The articles in this work describe and discuss the results of a series of microcosmexperiments that tried to untangle the mechanical, physical and chemical impacts ofdifferent soil covers, i. e. litter and moss, on seedling emergence under differentenvironmental conditions.The results presented in chapter 2 reveal positive effects of dead plant remains(i. e. litter) on seedling emergence and establishment. These positive interactionsbetween living and dead plants do not only act in xeric or otherwise extreme ecosystemsbut also under mesic site conditions. Still, also in the latter the direction and size of littereffects on seedling emergence is regulated by soil humidity. In addition, the thresholdvalue of litter amount, above which litter effects turn negative, is species- and family-specificand seems to be related to seed size.Chapter 3 shows that in dry grassland secondary chemical compounds of thedominant grass species Stipa pulcherrima affect seedling emergence of co-occurringspecies at various stages. Litter leachate delayed seed germination and also reducedradicle elongation. In dry grassland, with a rather narrow temporal gap for successfulseedling emergence, the delay of germination and reduction of radicle elongation willjeopardize species regeneration. These negative chemical effects in concert with bioticand abiotic interactions may enhance and maintain species loss.Litter shows also ecosystem-specific effects (chapter 4). The experimental resultspresented here suggest that species from woodland habitats are more negatively affectedby litter than species from grassland. Across species, woodland plants showedsignificantly lower seedling emergence from beneath grass litter than from beneath thelitter of their original habitat. In contrast, grassland species emerged equally well frombeneath both litter types. Thus, the response of species towards the presence of litter isecosystem-specific. These results highlight the potential influence of ground cover onspecies composition of grassland and woodland habitats through a complex interplaybetween ecosystem specific litter types, amounts of litter, environmental conditions andspecies adaptation.Chapter 5 confirms the dependence of ground cover effects (grass litter vs. moss) onspecies-specific traits such as seed size. In this experiment the effect of relative seedposition (on top vs. beneath) was explored, too. It seems that large-seeded species have aselective advantage at moderate amounts of grass litter cover, whereas some small seededspecies may adapt to these litter amounts through delayed germination. If litteramounts increase, diversity decreases because there will be selection against small-seededspecies, commonly not able to persist under such conditions. The results alsosuggest that - irrespective of cover type - large-seeded species have an advantage oversmall-seeded ones when seeds are situated beneath a cover. In contrast, when seeds areshed on top of a cover, the advantage of large seed size remains only on grass litter butnot on mosses. Thus, bryophytes may decrease plant diversity irrespectively of the plant sseed size.The influence of soil invertebrates, e. g. earthworms, on after-death plant‐plantinteractions is highlighted in chapter 6. These results indicate that the impact of litterlayers on seedling emergence cannot be determined solely by the assessment of theirchemical, physical and mechanical effects. This study shows that natural disturbance, e. g.through animals, have the potential to even change the direction of the seed positioneffect on seedling emergence. While in general, earthworms reduce seedling emergence,their presence did increase successful seedling establishment of large seeded speciessown on top of the litter up to three times compared to pots without earthworms.The studies included in this compilation show that a soil cover has chemical,mechanical and physical effects on the invasibility of plant communities and their speciescomposition. The effect size and direction depends on the prevailing abiotic and bioticconditions. These general litter-related processes that influence seedling emergence arealso of practical relevance in restoration ecology. Concerning the restoration of species-richsemi-natural grassland through reintroduction of regular management in abandonedgrasslands or transfer of seed-containing plant material, the type and quantity of litterhas decisive effects on the success of restoration efforts.
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