Mean annual warming 0 C

Fig. 9.24 Salix polaris and possible responses to seasonal differences in relation to climatic warming. For explanation see Fig. 9.21.

than usual winter this species suffered lethal injuries. During a mild winter it was found that rehydrated shoots were at their greatest degree of cold-hardiness when tested early in winter and that they gradually lost their frost tolerance as the mild winter weather progressed. This loss of frost tolerance was accompanied by a decrease in the solute content of the shoots, suggesting a progressive respiratory loss of cryo-protective sugars. Gas exchange measurements estimated that the initial carbohydrate reserves would have lasted for only four months if tissue water content remained high. When thin snow cover was coupled with clear skies then shoot dehydration could improve cold tolerance by 5-10 0C. However, in mild winters, long-term dehardening due to recurrent periods of mist and rain increased metabolic activity and resulted in a 100% increase in shoot damage.

9.8.3 Historical ecology of heathlands

The expansion of heathlands in north-western Europe began with the spread of Neolithic farming. The problems of maintaining soil fertility in an oceanic environment before the advent of deep ploughing and the use of inorganic fertilizers (see Chapters 1 and 10) inevitably led to soil impoverishment and the development of podzols, particularly in the Iron Age with the advent of more efficient ploughing. Consequently, more and more land was left to become either heath or bog depending on local conditions. This did not necessarily mean that no further agricultural use was made of the developing heaths. In many cases the moorlands became a significant agricultural asset for summer grazing as well as providing winter pasturage for hardy stock.

Given adequate manuring, excellent crops can be obtained from heathlands, usually through creating fertility by transporting turf and peat from other parts of the moor to the area for cultivation and incorporating as much manure as possible. This technique was practised throughout Europe and created plaggen soils (Dutch, layered). The creation of increased productivity in small areas, however, led to the further destruction of the agricultural potential of the region as a whole. Large barren areas were created where turf was taken from outfield areas such as moorlands and common-land forests, for bedding for livestock and then spreading the slurry-soaked bedding on the arable fields as fertilizer. Over time, this created very rich agricultural soils which in places could be over 1 m in depth, in contrast to many modern arable soils, which normally have a plough horizon depth of about 30 cm. However, the very extensive removal of turf over long periods of time led to the eventual destruction of the grazing value of the outfields (see Section 11.4.1).

In north-western Germany the plaggen method was used in the Liineburger Heide (heath) from the

Middle Ages with the result that over a very extensive area the soil was destroyed and the forest prevented from regenerating. Consequently severe erosion took place before strict laws were introduced to stop the practice. In Orkney and Shetland plaggen soils were created already in the twelfth to thirteenth centuries, and on some islands in Shetland these methods continued to be used until the 1960s.

9.9 NEW ZEALAND: A HYPEROCEANIC CASE STUDY

In New Zealand, there is a remarkable alpine and subalpine flora including families which in the northern hemisphere are mainly herbaceous, e.g. Rubiaceae, Asteraceae, Malvaceae and Scrophulariaceae, but in this southern location occur as woody-scrub species. A wealth of other genera create a woody flora which varies from creeping mat-forming species to almost tree-like branching woody shrubs with various species of Dracophyllum, Halocarpus, Pittosporum and other genera. Given the relatively recent uplift of the mountain chains in New Zealand the existence of this isolated yet species-rich alpine flora is a matter for botanical wonder.

Sixty million years ago, New Zealand was a low-lying, subtropical archipelago with a mild climate. The transformation to the mountainous topography that is now such a predominant feature of New Zealand's South Island is a result of a series of very rapid mountain-building events occurring mainly over the past three million years. In some places, the uplift rate is estimated to be as much as 100 m in 10000 years. Over a three million year period this would have raised Mount Cook to five times its present elevation of 3764 m - three times that of Mount Everest were it not for the very high erosion rate which has removed two thirds of the uplift (Fleming, 1980).

As the land has risen, so has the vegetation changed to produce the modern species-rich alpine flora. Opinions vary as to whether this flora has evolved in situ from plants that inhabited the cool mist-covered rolling plains of an oceanic environment or whether they are recent immigrants from Asia via the mountains of New Guinea and Australia.

The isolation of the New Zealand Gondwana flora and the low number of genera, combined with a high number of endemic species, suggest that there has been rapid and recent evolution within certain groups. This is particularly the case with the alpine species where over 93% of the species are endemic to the New Zealand biological region, including the more southern and very isolated Sub-Antarctic Islands (Halloy & Mark, 2003). There are also extensive variations within a relatively small number of genera, with many closely related species which support further the in situ, recent-evolutionary hypothesis. One of the most striking examples is the dwarf woody mountain totara (Podocarpus nivalis) which with a stature of 1-3 m contrasts with its presumed ancestors in the giant Podocarpus forests of the lowlands (Fig. 9.25).

