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Persistence in Sarcopoterium spinosum dwarf-shrub communities
Authors:Seligman  No'am  Henkin  Zalmen
Institution:(1) Qiryat Shemona, MIGAL – Galilee Technology Center, Rosh Pinna, 12100, Israel
Abstract:Our study aimed at identifying mechanisms that permit the long-termpersistence of Sarcopoterium spinosum, a dwarf-shrub thatdominates large areas of hilly landscape in eastern Mediterranean countries.Data were obtained from 16 1×1 m quadrats distributed amongwell-established S. spinosum communities in Israel thathadnot been burned or otherwise disturbed for well over 20 years. We sampled threecommunities that represent different climatic and habitat conditions andanalysed the growth rings in the root crowns of 604 uprooted plants, todetermine the age structures of the sampled stands. Many older'shrubsrsquo that appeared to be individual plantswere in fact clusters of ramets with their root crowns concentrated in a verysmall area. The morphology of these ramets indicates that they arose by rootingof peripheral stems. Seedlings and young plants were rare. Ramets seldomexceeded 17 years of age, although a few were 18–21 years old and one had34 growth rings. The average ramet age on the different sites varied between 9and 11 years. The ramets appeared to separate from the mother plant when theywere older than 7 years, after which they had an estimated average half-life ofca. 2.8 years. Young shrubs grew rapidly in height, but their basal cover waslow; it increased when they were much older, presumably as the older plantsdied. Consequently, three or four cohorts of older ramets between 12 and 17years old often dominated stand cover. The continuing profuse production ofseeds even where seedlingrecruitment is normally rare could be related to the relatively short averagelife-span of the ramets. We conclude that where resources are irregularlydistributed and establishment sites are fully occupied by perennial species,localised phalanx-type clonal proliferation can reduce the risk of genetextinction. On favourable sites this strategy confers upon S.spinosum genets the tenacity required to hold off replacement byother species and to dominate large areas of the landscape for much longer thanthe average life span of individual ramets.
Keywords:Clonal shrubs  Encroachment  Growth rings  Population biology  Rendzina  Tenacity  Terra rossa
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