杏吧原创

FOR the past two million years or so, what鈥檚 called the Quaternary period,
the Earth鈥檚 climate has blown both hot and cold. So far, it has fluctuated from
cold ice age or glacial period to warm interglacial period some 50 times during
the Quaternary, with the last ice age ending 10 000 years ago.

Vast ice sheets formed over huge areas of Northern Europe and North America
during the cold phases. These left many telltale marks, typically deep U-shaped
valleys with scarified rock surfaces and basins filled with water, forming deep
lakes. Much of the land that was once blanketed with ice is now covered with
rock debris including boulders scoured from the rock and first carried forward
by ice sheets as they spread and then left abandoned as they retreated.

Frozen habitats

Glacial refugia

At various times before the last glacial period, straight-tusked elephants,
aurochs, bison, fallow deer, hippopotamuses, cave lions and hyenas roamed
southern Britain. Their habitats were typically deciduous woodlands, largely
oak, elm, hazel, lime, birch and alder. But when the ice came back, it was
impossibly cold for these animals and their habitats. And during the most recent
glacial period, an ice sheet covered most of Britain except the South and parts
of the Midlands. Frozen ground, or permafrost, extended far into France
(see Figure 1).

When ice began to retreat 10 000 years ago, few if any of these animals or
plants were still living in the same places. In Britain, trees such as birch and
juniper had invaded the hitherto barren zone between the ice and the timber
line, the so-called tundra zone. We know, though, from fossil remains that polar
bears, musk ox, reindeer and mammoths lived at least as far south as the Thames
Valley during that ice age. So, we are confronted by a major ecological problem:
what happened to the trees, other plants, animals and micro-organisms when
the ice age set in? It鈥檚 easy to think of animals such as birds moving in
response to any large-scale climate changes, but plants are literally rooted to
the spot, or are they? Plants do move between generations by dispersing their
seeds but could this account for their widespread movement?

Brian Huntley of the University of Durham and Tom Webb of Brown University,
Rhode Island, have suggested that trees respond to climate change much as birds
do. As the climate changes from summer through autumn to winter, many bird
species migrate to areas with more favourable climates (see Inside Science No.
56). Think of plant populations responding not to these short time scales but
instead to the larger time scales of glacial/interglacial cycles. So if the
trees and many other plants of north and central Europe could not survive the
big freeze where they were, where did the ancestors of, say, Britain鈥檚 oaks,
limes and elms wait out the ice age?

Palynology, the analysis of fossil pollen, is beginning to provide an answer
to this question. It seems that these tree species were able to
鈥渉ide out鈥� in the Mediterranean peninsulas鈥擨beria (Spain and Portugal),
Italy and the Balkans. It would be wrong, though, to think of such areas at the
time of the northern glaciation becoming covered with large areas of woodland.
All evidence points to tree species surviving in small pockets on the mountain
sides in southern Europe. There are two main advantages for the trees in taking
to the hills on such occasions. The greater rainfall in the mountains would have
been important, as much of southern Europe was dry because vast amounts of water
was by then locked up in the northern ice sheets. Also, as any hill walker
knows, it becomes warmer as you climb back down from the summit of a mountain.
Living in the hills would let tree populations handle minor climate change by
moving up or down the hillside to find the best growing conditions.

The areas where these species managed to survive the ice age are known as
refugia; they provided refuges from the arctic conditions further north. Some
species of trees, such as oak and pine, seem to have been present in all the
Mediterranean refugia. Others seem to have been more local, for example lime was
found mainly in the more eastern refugia
(see Figure 1).FIG-mg21788601.JPG

While most evidence for these refugia comes from fossil pollen, the genetics
of modern populations can also cast some light on where the ancestors of these
trees spent the last ice age. For example, Colin Ferris and his colleagues at
the University of East Anglia have studied the DNA in the chloroplasts of
European oaks. Chloroplasts are the part of a plant cell where photosynthesis
occurs (see Inside Science No. 18). They found that there were two distinct
types of chloroplast, one in eastern Europe and one in western Europe. They
believe these are the result of mutations that occurred in two different glacial
refugia, one in the eastern Mediterranean and one in the western Mediterranean.
Recent studies of chloroplast DNA from the European beech have raised the
possibility of refugia in central Europe or western Asia for this species.

