Ecological Sensitivity Of Woody Plants To Oceanic Conditions

Sensitivity to oceanic versus continental conditions is found in many northern dwarf woody species (Crawford et al, 2003; Crawford & Jeffree, 2007). Oceanic conditions have long been known to lower both the altitudinal and latitudinal position of the treeline. It appears that this is also the case for a number of shrubby, woody species. The blanket bogs of Scotland and Ireland stand out as highly oceanic habitats. The basis on which the blanket bog vegetation is distinguished from less oceanic mire associations in the British National Vegetation Classification is not from the plants that are present in this community, but by the species that are absent (Rodwell, 1991). In particular, it is the absence of woody shrub species such as Vaccinium vitis-idaea, V. uliginosum and Empetrum nigrum ssp. hermaphroditum that typifies blanket bogs with bog myrtle (Myrica gale) as the most distinctive woody species.

In these blanket bogs, although bryophytes flourish, oceanicity is nevertheless predominantly a negative influence for many higher plants in that it gives rise to communities that are species poor, particularly in woody species, with the ericoids being represented mainly by the flood-tolerant Erica tetralix while the presence of Calluna vulgaris is much reduced. With Betula nana there is a marked difference in Scotland between the oceanic west and the continental east coast. In the warmer, wetter, west coast, B. nana is restricted to higher ground but descends to nearer sea level in the colder, drier east coast (McVean & Ratcliffe, 1962).

Neither B. glandulosa nor B. nana is tolerant of continuous flooding. They do, however, survive on blanket peat but tend to occupy elevated hummocks when the ground is prone to prolonged waterlogging (Rodwell, 1991). Betula glandulosa is similar to B. nana in its ability to become re-established after fire. Glasshouse experiments on the effects of fire of varying severity on plant populations from northern and western Canada have shown that burning significantly increased growth at all temperature treatments and that this effect was most apparent at the highest growth temperature. The warmer growing conditions of post-fire microsites appear to provide this shade-intolerant plant with a competitive advantage over other invading pioneer and resprouting species by enhancing the fire-stimulated, height-growth response. Due to its fire ecology, Betula glandulosa populations might therefore be expected to expand and thrive under a future warmer climate regime (DeGroot & Wein, 1999).

Another aspect of how B. glandulosa may profit from fire has been illustrated in a number of studies on the causes of spruce decline in northern Quebec where the environmental changes associated with fire have induced a shift from old-growth lichen-spruce krummholz to lichen-tundra. Tree-ring measurements, together with growth-form patterns from black spruce (Picea mariana) remains lying on the ground in a lichen-tundra community, have enabled a reconstruction of the structure of a conifer stand at the time that a burn that took place at approximately AD 1750 (Arseneault & Payette, 1992). Before the 1750 fire event, the spruce krummholz was predominantly maintained by layering, and overwintered under a considerable snow cover due to the severe climatic conditions that have persisted in this region since the beginning of the Little Ice Age (approximately AD 1580). However, after the fire, the site remained deforested due to the limited regenerative potential of stunted spruce. The post-fire shrubs, mostly dwarf birch (Betula glandulosa), of the lichen-tundra community were unable to trap sufficient drifting snow to permit spruce regeneration and consequently lichen-heath and birch have remained the dominant vegetation for the past 250 years.

The future ecological success of the dwarf birches does not appear to be in doubt. There may be expansion in northern Canada if fire damage continues to increase. In oceanic climates, a tendency to wetter, warmer winters may cause the species to retreat both inland and upwards to regions in which the winter temperatures are lower.

Bog rosemary (Andromeda polifolia; Fig. 9.17) is a small heath species (Ericaceae) growing generally up to

Fig. 9.17 Bog rosemary (Andromeda polifolia), an inhabitant of cold north temperate bogs which can store 75% of its fixed carbon below ground (see Chapter 3). Bog rosemary has also been found to have high antioxidant activity and high total phenolic content (Kahkonen et al, 1999).

no more than 10-20 cm, and widespread across much of the northern hemisphere. Despite its circumpolar distribution bog rosemary appears nevertheless to have a marginal existence, as witnessed by its current decline in many of its former habitats. The circumpolar distribution is achieved by two subspecies: Andromeda polifolia var. polifolia in northern Europe and Asia and A. polifolia var. glaucophylla in north-eastern America. Bog rosemary is only found in cold climate bogs with accumulating peat. It is present throughout the whole of Scandinavia including Finland but, curiously, is absent in bogs from northern Scotland and is rare in northern Ireland, possibly due to peat cutting and drainage (Godwin, 1975). It is also absent in Faroe and Iceland. The species is showing a decline in many habitats, probably due to increased drainage and in some places to greater forest growth. By comparison, a two-year study of flowering which compared temperature responses in cloudberry (Fig. 9.18) and bog rosemary in Swedish Lapland found that both species are very responsive to climate change, with warmer

Fig. 9.18 Cloudberry (Rubus chamaemorus) in fruit. Cloudberries are responsive to rising temperatures and when ripe the fruits turn from red to yellow. The fruits ripen readily in Scandinavia and also in Labrador and Newfoundland where they are much prized. However, in Scotland's oceanic climate they do not fruit consistently and the berries only ripen beyond the red stage in rare favoured places. (Photo Professor R. M. Cormack.)

Fig. 9.18 Cloudberry (Rubus chamaemorus) in fruit. Cloudberries are responsive to rising temperatures and when ripe the fruits turn from red to yellow. The fruits ripen readily in Scandinavia and also in Labrador and Newfoundland where they are much prized. However, in Scotland's oceanic climate they do not fruit consistently and the berries only ripen beyond the red stage in rare favoured places. (Photo Professor R. M. Cormack.)

weather advancing flowering by two weeks. This response appears, however, to be dependent on specific climate events. In both species flower production was only stimulated by the spring-warming treatments, which suggests that more attention in high-latitude climate change experiments should be given to winter and spring events than has been the case so far (Aerts et al., 2004). The fact that bog rosemary occurs at altitudes up to 1250 m in Eidfjord and Ullensvang (60° N) in Norway (Lid & Lid, 1994) makes it curious that it does not occur even on lowland bogs north of Perth (56° N) in Scotland.

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