Trees by the sea: advantages and disadvantages of

oceanic climates R.M.M. CRAWFORD

Sir Harold Mitchell Building, St Andrews University, St Andrews, KY16 9AL, UK

e-mail rmmc@st-and.ac.uk

ABSTRACT ___________

Proximity to the ocean can have both positive and negative effects for tree survival. Across the world relict forests that were once trans-continental in their distribution depend

for their continued survival on coastal refugia. The tree species that dominate the cloud-

zone forests of Maceronesia, the coastal redwoods of California, the Atlantic Forests of Brazil, and the Podocarp forests of New Zealand’s South Island are all examples of

palaeoendemic species which once had a much wider distribution and appear to owe their survival to the particular environmental conditions that are provided by coastal sites and

oceanic islands. By contrast, in the islands of the North Atlantic, oceanic conditions

appear to limit tree regeneration and make forests vulnerable to human disturbance. Paradoxically, winter warmth appears to be harmful to trees in northern cool and moist

oceanic conditions. There may be many reasons as to why warm winters of maritime environments can be detrimental for woody species. Insect attack, pathogenicity and

metabolic dysfunction with loss of frost hardiness and over-wintering carbohydrate

reserves are all possibilities. Where long, mild winters are combined with wet soil

conditions, metabolic dysfunction brought about by prolonged periods of oxygen

deprivation can deplete root meristems of carbohydrate reserves that are essential for

avoiding post-anoxic injury when the soil profile once again becomes aerated in spring. In

areas where the temperature remains below zero for the greater part of the winter this is

not a danger. However, in oceanic habitats there is always the risk, even in the far north,

of temperatures rising above zero. Model predictions and field observations confirm the

potential dangers of warm winters by suggesting that any significant degree of winter

warming will cause a retreat of Pinus sylvestris and other woody species from oceanic

regions at northern latitudes.

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ADVANTAGES AND DISADVANTAGES OF COASTAL HABITATS

Coastal regions present both opportunities and challenges for plant survival. On

one hand, amelioration of temperature extremes combined in many instances with freedom

from drought, extends the distribution of species which otherwise could not survive in

more extreme continental climates. On the other hand, for many species with continental

distributions, coastal regions are marginal habitats (Crawford, 2000). Maritime

environments combine the constant threat of habitat destruction with the physical stresses

including wind exposure, salt drenching, and sometimes even flooding. Islands in

particular, wherever they occur, appear to be particularly susceptible to losing their tree

cover as a result of human disturbance. Regions where tree cover appears to be

disadvantaged by an oceanic environment have to be compared with other areas where the

maritime situation favours the growth of trees. In many parts of the world there are forests

which are restricted to coastal habitats (Laderman 1998a). Such forests include the cloud-

zone forests of Madeira, the Azores and the Canary Islands, the coastal redwoods of

California, the Atlantic Forests of Brazil and the Podocarp forests of New Zealand, as well

as the uninhabited sub-Antarctic islands that lie 600 km south of New Zealand, which

preserve an ancient forest with an extraordinary resilience to the severity and fluctuations

of the variable and violent weather emanating from Antarctica. Most of the tree species of

these coastal refugia are palaeoendemics which once had a much wider distribution and

are restricted now to scattered coastal sites with many biogeographical disjunctions (Table

1).

Coastal environments now provide the only refuge for some of the coniferous trees

that once had a worldwide distribution. The Californian redwood (Sequoia sempervirens),

in common with several species of Chamaecyperus and Taxodium all had trans-

continental distributions during the Tertiary period but are now found only in very

restricted coastal habitats. Apparently the oceanic niche, with its diminution of climatic

extremes, together with a reduction in competition from more recently evolved tree

species, provides many relict tree species with an environment in which they are still

viable. As with many of the other dominant species of these coastally restricted forests,

survival appears to be due in part to the longevity of these species which can exceed 1,200

years for S. sempervirens and 3000 years for the Alaskan Cedar (Chamaecyperus

nootkatensis) (Laderman 1998a).

