Reproduction In Hot Deserts

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4.7.1 Diversity of plant form in drought-prone habitats

The flowering plants of warm deserts include every variety of life-form (Figs. 4.21, 2.22). Desert floras contain trees and shrubs, as well as annual and perennial herbaceous species. Although none of these groups is excluded as a result of drought not all are visible at any one time (see also Section 3.6.2). The relative contribution of each life-form to desert communities differs depending on the particular conditions of each desert in relation to rainfall, seasonality, and temperature extremes. The flowering of perennial herbaceous species, succulents, cacti and trees is less affected by variation in the rainfall than annuals. Above all, it is the annual species that provide the 'desert in bloom' events that attract public attention due to their sporadic nature and the visual transformation of the landscape

Fig. 4.19 Cottonweed (Otanthus maritimus), a common species on the Atlantic shores of south and western Europe but now almost extinct in the British Isles. This species might become more common with climatic warming. (Photograph taken in south-west Spain.)

Fig. 4.19 Cottonweed (Otanthus maritimus), a common species on the Atlantic shores of south and western Europe but now almost extinct in the British Isles. This species might become more common with climatic warming. (Photograph taken in south-west Spain.)

Fig. 4.20 Hillside infestation of bracken (Pteridium aquilinum) in the English Lake District. The upper limit to the spread of bracken is the frost line and the lower limit the grazing efficiency of the upland farms. The former is likely to rise and the latter to deteriorate, which will result in an increase in the area of ground covered by bracken.

Fig. 4.20 Hillside infestation of bracken (Pteridium aquilinum) in the English Lake District. The upper limit to the spread of bracken is the frost line and the lower limit the grazing efficiency of the upland farms. The former is likely to rise and the latter to deteriorate, which will result in an increase in the area of ground covered by bracken.

that takes place with the mass flowering of a wide range of species.

Desert annuals are highly dependent on autumn and winter rains. Consequently, they are by far the most variable group both in terms of their presence or absence in the landscape and for their floral display. In many deserts a massive flowering of the winter annuals in early spring is a rare event and may take place only every three or four years.

In the more climatically variable deserts, massive flowering events can be as much as 20 years apart. The occurrence of peak flowering episodes depends on a number of environmental variables and cannot always be predicted with any certainty. The spring-flowering annuals are dependent on a substantial amount of rain in the previous autumn for germination. The timing of this autumn rain and its intensity, as well as temperature and soil type, all interact in various ways with different species. Usually, heavy rain in autumn followed by regular periods of rain throughout the winter will lead to massive spring-flowering provided there are no unexpected or prolonged cold periods.

Many desert species require a period of 'after ripening' for the successful germination of their seeds. Such seeds will not germinate even when there is sufficient moisture in the soil. Thus, in winter annuals, when the seeds are shed in early or mid summer there is no germination until the seeds have dried further. In the autumn when soil moisture rises again germination takes place rapidly. In hot deserts only 10% of species have non-dormant seeds compared with over 60% in tropical rainforests (Baskin & Baskin, 1998).

It is one of the marvels of nature that a minute dormant seed, sometimes only a few millimetres in length, is able to sense and respond to a wide range and time sequence of environmental signals and thus optimize its germination responses for maximum survival. In many desert species the seeds are able to sense the difference between sporadic rainfall and precipitation of sufficient intensity and duration to provide sufficient reserves of water for the successful completion of their life cycle. This is achieved in a variety of ways. Post-maturation dormancy mechanisms may respond to the quantity of rain, relative humidity, temperature, light, and soil salinity through germination inhibitors which influence the percentage of seeds that are ready to germinate. In some cases it is simply a certain minimum rainfall that is necessary to remove soluble germination inhibitors. In other cases a period of high relative humidity is needed before the micropyle pore will open sufficiently to allow the ingress of liquid water to initiate germination.

Fig. 4.21 Sagebrush steppe in Nevada in spring. Among the tussocks of the dominant sagebrush (Artemisia spp.) can be seen (inset) the scarlet flowers of the Indian paintbrush (Castilleja linariaefolia).