The obvious question now is: how did the trees return to places like Britain
when the ice melted? Think of an oak tree. An acorn falls from a tree, bounces
and rolls a few metres. Then, if it鈥檚 lucky, it germinates and begins to grow
into a new tree. Between 15 and 20 years later it starts to produce acorns of
its own and the process is repeated. It is clearly a slow process, but would it
be enough for oaks to make their way back from the Mediterranean refugia to
Britain in the 10 000 years since the end of the last ice age? To determine this
you have to map the spread of trees across Europe from the samples of fossil
pollen collected so far: the results are striking.

Speedy trees

Wind or bird transport

Most of the trees had maximum rates of movement of at least 500 metres a
year, and many moved even faster .

This is far faster than botanists had calculated British oak could move
(around 50 metres every 20 years at best). It is even faster than you would
expect for wind-blown tree seeds, such as ash or lime, whose seeds weigh about
10 milligrams (which compares markedly with the average acorn of around 3000
milligrams). They would not be blown far enough to match the rates from the
fossil data. So what else was going on?

Several botanists noticed that wind dispersed trees and nut trees, such as
oak, moved at similar rates in both Europe and North America at the end of the
last ice age. It led them to suggest that the nuts of trees such as oak and
beech must have been transported by birds, such as jays. And they went on to
conclude that this was so effective that it was possible even for trees with
heavy nuts to move as quickly as the wind-dispersed species. Yet, this is the
wrong way of looking at the problem. People expected wind-dispersed
seeds to move faster that other seeds, so they assumed that bird transport
enabled acorns and the like to move as fast as wind-blown seeds. But to explain
how the wind-blown seeds moved as fast as they did required that they too needed
help from birds to do so. Speeds recorded in the fossil data seem too fast even
for the wind dispersal of tree seeds. So both the tree species that botanists
usually think of as wind-dispersed (for example, ash, lime and elm) and species
that are normally dispersed by animals (for example, oak, beech and hazel), must
rely on help from birds to achieve this speed.

Many crow species, such as jays, are known to carry nuts several kilometres
before they hide them for future use. Not all these seeds will be recovered by
the birds and some can grow into mature trees鈥攕omething that Greek
philosopher Theophrastus knew as long ago as 300 BC. Squirrels have a similar
habit yet they don鈥檛 move anything like as far or as fast as crows and so could
not have helped the trees to migrate fast enough. (An obvious implication of
this idea is that plants could not have responded to climatic changes as quickly
before the birds evolved as they could when birds were about to carry them.
Plants would have had to rely on other, slower means of seed dispersal.)

On rare occasions, birds can enable seeds to make a jump of hundreds of
miles. With thousands of years to play with, rare events become very important.
The problem is that biologists don鈥檛 often stumble across fossil evidence for
such rare events, so it makes them difficult to study. However, Richard Bradshaw
of the Swedish Forest Research Centre has radiocarbon dated fossil wood and
found that Norway spruce reached very limited areas of Sweden about 8000 years
ago鈥�3000 years before the spruce became widespread in the area. Recently,
Lief Kullman of Ume氓 University, Sweden, found seed and leaf remains of several
species of temperate broad-leaved tree species (for example alder, hazel, oak
and elm) in the Scandes Mountains in Sweden. Radiocarbon dating told him that
these trees lived around 8500 years ago. This shows that the trees migrated very
rapidly from their southern refugia. Were these early northern occurrences of
the species due to rare long-distance bird dispersal? It鈥檚 difficult to think of
any other mechanisms which would be sufficiently fast.