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Table 1. Examples of tree species occurring naturally only in coastal habitats. Species Location Habitat Alnus japonica1 China, Taiwan Japan, Korea

Russia Swamp forests, peatlands and other poorly drained soils from coastal to montane cloud-zone sites

Chamaecyparis formosensis2 Taiwan Montane, moist coniferous forests

C. taiwanensis2 Taiwan Higher montane, well-drained coniferous forests in cloud zone

C. lawsoniana2 Western USA Dry and wet poor soils, dunes and bogs and stream margins in coastal fog zone

C. nootkatensis2 Western North America Poor soils and bogs from sub-alpine sites in the south to coastal sites in Alaska

Picea glehniss2 Japan. S.Kurile Is. and S. Sakhalin Is

Raised bogs and moors in regions with cool summer fogs

Pinus muricata2 Mexico, California Coastal forests, steep slopes on forests, in or near bogs

P. radiata2 Mexico, California Rocky volcanic ridges or slopes in coastal fog belt

P. serotina2 Eastern USA Flooded coastal bottom lands, shrub-dominated coastal wetlands (pocosins)

P. pumila2 China, Japan, Korea, Kurile Is. Sakhalin Is. Siberia

Sub-alpine to sub-Arctic sites on poor soils, montane to sea-level

P. canariensis5 Westernmost Canary Islands; Gran Canaria, Tenerife, La Gomera, La Palma and El Hierro

Upper montane regions on dry slopes in cloud-cover zone

P. pinaster2 Western Mediterranean Coastal sand dunes and neighbouring mountains

Sequoia sempervirens4,6 Northern California and South Oregon

Coastal fog belt beginning a few km from the coast but with an eastern limit 35 km or more inland

Taxodium distichum7 Coast of the south-eastern United States

Coastal freshwater wetlands

Nyssa aquatica7 Coast of the southeastern United States

Coastal freshwater wetlands

Podocarpus totara8 Throughout New Zealand In lowland and montane forests Metrosideros umbellata8 Throughout New Zealand Sea level to 760 m- rare in north Dracophyllum longifolium8,9 Throughout New Zealand Widespread - coastal lowland

and sub-alpine scrub and sub-antarctic islands.

Arbutus unedo11 S.W. Ireland and Mediterranean littoral

Common in coastal matorral and cork oak woods

Laurus azorica12 Azores, Madiera and Canary Islands

From 175-500 m in laurel-juniper native cloud-zone forests

Persea indica12 Azores, Madiera and Canary Islands

Generally above 200 m in native cloud-zone forests

References 1Fujita (1998); 2Laderman (1998b); 3 Johnson, 1973; 4 Barbour & Billings, 1988; 5Gomez et al., 2003; 6Dawson (1998), 7McWilliams et al., 1998; 8Wardle (1991); 9 McGlone & Moar (1997); 11Mitchell (1993); 12 Sjögren (2001).

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Fig.1. Looking down on forest of Canary Pine (Pinus canariensis) emerging through cloud zone at 1800 m on Pico de Teide, Tenerife, Canary Islands.

Fig. 2. Close up view showing needle length (up to 270 mm) in clusters of Canary Pine needles which enhance the capacity of the forest to condense dew from the NW trade winds.

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Many of the tree species in these maritime forests share characteristics which have

similarities with the oceanic heathlands of N.W. Europe, particularly with regard to being

evergreen, possessing sclerophyllous foliage, inhabiting nutrient-poor soils, and having a

requirement for high levels of aerial moisture. They also illustrate the limiting effects of

such oceanic environments, which have been described as “success through failure”

(Laderman 1988b), in that these coastally restricted forests survive in regions where other

species have found it impossible to survive.

It is not necessary to have to search the shores of the Pacific Ocean for examples of

arborescent refugia species with an ericoid growth form. The strawberry tree (Arbutus

unedo) is a member of the Ericaceae that has long been a member of the Irish Flora. There

has been debate as to where the strawberry tree is truly indigenous to Ireland or whether it

was introduced by monks during their travels from the Continent in the early Christian

period. However, investigations of the pollen-record have now shown it to have been

present for over 3000 years in Co. Kerry and 2000 years in Co. Sligo (Mitchell 1993) and

therefore that this tree can be added to the world-wide list of ericoid tree forms that

survive as relict forests on islands.

ISLAND FOREST CASE HISTORIES

___________________________

MACERONESESIA

The islands of Maceronesia (The Canary Islands, Madeira and the Azores) have all had

their tree cover drastically altered by human settlement. Madeira, and the Azores in

contrast to the Canary Islands had no autochthonous human populations and were first

settled with the arrival of Europeans in the early 15th century. The immediate effect of the

human settlement was the large-scale removal of this forest. In Madeira there is evidence

that much of the forest was removed by devastating fires (Sziemer, 2000). The forests that

remain are in general severely limited by drought. Fortunately, the Maceronesian Islands

lie in the path of the NW trade winds and this combined with the presence of high

mountains is hydrologically advantageous. When these winds meet the northern sides of

the mountains they rise and form a cloud-zone from which the remnants of the ancient

