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Martin-Luther-Universität Halle-Wittenberg (2011)

Population ecology of Stipa species in Mongolian drylands

Ronnenberg, Katrin

Titre : Population ecology of Stipa species in Mongolian drylands

Auteur : Ronnenberg, Katrin

Université de soutenance : Martin-Luther-Universität Halle-Wittenberg,

Grade : Doctor rerum naturalium (Dr. rer. nat.) 2011

The ecology of Stipa species (S. krylovii, S. gobica and S. glareosa), which are widespread and often dominant in the dry Central Asian steppes, was investigated in southern and central Mongolia. For comparison, some co-dominant taxa were also taken into account. The Mongolian steppes host numerous herds of domestic and wild herbivores and herewith provide the means of livelihood for the majority of nomadic Mongolians. Biomass productivity is known to be water limited in the dry steppes, but grazing leads to nutrient withdrawal and may thus indirectly constrain plant growth. In a fertilization and irrigation study, we found that nutrients accumulate in the soil. Biomass and number of inflorescences increased dramatically in all species groups after fertilization but irrigation showed little impact. The contents of nutrients in plant tissue were higher on fertilized plots. All studied species benefited from fertilization, although the magnitude of responses varied among species-groups. The effect of irrigation on biomass production was very limited. Nutrient depletion is an indirect effect of over-grazing, but can eventually lead to severe rangeland degradation. Effects on the community level do not necessary reflect effects on the population level and data on population ecology were extremely rare. Thus, we also tested the effect of fertilization and water-surplus on seed viability of S. krylovii. The number of viable seeds per area was vastly increased by fertilization. Seed viability was negatively correlated to precipitation along a large-scale gradient. Experimental irrigation increased the number of seeds per inflorescence but reduced the percentage of viable seeds. These results were entirely unexpected and contradict ecological theory, which predicts that reproduction in a dryland is mainly limited by moisture availability. In dry years, S. krylovii pollinates cleistogamous, however in wetter conditions partly chasmogamous. In a study on pollen characteristics, we found that pollen longevity is very short under ambient conditions. Thus, reduced pollen vigour may explain lower seed viability under chasmogamous pollination and cleistogamy, due to higher resilience to abiotic stress, increases pollination efficiency and herewith seed viability. Seed germination of the three Stipa species differed slightly. The earlier flowering species, S. gobica and S. gobica germinated to 100% without pre-treatment. Stipa krylovii’s germination rate was increased after cold-stratification, and even more so in the more mesic central Mongolian sample regions. Germination was, however, near zero until extraordinarily heavy rains triggered seedling emergence. The study showed the importance of in-situ studies and highlighted the disproportionably large effects of extreme rainfall events. To put results into perspective we also analysed five co-dominant species. All five species germinated best at warm temperatures and low osmotic pressure. The dicots Arenaria meyeri, Artemisia frigida and Art. santolinifolia showed almost instantaneous germination, whereas Allium polyrrhizum and Agropyron cristatum were more similar to the Stipa species. Our studies show the importance of preserving the Stipa steppes. Sexual reproduction is rarely possible, which would impede natural expansion into degraded areas or artificial restoration measures. Thus, soil nutrient depletion must be stopped to prevent further steppe degradation.

Mots clés  : Dryland ; Mongolia ; nutrient limitation ; primary productivity ; reproduction ; restoration ; seed viability ; steppe ; Stipa ; water limitation


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