Not all return home

Migrating herbs

If trees were able to move north at such speed at the end of an ice age, did
the opposite happen at the beginning of the big freeze? Possibly not. Keith
Bennett of Uppsala University has pointed out that most of northern Europe鈥檚
tree species are still also found somewhere near the Mediterranean. This seems
to suggest that when the climate cools the northern trees just die off, to be
replaced by new stock from the south in the next warm period. If he is correct
(and not every one is convinced) then trees with Mediterranean ancestors have
made repeated forays to Britain during the past million years, but trees with
British origins never made it to the south. This would mean that the trees of
northern Europe are an evolutionary dead end. Anything of importance in the
evolution of European trees, and many other plants, in the past 1 or 2 million
years must have happened in southern Europe.

Because of their size, trees play an important part in altering the
environment for other species living around them. But what of the smaller
species of plant, such as the herbs which live on woodland floors? It is often
claimed that such herbs 鈥渕igrate鈥� only very slowly, so are good indicators that
woodland has existed undisturbed on a site for a long time. However, pollen
analysis shows that this is not always true.

Michael Cain of New Mexico State University, working with Hans Damman and
Angela Muir of Carleton University, Ottawa, have studied the modern seed
dispersal of the North American wild ginger, Asarum canadense. They
found that ants can move an individual seed of wild ginger up to a some 35 metres
(see Figure 2).
This would mean that in the 16 000 years since the height
of the last ice age (glacial maximum) wild ginger ought to have migrated only 10
or 11 kilometres from its refugia. In reality the plant has moved something like
450 kilometres in this time. So again there is evidence of some rare events
orchestrating long-distance movements. The moving force might well have
involved, for example, birds or tornadoes. Cain and his team decided to search
the literature on herb species for any signs of other such rare events. They
found many instances of disparity between the distance covered by the herbs
since the last ice age and the speed botanists have seen them move year-to-year
in the wild.

In their report, Cain and his colleagues compared these results for herbs
with the idea that birds played a part in the rapid migrations of trees
following glacial periods. They concluded that occasional 鈥渁ccidental鈥� events
may provide the means by which most plants move in response to climate change.
In the 19th century, Charles Darwin reached similar conclusions. He referred to
the importance of 鈥渙ccasional means of distribution鈥�.

In recent years, botanists have become increasingly interested in how plants
have responded to past climate changes in the hope of throwing light on how
plants may cope with future climate changes. Changes in temperature, water
availability and the concentration of carbon dioxide all have major effects on
the species of plants that are able to grow at a particular site (see Inside
Science No. 21). A rise in temperature of 2.5 掳C is roughly equal to moving
400 metres up a mountain or 300 kilometres in latitude towards the equator.

One of the main lessons that seems to emerge from looking at such past
changes is that plants mainly respond to changing climates by
migration鈥攏ot evolutionary adaptation. Evidence from fossil pollen also
allows us to estimate these migration rates.

Understanding the way in which plants move in response to changing climate is
of great importance if we are to make accurate predictions on the effects of
global warming. Birds can carry seeds much greater distances than seeds could
travel unaided. James Dyer of the University of Ohio has developed a
mathematical computer-based model of plant dispersal which illustrates this
well. Such modelling, although a gross simplification of the reality, means a
researcher can run a series of experiments in a virtual world with different
rates of climate change or different landscapes through which the plants
migrate. These experiments would be impossible in the real world. Many changes
caused by an enhanced greenhouse effect are predicted to happen much faster than
those at the end of the last glacial period (see Inside Science No. 92). And
there is at least an outside chance of ending up with a climate warmer than it
has been for 100 million years. If this were so then Dyer鈥檚 modelling suggests
that even the means of transport that birds offer seeds may be too slow to
safeguard many of our trees and herbs. In a greenhouse world, we human beings
may have to take on the role that hitherto has been left to the birds. We may
have to take the initiative and move plant seeds to areas where the climatic
conditions are better suited to them.