Tertiary laurel and pine forests are able to augment their water supply by the condensation

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of dew (Aboal et al., 2000). A feature common to all the Maceronesian islands are the

relict stands of laurel forests which thrived in these islands in the Tertiary period. As a

result of human occupation these relict forests are found now mostly at high altitudes in

mountain retreats. In the Azores this is generally above 500 m where the cloud-zone is

most persistent and the humidity almost permanently above 80-90 per cent (Sjögren,

pers.com.). The north-facing cloud-covered slopes are also the favoured sites for the

indigenous canary pine (Pinus canariensis) which can be seen typically in Tenerife on the

northern slopes of the Pico de Teide (3718 m). This canary pine has the longest needles of

any pine species (up to 270 mm) which provide a large surface area for condensing dew

out of the mountain cloud cover (Figs. 1-2).

NORTHERN MARITIME FORESTS

The warm-temperate and sub-tropical Atlantic Islands clearly benefit from the

humidity that can be provided by moisture-laden on-shore winds. Less fortunate however,

are the islands of the North Atlantic where prolonged winters, high rainfall and short

growing seasons, often interrupted by periods of adverse weather and strong winds, can

have a negative effect on tree growth and regeneration. Consequently, Iceland, the Faeroe

Islands, the Hebrides, Orkney, Shetland and much of western Ireland, have landscapes that

are largely treeless. Furthermore, the length of time that they have been treeless has lead to

a common acceptance that it is the adverse climate of these coastal regions that causes the

lack of trees. However, it is necessary to remember that in these tree-impoverished North

Atlantic islands, even the remotest locations, can have a history dating from the early

Neolithic, of human disturbance through felling, burning, and livestock grazing. In the

cool, moist, conditions of the North Atlantic the removal of trees has frequently lead to

soil deterioration resulting in many cases in the growth of moorlands. The reduction in

evapotransporation through the lack of trees, together with iron-pan formation in leached

soils has led to the expansion of bogs (paludification). In these denuded landscapes, long

winters and the highly variable climatic conditions of the north Atlantic coupled with

centuries of landscape degradation and extensive peat formation, greatly hinder both

natural and planted forest regeneration. Re-afforestation, in these hyper-oceanic habitats

although not impossible, requires careful management in terms of providing drainage,

shelter and protection from grazing.

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Fig. 3. Comparison of present and past northern limits for tree survival in northern Siberia. Present day distribution of the Boreal forest (brown) is based on the vegetation map produced by Grid Arendal and published by the World Wildlife Fund for Nature. Mid-Holocene limits to forest trees are regional generalisations from locations of fossil remains (green = evergreen - pine and/or spruce spp; red = tree-birch; purple = larch) based on Kremenetski et al., 1998. Modern limits for the northern survival of individual tree species (colours as above) are also taken from Kremenetski et al., 1998 as drawn by Callaghan, et al, 2002.

It is probable that the natural woodlands of the North Atlantic islands were already

being affected adversely by an increase in oceanicity before there was any substantial

human settlement in the region. In the Northern Isles of Scotland some opening of the tree

canopy on the island of Unst (Shetland) is detectable before the arrival of the first

Neolithic settlers approximately 3500 years ago (Bennett et al. 1992). A general increase

in oceanicity at this time can be detected from the Maritime provenances of Canada across

the British Isles and northern Scandinavia to Russia (Crawford 2000, 2003). Similarly, the

forests of Cananda’s maritime provenances, although not islands, are profoundly

influenced by the sea. In maritime sub-arctic Québec, forests of black spruce (Picea

mariana) have been in retreat since the inception of active peat growth from about 6000

BP (Payette and Lavoie 1994). The Russian West Siberian Lowlands provide another

example of the adverse effects of oceanicity on tree survival in a region with minimal

human settlement. Here again, an increase in oceanic conditions beginning 6000 years ago

is thought to be the main cause of forest retreat. At the time of the hypsithermal climatic

optimum in this region (approx. 8-6000 BP), tree cover in Siberia extended almost to the

shores of the Arctic Ocean (Fig. 3). However, since then the boreal treeline in the West

Siberian Lowlands has retreated southwards by 300-400 km (Kremenetski et al. 1998).

