By Nancy Ross-Flanigan / News and Information Services
With the help of rock-eating fungi, some types of trees are able
to "mine" calcium, a nutrient essential to their growth, a research
team led by geological sciences Prof. Joel D. Blum has found.
The finding, published in the June 13 issue of Nature, has implications
for forest ecology and management and should help scientists better
understand the effects of acid rain on forest ecosystems, Blum says.
Until now, scientists thought trees got most of their calcium by
root uptake of calcium that is loosely bound to the surfaces of
soil particles, known as the "plant-available pool." A large pool
of calcium also exists in silicate minerals in the soil that slowly
weather over thousands of years and release it to the ecosystem,
but scientists have assumed that it is not available directly to
"What we discovered through the experiments that are reported in
this paper is that there's not simply the plant-available pool and
the pool of calcium in the silicate minerals, but there's also an
intermediate pool of calcium contained in the common calcium phosphate
mineral called apatite, which previously hadn't been recognized
as being available to plants," Blum says.
Trees do not take up calcium directly from this source; instead,
they rely on fungi that live symbiotically on tree roots. Previous
research showed that these ectomycorrhizal fungi send out projections
(hyphae) that release organic acids and penetrate mineral particles.
The acids dissolve the mineral material around the hyphae, releasing
essential nutrients such as calcium, phosphorus and potassium. Blum's
study was the first to show that the trees are using calcium that
the fungi have obtained in this way.
"The previous view was that all the trees in the forest get their
nutrients from the plant-available soil pool. The conclusion we
came up with from this study was that, while some trees are only
able to use the plant-available soil pool, others can send down
fungal hyphae and find the apatite. Instead of drinking their calcium,
they mine it," he says.
The researchers were able to determine where individual trees were
getting their calcium by analyzing the calcium-to-strontium ratios
in their leaves and needles. Calcium and strontium behave similarly
in living systems, but not during the process of mineral formation
in crystalline rocks. Consequently, different minerals have different
signature ratios of calcium to strontium. The ratios in the foliage
revealed that spruce and fir trees were tapping into apatite via
fungal hyphae for their calcium, whereas sugar maples, which lack
ectomycorrhizal fungi, could only sip it from the soil solution.
The question of how much calcium is available to trees is crucial
in understanding the effects of acid rain on forest ecosystems.
Acid deposition leaches calcium from the soil, and because soils
take thousands of years to develop, such damage is not easily repaired.
Finding that some trees can bypass the soil solution and pump up
calcium from deep in the soil is good news, but it does not mean
that acid rain is not harming forests.
"We don't want to suggest that there is no longer a need for concern
about acid rain depleting calcium from forests," Blum says. "Our
research shows that some trees have other strategies for getting
their calcium, which may be important when the plant-available pool
starts running out, but trees that don't have the ability to go
deeper are still in the same dilemma they've always been in."
Indeed, sugar maples are declining dramatically across New England,
and while no one is entirely sure why that is happening, some researchers
believe it is because maples are particularly sensitive to loss
of soil calcium from acid rain.
"It's just one of many theories, but our work strengthens that
argument a little by saying that maples in particular don't have
access to this other source of calcium, so they might be more sensitive
to the loss of soil calcium from acid rain," says Blum, who collaborated
on the project with researchers from Syracuse University, Yale University,
Cornell University, the USDA Forest Service and the Institute of
The findings also have implications for forest management. Typically,
forest researchers sample soil near the surface and measure the
amount of calcium easily removed by shaking the soil in a salt solution,
to determine whether there is enough calcium for a forest to re-grow
after logging. "But now we realize that's not as good a measure
of what's available to trees as we thought," Blum says. Like the
trees, researchers may have to go deeper in search of apatite to
get a true picture of a forest's calcium reserves.
The research was funded by the National Science Foundation and
was carried out at the Hubbard Brook Experimental Forest in New