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During interglacial phases, the coldvanna becomes the largest biome in Antarctica. While not as diverse as the trample steppe, this habitat hosts a multitude of plant species adapted to harsh conditions. Heavy snowfall reduces grazing impact on herbaceous species, leading to reduced overall dicots diversity in favor of a few dominant grass species. The lack of grazing, however, also favors the presence of woody taxa, reaching a diversity peak in the coldvanna.
Shrub associations are dominated by various birch family plants such as birchbushes (Cespibetula spp.) and stocky alders (Magnialnus spp.), forming impenetrable thickets. In the densest thickets, relictual trees like Altidryopteris vagus, the last arboreal fern in Antarctica, can still be found, reaching no more than two meters in height. Altidryopteris vagus persists in Antarctica due to its ability to thrive in microclimates created by thickets and the unpalatability of its leaves, rich in tannins.
Tree associations are diverse, with angiosperms like snow willows (Nivisalix spp.) and conifers like the western polar fir (Uniabies velox), the fastest-growing Antarctic gymnosperm. While these species are still bush-like, they can reach up to 15 meters in less browsed areas. However, their lifespan is relatively short, rarely exceeding 50 years due to debarking by herbivores. One exception to this trend is a unique group of woody plants known as brumbletrees or cancerthorns (genus Carcinicortex). Although distantly related to dandelions, their appearance is entirely different, featuring bizarre and unaesthetic forms shaped by cold winds and herbivore browsing.
Despite the challenging conditions, brumbletrees exhibit unique adaptations. They have a thick cork layer, reaching up to 10 cm in old individuals, acting as a defense against herbivores and cold temperatures. In response to browsing, brumbletrees produce a so-called "witches' broom," a dense mass of shoots that grows from a single point, providing a shield against predators. While they don't produce suckering in response to browsing, brumbletrees endure injuries through the production of epicormic shoots, ensuring plant recovery.
Brumbletrees have an unusual way of dispersing seeds through cauliflory, where flowers grow directly on woody trunks. The flowers are partially sunken in the cork layer to reduce mortality from freezing temperatures and winds. After pollination, the cork layer blocks the entry of the flower, and a hardy fruit develops, protected by the cork layer. The seeds remain viable inside the bark for up to 100 years, even after the plant's death and in some cases they can grow directly on the bark. Seed dispersal occurs through herbivore ingestion, unintentionally carrying seeds in their feces, providing nutrients for new seedlings.
This complex defense and dispersal strategy has made brumbletrees one of the most successful tree species in Antarctica, with the genus Carcinicortex comprising over 50 species across the continent, predominantly in the coldvanna but with some shrubby species found in the trample steppe. The intricate stems of brumbletrees create ideal habitats for woodclimbers, which have developed symbiotic relationships with these trees and doorpeas.
A frontal section of a brumbletree trunk. The two insects shown are a weeeevil and a waxant
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