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Given the sensitivity of the tree-cover in many maritime forests to increasing

oceanicity it is not surprising that there is a rapid deterioration in forest cover when human

disturbance is super-imposed in addition to the already sub-optimal conditions of low-

temperature oceanicity. The late Norse settlement of Iceland (ca. 870 AD) provides a

relatively recent example of human colonisation in a North Atlantic Island and illustrates

the speed with which deforestation can take place in an oceanic environment. Studies of

pollen and plant macrofossils show that dense birch forest was present from 6900 BP

onwards (Hallsdóttir 1987; Rundgren 1998). Ari the Learned, writing in 1120-30 AD, in

Islendingabók , states that “at that time (the time of the settlement) Iceland was covered by

woodland from the mountains to the coast.” It is unlikely that the tree-line in Iceland ever

exceeded 300-400m (Hallsdóttir, 1987), yet Ari’s description nevertheless testifies to a

marked change by the 12th century from the earlier situation. The Landnámabók also

records that the early settlers had to clear trees to make it suitable for farming (Palsson

and Edwards 1972). Icelandic land-claim laws contained in the Grágás laws (as recorded

in codexes from about 1270) and pertaining certainly to the twelfth century and probably

also the eleventh (Foote, pers. comm.) define very precise rules governing forest

utilisation for fuel and building. Careful distinctions are made as to whether wood is to be

taken by cutting or pulling and whether or not meadow is allowed to replace woodland.

Conservation is also an issue as there are rules in place where joint owners of woodland

may have disputes with regard to woodlands being over-used. There are even penalties for

hacking notches in a tree or causing scrapes that result in damage. Similarly, the penalties

for browsing another man’s trees are greater than those exacted for grazing his grassland.

There are also precise laws governing the exploitation of new growth from old trees

(Dennis et al. 1980). The detail of these laws strongly suggests that by the 12th century

woodland was regarded as a very important asset and no longer something which farmers

had an automatic right to remove.

Accurate dating of vegetation changes just before and just after the time of the

Landnám has been possible due to the tephra (LNL) deposited from the eruption of the

Veidivötn volcanic system now dated from the Greenland GRIP ice-core to AD 871 ± 2

(Grönvold et al. 1995). The differences in the pollen record below and above the tephra-

layer show that the decline of birch was greatly accelerated after the settlement and was

accompanied by an expansion of waterlogged soils and mires (Hallsdóttir 1987). Thus,

Iceland retained much of its tree-cover despite exposure to the hyper-oceanic conditions of

the North Atlantic throughout most of the Holocene. It was only after the Norse

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settlement, that it disappeared with great rapidity. Throughout the islands of the North

Atlantic natural stands of woodland can be found in habitats free from grazing, such as

steep gullies and islands in inland lochs which demonstrates that even in exposed areas,

trees can survive under strongly maritime conditions provided that they are undisturbed by

pastoral activity. It is when human disturbance is coupled with the maritime environment

that tree-cover becomes greatly reduced.

TREE REPLACEMENT BY HEATH AND BOG

In many parts of Ireland, Scotland, and Western Norway, grazing and burning have

hindered tree-regeneration. Where the annual potential precipitation deficit is low the

removal of trees can lead to the soil profile remaining saturated for much of the year and

can gradually promote paludification (bog growth). In Ireland and Scotland this has

frequently been accelerated due to the impoverishment of the soil by the activities of early

farmers. In oceanic environments soils are seldom frozen and consequently nutrient

leaching takes place throughout the year, which accelerates soil impoverishment. Iron pan

formation can also contribute to water-logging which hinders mineralization and nitrogen

fixation. The development of ploughing technology in the Iron Age, would have increased

soil leaching and in oceanic areas this would have lead to the podzolisation of many soils -

a process which had already begun in many western European locations with extensive

production of heathlands in the early Neolithic (Behre 1988). A particular case of oceanic

heathland development is found in the 25 km wide belt of heathlands that extends from

western Norway to the Arctic Circle with the oldest heaths occurring in the extreme west

of Norway and dating to the Neolithic (4300 BP) with later extensions around 0 AD, and

again during the Viking age (Kaland 1986). The inevitable formation of iron pans in

nutrient-poor leached soils, aided by primitive agriculture with repeated shallow

ploughing, can lead to the development of gley soils. This is followed by bog growth

which further decreases the inputs of nitrogen and phosphorus and creates a deepening

nutrient-flow trap in oceanic areas (Dodgshon 1994). In many maritime regions this entire

scenario can be described as paludification, as former mineral soils with unimpeded

drainage were gradually converted to bogs resulting in the abandonment of Neolithic and

Iron Age farms. Frequently, after peat cutting signs of this early farming activity on

mineral soils can be seen in the remains of ancient walls from the Iron Age, or earlier, as

the farmers retreated before the unrelenting advance of the bog. In Ireland there are over

50 known locations of pre-historic farms, which became engulfed by peat through

paludification. The most famous is the early Neolithic site at Céide Fields (Co. Mayo)

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where an agricultural landscape with walled fields from 4000 BP was eventually buried by

4 m of peat (Mitchell and Ryan 1998).

