Mycorrhizal Association in Dwarf Shrub Species Growing in Soft Cretaceous Rocks

Abstract

In the course of some ecological studies on dwarf shrub communities in the Judean hills of Israel it was noticed that areas in which strongly calcareous rendzina overlies soft chalk or marl strata are characterized by a highly specific vegetation. This forms a sparse cover of dwarf shrubs consisting of Thymus capitatus as dominant together with Cistus villosus, Fumana thymifolia, and-among others-Poterium spinosum, which tends to occur here in the more favourable habitats, while it is dominant on adjacent areas of terra rossa (Litav & Orshan 1963). Annual plants are extremely rare. The unique nature of the vegetation, which is especially marked where the soil cover is shallow (20 cm or less), reflects probably to a large degree the low level of nutrients in the soil, which consists of 50-80 Y of calcium carbonate. However, it should be emphasized that the underlying rock, which is friable, porous and varve-like, is often densely permeated with roots concentrated mainly between the adjoining surfaces of adjacent layers (Litav & Orshan 1963). Thus, individual plants may have a considerable portion of their root systems developing within the rock itself. Although soil and rock largely resemble each other in this type of habitat (exposed rock surfaces are sometimes ploughed and converted into arable land), deterioration of vegetation, as reflected both in composition and in percentage cover, is evident wherever soil cover is missing. It may now be enquired whether lack of nutrients in the rock is sufficient to account for its being an unfavourable medium for plant growth, or whether, perhaps, other factors such as mechanical impedance to root growth, insufficient aeration or effect of soil micro-organisms should be considered too. In the following experiment an attempt was made to find out to what extent the low nutrient content of the rock and mechanical impedance retard the development of plants of Thymus capitatus and Poterium spinosum. Blocks of soft rock were inserted into plastic containers having a diameter of 115 cm. The blocks were cut to be held firmly in place in the containers but not to fill up all the available space. A vertical crack, conforming to the natural splitting planes of the stone, was produced in the middle of each block by hitting it with a hammer and chisel. Other containers were filled with a light-grey rendzina (Litav & Orshan 1963) which overlies the soft rock, while a third batch was filled with crushed rock material. Thymus capitatus and Poterium spinosum were sown separately in early winter 1962 within the cracks of the stone blocks and in the other two substrates. At c. 20 days intervals one half of the containers received 60 ml of half-strength Hoagland solution containing 10 ppm of iron. The other half served as controls. Each treatment was replicated five times. Ordinary tap water was used for irrigation of those pots containing soil, and distilled water for the pots containing stones which were not provided with drainage holes. Thorough moistening of the stone blocks was achieved each time by letting water (or a nutrient solution) soak into them for 2 days. Following this the blocks were left dry for 2-5 days-depending upon the ambient temperature and humidity.