Picea mariaria, 10 - 20 kg C, moderate fire frequency

Fen, 100 kg C, low fire frequency

the growing season because they are annual species which form their seeds before the drought starts and then die. The high water use of Bromus means that the perennial Arte-mesia, which must survive the dry period, does not have enough water later. Artemesia dies, often without forming seeds and without having the ability to regenerate as fast as Bromus; also, seedlings of Artemesia are not able to compete with Bromus. The effect of differential water use is enhanced by fire. Dry stalks of Bromus increase the frequency of fire, whereby Artemesia is not fire-tolerant. Grazing also plays a role as Artemesia is very sensitive to mechanical disturbance from trampling by animals which is not the case for Bromus.

2. Grazing: Disturbances causing a change in the assembly of the vegetation are particularly caused by grazing. Grazing animals have par

| Fig. 3.2.14. A Spread of invasive species is generally along roads. Salsola kali (Chenopodiaceae) can be seen close to roads in semi-deserts of Utah, western North America. Salsola is a salt-tolerant species from Asian deserts. Away from the edge of the road the Mediterranean Bromus types dominate. B Mediterranean Bromus types in the Artemisia steppes of Wyoming. Artemisia cover is already broken. (Photo E.-D. Schulze)

ticular effects because they selectively eat species which may be destroyed if they are not adapted to grazing pressure. Bouteloua cluster grasses in North America are not adapted to grazing by horses, as they remove the whole clump and pull the roots out of the soil, so the plant dies off. Similarly, grasses from Australia, with auxiliary buds high on the stem, are adapted to kangaroos but not to grazing by cows or sheep; kangaroos pick individual leaves from shoots, whilst cows and sheep bite the above-ground shoots completely off and thus stop re-sprouting of the plant. In both cases, grazing with European domestic animals caused far-reaching changes in the flora. Additionally, there are indirect disturbances, e.g. trampling by animals compacts the soils and there is relocation of resources with the deposition of faeces, e.g. in cow pats, or places where animals congregate.

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