More recent examples of the susceptibility of northern regions to climatic

fluctuations during the Little Ice Age in the oceanic areas of north-western Europe are

found in the closing years of the 17th century which were marked by a series of volcanic

eruptions; Hekla in Iceland and Serua in Indonesia in 1693 followed by Aboina in

Indonesia in 1694. These presumably large extrusions of dust into the atmosphere may

have been responsible for the series of harvest failures in Finland, Estonia, and Iceland

with widespread famine leading to large reductions in the human population. The period

is remembered in Scotland as the ‘seven ill years’ (1693-1700) when a succession of

disastrous summers caused widespread starvation soon followed by political union with

England (Morrison 1990). More recent evidence for the effect of volcanic eruptions in

cooling yet further the already cool oceanic climates of North Atlantic islands can be

found in dendrological studies. The sensitivity of northern climates to the adverse affects

of volcanic eruptions is visible in the effects that are left recorded in tree rings in

continental and oceanic areas after an eruption. After the Tambora (Indonesia) eruption of

1815 the oak trees in the great French forests of Fontainebleau and Tronçais benefited

from the cooler, wetter summers while in the oceanic conditions of Ireland, cooler, wetter

summers reduced tree growth (Pilcher 1997).

NEW ZEALAND AND THE SUB-ANTARCTIC ISLAND FORESTS

In terms of oceanicity few countries can compare with New Zealand. High average wind

speeds, heavy rainfall, and rapid soil erosion, are all consequences of a strongly maritime

environment. New Zealand is an unsurpassed example of how climate and

biogeographical isolation have preserved ancient forests. The montane Southern beech

(Nothofagus spp.) and coastal Podocarp forests (e.g. Podocarpus tortora) now sadly much

depleted, still thrive particularly in the more oceanic regions of South Island (Wardle,

1991). It is however in the uninhabited sub-Antarctic islands, lying 600 km south of the

mainland of New Zealand (Fig. 4) away from the complexities of human disturbance that

the effects of oscillating oceanic conditions on island forests can be best studied. Auckland Island, a small uninhabited sub-antarctic island south of the New Zealand

mainland (c.51˚S), is the

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Fig. 4. Map showing location of New Zealand’s sub-antarctic islands. The Auckland Islands are the most southerly islands in the Tasman Sea to have forest cover.

Fig. 5. Coastal forest fringe on northern Auckland Island, showing wind-pruned forest canopy dominated Metrosideros umbellatai (photo by courtesy of Dr. M.S. McGlone).

Fig. 6. Enderby Island, the most northerly of the Auckland Islands (50˚ 30’ S). View taken near sea level. Solitary tree of Dracophyllum longifolium emerging from Myrsine divaricata scrub (photo by courtesy of Dr. M.S. McGlone).

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southernmost outpost of tall forest in the southwest Pacific (McGlone et al. 1997;

McGlone et al. 2000). Pollen analyses studies of two closely adjacent peat cores near the

treeline on this island have revealed a significant relationship between climate change

and forest dynamics during the Late Glacial and Holocene. Forest tree species

(Metrosideros umbellata, Dracophyllum longifolium and Raukaua simplex) spread slowly

from 10-000 C14 yr BP, but did not form tall forest until 5500-4000 C14 yr BP, despite

southern ocean temperatures being warmer in the early Holocene (Figs.5-6). It appears

from these studies that warm, cloudy, low-radiation environments inhibited forest growth

during the early Holocene in this intensely oceanic setting, through promoting saturated

soils and reducing net photosynthesis. It was only in the later Holocene, when increased

westerly wind flow brought sunnier, although cooler and windier climates, that forest re-

expansion occurred on sheltered lowland sites. 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 and Moar 1997).

FUTURE MIGRATION TRENDS IN RESPONSE TO CLIMATIC WARMING

Climatic warming in northern latitudes will undoubtedly create new ecological

opportunities for vegetation advance as ice sheets retreat and permanent snow cover is

reduced. Depending on proximity to the oceans, and this includes the Arctic Ocean, the

degree of winter versus summer warming is likely to differ. Using an objective manner of

comparing species distribution with the interaction between winter and summer

temperatures (Jeffree and Jeffree 1994), it has been possible to plot the climatic

temperature preferences in terms of the temperatures of the coldest (tx) and warmest (ty)

months of the year recorded at locations within a species’ geographical distribution. This

approach enables the preparation of maps, which show the probability of occurrence of the

species in relation to temperature. Using this method it has been possible to model some

of the commonest woody species. The model suggests that in oceanic regions increased

winter warming may in some cases cause a significant retreat while in other areas the

species may make significant advances.

