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The trample steppe presents the highest level of biodiversity for both animals and plants in Antarctica. Such an outcome is possible only because of the synergistic action of climate and animal grazing. Megaherbivores' grazing favors the presence of a more biodiverse herbaceous association, indirectly allowing the presence of more animal species. As an auto-catalytic process, the high number of animals increases the nutrient supply in the terrain, which boosts further plant biodiversity and biomass.
The effects of megafauna are favored by the climate of the steppe, which is dry but cold. Despite the few rainfall and snowfalls across the year, evapotranspiration from the soil is so insignificant that a lot of humidity is preserved inside the soil, creating a deep permafrost layer underground that acts as a carbon sink. With less snow in winter, the spring snowmelt lasts for just a few weeks, enabling the start of plant germination relatively earlier than usual. Both animals and climate create challenging conditions for forests and trees in general, which would damage the integrity and productivity of the steppe . Only a few highly palatable or hardy species are able to resist, creating a mosaic of grassy habitats in lowlands alternated by low shrubs in hilly areas or near river banks.
At the start of the warm season, the density of megaherbivores has reached its lowest value, due to a great winter mortality. The temporarily reduced biomass gives time to grasses to recover. If the cold season would be shorter, plants wouldn't capable of withstanding the overgrazing effect of these herbivores, but at the same time a prolonged winter would means less time to germinate and make use of the great nitrogen intake by megafauna.
At the end of the warm season, most of the trample steppe has been completely exploited by grazers, becoming a short grass environment. Large herbivores must have required enough fat for the winter: during this season, the calories intake from the few plants eaten under the snow cover is low and not enough to sustain their bodies, which goes in deficit. A tramplerat for example can lose more than 20% of its weight after the entire cold season.
Among the most common shrubs, we can't fail to mention a group of legumes that have monopolized the woody associations of the trample steppe: the doorpeas (genus Forilathyrus). The genus includes over 30 species of small size, from the large doorpea of up to 6 meters in height to the snaking doorpea of just 10 cm, which develop parallel to the ground rather than in height. Leaves and stem robustness vary among the species, but all share common basic characteristics: they are all evergreen shrubs with short roots and a foliage rich in alkaloids, which act as a defense against herbivores.
They are moderately fast-growing plants with a curious habit: their buds bloom in late winter, instead of early spring, under a snow cover of up to 30 cm. The risk of frost damage for plant tissues is avoided thanks to the capacity of these plants to produce heat: they are, in fact, thermogenic plants, which produce heat thanks to specialized mitochondria.
While outstanding, this adaptation is not unique in plants: several holocenic thermogenic plants are known, although mostly found in tropical regions. Heat production in plants can be useful for attracting pollinators and, as in the case of doorpeas, for enduring cold temperatures.
Thanks to their early germination, doorpeas are competitively advanced compared to other shrubs and grassy plants, which explains their cosmopolitan distribution in Antarctica. Heat production is maintained throughout the entire year, with a peak during the winter: leaves close on themselves protecting the stem and keeping it warm, producing temperatures as high as 20 °C. The energy to produce warmth is possible thanks to the symbiosis with root fungus, which helps collect as many nutrients as possible during the warm season.
The heat production of doorpeas indirectly favors the creation of microclimates that are exploited by many plants but also animals (especially lipidragons, the only cold-blooded vertebrates in Antarctica). Among plants, a successful group of trees called brumble trees take the most advantage of doorpeas: the heat allows this tree species to grow faster and earlier, even in the trample steppe, rapidly displacing other woody plants and creating small thicket associations called Carcinocortex-Forilathyrum.
An illustration showing the thermoregulation system of a doorpea
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