Re: do mountain pines release nitrogen via their roots and help hemlocks grow?

Dear Richard,

Nitrogen does move from plant to plant in an underground network. The roots of nearly all plant species are connected by a network of fungal strands known as mycorrhizae (from the Greek myco, fungus, and rhizus, root; Allen 1992). Mychorrhizal fungi surround the fine roots of the plant and actually penetrate into the cells of the root cortex. The fungal hyphae (long strands just one cell wide that make up the bulk of the fungus) extend out into the soil, and when they reach the roots of another plant, they can form connections with it as well. Both plants and fungi benefit from the mycorrhizal association: plants provide the fungus with sugars and other carbohydrates, and the fungus greatly extends the root surface area and facilitates uptake of nitrogen and phosphorus (Allen 1992, He et al. 2004). The fine white hairs you can see in this photo are mycorrhizae surrounding the roots.

Scientists have shown, through a series of elegant experiments, that the mycorrhizal network can transfer nutrients from one plant to another. In most experiments, the plants are separated by fine mesh which allows the tiny fungal hyphae to pass through, but does not allow roots to grow from one side to the other. The nutrients are either radioactively labeled, or labeled with stable isotopes(Allen 1992, He et al. 2004) It is possible to quantify exactly where the nutrients in each plant are coming from, and how much is transferred through the mycorrhizal network. In a recent study, He et al. (2006) showed that pines can serve as "donors" to the mycorrhizal network: labeled nutrients applied to pines could travel through fungal hyphae to plants of other species.

However, pines are unlikely to be an important source of nitrogen in the ecosystem. Only a few groups of plants have the ability to fix nitrogen; that is, to take nitrogen from the atmosphere, where it exists as an inert gas, and make it available in a form that living organisms can use. Legumes and a handful of other plants form associations with bacteria or cyanobacteria that are capable of this process. In a recent experiment, He et al. (2004) showed that 20-50% of the nitrogen fixed by legumes is transferred to other plants through the mycorrhizal network. Pines are not capable of fixing nitrogen, so they are not likely to be an important source of nitrogen for hemlocks.

Finally, the observation that hemlocks and pines often grow together does not necessarily mean that one species facilitates the presence of the other. Perhaps hemlocks and pines have similar environmental preferences, such as temperature or soil conditions. Perhaps both species respond to disturbance in a similar way. There are many possible explanations for their co-occurrence that do not necessarily imply that one species “needs” the other. One of the challenges of ecological research is to understand the underlying factors which determine the coexistence of species.

Literature Cited

Allen, M.F. 1992. Mycorrhizal functioning: an integrative plant-fungal process. Springer, Berlin.

Miller, S.L., and E.B. Allen. 1992. Mycorrhizae, nutrient transfer, and nutrient interactions between plants. pp. 302-332 in M.F. Allen (ed.)Mycorrhizal functioning: an integrative plant-fungal process. Springer, Berlin.

He, X.-H., C. Critchley, and C. Bledsoe. 2004. Nitrogen transfer within and between plants through common mycorrhizal networks (CMNs). Critical Reviews in Plant Sciences 22: 531-567.

He, X.-H., C. Bledsoe, R.J. Zasoski, D. Southworth, and W.R. Horwath. 2006. Rapid nitrogen transfer from ectomycorrhizal pines to adjacent ectomycorrhizal and arbuscular mycorrhizal plants in a California oak woodland. New Phytologist 170: 143-151.