Montane And Arctic Willows

As a genus, the willows are a widespread group containing approximately 400 species occurring mostly in the northern hemisphere. In the arctic and subarctic regions of Europe and North America there are approximately 28 boreal species with numerous subspecies and hybrids. However, only five of these species achieve circumpolar distribution, namely S. arctica, S. glauca, S. lanata, S. phylicifolia, and S. reticulata (Figs. 9.9-9.12; Hulten & Fries, 1986). The genus Saltx also stands out for its unsurpassed ability to maintain the woody form higher up mountains and further north than any other genus. In the Arctic, the polar willow (S. polaris; Fig. 9.10) can be found as carpets of vegetation at Biskayerhuken (79° 50' N) in Spitsbergen while its North American vicarious counterpart S. polaris subsp. pseudopolaris reaches 75° N in the Canadian arctic archipelago. In Greenland, Salix arctica reaches 83° N in Peary Land where along with Papaver radicatum and Saxifraga oppositifolia it belongs to the most northerly terrestrial plant community in the world. In terms of high-altitude survival, the least willow (S. herbacea) occurs at 2170 m in central Norway (Lid & Lid, 1994) while in the Alps it can be found at over 3000 m. In the hyperoceanic climate of Shetland (Scotland's Northern Isles - 60° N) it occurs at much lower altitudes from 150 m down to sea level (Beerling, 1998).

In Scotland montane willows are much depleted due to grazing. Species such as the woolly willow (Salix lanata; Fig. 9.12) are now restricted to steep slopes where they are more vulnerable to chance events such as erosion, rock falls and avalanches.

All willows share a number of common characteristics. They are dioecious, developing male and female flowers in different individuals. Flowering is commonly precocious with the catkins appearing before the leaves. However, in the most northern species this is less marked. In S. polaris and S. herbacea leaves and catkins appear almost simultaneously. The species are insect-pollinated, even in the Arctic where small flies are attracted by the ability of the willows to secrete nectar. Seeds are widely dispersed by wind. Vegetative reproduction is common. In larger willows rooting can take

Fig. 9.8 Site preferences for crowberry (Empetrum nigrum) on sand dunes on the island of Spiekeroog. (Above left) Location of Spiekeroog in the East Friesian Islands (North Germany). (Above right) Aerial view of Spiekeroog. (Photo Dr A. Gerlach.) (Below) View of crowberry colonies (dark coloured areas of dune slopes). Although Empetrum nigrum is a very widespread species geographically it is restricted in these dunes to north-facing slopes.

Fig. 9.8 Site preferences for crowberry (Empetrum nigrum) on sand dunes on the island of Spiekeroog. (Above left) Location of Spiekeroog in the East Friesian Islands (North Germany). (Above right) Aerial view of Spiekeroog. (Photo Dr A. Gerlach.) (Below) View of crowberry colonies (dark coloured areas of dune slopes). Although Empetrum nigrum is a very widespread species geographically it is restricted in these dunes to north-facing slopes.

place whenever the branches touch the ground. In the creeping arctic forms such as S. herbacea, S. polaris, S. arctica, and S. reticulata (Figs. 9.9-9.10) underground creeping stems ensure vegetative reproduction and lead to the establishment of extensive and long-lived clones.

The potential hazard of biomass loss by herbivory can be countered either by maximizing resource acquisition and compensatory growth, or by minimizing loss of resources by grazing deterrents. The latter strategy depends on investing in structural defence or synthesizing secondary metabolites that render the plant unpalatable, toxic or non-nutritious to the herbivore. A study of Salix polaris, which is regularly browsed by reindeer in Spitsbergen, examined the response one year after simulated browsing (by repeated clipping) in early, mid, and late summer. The simulated grazing greatly reduced the number of leaves as well as the total and individual biomass of leaves, and the number of catkins (Skarpe & van der Wal, 2002). There was no increase in phenolics but a tendency to an increase in nitrogen content in the leaves one year after the treatment. It appears that S. polaris responds to summer browsing the previous year by allocating resources to compensate for loss of biomass rather than stimulating the synthesis of secondary metabolite grazing deterrents.

The final result is a herbage that is reduced in quantity but may have some increase in quality.

