Alpine Floral Biology

No account of mountain vegetation is complete without some discussion of alpine floral biology. Alpine flowers attract attention even from the most casual mountain visitor. A similar situation exists in the Arctic where the flora is less diverse and individual plants are somewhat smaller than those in alpine regions. Nevertheless, the visual impact of the Alps and tundra in flower in early summer is an impressive tribute to the sexual activity of plants in cold climates. Why plants of these thermally limited marginal areas have such outstanding floral displays has long been a source of wonder and speculation. In many habitats, such as cliffs and screes, the plant cover in general is sparse and therefore the floral parts of individual plants and clonal patches are vividly displayed against a background of bare rock or gravel.

Visibility of the flowers to the human eye is enhanced by the reduction in size of the vegetative organs in relation to the flowers (Fig. 10.31). It would appear that reduction in leaf size reduces the hazards of exposure but a similar reduction in flower size would result only in a flower that would not fulfil its primary function in attracting insect pollinators. To the human observer and also to insects the visibility of alpine and arctic flowers is enhanced as they are frequently borne in clustered displays on creeping and cushion plants.

The ability of flowering parts to fulfil their normal function of facilitating outcrossing and seed production could be problematical in many marginal alpine and arctic habitats. As already discussed, there are limits to the ability of plants to reproduce sexually, which may arise from numerous causes, grouped under two headings, pre-zygotic and post-zygotic limitations (see Chapter 4).

In pre-zygotic limitations there is the possible lack of suitable vectors for pollination and even if pollination does take place, fertilization of the ovule may not be achieved, either because the pollen grain does not germinate or else the pollen tube fails to reach the ovary. Under post-zygotic causes of reproductive failure there may be low seed viability, failure to germinate and destruction by predators. In marginal areas seeds frequently do not mature in short growing seasons, and even if they do eventually germinate, the young seedlings may fail to become established in the plant community.

In cold regions it might therefore be expected that there may not be sufficient insect pollen vectors and that wind might provide a more reliable form of pollen dispersal. Surprisingly, wind pollination is not as prevalent in alpine and arctic floras as might be imagined. However, insects are ubiquitous. They succeed in reaching some of the coldest regions of the Earth, where they adapt their lifestyles to mitigate the effects of the diurnal temperature oscillations of mountain habitats or seasonal activities to profit from the brief periods of summer warmth in the arctic and alpine habitats.

Pollen transport that is targeted in its delivery is much more efficient than that which is thrown into the wind. It is sometimes suggested that the open nature of high mountains and tundra is ideal for the wind dispersal of pollen. However, the ferocity of the winds and the open expanses of rock and water that make up so much of these terrains will inevitably make wind pollen dispersal a very wasteful process. The thermal climate of alpine and arctic environments when viewed from the standpoint of insects and vegetation is not so adverse during the flowering season as might be expected. The diminutive vegetation lives in a microclimate close to the ground that is noticeably warmer than the air temperature. This is the case especially for bulky organs such as stems, cones and buds, which heat up significantly in bright sunshine, as has been shown

Fig. 10.31 The alpine forget-me-not or king of the Alps (Eritrichium nanum). A species of high mountains occurring between 2500 and 3600 m in the Alps and a noted example of maintaining the alpine habit of having a highly visible flower display which when in full flower almost totally obscures the diminutive foliage. (Photo by courtesy of Professor R. M. Cormack.)

Fig. 10.31 The alpine forget-me-not or king of the Alps (Eritrichium nanum). A species of high mountains occurring between 2500 and 3600 m in the Alps and a noted example of maintaining the alpine habit of having a highly visible flower display which when in full flower almost totally obscures the diminutive foliage. (Photo by courtesy of Professor R. M. Cormack.)

in pine trees growing near the treeline in Scotland by inserting extremely fine thermocouples into the tree buds in their natural environment. For mature pines the bud temperatures were 4 °C warmer than the air by day, and even warmer in the dwarf krummholz pines near the treeline (Grace, 2006). The insects of these regions also inhabit this same thermal space, where they crawl over the plants rather than fly. This restricted movement not only aids their metabolic activity, but also reduces the risk of being blown away by wind. Sun-tracking flowers such as the glacier buttercup (Ranunculus glacialis) maximize their sun trapping capacity with the parabolic reflectance of the flowers concentrating radiation on the reproductive organs and thus enticing insect pollination (Fig. 10.32). The flowers are white while they are required as reflectors but after fertilization the petals close to protect the developing seed and turn red, which will increase radiation absorbance (Fig. 10.33). Some controversy has taken place as to whether there are both red and white forms of R. glacialis or whether the red colour is a post-fertilization phenomenon when the

Fig. 10.32 Sun-tracking (heliotropic) flowers of Ranunculus glacialis. The flowers act as parabolic reflectors concentrating radiation in the centre of the flower and thus attracting pollinators. (Photo Professor R. M. Cormack.)

petals close around the flower and absorption rather than reflectance has a greater efficacy in absorbing heat. In Fig. 10.33 the plant on the left is still showing some white open flowers while the closing flowers are turning red. It appears, however, that both cases exist with some plants having red flowers before they are fertilized. (See also Chapter 4.)

Although insect pollinators are present at astonishingly high altitudes and latitudes, and even the purple saxifrage (Saxifraga oppositifolia), the earliest flowering plant in many high mountain sites, is an insect-pollinated species, their abundance is much diminished compared with temperate habitats. It is therefore not surprising that the flowering plants of these regions have a number of adaptations that can compensate for the potential dangers of this deficiency. Among the most common is self-compatibility. However, persistent self-fertilization will reduce genetic variation which could be unfavourable for population persistence in highly stochastic environments. An evaluation of these contradictory scenarios for the advantages of self-pollination as opposed to out crossing was carried out with in situ pollination experiments on the predominantly out crossing species Saxifraga oppositifolia in the Swiss Alps at sites where pollinator limitation had been detected (Gugerli, 1998).

Fig. 10.33 Diversity of colour in Ranunculus glacialis. Some controversy has taken place as to whether there are both red and white forms of R. glacialis or whether the red colour is a post-fertilization phenomenon when the petals close around the flower and absorption rather than reflectance has a greater efficacy in warming the developing seeds. The plant on the left is still showing some white open flowers while the closing flowers are turning red. It appears, however, that both cases exist with some plants having red flowers before they are fertilized. (See also Section 3.1 and Fig. 3.3.) (Photo by courtesy of Professor R. M. Cormack.)

Fig. 10.33 Diversity of colour in Ranunculus glacialis. Some controversy has taken place as to whether there are both red and white forms of R. glacialis or whether the red colour is a post-fertilization phenomenon when the petals close around the flower and absorption rather than reflectance has a greater efficacy in warming the developing seeds. The plant on the left is still showing some white open flowers while the closing flowers are turning red. It appears, however, that both cases exist with some plants having red flowers before they are fertilized. (See also Section 3.1 and Fig. 3.3.) (Photo by courtesy of Professor R. M. Cormack.)

Hand-crossings and hand-selfings yielded seed set at both elevations that were studied. It was concluded that this constant pattern of the breeding system in S. oppositifolia indicated that selective factors must exist that lead to the maintenance of a high level of out crossing even in high-elevation populations. Such findings can be claimed as contradicting the hypothesis that inimical environmental factors in alpine or arctic habitats necessarily select for increased selfing rates in a preferentially out crossing species. Further discussion of the capacity for sexual reproduction in marginal areas including alpine and arctic habitats can be found in Chapter 4.

0 -1

Post a comment