快猫短视频

THE only woody plant on Axel Heiberg Island is the dwarf arctic willow. Even
with its catkins flapping upwards in the chilly summer winds, the tiny tree
barely tops a hiking boot. But beneath the willow鈥檚 roots lies evidence that 45
million years ago the island鈥檚 vegetation was very different. Then, the hills
were covered in a lush forest of oak, sycamore and larch鈥攈ome to flying
lemurs and the rhino-like Titanothere. The boggy lowlands were thick with swamp
cypresses and crawling with crocodiles and terrapins.

This isn鈥檛 just another example of plate tectonics changing the face of the
world. Axel Heiberg Island, in the Canadian Arctic, has moved no more than 2
degrees in tens of millions of years. But the transformation from temperate
woodland to arctic tundra holds the key to the disappearance of a primeval
forest that once extended throughout Asia, Europe and North America. It is also
the story of how the redwood family, which once dominated northern temperate
zones, was driven close to extinction, so that now all that remains is a handful
of giant sequoias and California redwoods in North America鈥檚 Pacific northwest
and a few isolated enclaves of dawn redwood and oriental swamp cypress in
China.

鈥淚t鈥檚 one of the most dramatic transitions in palaeobotany and I wanted to
know why it happened,鈥� says Ben LePage, who has spent the last decade trying to
solve the mystery. In his cluttered office in the geology department at the
University of Pennsylvania, LePage holds up a sheet of rock containing the
fossilised remains of a tree. Fossils like this helped him discover that the
range of a whole group of trees, including redwoods, larches, ginkos and
sycamores began to shrink around the same time鈥攂etween 40 and 35 million
years ago. He realised that the entire structure of the woodlands changed as
pines came to dominate the ecosystem. While species such as sycamores seem to
have survived the transition with most of their range intact, redwoods took a
body blow.

Exquisite clues

鈥淓vidence from fossil wood, cones and pollen clearly shows that the pines and
redwoods coexisted for at least 100 million years,鈥� says LePage. Throughout this
period pines were always a relatively minor component of the forest. Then 15 to
20 million years ago pine species suddenly took off. But this wasn鈥檛 a case of
an ancient group being ousted by a new competitively superior one. 鈥淭he redwood
design worked well for over 60 million years,鈥� he says. 鈥淲hat changed?鈥�

LePage believes he now has an answer. And the first clue comes from Axel
Heiberg Island. At the base of the island鈥檚 Princess Margaret Mountain Range
lies Geodetic Hill, a hump shaped like a single teardrop. About two kilometres
long, it contains some of the most remarkable deposits of fossil plants in the
world. 鈥淭he preservation is exquisite,鈥� says LePage鈥檚 colleague, James Basinger
from the University of Saskatchewan. 鈥淚t鈥檚 like looking at leaf litter on the
forest floor today.鈥� They may be 40 million years old, but these remains are not
fossils. They have been effectively mummified. As a result of millions of years
of burial in an acidic, oxygen-free environment they are still organic, not
turned to stone but chemically and biologically intact.

Twelve seasons excavating these leaves, roots and tree stumps has let LePage
and his colleagues reconstruct the ancient landscape. 鈥淭he plant communities
resembled those around a Louisiana bayou of today,鈥� says Basinger. But there is
one crucial difference鈥攖he island鈥檚 trees would have endured complete
darkness for several months each Arctic winter. 鈥淚t doesn鈥檛 seem to have harmed
them,鈥� says Basinger. 鈥淭ree ring analysis shows their growth rates were as fast
as any temperate tree today.鈥� Moreover, the abundance of swamp cypresses, which
won鈥檛 survive in freezing conditions, indicates that the climate was much warmer
then.

What once warmed this now frozen island? Throughout the late Cretaceous and
early Tertiary鈥攂etween 70 and 45 million years ago鈥攐cean circulation
patterns of the proto-Gulf Stream in the young northern Atlantic kept
temperatures high. Ocean currents brought warm water and moist warm air to
northern latitudes, much as the Gulf Stream does today. But because the land
masses had a different shape then, ocean circulation pushed tropical waters much
further north (see map).
The effects of this penetrating heat pump were
widespread. In Siberia, for example, much of what is now tundra was once a lush
forest, rich with redwoods. Then came some critical changes.