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Fig 7. Comparison of possible contrasting changes in distribution of Pinus sylvestris with varying climate climatic conditions as compared with 1961-1990. (a) Winter 4C˚ colder, summer 4C˚warmer; (b) winter 4C˚ warmer, summer 4C˚ colder. In plots (a-b) the colours represent bands of increasing probability of the (x), winter and (y) summer temperatures being suitable for the species. Red is most suitable (see inserted scale); note the retreat from western Europe with the imposition of warmer winters (adapted from Crawford and Jeffree, 2005).

Examination of potential changes in distribution for Pinus sylvestris (Fig.7a-b) has

shown that winter warming is likely to cause a retreat from western oceanic areas. Similar

trends have been found in woody scrub species e.g. Cassiope hypnoides, Vaccinium

myrtillus, Calluna vulgaris and Salix polaris (Crawford and Jeffree 2005). Such a trend

mirrors the existing tendency for certain species to retreat from maritime habitats in

response to increases in oceanicity.

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PHYSIOLIOGICAL DISADVANTAGES OF WARM WINTERS IN OCEANIC

CLIMATES _____________________________________

Some physiological explanation is required to account for the deleterious effects of milder

oceanic conditions on woody species. The Norwegian plant ecologist Eilif Dahl was one

of the first to distinguish between the positive and negative effects of oceanic conditions

on mountain plants. The relatively species-poor montane floras of the Scottish Highlands

and south-west Norway, were considered by Dahl to be due to mild periods of winter

weather that encouraged premature spring growth causing severe die-back of non-hardy

shoots.

It may seem counter intuitive, but there is even an argument for suggesting that in

oceanic areas climatic warming may lead to a retreat rather than an advance of the treeline.

Examination of temperature variations over the past century for Europe and the Arctic

from Northern Norway to Siberia suggests that variations in the North Atlantic Oscillation

are associated with an increase in oceanicity in certain maritime regions. A southward

depression of the treeline in favour of wet heaths, bogs and wetland tundra communities is

also observed in several northern oceanic environments. The heightened values currently

detected in the North Atlantic Oscillation Index, together with rising winter temperatures,

and increased rainfall in many areas in Northern Europe, present an increasing risk of

paludification with adverse consequences for forest regeneration, particularly in areas with

oceanic climates. Climatic warming in oceanic areas may increase the area covered by

bogs and thus, contrary to general expectations, may lead to a retreat rather than an

advance in the northern limit of the boreal forest (Crawford et al. 2003).

A detailed dendro-climatological study of Scots Pine (Pinus sylvestris) in northern

Norway (69˚N) has shown that high winter temperatures represent a stress factor at the

limit of pine in oceanic habitats. Consequently, a period around 1920, with low winter

temperatures, coincided with a marked rise in growth (Kirchhefer 2001). Similarly, studies

on Vaccinium myrtillus (Ögren 1996), have shown that mild periods in winter, cause

carbohydrate levels to fall and progressively reduce frost hardiness. Likewise in northern

Norway Pinus sylvestris has been found to have a tendency for early bud- break and

increasing heavy needle loss after mild winters and this appears to be associated with poor

growth in summer after a series of mild oceanic winters (Kirchhefer 1999).

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There may be also be many other reasons, metabolic, phenological, pathogenic,

(fungal and insect attacks) as well as ecological consequences through competition, that

may account for oceanic conditions being detrimental for the survival of Scots Pine and

other woody species in areas with warm-wet winters. The oceanic habitat and northern

latitudes is very favourable for the growth of bryophytes. Consequently, climatic warming

may induce increased bog growth which would be disadvantageous for the regeneration of

trees to an extent that did not occur at the time of their the early Holocene migrations.

Warm weather in autumn and early winter will delay the onset of dormancy in tree roots.

When this is followed by soil-saturation or flooding it can induce anaerobic conditions

which result in a rapid consumption of winter carbohydrate reserves in root meristems

which are crucial for the successful initiation of metabolic activity and new root growth in

spring and defence against the risk of post-anoxic injury when soil aeration is restored

after prolonged winter-flooding (Crawford 2003). It is therefore not necessarily

appropriate to relate the carbon balance of the whole tree in assessing the possible dangers

of subjecting plants to wet soils in winter. Instead, it is the vulnerability of certain specific

sensitive tissues to repeated environmental stress that determines the long-term probability

of survival. If the roots die back each year due to winter-flooding the tree may survive for

a while but will eventually become unstable and will be eliminated by premature wind-

throw (Coutts et al., 1999). These possible physiological explanations may underlie the

predicted effects of warmer winters as shown in Fig. 7, which suggests that in the event of

warmer winters prevailing there could be a marked retreat of Scots pine from Western

Europe.

