Grass tussocks in arid Spain

The south-east of Spain is said to be the most arid part of Europe. Rainfall is low and summer temperatures may reach 50° Celsius for extended periods in some areas. In this region bryophytes (and lichens) exploit the protection offered by the larger grass tussocks, mostly Stipa tenacissima and Lygeum spartum. A group of Spanish researchers investigated the bryophyte and lichen communities growing with grass tussocks on gypsum-bearing soils in south-east Spain. These tussocks provide protection against direct sunlight and around their bases soil moisture will be held for longer since evaporation would be slower in the shade of the tussock. The area surrounding each tussock is not uniform and the research team distinguished three micro-habitat zones around each tussock. The following stylized diagram represents one tussock, with grey-green leaves and several flower spikes shown in grey and black.

Moving inwards the first zone is represented by the letter A. The outer boundary corresponds roughly to the greatest reach of any part of the grass tussock and the inner boundary is marked roughly by the reach of the tussock leaves. In this area the shade varies considerably both during the day and through the year. There is a low level of organic material with a carbon/nitrogen ratio in the range 4.3-5.1. The soil is alkaline with the pH in the range 8.0-8.2 and the soil's electrical conductivity was 2.5-2.6 mmohs/cm.

In zone B shade is permanent, though the leaf cover by the tussock is not complete. There is a higher organic soil content, with the carbon/nitrogen ratio in the range 12.4-13.5. The soil is alkaline, but less so than in A, with a pH of 7.8-7.9 and electrical conductivity around 2,6 mmohs/cm, much the same as that in zone A.

Zone C is the area around the base of the tussock. Here the tussock cover is complete and shade is permanent. The carbon/nitrogen ratio was measured at about 15 to 16, indicating only slightly mineralized organic matter. The soil was slightly less alkaline than in B, the pH now being in the range 7.4-7.6 and electrical conductivity was in the range 2.1-2.3 mmohs/cm.

The bryophyte and lichen community in A differs little from that of the fully exposed areas. One of the mosses found in the exposed areas was Aloina aloides, a very dry-adapted species, which was also found in zones A and B but never in zone C. The leafy liverwort Cephaloziella baumgartneri is a good example of a niche specialist, being found only in zone B. This species can neither survive in the more exposed zone A nor tolerate the higher levels of organic matter in zone C. The mosses Dicranella howei and Gymnostomum viridulum and the leafy liverwort Southbya nigrella often grow as communities along well-shaded soil banks. the Spanish team found these three associated under tussocks, in zone B. The team commented that it was very unusual to find them under Stipa or Lygeum tussocks and that this could only be explained by the creation of the appropriate micro-climate under such tussocks. The characteristic moss in zone C was Pleurochaete squarrosa. One noteworthy find was that a couple of foliose and large squamulose lichen species, rarely found in arid areas, grew in this innermost zone.

Reference

Martínez-Sáchez, JJ; Casares-Porcel, M; Guerra, J; Gutiérrez-Carretero, Ros, RM; Hernández-Bastida, J & Cano, MJ. (1994). A special habitat for bryophytes and lichens in the arid zones of Spain. Lindbergia, 19, 116-121. [The grass tussock diagram given above is an adaptation of Figure 2 in this paper.]