Strategies to Cope with Unpredictable Water Resources

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A wealth of adaptations arose in desert organisms that allows them to utilize the pronounced spatiotemporal stochasticity of water availability typical to deserts. As detailed before, mobile organisms are able to use spatially patchy water sources that are not available to less-mobile organisms. These sessile organisms often have dormant dispersal units that can reach good microsites where they can establish, reproduce, and eventually send their own diaspores onto other favorable microsites. Such life cycles are typical for short-lived plants (annuals, ephemerals) and some invertebrates. These diaspores typically remain viable for long periods and can 'sit and wait' for years with sufficient precipitation. Most annual desert plants form such extensive seed banks. Seeds within such seed banks tend not to germinate equally and even after strong precipitation events, a fraction of the seed will remain dormant. Such fractional dormancy might serve as avoidance of sibling competition as it will reduce densities, but more importantly has been explained as a bet-hedging adaptation in order to cope with rainfall stochasticity. When no supplemental rainfall follows an initial germination triggered by a rainfall event, at least a fraction of the seeds will be available in the following years, thereby ensuring the long-term survival of the population.

Figure 12 Dry dead plant of the rose of Jericho (Anastatica hierochuntica - mustard family Brassicaceae). Seed pods of this annual plant are contained within curled branches forming a ball that opens when moistened and seeds are released only after rainfall events. Dead Sea region, Israel, March 1987. Photograph by C. Holzapfel.

Figure 12 Dry dead plant of the rose of Jericho (Anastatica hierochuntica - mustard family Brassicaceae). Seed pods of this annual plant are contained within curled branches forming a ball that opens when moistened and seeds are released only after rainfall events. Dead Sea region, Israel, March 1987. Photograph by C. Holzapfel.

In addition to dormancy, many desert plants develop some water-sensing adaptation (so called 'water clocks') that controls both dispersal and germination. Dry inflorescences of the famous rose of Jericho (Anastatica hierochuntica) and other annual plants (e.g., the New World Chorizanthe rigida) open up only after abundant rainfall and release only some of their seeds (Figure 12). Many desert plants have morphologically different seeds that differ in dispersal ability and have different germination requirements (amphicarpic plants). In general, a high proportion of desert plants suppress seed dispersal altogether (atelechory). This has been interpreted as an adaptation to remain on the mother site, as it has already been proved to be a favorable location.

Most perennial plants suppress flowering (aridopassive shrubs) or sprouting altogether (e.g., geophytes) in drought years. This is analogous to many desert animals that shift sexual maturity and mating to synchronize with favorable conditions. Similar to plants, sterility is typical for extreme drought years and dispersal and migration (nomadism) are triggered by precipitation regimes. There is some indication that insects and desert shrubs can shift their sex expression with changing rainfall regimes. Especially, monoecious shrubs, plants that have male and female reproductive units on the same individual, can shift their sex ratio with water availability. The male function requires fewer resources from the plant ('cheaper sex'), and is typically the predominant sex in dry years.

Many desert shrubs tend to break apart into separate shoot sections over time (axial disintegration). This so-called 'clonal splitting' is very common for desert shrubs worldwide and has been explained as a risk-spreading adaptation. In time of severe drought, instead of the death of the whole original individual, some segments of the original shrub may survive. The consequence of this growth strategy is often the formation of shrub rings that grow outward and have a dieback zone in the center. Age estimations have been made based on this growth form. Large creosote bush (Larrea tridentata) rings in the Mojave Desert, for instance, have been determined to be of an age exceeding 11 000 years (e.g., the famous 'King Clone' located by Vasek in 1980).

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