CONCLUSIONS

The effects of proximity to the ocean on tree growth and survival vary with

geographical location. In warm, temperate and sub-tropical regions many coastal and

island habitats provide refugia for the once circum-polar species of the evergreen circum-

polar forests of the Tertiary period. By contrast in northern oceanic climates, where

winters can be long and variable, warm oceanic interludes can prove physiologically and

ecologically disadvantageous. In particular there can be certain parts of the tree, such as

exposed buds or non-dormant root meristems that may be affected. It is possible that the

adverse effects of winter conditions act specifically on certain vulnerable tissues and that

this determines the probability of long-term success or failure of trees rather than any

overall effects of carbon balance as determined for the whole tree. There is therefore a

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delicate ecological balance for woody plants, and in particular trees, in relation to growing

near the sea. The advantages and disadvantages of warm winters can depend greatly on the

variability and length of the winter season, as well as on the extent to which short-term

temperature fluctuations occur both above and below the freezing point.

ACKNOWLEDGEMENTS

_________________________

I am much indebted to Dr. M.S. McGlone for information and photographs in relation to

forest cover in New Zealand’s sub-antarctic islands, to Dr. E. Sjögren for information on

the trees of the Azores and to Dr. C.E. Jeffree for modelling the effects of winter

temperatures on the distribution of Pinus sylvestris

REFERENCES

_________________

Aboal, J.R., Jimenez, M.S., Morales, D., and Gil, P. 2000 Effects of thinning on

throughfall in Canary Islands pine forest - the role of fog. Journal of

Hydrology, 238, 218-230.

Barbour, M.G. and Billings, W.D. (eds) 1988 North American terrestrial vegetation,

pp 434. Cambridge University press, Cambridge.

Behre, K.E. 1988 The role of man in European vegetation history. In B. Huntley and

T. Webb III (eds), Vegetation History, 633-672. Kluwer, Dordercht.

Bennett, K.D., Boreham, S., Sharp, M.J., and Switsur, V.R. 1992 Holocene history of

environment, vegetation and human settlement on Catta Ness, Lunnasting,

Shetland. Journal of Ecology, 80, 241-273.

Callaghan, T.V., Werkman, B.R., & Crawford, R.M.M. (2002) The tundra-taiga interface and its dynamics: Concepts and applications. Ambio, 6-14.

Coutts, M.P., Nielsen, C.C.N., and Nicoll, B.C. 1999 The development of symmetry,

17

rigidity and anchorage in the structural root system of conifers. Plant and Soil,

217, 1-15.

Crawford, R.M.M. 2000 Ecological hazards of oceanic environments. New

Phytologist, 147, 257-281.

Crawford, R.M.M. 2003 Seasonal differences in plant responses to flooding and

anoxia. Canadian Journal of Botany-Revue Canadienne de Botanique, 81,

1224-1246.

Crawford, R.M.M. and Jeffree, C.E. 2005 Northern climates and woody plant

distribution. In J.B. Oerbaek (ed), Environmental challenges in Arctic-Alpine

regions, 1-20. Springer Verlag (in preparation), Berlin, Heidelberg.

Dawson, T.E. 1998 Fog in the California redwood forest: ecosystem inputs and use by

plants. Oecologia, 117, 476-485.

Dennis, A., Foote, P., and Perkins, R. (eds) 1980 Laws of early Iceland, University of

Manitoba Press.

Dodgshon, R.A. 1994 Budgeting for survival: nutrient flow and traditional Highland

farming. In S. Foster and T.C. Smout (eds),The History of Soils and Field

Systems, 83-93. Scottish Cultural Press, Aberdeen.

Fujita, H. 1998 Characteristics of the soil and water table in an Alnus japonica (Japanese alder) swamp. In A.D. Laderman (ed) Coastally restricted forests

pp. 187-198. Oxford University Press, Oxford.

Gomez, A., Gonzalez-Martinez, S.C., Collada, C., Climent, J., and Gil, L. 2003 Complex population genetic structure in the endemic Canary Island pine

revealed using chloroplast microsatellite markers. Theoretical and Applied

Genetics, 107, 1123-1131.

