Reproductive diversity of high mountain plants

Plants inhabiting high altitudes must to be adapted to grow and propagate within a short period of time at low temperatures. In mountains of the temperate zone, the length of the growing season decreases from 4-5 months at the treeline to 2 months and less at the upper limits of higher plant life. Single investigations on flower and seed development of high mountain plants suggest much variation in the pattern and dynamics among different species. The developmental pattern becomes additionally shaped by environmental conditions, temperature and day length being the most important factors. The reproductive development of high mountain plants and the response to environmental factors are largely unexplored. Therefore, the proposed project aims at investigating the whole reproductive cycle of certain high mountain species representative for the alpine and nival belt in the European Alps. Quantitative investigations and transplant experiments will be carried out on climatically contrasting sites. The main questions being addressed are: (1) Which structural and functional traits of sexual reproduction exhibit high mountain plant species? (2) Are there certain functional types of sexual reproduction which are better adjusted to manage the requirements in the nival zone? (3) How flexible are the different species with regard to different environmental conditions as temperature, day length and length of the growing season? Information about the species-specific pattern and the flexibility of reproductive development is important, in order to judge how plants growing at the limits of plant life may response to changing environmental conditions. Species with a high degree of developmental flexibility have a better chance to remain in their habitat than less flexible species which have to migrate to more suitable habitats in the case of climate change.

High mountains are climatically extreme environments. Short growing seasons and low temperatures are the most important factors limiting plant life at higher altitudes. The period available for growth, flowering and seed production in the European Alps varies with relief and snow accumulation in winter. In the alpine belt it is 3 to 5 months and in the nival belt it is 1 to 3 months. Accordingly, species richness decreases from several hundred species in the alpine belt to about 30 species in the nival belt. Only a dozen species still occur above 4000 m a.s.l. The present study investigated strategies of sexual reproduction in common plant species representative of the alpine and nival vegetation belt in the European Alps. Successful seed production is essential for a species to persist at an existing site or to colonize new sites. Our scientific hypothesis was that nival plants have developed particular strategies to ensure reproductive success even under the most unfavourable climatic conditions. The majority of the species investigated were saxifrages. We also investigated Ranunculus glacialis (glacier buttercup), Cerastium uniflorum (alpine form of chickweed) and Androsace alpina (rockjasmine). Investigations were carried out at different altitudes between 2300 and 3450 m a.s.l. in the Tyrolean Alps (Austria). The reproductive cycle of a plant starts with floral induction and ends with seed maturation. The majority of the investigated species extend flower formation over two growing seasons, but there are marked differences among species in the timing and extent of flower preformation in the first year. This has consequences for the date of flowering in the second year. Only two species initiate flower buds in the year of anthesis and accordingly they bloom late. In contrast to the general view that plants at higher altitudes promote selfing and asexual propagation, we showed that the nival species also depend on pollination by insects to successfully form seeds. From fertilization to seed maturity takes about one month. However, some species need longer for seed development. Among these species is Saxifraga biflora (6-7 wks), one of the species reaching the upper limits of higher plant life in the Alps. The time span between snow-melt and first seed maturity gives the minimum period required for reproductive development which, depending on the species, ranges from 40 to 110 days. Our investigations have shown that there is not just one reproductive strategy which is suited to the particular climatic conditions at higher altitudes. There are species restricted to the alpine belt that reproduce quickly and effectively and in contrast typical nival plant species that require a surprisingly long period for reproduction. However, these species show traits which might be beneficial for seed production in a nival climate, such as the ability to mature seeds below snow, or a high seed output per flower.