Results

After seed maturation, but before the first rains of the following growing season, the soil is dry and the soil crust (about 1 cm deep) has a pattern of cracks and splits of different widths and depths. At this time, seeds are collected from the different plots. There is a correlation between the location of seed collection of the different species in the seed bank and the different strategies of seed dispersal of the four most common species (Table 42.1). The tiny seeds that are dispersed by wind in summer escape into the cracks formed in the soil crust, mostly in the upper 1 cm layer of soil. In other plant species, seeds remain in lignified structures on the mother plant during summer and are dispersed during rain in winter. The seeds of many of these species are mucilaginous, and hence adhere to the wet soil surface immediately after their dispersal. These seeds may then germinate from this position.

Table 42.1. The seed bank distribution of four common plant species found in the Negev Desert.

Number of seeds

Seed bank

Carrichtera

Spergularia

Schismus

Neotorularia

location

annua

diandra

arabicus

torulosa

Aerial

3110

0

10

630

Crust (0-1 cm)

123

896

515

177

Soil (1-2 cm)

10

99

152

147

Total

3243 (2375)

995 (730)

677 (496)

954 (699)

The numbers in brackets after the totals are the estimated number of seeds per square metre.

The numbers in brackets after the totals are the estimated number of seeds per square metre.

S. diandra has an 'escape' strategy of seed dispersal. The great majority of its tiny seeds were found in the upper layer of the soil crust (i.e. in the upper 0-1 cm) and a relatively small amount reached the 1 cm layer below this. None of the seeds remained in the dry capsules of the dead plant. The opposite was found in C. annua, which has a protection strategy of seed dispersal. All the seeds of the last season remained enclosed in the dry lignified siliquae (i.e. fruit) on the dead plants. Relatively small numbers of seeds were found in the upper soil layer, and even less were found in the lower soil layer. Most of the seeds in the lower layer of the soil were the seeds that remained from previous years. The seeds of this plant are dispersed by rain and adhere to the soil surface by the aid of their mucilaginous seed coats or the seeds may be washed into cracks in the soil. In N. torulosa there is a dual strategy of seed dispersal: (i) seeds from the unlignified upper fruit disperse their seeds by wind during the summer after maturation and escape into cracks in the soil; and (ii) seeds that mature in the lower lignified fruit remain enclosed and protected during summer and are dispersed by rain in the following winter.The majority of the seeds are located in the aerial seed bank, enclosed in the lignified fruit, while a higher proportion of seeds with escape strategy are located in the upper 1 cm layer of the soil with the rest in the lower level (Table 42.1). In these soils, it was found that the number of seeds below the 2 cm level is negligible. When samples of seeds from the soil and aerial seed bank collected on 3 August 2004 were germinated, it was found that 60% of C. annua seeds from the aerial seed bank germinated, but none of the seeds that were separated from the upper (0-1 cm) or the lower (1-2 cm) layer of the soil germinated (Fig. 42.2). In contrast, in S. diandra, ~70% of the seeds from both soil layers germinated. In S. arabicus, only a few seeds germinated. In N. torulosa, a small percentage of seeds germinated from the seed samples were separated from the aerial seed bank as well as from the upper layer of the soil. No germination was observed in the seed samples separated from the lower layer of the soil.

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