In other Spitsbergen studies, the length of the growing season was manipulated by adding or removing snow so as to effect a two-week difference in snowmelt. This was equivalent to an approximately one-sixth alteration of the growing season between advanced (first to be snow-free) and delayed (last to be snow-free) treatments. The shift in 'phenological time', led to the Spitsbergen reindeer (Rangifer tarandus platyrhynchus) selecting the advanced, longer-exposed snow-free plots, presumably because of the greater biomass of Salix polaris and Luzula confusa, both major components of reindeer diet at the early part of the year. By contrast, plant quality, measured as nitrogen content and C:N ratio of leaves, was lowest in the preferred plots. Here again phenolic content did not differ among treatments. It would appear that in this northern arctic region, grazing preferences are for quantity rather than quality, which is the reverse of the usual tendency in temperate regions. This grazing habit of consuming large quantities of herbage is common to many lactating mammals in the Arctic and may be due to the need to secure a sufficient supply of minerals and in particular calcium for their lactation periods.

Fig. 9.10 The polar willow (Salix polaris) showing pairs ofleaves (length 5—10 mm) emerging through moss from a buried rhizome. (Upper) Male plants. (Lower) Female plants — the predominant sex, 59% female; see Fig. 4.33). Photographed in Moller Fjorden, Spitsbergen (79° N).

A curious feature of willow populations that is very noticeable in the Arctic is the bias towards females which is found in a number of species and appears to be a circumpolar phenomenon. Various explanations have been postulated to explain the skewed sex ratio in arctic willows in favour of female plants first noted in S. polaris in Spitsbergen and S. herbacea in Iceland (Crawford & Balfour, 1983). On one hand it might be due simply to differences in growth rates with more aggressive females outcompeting male plants, as has been observed in sea buckthorn (Hippophae rhamnoides; Fig. 4.36). On the other hand, variation in growth rates might lead to differences in the relative investment in metabolic grazing deterrents between the sexes bringing about a skewed sex ratio due to differential herbivory (see also Chapter 4). An experiment in Spitsbergen where there are no lemmings, and therefore where reindeer are the main grazers, found that excluding reindeer for three years increased the abundance of male flowers in one of two vegetation types investigated (Dormann & Skarpe, 2002). However, growth rates differed only slightly between the sexes, with females investing more in inflorescences. The concentration of chemical defence compounds (phe-nolics and condensed tannins) did not differ between the sexes. Consequently it was concluded that the

Fig. 9.12 The woolly willow (Salix lanata), a common mountain willow in Scandinavia but now rare in Scotland and proving difficult to re-establish in former habitats. Photograph taken in Iceland.

Fig. 9.11 Light saturated photosynthetic rates (Amax on a leaf-weight basis) in relation to leaf concentration of (a) nitrogen and (b) phosphorus. The differences in nutrient concentration have been induced by feeding sustained treatments. Open circles,

Cassiope tetragona; squares, Dryas octopetala; triangles, Salix polaris. (Reproduced with permission from Baddeley et al., 1994.)

hypothesis was not tenable that growth rate-dependent herbivory caused the unbalanced sex ratio in S. polaris. The universality of this skewed ratio in northern willows, irrespective of the degree of grazing, suggests that the bias in favour of females is an intrinsic property of montane and arctic willows. The skewed sex ratio may be connected in some as yet unknown manner to inherently different survival rates as young populations have sex ratios near equality, with the sex bias developing only as the populations age (Crawford & Balfour, 1990).

Arctic and montane willows do not appear to thrive in oceanic areas with warm winters and may therefore be forced to retreat from certain parts of their present distribution should climatic warming continue. Already in more oceanic subarctic regions, as in Scotland, great difficulty is being experienced in restoring the mountain willows that were formerly more widespread but are still such a characteristic feature of the colder mountain

Fig. 9.12 The woolly willow (Salix lanata), a common mountain willow in Scandinavia but now rare in Scotland and proving difficult to re-establish in former habitats. Photograph taken in Iceland.

regions of Norway. In Scottish mountains the long-term survival of the montane shrubby willows (S. lapponum, S. lanata, S. arbuscula, S. myrsinites and S. reticulata) is giving particular cause for concern. Seedling establishment is low even when grazing is absent. The populations are small and fractionated and frequently the separate male and female plants are so distanced from each other that pollination is inefficient. Why these Scottish willows lack the vigour of the Scandinavian populations is not clear. There may be an adverse effect from the hyperoceanic Scottish environment. The possibility that woody species in general in northern habitats are disadvantaged by oceanic conditions is discussed further below.

0 0

Post a comment