鈥淏etween 55 and 33 million years ago a seafloor rift was opening and
propagating northwards,鈥� says Olav Eldholm from the University of Oslo. Until
about 38 million years ago the Arctic and Atlantic Oceans were separated by the
Greenland-Fennoscandian Ridge under the sea, and the Arctic鈥檚 isolated waters
were a thermal law unto themselves. Eldholm and Jorn Tielde from the Institute
for Marine Geoscience in Kiel have shown that the rifting created deep channels
on the seafloor, allowing cold, dense water from deep in the Arctic Ocean basin
to drain into the north Atlantic. The transition occurred over less than a
million years, according to evidence from the temperature-sensitive
ratio of the oxygen-16 and oxygen-18 isotopes in tiny fossil shells from the
seafloor. With this huge lens of dense cold water in place, patterns of ocean
circulation altered and the proto-Gulf Stream took on something approaching its
present circulation. The climate rapidly became cooler and drier. 鈥淔or the
forests on Axel Heiberg the effects would have been catastrophic,鈥� says LePage.
鈥淭he entire ecosystem probably collapsed in a short space of time.鈥�

The cooling climate also affected the forests in the rest of North America
and Europe. Plants with a penchant for moist temperate climates that couldn鈥檛
adapt to the changing environment had nowhere to go but extinct because the
rifting also sundered a series of land bridges joining North America and Europe.
So immigration could no longer supplement declining North American plant
populations, nor could they escape to the warmer climate of Europe from the
cooling regions in the north of the continent. The redwoods began a long slow
retreat.

Body blow

Trouble loves company, and the reeling redwood lineage was soon hit by
another body blow, the rise of the Rockies. 鈥淭his had four major phases,鈥� says
LePage鈥檚 departmental colleague Gomaa Omar. 鈥淭he biggest occurred between 60 and
65 million years ago.鈥� This would rapidly have added altitude to a predominantly
flat landscape, leaving large areas in the rain shadow of new mountains. LePage
believes these changes effectively trapped the American redwoods. 鈥淲est of the
Rockies, their narrow environmental tolerance would have made the redwoods
unable to colonise or survive at the new mountains鈥� higher altitudes which were
colder and drier,鈥� he says. 鈥淭o the east, the new rain-shadow lands of the
Midwest would also have been unsuitable and tracts of redwood forest would
simply have shrivelled away.鈥� In Asia, the final rise of the Himalayas, around
20 million years ago, likewise forced populations of dawn redwood and oriental
swamp cypress into ever smaller areas.

The rise of the Himalayas wrought immense changes which may have reshaped the
world climate鈥攃ausing a cooling trend 55 million years long and initiating
the ice ages of the past 2 million years, according to MIT鈥檚 Maureen Raymo and
Bill Ruddiman of the University of Virginia. One important consequence was the
intensification of the monsoon weather system around 8 million years ago.
Gabriel Filippelli, a palaeoclimatologist at Indiana University, Pennsylvania,
has recently shown that an intensification of the Asian monsoon in the late
Miocene around 8 million years ago, triggered an upsurge in chemical weathering
on the Himalayan-Tibetan Plateau. This would have increased the drawdown of
atmospheric carbon dioxide which may well have caused the worldwide cooling from
the late Pliocene to the present and would, LePage believes, have hastened the
redwoods鈥� retreat.

Fatal flaw

But fate had not finished with the redwoods. The forest鈥檚 demise may have
triggered a feedback loop of increasing cooling and desiccation and further
forest loss. Computer modelling by Jan Dutton and Eric Barron of Pennsylvania
State University recently revealed that as trees were replaced by low-growing
grassland and tundra vegetation, more of the Sun鈥檚 energy would have been
reflected off the Earth鈥檚 surface. 鈥淚n the summer the vegetation itself would
have had a higher reflectivity. In the winter it would have been the
snowfields,鈥� says Barron. Snow would be slower to melt in spring and, with
moisture locked up in the snow and ice, drier conditions would prevail. Such
conditions would be a perfect breeding ground for grasses and hardy shrubs.

鈥淎ll of which was obviously bad news for redwoods and their relatives,鈥� says
LePage, 鈥淚t partly explained the contraction of their range, but it didn鈥檛
explain why pines replaced redwoods as the major northern temperate trees.鈥�
Given the extent of the redwoods鈥� former range, LePage could not accept that
these mighty trees were incapable of genetic innovation. 鈥淵ou don鈥檛 last for
over 100 million years without some capacity to duck and dive. There had to be
some kind of Achilles heel that no one else had thought of.鈥� He believes he has
now found the fatal flaw and, like many of the best discoveries, it involved
some serendipity.

