Persistence in Sarcopoterium spinosum dwarf-shrub communities

Our study aimed at identifying mechanisms that permit the long-term persistence of Sarcopoterium spinosum, a dwarf-shrub that dominates large areas of hilly landscape in eastern Mediterranean countries. Data were obtained from 16 1x1 m quadrats distributed among well-established S. spinosum communities in Israel that had not been burned or otherwise disturbed for well over 20 years. We sampled three communities that represent different climatic and habitat conditions and analysed the growth rings in the root crowns of 604 uprooted plants, to determine the age structures of the sampled stands. Many older 'shrubs' that appeared to be individual plants were in fact clusters of ramets with their root crowns concentrated in a very small area. The morphology of these ramets indicates that they arose by rooting of peripheral stems. Seedlings and young plants were rare. Ramets seldom exceeded 17 years of age, although a few were 18-21 years old and one had 34 growth rings. The average ramet age on the different sites varied between 9 and 11 years. The ramets appeared to separate from the mother plant when they were older than 7 years, after which they had an estimated average half-life of ca. 2.8 years. Young shrubs grew rapidly in height, but their basal cover was low; it increased when they were much older, presumably as the older plants died. Consequently, three or four cohorts of older ramets between 12 and 17 years old often dominated stand cover. The continuing profuse production of seeds even where seedling recruitment is normally rare could be related to the relatively short average life-span of the ramets. We conclude that where resources are irregularly distributed and establishment sites are fully occupied by perennial species, localised phalanx-type clonal proliferation can reduce the risk of genet extinction. On favourable sites this strategy confers upon S. spinosum genets the tenacity required to hold off replacement by other species and to dominate large areas of the landscape for much longer than the average life span of individual ramets.