Size and longevity of seed banks in Antarctica and the influence of ultraviolet-B radiation on survivorship, growth and pigment concentrations of Colobanthus quitensis seedlings

Populations of Colobanthus quitensis and Deschampsia antarctica, the only two vascular plant species native to Antarctica, are increasing. We performed a seed bank assay to determine the persistence of seeds from intact vegetation/soil cores collected near Palmer Station on the west coast of the Antarctic Peninsula. Vegetation/soil cores were cold stratified at 3°C for >4 years. Subsequent seed bank densities, estimated from seedlings germinated, averaged 847 and 5645 seedlings m^-2 for C. quitensis and D. antarctica, respectively. We also conducted germination trials on C. quitensis seeds collected at our field site and stored for either 120 days or > 4 years at 3°C. Germination rates ranged from 6% after 120 days of cold storage to 38% after > 4 years of cold storage. These findings show that previous estimates of seed bank densities and germination rates in these species, based on short-term laboratory stratification experiments, may underestimate those found in the field. Stratospheric ozone depletion has lead to increases in ultraviolet-B radiation (UV-B; 280-320 nm) along the Antarctic Peninsula during the austral spring. In a separate experiment we manipulated levels of biologically effective UV-B (UV-B_BE), over current-year C. quitensis seedlings near Palmer Station on the west coast of the Antarctic Peninsula by placing frames over them that either held filters that absorbed most UV-B_BE ('reduced UV-B_BE'), transmitted most UV-B_BE ('near-ambient UV-B_BE') or had no filters ('ambient UV-B_BE'). We monitored seedling survivorship over the course of the growing season (January-March) and growth and pigment concentrations at the end of the season. There were no UV-B_BE treatment effects on seedling survivorship over the course of the season and overwinter survivorship averaged 12%. However, seedlings growing under near-ambient and ambient UV-B_BE had 25 and 48% smaller total leaf areas, 7 and 16% fewer leaves and 65 and 82% fewer branches, respectively, than those growing under reduced UV-B_BE. In addition, concentrations of methanol-soluble UV-B-absorbing compounds were 26% higher and concentrations of chlorophyll b were 26% lower in leaves of seedlings growing under ambient UV-B_BE compared with those under reduced UV-B_BE.