In the early 1990s LePage鈥檚 first postdoctoral appointment was a study of
fossil fungi. 鈥淚t certainly wasn鈥檛 my first love,鈥� he says wryly. But the
experience proved fortuitous, giving him the background knowledge to explain why
redwoods have been overtaken so convincingly by pines. The answer, he believes,
lies in their roots. 鈥淎long with about 95 per cent of plants both pines and
redwoods are mycorrhizal, that is their roots possess a symbiotic fungal
association which is critical when getting nutrients out of the soil,鈥� he says.
But LePage鈥檚 key realisation was that the two lineages have different types of
mycorrhizae. In redwoods the fungal strands have a deep and intimate association
with the tissues of the root鈥攖hey are endomycorrhizal鈥攚hereas pines
are ectomycorrhizal, the fungal web sits near the root鈥檚 surface.

The next bit of the puzzle fell into place when LePage realised that these
two associations have different capacities for removing nutrients from the soil.
鈥淚n warm rainy environments phosphorus becomes very tightly bound to the
aluminium and iron hydroxides in the soil and is effectively unavailable to
plants. Then it is the limiting nutrient,鈥� says LePage. 鈥淯p to 90 per
cent of a soil鈥檚 phosphorus can be locked up in this way.鈥� The endomycorrhizal
fungi can ferret out this hidden bounty. Nitrogen becomes more difficult to
extract when the soil is acidic, or when the climate is very dry and or very
cold. This is when the pines come into their own because their ectomycorrhizae
excel at obtaining nitrogen.

鈥淭here is also a difference in the pH sensitivity,鈥� explains LePage.
The fungi that form ectomycorrhizae perform best in acid soils where the
pH is 5.5 or lower. These conditions are common in cold areas where slow
decomposition rates result in the accumulation of humic acids which make the
soil more acidic. 鈥淚n contrast, endomycorrhizal fungi commonly do best at
pHs of between 6 and 8,鈥� says LePage. 鈥淪uch alkaline soils are found in
places where decomposition is rapid, like redwood forests.鈥� LePage concludes
that these mighty trees may have been brought low by their fungal associates.
鈥淭he two have probably been in an intimate physiological association for the
whole of their 100-million-year history. That鈥檚 a difficult bond to break.鈥� The
more recently evolved pines, on the other hand, could benefit from
ectomycorrhizal associations which first appear on pine roots around 48 million
years ago. 鈥淎t first it probably meant that they could inhabit the marginal
habitats unavailable to the more dominant redwoods. But, as the climate changed,
what was once marginal became mainstream, and the pine鈥檚 stock rose as a
result,鈥� he says.

Limiting factor

It鈥檚 a very neat explanation. But there are some dissenting voices. David
Read, an expert on plant-fungal associations at the University of Sheffield,
agrees that ectomycorrhizae have been vital to the success of pines. 鈥淏ut I鈥檓
not so sure about this idea with the redwoods,鈥� he says. 鈥淚 think the best you
can say is that it would be a contributing factor.鈥� He points out that
endomycorrhizal associations don鈥檛 necessarily restrict a tree鈥檚 distribution.
The western red cedar, for example, extends right up into Alaska. 鈥淭rue, it
tends to occur on the better soils,鈥� he admits, 鈥渂ut it does show that endos can
do it.鈥� Read believes that the limiting factors for redwoods were mainly above
ground. As the climate became colder they would have found it increasingly
difficult to move water from the soil up into their high canopies, perhaps due
to a decreased rate of transpiration in the leaves.

鈥淥f course, there will be exceptions,鈥� responds LePage, 鈥渁ny flora comprises
a mosaic of plants with different nutrient acquisition strategies. Nothing in
biology is absolute. But cooler regions do tend to be dominated by
ectomycorrhizal species.鈥� Neither does he accept that height was a critical
factor because on average, pines are taller than redwoods. 鈥淪equoiadendron, the
giant redwood, and sequoia, the Californian redwood, are the only exceptions,鈥�
he says. 鈥淚f Read were right these once-widespread redwoods grew taller as their
ranges contracted.鈥�

Despite such complications, LePage鈥檚 explanation for the fate of the redwoods
is generating excitement. 鈥淚ts an elegant multi-disciplinary synthesis,鈥� says
palaeobotanist Lisa Boucher, from the University of Nebraska. 鈥淭his work has
revealed how key `choices鈥� in symbiotic relationships can strongly influence a
group鈥檚 evolutionary and ecological success.鈥� It pays to choose your associates
wisely, it seems, because when the going gets tough they may let you down.