There are huge implications in all this for nature conservation. In the long
term, merely relying on a small number of nature reserves as a kind of
鈥渁rtificial refugia鈥� will be of limited use. Many of our woodland and forest
reserves may no longer be suitable for the organisms they were set up to protect
if the climate changes too much or too quickly.

It is clearly vital to preserve the countryside in a state that allows plants
and other organisms to migrate through it unimpaired when major climate changes
come. Fewer acorn-carrying jays and magpies now fly across major urban areas
such as London or Paris than in the past when these areas were more rural, so
our changes to the urban landscape are making it harder for plants to move in
response to climate change. The problems that modern plants face may be far
greater that those in the past.

Pollen grains and spores are highly resistant to decay, especially in lake
sediments and peat bogs. In these sites an absence of oxygen, which can diffuse
only slowly through waterlogged sediments, greatly reduces the rate of any
microbial decomposition. So when pollen becomes trapped in sediments as they are
laid down the grains provide a record of the vegetation that the pollen
originally came from.

If a core of sediment is collected and taken to the
laboratory, you can dissolve the sediment, leaving behind the pollen grains. If
you can identify the pollen under a microscope, you can reconstruct past
vegetation. It鈥檚 often a time-consuming task, with between 300 and 1000 pollen
grains to be identified from each layer of sediment in the core. And you may
need to look at as many as 100 levels to cover the last 10 000 years in any
detail. Because pollen can blow over large distances you need to interpret the
data with care. Larger plant remains such as seeds, leaves or wood can often be
found in the sediments as well as pollen. These macrofossils are much larger
than pollen, and provide useful extra evidence because they are less likely to
have started out a long way from the site. Bradshaw鈥檚 and Kullman鈥檚 studies,
already described, provide good examples.

If you can date the sediments containing the fossil pollen (for example, by
using radiocarbon dating) then you can find out when a plant species first
arrived at a particular site. The chief problem is deciding how much pollen you
need to find to be certain that the plant was growing locally. One or two pollen
grains could well have blown in from some distant site. There are a number of
different criteria for defining this, the two most common are illustrated
in Figure 3. Once you have worked out the occurrence of plant species for a
number of sites, you can draw up a map of past distribution.

Seeds are normally classified by their adaptations for dispersal.
But when we are looking for large-scale changes to track climate fluctuations,
signs of 鈥渞are events鈥� become important, such as wind-dispersed seeds that have
obviously travelled long distances and must have been carried by birds

CHARLES DARWIN is best remembered for his theoretical work on evolution and
natural selection and also for his studies on geology and biology made during
the voyage of the Beagle. However, he was also ahead of his time as an
experimental biologist. He was fascinated by the way such immobile organisms as
plants and snails managed to travel long distances and colonise new areas. To
study this, he carried out many experiments鈥攖echnically easy to perform
but producing highly important results. They require no complex equipment and
can easily be repeated.

Darwin thought that mud on the feet of birds could be important in the
movement of small seeds, so to test this he collected mud from several ponds and
experimented to see if he could grow any seeds from it. In one case, from as
much mud as 鈥渨ould fill a very large breakfast cup鈥� he grew 537 plants! So a
small amount of mud on the feet of a bird could transfer lots of seeds. Bringing
this up to date, Dunmail Hodkinson and Ken Thomson, at the University of
Sheffield, have shown that mud on cars transports large numbers of seeds. (Try
looking at samples of mud from under wheel arches.)

Darwin describes in On the Origin of Species how he would walk
around his garden looking for bird excrement and the seeds that it contained. In
the course of two months, he picked up 12 kinds of seeds which he was able to
germinate.

Darwin also carried out a series of experiments on how long different types
of seeds would float in seawater. He found that many small seeds soon sank or
were injured by the salt water. However, when he experimented with dried
hazelnuts they floated for 90 days and afterwards, when planted, germinated. He
records that of 94 plants most sank quickly but 鈥�18 floated for above 28 days,
and some of them very much longer . . .鈥