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Lack of tree regeneration and persistency of species-poor shrublands represent a growing problem across Mediterranean evergreen oak forests. What constrains forest regeneration is poorly understood, and restoration attempts have been largely unsuccessful. We assessed the contribution of four different mechanisms of tree recruitment limitation (that is, source, dispersal, germination, and establishment) in a cork oak (Quercus suber) system in southern Portugal. Using a combination of field studies and experiments, we quantified seed production, seed removal and dispersal, seed survival and germination, seedling establishment and survival, as well as cork oak natural regeneration for the three dominant vegetation types in this system (Cistus ladanifer shrubland, oak forest, and oak savanna). We found that all four forms of cork oak recruitment limitation were significantly more severe in shrublands than in oak forests and savannas, so that oak seedling recruitment in shrubland was impeded in multiple ways. Our results explain why transitions from shrublands to oak savannas and forests are extremely difficult, and that the release from arrested succession in this system requires the simultaneous relief of multiple constraints on recruitment limitation in the early life history of oaks. These results have important implications for the restoration and conservation of Mediterranean oak systems.  相似文献
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Identifying the mechanisms and interactions that influence the spatial structure of vegetation is important for both scientific and practical purposes. Grazing is one of the most fundamental interactions in ecology but so far its effect on vegetation spatial pattern received little attention. In this study we propose a conceptual model that can be used to predict the effect of grazing on shrub spatial pattern in water-limited ecosystems where shrubs grow within a matrix of annual vegetation. According to the model, grazing may increase or decrease clumping in shrub distribution, depending on (1) the relative palatability of shrubs vs. annual plants to the herbivores, and (2) the manner (negative or positive) by which adult shrubs and annual plants affect the establishment of shrub seedlings. We tested our model in a Mediterranean scrub ecosystem by analyzing the development of shrub spatial pattern over a period of 40 years in plots characterized by contrasting intensities of cattle grazing. As predicted by the model, all plots showed a clumped pattern of shrub distribution in the absence of cattle grazing while intense cattle grazing reduced the clumpiness of the vegetation and generated a more random pattern of shrub distribution. Interestingly, plots representing the two grazing regimes did not differ significantly in their shrub cover, suggesting that shrub spatial pattern may be more sensitive to grazing than overall shrub cover.  相似文献
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Woody plant encroachment into semiarid ecosystems has become a global trend in recent decades. Due to stream channel incision, the semiarid riparian montane meadows of the southern Sierra Nevada Mountains, USA are experiencing long-term declines in soil moisture. A woody shrub, Artemisia rothrockii A. Gray (Rothrock sagebrush, Asteraceae) is invading these herbaceous meadows. We used an analysis of the stable oxygen isotope ratios of plant and soil water to measure the depth of plant water acquisition during the early stages of this woody plant encroachment. Sagebrush used deeper water on average than most herbs, but it also acquired 10–30% of its water from shallow (<30 cm) soil. Most of the young sagebrush seedlings (1–3 years old, <15 cm) that we sampled used deep water like the older shrubs. Many, but not all of the herb species we sampled were also able to acquire deep water. These findings are consistent with a scenario of shrub encroachment in which channel incision causes shallow-water-dependent herbs to die back, allowing shrub seedlings to establish in disturbed areas during wet years. At least during the early stages of the invasion, some herbs appear to coexist with sagebrush by using deep root systems to cope with the declining shallow soil moisture.  相似文献
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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'shrubs 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.  相似文献
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