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 calcium to the ecosystem, but scientists have assumed that it is not available directly to plants.
"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," says Blum. Trees do not take up calcium directly from this source; instead, they rely on fungi that live symbiotically on tree roots. Previous research had shown 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."
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 very 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," says Blum. "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 Ecosystem Studies.
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," says Blum. 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 Hampshire.
The University of Michigan
Ann Arbor, MI 48109-1399