Phenological responses of mountain plants

A question that always arises in relation to climatic warming, particularly in areas where spring can be late and the growing season short, is whether or not photoperiodic control of phenology will limit the use that plants might be able to make of earlier warmer springs. An experiment to answer this question in Austria on a number of high-elevation species (26003200 m a.s.l.) found that the number of days between soil thawing and flowering was sensitive to photoperiod in just under half of the species. However, Cerastium uniflorum, Ranunculus glacialis, Kobresia myosuroides, Saxifraga oppositifolia and S. seguieri were found to be insensitive to both photoperiod and temperature and flowered as soon as released from the snow. These results suggest that about half of the tested alpine species are sensitive to photoperiod and may therefore not be able to utilize fully periods of earlier snowmelt (Keller & Korner, 2003).

Among the species that are insensitive to temperature or photoperiod are those that are found at the highest elevations (e.g. Ranunculus glacialis). In species adapted to the more extreme environments of high mountains, flowering proceeds as rapidly as possible after snowmelt and is not delayed for any further environmental signals.

Rapid flowering after snow and ice melt is a phenomenon that is common to both alpine and arctic floras (see Section 4.8). The purple saxifrage (Saxifraga oppositifolia) will flower 5-8 days after being released from snow (Larl & Wagner, 2006). This ability to produce flowers quickly is dependent on the possession of pre-formed flowering buds which are usually initiated as the day length shortens during the previous growing season. The speed with which these tissues can develop is remarkable.

A study of the dynamics of reproductive development from floral initiation to fruit maturation, as well as leaf turnover in vegetative short-stem shoots, of Saxifraga oppositifolia in the Austrian Alps found marked differences in the timing and progression of reproductive and vegetative development depending on altitudinal location. In an alpine population a four-month growing season was required to complete reproductive development and initiate new flower buds, whereas individuals from later thawing subnival sites attained the same structural and functional state within only two and a half months (Larl & Wagner, 2006; Fig. 10.21). Reproductive and vegetative development were not strictly correlated because timing of flowering, seed development, and shoot growth depended mainly on the date of snowmelt, whereas the initiation of flower primordia was evidently controlled by photoperiod the previous season. Floral induction occurred during June and July, from which a critical day length for primary floral induction of about 15 hours could be inferred. Pre-formed flower buds overwinter in a pre-meiotic state and meiosis started immediately after snowmelt in spring. This study illustrates a striking differentiation between alpine and subnival populations in the timing and progression of reproduction.

As this study was carried out over a three-year period, during which there were differences in the degree of summer warmth, it was also possible to draw some conclusions as to how increasing climatic warming might affect the phenology of mountain plants. In 2003 when there was a higher number of day degrees accumulated at the subnival sites there was no further acceleration in the rate of development of these

Fig. 10.21 Dynamics of reproductive growth of Saxifraga oppositifolia at alpine and subnival sites with early (E) and late thawing (L) over two years of observation. Values are the mean lengths of flower buds, carpels, and stamens. Light dotted shading, temporary snow cover; heavier shading, winter snow cover. (Reproduced with permission from Larl & Wagner, 2006.)

Fig. 10.21 Dynamics of reproductive growth of Saxifraga oppositifolia at alpine and subnival sites with early (E) and late thawing (L) over two years of observation. Values are the mean lengths of flower buds, carpels, and stamens. Light dotted shading, temporary snow cover; heavier shading, winter snow cover. (Reproduced with permission from Larl & Wagner, 2006.)

high-altitude populations. Such findings are in agreement with arctic studies on this same species where it was found that enclosing plants of S. oppositifolia in open-top chambers had little influence on phenology, flowering frequency, and reproductive success (Sten-strom et al., 1997). It seems that for cold-tolerant plants that are indifferent to higher temperatures, e.g. species such as the purple saxifrage, which is capable of growing into late autumn even when covered with snow, climatic warming will be unlikely to evoke any phenological response.

For further discussion of reproduction in marginal areas see Chapter 4.

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