Mountain Birches

The term mountain birches encompasses a diverse taxo-nomic group which can be defined most easily in terms of growth habit and location. Mountain birches are distinguished from all other birches by their ability to live in the alpine or subarctic zone above the treeline and have the potential to adopt the many-stemmed growth (polycormic) habit as opposed to the pole form

(monocormic). The silver birch (Betula pendula) grows only as the pole form, which in Scotland requires a minimum of 1100 day degrees for survival. Consequently, silver birch fails before the treeline is reached. By contrast the downy birch (B. pubescens), which has a capacity to grow also in the bush form, can exist in Scotland with as little 700 day degrees (Forbes & Kenworthy, 1973). In the Nordic regions of Europe mountain birches (Figs. 9.13-9.15) are treated as a subspecies of Betula pubescens ssp. czerepanovii (formerly ssp. tortuosa) which grows today from Iceland to the central Kola Peninsula (Wielgolaski & Nilsen, 2001). The species is highly polymorphic as it has probably arisen on more than one occasion from hybrids between downy and dwarf birch (Betula nana) which can then backcross with downy birch. The Iceland form has most probably arisen independently of the Nordic forms, as have also the Scottish populations of B. pubescens ssp. carpatica which is distinct in having the round odoriferous leaves characteristic of B. nana (Fig. 9.16).

In Iceland it has been shown that hybridization between diploid (2n — 28) dwarf birch Betula nana L. and the tetraploid (2n — 56) downy birch B. pubescens Ehrh. has occurred in natural populations (Anamthawat-Jonsson & Thorsson, 2003). About 10% of birch plants randomly collected for this study were triploid hybrids (2n — 42) as confirmed by ribosomal gene mapping. The triploid hybrids are morphologically distinct from the diploid and tetraploid parental plants with an intermediate morphology. It appears that the triploid hybrids have played an important role in driving bidirectional gene flow between these two species.

In Greenland B. glandulosa and B. pubescens form hybrids, as do B. glandulosa and B. papyrifera in Alaska. Mountain birches therefore are not a coherent taxon and show much clonal variation (Vare, 2001). For the purposes of this present discussion they will therefore be defined simply, as stated above, as those birches that can live in the alpine or subarctic zones above the treeline and possess the ability to adopt the many-stemmed (polycormic) as opposed to the pole (monocormic) form (Figs. 9.13-9.14).

The woody habit of the mountain birches is highly variable even within specific sites with mixtures ofmonocormic and polycormic individuals. A third type can also be found in some situations which has a semi-prostrate undulating main stem which extends in a general horizontal direction supporting vertical shoots

(Fig. 9.14). The causes of polycormic growth in birch have been much discussed (Wielgolaski & Nilsen, 2001). Hybridization between B. pubescens and B. nana is one possible reason for polycormic growth and may explain why mountain birches have more basal shoots than the lowland birches. The hybrids normally also grow at somewhat lower temperatures than B. pubescens.

The prevalence of polycormic as to monocormic forms of mountain birch varies regionally and may be due to various causes including soil conditions, flooding and herbivory. Severe outbreaks of insect defoliation are often cited as one reason for the prevalence of the polycormic form in mountain birch. The most frequent defoliators are Epirrita autumnata and Operophtera brumata, the autumn and winter moths, which show marked cyclical activity particularly in coastal regions. In the more continental areas, the outbreaks are more irregular but when they do occur can have catastrophic results and leave a mark on the landscape that is visible for decades. In the Abisko valley of northern Sweden (68° N), the mountain birches were completely defoliated by Epirrita autumnata caterpillars during an outbreak in 1954-55. The defoliation resulted in an 80-90% mortality of the leaf-carrying shoots, which then triggered extensive vegetative stand rejuvenation with very little regeneration from seed. The surviving plants regrew and increased production of long shoots from surviving shoots and basal sprouts, with the result that damaged stands developed a higher proportion of polycormic individuals than comparatively undamaged stands (Tenow & Bylund, 2000). The larvae of the geometrid moths also eat the foliage of the bilberry (Vaccinium myrtillus) and bog bilberry (V. uliginosum) to such an extent that these heath species can be replaced with wavy-hair grass (Deschampsia flexuosa). A similar outbreak in northern Finland in 1965 destroyed over 1000 km2 of birch forest (Kallio & Lehtonen, 1973).

Other reasons for strong development of the polycormic form (sometimes referred to as coppicing), may be browsing by sheep or reindeer. Some field observations also indicate that mountain birch trees tend to develop many stems in poor and dry soils. This seems particularly to be the case in cold, wind-exposed habitats with poor snow protection. By contrast, in moister, nutrient-rich soils, monocormic forms are more frequent (Wielgolaski & Nilsen, 2001). There may also be a connection between local climatic conditions and insect attack as the topo-climatically favourable,

Mountain Forest
Fig. 9.13 Mountain birch (Betula pubescens spp. czerepanovii) growing with typical twisted stem krummholz form in the Lofoten Islands (68° N, Norway).

warmer, earlier areas provide better overwintering conditions for insect defoliators. In landscapes with marked relief, as in the areas around Abisko where many studies on mountain birch have been made

(Neuvonen et al., 2001), there are differences in the location of tree forms, climate and insect attack. Winter moth (Operophtera brumata) outbreaks are more frequent and usually start on south-facing slopes, as

Fig. 9.14 Varying forms of the mountain birch (Betula pubescens spp. czerepanovii). (Left) Creeping krummholz form (with knees) growing at the treeline in central Norway. This is sometimes referred to as var. appressa. (Right) Many-stemmed form (polycormic) growing on the Dovrefjell, Norway.

