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FIELDWORK
KEW SCIENCE
Hidden in the soil, myriad fungi form complicated webs, in partnership
with the roots of trees. But as Kew’s recent research shows, these vital
connections are under threat, as Rachel Mason Dentinger explains
underground
connections
Photos: Alamy, Laura Martínez Suz
It’s commonplace to describe forests
as a great set of global lungs. This vast,
dispersed organ draws in damaged
atmosphere, saturated with carbon dioxide
and pollutants, and exhales clean, oxygenated
air. We revere forests for the benefits they
provide, as well as for their beauty, while the
tree itself has become both an icon and an
indicator of environmental health.
But what if the trees towering above us were
only half the story? What if the familiar form of
a forest was the reflection of a subterranean
geography, an underground world fundamental
to the health of the trees overhead?
In fact, this is the case for all the world’s
forests and about 80 per cent of plants. Below
the surface of the soil, the forest meets the fungi.
And the symbiotic union they form is vital to the
forest’s ability to withstand and recover from
environmental damage.
For the past two years, Laura Martínez Suz
has been collecting data on the ectomycorrhizal
partners of oak trees from all over Europe (ecto
meaning outside, myco meaning fungus, and
rhizal meaning roots). A Marie Curie postdoctoral
fellow at Imperial College London and Kew,
she’s part of a group trying to understand how
environmental change affects this special
relationship between plant and fungus.
The shared life of fungi and plants has
been recognised since the mid-19th century,
when many naturalists believed that root fungi
parasitised plants. Today, however, scientists
understand that mycorrhizal fungi are
indispensable to plants and believe that this
symbiosis enabled plants to colonise land
roughly 500 million years ago. Far from
parasitising trees, mycorrhizal fungi confer a
great range of benefits, supporting the trees’
ability to take up water and essential minerals
from the soil, resisting attack by pathogens and
predators, or helping trees to tolerate harsh
environmental conditions, like high soil salinity.
But the fungi don’t stop there. They grow
outward from their tree partners by means of tiny
thread-like hyphae that intermingle in a complex
network throughout the soil. The mushrooms
you see above ground are only fruiting bodies
– the relatively small reproductive structures –
of an organism whose size must be considered
on a much larger scale. The hyphae of a single
fungus may extend up to 30 m underground,
and a single cubic centimetre of forest soil may
contain miles of densely interwoven hyphae.
Laura and her colleagues seek to
understand the diversity of these fungi and
how they change in response to environmental
factors like pollution. Because different fungi
specialise in different functions, the diversity of
the fungal community has direct implications
for the health of the forest itself: if a decrease
in fungal diversity means that the benefits to
plants also decrease, then the overall health
of the forest suffers.
With help from Martin Bidartondo, a
researcher at Kew and Imperial College, Laura
has made use of a project established in 1985
by the International Co-operative Programme
Left: Individual trees, such as these
oaks, live in partnership with many
different fungi that form mycorrhizas
and extend in the soil through a
vast network of hyphae
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Photos: Laura Martínez Suz
To conserve these
habitats, we should think
about the subterranean
half of the story
Below: Since 1985, ICP Forests has been
monitoring the possible adverse effects
of air pollution on forests across Europe
Bottom: Mycorrhizas are extracted from
soil cores that are taken at evenly
spaced distances from the trees
on Assessment and Monitoring of Air Pollution
Effects on Forests (ICP Forests). ICP Forests has
been monitoring a large network of European
forests, recording everything from soil condition
to the nutrient content of forest leaves, producing
a goldmine of data for studying forest health.
In 2005, ICP Forests approached Martin,
concerned that even with all the data its scientists
were collecting, biodiversity below ground was
not being assessed. When Laura came to Kew
in 2010, Martin’s team were already surveying
fungi in plots of pine forest in the UK and
Germany. Laura’s work has now expanded on
this, covering 22 oak forests in nine different
countries. From each of these plots, Laura and
an extensive network of colleagues, students,
foresters and members of ICP Forests have
collected numerous soil cores, from which
they sampled mycorrhizal roots.
Selecting 288 mycorrhizas per plot, Laura
has spent many hours in the lab staring at roots.
Luckily for her, ‘they are beautiful’. Describing
the purple root tips produced by the species
Laccaria amethystina, she marvels that some
fungi maintain the same colours below ground
that their mushrooms produce above ground.
She avoids letting her appreciation of beauty
influence her selection of root tips, however,
as sampling must be random. Fungal DNA will
be extracted and sequenced from the roots
she chooses. By comparing her sequences
with those in DNA sequence databases, Laura
identifies the fungi she finds.
Laura is reaching the end of her fellowship,
but tells me she is busily ‘still putting names on
all of the species’. She is challenged by the huge
diversity in the fungal community, which
swamps the diversity of forest plants many times.
Knowledge of these fungi is still quite poor and
existing DNA databases are under-populated,
so it’s likely she’s turning up many new species.
Martin and Laura prepare to
take a series of soil samples
to assess the number of fungal
species present, and how
abundant each species is
But once Laura has finished identifying
her fungi, she’ll have the first measure of
fungal diversity from oak forests across Europe.
The research also traces a gradient of nitrogen
in the soil, deposited by pollution following the
industrialisation of European countries. Laura’s
plot in the Netherlands represents the apex
of nitrogen pollution, at more than 30 kg of
nitrogen per hectare per year. By comparison,
she says, ‘a clean plot contains about 5 kg per
hectare,’ which Laura found in Spain and France.
‘But we don’t even know if the fungal communities
there have already been affected,’ she says.
Despite their lack of truly ‘clean’ plots (containing
less than 3 kg per hectare), Laura and Martin will
establish a baseline against which to measure
future changes in fungal diversity.
They also expect to see a strong correlation
between high pollution and low fungal diversity.
This change in fungal diversity, not apparent
to you or I as we walk through a wood today,
may become more obvious in the near future.
As fungal diversity changes, so does the panoply
of benefits that fungi offer their plant partners.
If plants have needed fungi to function on land
for the past 500 million years, what will happen
now as this critical symbiosis is undermined?
Moreover, the economic crisis facing
Europe today has provided a new challenge
for those concerned about forests. Even in
countries where trees are a cultural touchstone
and way of life, monitored plots are being
reduced. But this is crucial work – time is running
out to get a baseline for the fungi, because they
are probably changing as well.
So, while many of us have the plight of
global forests in mind, if we’re truly to conserve
these habitats we should always be thinking about
that other ‘beautiful’, subterranean half of the story.
––
Rachel Mason Dentinger is a freelance science writer
Support Kew’s science
FIELDWORK
You can help Kew scientists find out more about the fascinating
world of plants and fungi, break new ground and inspire generations
of young people about the natural world by donating to the Kew
Fund. Kew’s scientific programmes are focused on understanding
and conserving the world’s plant life, fungi and habitats at risk.
For details, see www.kew.org/kewfund or call 020 8332 3348.
52 KEW AUTUMN 2012
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