The alpine and subalpine scrublands of New Zealand are also the home of the world's smallest conifer, the pigmy pine (Lepidothamnus laxifolius), which grows as a trailing mat spreading over stones and gravel. An evaluation of these competing views (McGlone et al., 2001) has suggested that the vascular plants reached New Zealand by long-distance transoceanic dispersal, probably during the Late Miocene to early Pleistocene period. Cooling climates and formation of a more mountainous and compact landscape after that time reduced the dispersal of woody plants and favoured herbaceous, wetland and highly dispersible species. Consequently, the woody alpine plants have evolved from a limited number of genera.

Podocarpus Totara

Fig. 9.25 The mountain totara (Podocarpus nivalis), a small prostrate New Zealand shrub, sometimes as large as a small tree that inhabits the upper forest margins and subalpine scrub from lat. 36° 50' S.

Fig. 9.25 The mountain totara (Podocarpus nivalis), a small prostrate New Zealand shrub, sometimes as large as a small tree that inhabits the upper forest margins and subalpine scrub from lat. 36° 50' S.

Pleistocene speciation has probably also been aided by the development of a differentiated terrain and climates which have provided isolation and distinctive environments as well as creating greater opportunities for niche specialization.

Ecologically, the Southern Oceanic Islands off the coast of New Zealand, and in particular those that lie south of 50° parallel, namely the Auckland Islands, Campbell Island, and Macquarie Island (Fig. 9.26), with their relative lack of lack of human disturbance, provide a unique opportunity for studying the dynamics of forest and scrub survival in relation to climatic fluctuation. These highly oceanic habits have considerable peat deposits which have enabled the reconstruction of the Holocene history of these remote regions (McGlone et al., 2000). Comparing the islands from north to south provides a comparative assessment of the response of woody vegetation to declining and variable climatic conditions.

The small, uninhabited subantarctic Auckland Island (c. 51° S) is the southernmost outpost of tall forest in the south-west Pacific while Macquarie Island at 54° S has no woody species (McGlone, 2002). The Auckland Islands are completely peat covered with extensive bogs. Low forest and scrub covers the lowland areas of the islands, with maritime tussock and herbfield associations on exposed coasts. Southern rata (Metrosideros umbellata) forest forms a coastal fringe in sheltered locations and is the southernmost limit of this typical New Zealand species. Dracophyllum longifolium forms a tall scrub at the forest margin that can be up to 5 m high, but this then grades into dense low shrubland less than 2 m in height. With increasing altitude low scrub and shrub-grassland predominate, while above 300 m, tussock grassland and fellfield are the norm (McGlone et al., 2000).

Campbell Island at 600 km south of the New Zealand mainland (52° 33.7' S) has an intermediate location between the Auckland Islands and the woodless scrub-deficient Macquarie Island. A series of photographic records starting from 1888, together with mid-nineteenth-century vegetation descriptions of the island, have made it possible to trace changes in the extent of scrub cover since the 1840s, and this together with pollen sampling of the peat has provided an in-depth analysis of the vegetation history of the island (Wilmshurst et al., 2004). Campbell Island was extensively glaciated and Dracophyllum was uncommon until

Fig. 9.26 (Above) Location of New Zealand's subantarctic islands. Auckland Island is the most southerly island in the south-western Pacific Ocean to have a forest cover. (Below) Enderby Island, a small island off the north coast of Auckland Island (500 300 S). View taken near sea level. Solitary tree of Dracophyllum longifolium emerging from Myrsine divaricata scrub. (Photo Dr M. S. McGlone.)

Fig. 9.26 (Above) Location of New Zealand's subantarctic islands. Auckland Island is the most southerly island in the south-western Pacific Ocean to have a forest cover. (Below) Enderby Island, a small island off the north coast of Auckland Island (500 300 S). View taken near sea level. Solitary tree of Dracophyllum longifolium emerging from Myrsine divaricata scrub. (Photo Dr M. S. McGlone.)

around 7000 BP and reached its greatest altitudinal and areal extent between then and 3000 BP. Warmer, drier summer conditions during the mid to late Holocene (+0.5 °C), favoured this scrub expansion (McGlone et al., 1997). However, cooling temperatures and increased south-westerly winds in the late Holocene were probably the cause of a reduction in both density and altitudinal range. The vegetation of Campbell Island now consists mainly of lowland Dracophyllum scrub and upland tussock grassland and tundra. Since the 1960s, the island has become warmer and drier. Dracophyllum scrub was restricted at the time of the first photographs (1888) and earlier written observations suggest that there had been little change between the 1840s and that time.

The recent spread of Dracophyllum scrub has been assisted by a pronounced shift to warmer, drier climates in the second half of the twentieth century. In terms of climatic limits to scrub survival, it is significant to note that the upper elevational limits of scrub have not increased, suggesting that factors other than summer temperature are controlling the altitudinal position of the scrubline in this hyperoceanic environment (Wilmshurst et al., 2004). It appears that in this intensely oceanic setting, warm, cloudy, low-radiation environments inhibited forest growth during the early Holocene, possibly by promoting saturated soils and reducing net photosynthesis. It was only in the later Holocene, when increased westerly windflow brought sunnier, but cooler and windier, climates, that forest expansion occurred on sheltered lowland sites. Pollen analysis has shown that the forest at the study site has collapsed to scrub at least twice within the last 2000 years, most likely because of extended periods of saturated soils (McGlone, 2002).

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