Fig. 9.15 Holocene forest line oscillations in central Troms, northern Norway. Open circles, forest dominated by mountain birch (Betula pubescens spp. czerepanovii); closed circles, dominated by pine (Pinus sylvestris); squares, dominated by grey alder (Alnus incana). (Reproduced with permission from Aas & Faarlund, 2001.)

Fig. 9.15 Holocene forest line oscillations in central Troms, northern Norway. Open circles, forest dominated by mountain birch (Betula pubescens spp. czerepanovii); closed circles, dominated by pine (Pinus sylvestris); squares, dominated by grey alder (Alnus incana). (Reproduced with permission from Aas & Faarlund, 2001.)

Fig Leaves The Annanite Mountains
Fig. 9.16 The dwarf birch (Betula nana) — note the rounder (orbicular) leaves, a character that this species brings to hybrid mountain birches.

this species is less cold-hardy than Epirrita autumnata. The periodicity and frequency of the attacks could be a significant long-term factor in the selection of the polycormic growth form in these stands. In areas where local climatic conditions ensure cold winters, the stands appear to be dominated by old monocormic trees. When warmer conditions lead to an outbreak in these areas many trees are killed and recovery is slower especially if grazed by reindeer (Tenow et al., 2001).

9.4.1 Biogeographical history of mountain birch

Mountain birch has a long history in Scandinavia and neighbouring North Atlantic islands. In Swedish

Lapland, megafossil wood remnants of Betula pubescens ssp. czerepanovii buried in peat at an altitude of 999 m a.s.l., provide evidence of an early Holocene birch belt at 68° 20' N in the Abisko-Lake Tornetrask area. Radiocarbon dating of these remains, which lie approximately 500 m higher than today's tree limit of Pinus sylvestris in this region, yielded values of c. 5400-4500 BP (Kullman, 1999). Figure9.15 illustrates the extent of fluctuations in forest altitude limits as observed in northern Norway (Aas & Faarlund, 2001). In other more southern areas the birch belt can be traced to earlier dates. In the south-central Norwegian mountains megafossils of birch and pine from altitudes of 900-1370 m have been found with datings between 2900 and 8660 BP, and in the eastern Jotunheimen Mountains at 1288 m from 8000 BP

and 1370 m from 6930 BP (Aas & Faarlund, 2001). The earliest macrofossil remains of birch so far reported in Scandinavia are from a study site at 1360 m a.s.l. close to the summit of Mt Areskutan (63° 26' N, 13° 06' E). In the alpine region of the southern Swedish Scandes, fossil remains of mountain birch dated to 14000 BP have been found 400-500 m above modern tree limits (Kullman, 2002a).

Elsewhere in the North Atlantic region, as mentioned above, mountain birch may have different historical and genetic origins. In Iceland, there is a suggestion that birch may have survived in glacial refugia or may have evolved more recently from parental immigrants of downy birch crossing with Betula nana that may have survived in situ or migrated in the early Holocene. In Greenland, the earliest record of birch forest is 4400 BP and was already in decline at the time of the Norse settlement in the late tenth century. Ruins of Norse farms have been found located at what appears to have been the forest altitudinal limit at the time of Norse settlement (Albrethsen & Keller, 1986). With the onset of the Little Ice Age both people and trees disappeared from these marginal sites.

9.4.2 Current migration

In modern times the possible effects of climatic warming and increased levels of atmospheric carbon dioxide have stimulated careful monitoring of the performance as well as the distribution of birch at altitudes above the main mountain forest belt. Among forest trees birch appears to be well adapted to take advantage of both higher levels of atmospheric carbon dioxide and soil nitrogen. Several reports have been published of seedlings being found well above the present birch treeline. Examination of the late-Holocene tree-limit history (Betula pubescens ssp. czerepanovii), on the east-facing slope of Mt Storsnasen in the southern Swedish Scandes (63° 13' N, 12° 23' E) has detected an upward migration of the tree limit by 75 m over the past century, which matches the meteorological records for summer warming (Kullman, 2003). The new tree colonies are, however, limited to small clumps of trees growing in minor depressions or else limited to young seedlings 5-20 cm high.

Treelines are not easily mapped and the presence of isolated seedlings above the main occurrence of trees has probably always taken place, particularly during warmer climatic intervals, with the plants failing to survive as they grew above the immediate shelter of their surroundings. Such ephemeral migrations could have happened many times in the past and might not have been detected at a time when investigators were less aware of climatic warming phenomena. Definite confirmation of an advancing treeline still awaits a demographic demonstration that can show that what has so far been observed is not just a temporary advance of ephemeral seedlings.

Was this article helpful?

0 0

Post a comment