HOOPER, ELISABETH A.1* and GEORGE YATSKIEVYCH2. 1Truman State University, Kirksville, MO 63501; 2Missouri Botanical Garden, P.O. Box 299, St. Louis, MO 63166. - Distribution of arid climates and drought-adapted pteridophytes, an introduction.
Approximately one-third of the Earth's land surface is classified as
arid or semi-arid. The classic 'hot' deserts of the world (e.g.,
Sahara) occur between 20° and 30° latitudes N and S, but arid
conditions are found across the globe. Most arid areas experience high
daily temperatures as well as low and unpredictable precipitation
(<200 mm per year). Water lost by evapotranspiration often exceeds
that gained by precipitation, resulting in water deficits. Factors
contributing to aridity include rain shadows, distance from water,
proximity to cold oceanic currents, and localized soil conditions.
Even in wet tropical habitats, some plants (e.g., many epiphytes)
experience periodic drought conditions because of microclimatic
factors. Aridity presents special challenges to plants. As a group,
pteridophytes are rare in extreme desert environments, but many grow
in persistently and/or seasonally dry habitats. A very few of these
are drought-avoiding ephemerals that complete their life cycle during
wet periods. The majority, however, are drought-tolerating perennials.
Typical xeromorphic adaptations include dissected or coriaceous
leaves, dense indumentum, and/or cuticular deposits. In addition, many
ferns and fern allies endure drought periods through physiological
dormancy (e.g., the poikilohydrous resurrection plant, Selaginella
lepidophylla). It is important to note that selection pressures
act on gametophytes as well as sporophytes, but little is known about
gametophyte adaptations to drought. Taxonomic diversity of xerophytic
pteridophytes is concentrated in relatively few families: e.g.,
Pteridaceae and Selaginellaceae. However, xerically-adapted species
occur in most lineages. Of note are several epiphytes in the
Polypodiaceae as well as some typically aquatic species whose habitats
are intermittently dry (e.g., Isoetaceae and Marsileaceae). Hotspots
of pteridophyte diversity can be summarized for arid regions with
reasonably complete floristic data.
Key words: arid climates, deserts, drought adaptations, pteridophytes