Grönvold, K., Oskarsson, N., Johnsen, S.J., Clausen, H.B., Hammer, C.U., Bond, G.,

and Bard, E. 1995 Ash layers from Iceland in the Greenland GRIP ice core

correlated with oceanic and land sediments. Earth and Planetary Science

Letters, 135, 149-155.

Hallsdóttir, M. 1987 Pollen analytical studies of human influence on vegetation in

relation to the Landám tephra layer in southwest Iceland. Ph.D. Thesis,

University of Lund, Lund.

18

Johnson, H. 1973 The international book of trees Mitchell Beazley, London.

Jeffree, E.P. and Jeffree, C.E. 1994 Temperature and the biogeographical distribution

of species. Functional Ecology, 8, 640-650.

Kaland, P.E. 1986 The origin and management of Norwegian coastal heathlands as

reflected by pollen analysis. In K.-E. Behre (ed.), Anthropogenic indicators in

pollen diagrams, 19-36. Balkema, Rotterbal.

Kirchhefer, A.J. 1999 Dendroclimatology on Scots pine (Pinus syllvestris) in northern

Norway. Ph.D. Thesis, University of Tromsø, Tromsø.

Kirchhefer, A.J. 2001 Reconstruction of summer temperatures from tree-rings of

Scots pine (Pinus sylvestris L.) in coastal northern Norway. Holocene, 11, 41-

52.

Kremenetski, C.V., Sulerzhitsky, L.D., and Hantemirov, R. 1998 Holocene history of

the northern range limits of some trees and shrubs in Russia. Arctic and Alpine

Research, 30, 317-333.

Laderman, A.D. (ed.) 1998a Coastally restricted forests, pp 334. Oxford University

Press, New York.

Laderman, A.D. 1988b Freshwater forests of continental margins: overview and

synthesis. In A.D. Laderman (ed.),Coastally restricted forests, 3-35. Oxford

University Press, New York.

McGlone, M.S. and Moar, N.T. 1997 Pollen-vegetation relationships on the

subantarctic Auckland Islands, New Zealand. Review of Palaeobotany and

Palynology, 96, 317-338.

McGlone, M.S., Moar, N.T., Wardle, P., and Meurk, C.D. 1997 Late-glacial and

Holocene vegetation and environment of Campbell Island, far southern New

Zealand. Holocene, 7, 1-12.

McGlone, M.S., Wilmshurst, J.M., and Wiser, S.K. 2000 Lateglacial and Holocene

vegetation and climatic change on Auckland Island, Subantarctic New

Zealand. Holocene, 10, 719-728.

McWilliams, W.H., Tansney, J.B., Birch, T.W., and Hansen, M.H. 1998 Taxodium-

Nyssa (Cypress-Tupelo) forests along the coast of the Southern United States.

19

In A.D. Lademan(ed.) Coastally resticted forests pp. 257-270. Oxford

University Press, Oxford.

Mitchell, F. and Ryan, M. 1998 Reading the Irish Landscape Town House, Dublin.

Mitchell, F.J.G. 1993 The biogeographical implications of the distribution and history

of the strawberry tree, Arbutus unedo, in Ireland. In M.J. Costello and K.S.

Kelly (eds),Occassional Publications of the Irish Geographical Society, 35-

44, Dublin.

Morrison, I. 1990 Climatic changes and human geography: Scotland in a North

Atlantic context. Northern Studies, 27, 1-11.

Ögren, E. 1996 Premature dehardening in Vaccinium myrtillus during a mild winter:

A cause for winter dieback? Functional Ecology, 10, 724-732.

Palsson, H. and Edwards, P.(eds) 1972 The book of settlements, pp 159. University of

Manitoba, Winnipeg.

Payette, S. and Lavoie, C. 1994 The arctic treeline as a record of past and recent

climatic changes. Environmental review, 2, 135-138.

Pilcher, J.R. 1997. The IGBP PAGES Core Project and the application of tree-rings

for chronological control in PAGES activities. In J. Sweeney (ed.) Global

change and the Irish environment, 9-16. Royal Irish Academy, Dublin.

Rundgren, M. 1998 Early-Holocene vegetation of northern Iceland: pollen and plant

macrofossil evidence from the Skagi peninsula. Holocene, 8, 553-564.

Sjögren, E. 2001 Plants and flowers of the Azores Jonas Sjögren (tel. +46 733 20 80 70), Uppsala.

Sziemer, P. 2000 Madiera's natural history in a nutshell Ribiero, Funchal.

Wardle, P. 1991 Vegetation of New Zealand Cambridge Univiersity Press,

Cambridge.