In certain leguminous species, e.g. the cutleaf medick (Medicago laciniata), water enters the seed through the hilar fissure which is coated with a water repellant. The degree of opening is hygroscopically controlled. Initially it is only wide enough for the entry of water vapour, as free water is excluded by the water-repellant coating. After a set time at a critical water-vapour pressure the hilar fissure opens wide enough for the entry of free water, which then initiates rapid germination. Thus, it is not merely the state of moisture in the soil that triggers germination, but rather the relative humidity of the soil atmosphere and the length of time that it remains saturated that allows seeds to estimate not only the intensity of precipitation but also its duration (Baskin & Baskin, 1998).

Temperature fluctuations, as well as the presence or absence of light, also provide the seed with information as to the depth to which they are buried. Once these conditions have been met, and they appear to act in a similar way on a wide variety of annual species, then rapid germination, growth and flowering then follow, with subsequent renewal of the long-lived soil seed bank (Koller, 1969). A variety of other means, some involving complex structural adaptations, occur in other leguminous species in hot drought-prone habitats. One example is the central and southern American wild tamarind tree (Leucaena leucocephala), an invasive thornless tree forming dense thickets; the seeds have specialized cells in the region of the hilum and micropyle which under favourable conditions for germination allow water to enter through a thin palisade layer (Serratovalenti et al, 1995).

4.7.2 Desert seed survival strategies

The desert survival strategies that are most frequently apparent are the various forms of diversification that serve to obviate the dual dangers of predation by herbivores and drought injury. Predation losses and desiccation injury are usually high risk factors in deserts, especially if a high proportion of the seed production germinates at one time. The more extreme the desert, the more unpredictable the minimal and annual precipitation, and the more important become these various adaptations and survival strategies. One prevalent adaptation is delayed germination in a proportion of the seed produced by any one plant. With such adaptations the history of each seed can differ sufficiently, through development and maturation and eventual seed wetting, so that there is a subsequent effect on the phenotypic plasticity of seed germination. In this way the seeds from any one plant avoid the risk of all germinating at one time. This phenomenon can be described as somatic polymorphism where the time of the germination of seeds is governed by a maternal influence which varies with the position of the seeds in the plant, inflorescence, or capsule (Gutterman, 2000). In the Fabaceae it has been shown that when seeds are observed separately in relation to their position in the pod (numbering the seeds from the base of the pod) then those in position one (nearest to the point of attachment to the parent plant) will germinate after a shorter period of weathering than those further from the point of attachment to the parent plant. In extreme cases, this somatic polymorphism can determine the probability of germination, even 30 years after seed maturation (Gutterman, 2002).

In common with other marginal areas the maintenance of variation in desert plants appears to be a common strategy for long-term survival. In the lesser sand spurrey (Spergularia diandra), a plant native to the Mediterranean region of Europe and Africa (and now a widespread weed), the seeds that develop on the first flowers are black and are usually heavier and germinate more promptly than brown seeds which are produced later on lateral branches. In addition, in some of the S. diandra populations there are three distinct genotypes, producing seeds with hairs, glabrous seeds and partially hairy seeds. Thus in one population there can be nine types of seeds differing in colour, size, dispersal and germinability. This strategy of variation within populations is also much evident in other areas where reproduction has to contend with short and variable growing seasons, as is discussed below in relation to arctic and alpine habitats.

Rodents and other seed-eating animals are plentiful in deserts and many species of desert plants do not rely on buried seeds but instead retain their seeds in an aerial seed bank where the accessibility of seeds to rodents may be reduced. This is especially the case with lignified plants and inflorescences (serotiny) which adds a further protection against herbivory by small rodents. In these species the seeds can be retained for many years and constitute an aerial seed bank through their retention in serotinous cones or pods. The seeds have a delayed dehiscence with portions of the aerial seed bank being released when wetted by rain. In investigations of their subsequent germination (Gutterman, 2000) it was found that in many cases the degree of dormancy in the seed also depends on their position in the dispersal unit, which is yet another example of somatic polymorphism.

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