor uptake (Scheidegger et al., 1995). Liquid water availability seems to be the key factor for the survival of this lichen species.
At present the microclimatic conditions in these habitats, as well as the productivity and growth rates of these important lichen species, remain almost unknown. The aim was, therefore, to investigate, under natural conditions, the roles of the different sources of water (rain, dew and water vapor) in determining both the activity and distribution of T. lacunosus. T. lacunosus thalli were studied at two neighboring sites that differed in aspect and lichen abundance: a pediment, where T. lacunosus is specially well developed, and an east-facing slope where only few small thalli are developed. Chlorophyll a fluorescence techniques were used to detect potential photosynthetic activity (i.e.: the thalli were wet), and activity was correlated with moisture status and microclimate conditions. Temperature, PPFD and relative humidity (RH) data were collected in both exposures, parallel to the fluorescence measurements, in order to detect the water sources available for the thalli previous to the periods in which metabolic activity was detected. Logging of the microclimate allowed periods of dew formation to be calculated for a complete year.
Section snippets
Material and site description
Teloschistes lacunosus (Rupr.) Sav. is a fruticose, cushion-shaped lichen. It has flat laciniae with coiled edges. Lecanorine apothecia can be absent in some thalli but in others numerous and distributed over the whole surface of the laciniae. The species is known to be xerophytic and heliophytic (Llimona, 1973). It is abundant on gypsum soils in Almeria and on the edges of small ponds of the inner part of the Iberian Peninsula (Crespo et al., 1980). It also inhabits the Negev Desert (Galun,
Results
The 12 days when intensive measurements were carried out can be subdivided into three types according to the form of moisture available to the lichen: Type 1: Only humid air available, transient or no dew formation (7 days); Type 2: Dew formation (4 days); Type 3: rainfall occurred (1 day). The activity of the lichens also varied according to these groups and location.
Type 1: This pattern occurred on all three-measurement days in November (25–27 November 1998), on 3 days in January (26–27,
Discussion
The aim of this research was first, to discover the relevance of form of moisture (vapor, dew or rain) in activating desiccated T. lacunosus thalli, and second, to see if the activity patterns found helped to explain the differences in abundance and distribution of the species in the Tabernas desert.
The field studies showed clearly that desiccated T. lacunosus, in its natural habitat, only became photosynthetically active after rehydration by liquid water (rain or dew). Thalli were not able to
Acknowledgements
The authors are especially indebted to Dr. T.G.A. Green for the review of the manuscript and his right suggestions, and to Dr. V. Souza and Dr. R. Lazaro for their support in the field. Special thanks are due to Dr. I. Schmitt for correcting the English. We thank the Ministerio de Ciencia y Tecnologia for a grant to R.D.P. Financial support was provided by a Grant number PB95-0067 from DGICYT.
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