Perfect partners – mycorrhizae and forestry
Katrin Walbert
New
Zealand Tree Grower August 2005
Forest establishment is a critical phase of the forestry cycle, and
success at this stage sets the scene for the whole of the rotation. The
survival and growth of nursery seedlings in plantations is greatly
improved by the presence of mycorrhizal fungi both in the nursery stage
and when planting out.
Fungus root
 Mycorrhizae
are fungal species that live in symbiosis with tree roots
and play an important role in supplying soil nutrients to trees as they
grow. The term mycorrhiza stems from the Greek terms mykes and rhiza,
literally meaning fungus-root. These organisms form a linkage
between plant roots and the soil by transferring inorganic nutrients to
the plant in exchange for carbon. In this two-way movement of nutrients
, up to 20% of plant assimilated carbon, can be transferred to the
fungus.
The benefits for the plant are increased growth and yield, and improved
fitness. Such is the inherent value of the partnership that 82% of land
plants are mycorrhizal, and nutrient deficiencies will result if the
fungal partner is not present.
Protecting against pathogens
 There
are seven types of mycorrhiza – ectomycorrhiza being the common
type for radiata pine. Ectomycorrhizal fungi, which can be seen with
the naked eye, protect the trees against disease by imposing a physical
barrier against pathogens. They can also produce protective antibiotics
and promote beneficial bacteria, which may be directly responsible for
guarding against root pathogens.
Those who aim for good seedling quality do well to consider the role of
mycorrhizae. Lack of mycorrhizal fungi on root systems is a leading
cause of poor plant establishment and growth in the forest. When
purchasing tree stocks of radiata pine and Douglas fir, look for the
white and brown fungal growths on the roots. They indicate a healthy
population of beneficial ectomycorrhiza.
Compatible inoculation
Good forest tree nursery practice and compatible inoculation programmes
can ensure that seedling roots are colonised by mycorrhizal fungi,
resulting in improved growth after planting. It is not only important
to have mycorrhizal fungi present in the nursery and plantation. The
selection of the appropriate species is a key point in every
inoculation programme. Sensible manipulation and conditioning in the
nursery are important factors influencing the survival rate of a tree.
A common practice in New Zealand nurseries is to inoculate with soil or
duff inoculum, sporocarps or spores. Established nurseries
often do not inoculate at all, relying on natural soil populations, and
inoculum material is usually collected from these sites. When
considering controlled inoculations, the choice of fungus should be
based on its position within mycorrhizal succession, its ability to
stimulate the growth of the host and by the persistence of the
association.
Further study underway
Each component of the tripartite system of host/mycorrhizal
fungus/other soil microbes is subject to environmental influences. The
soil environment influences symbiosis, and mycorrhizal health is
affected by pH and soil fertility. Too much fertilisation is known to
reduce the abundance and presence of
mycorrhiza. As trees grow and mature, it is believed that the
associated mycorrhizal populations change. A study is underway by Ensis
scientists to understand the nature of mycorrhizal succession and to
identify indicator species at the investigated stages.
Over the past year, samples were collected from nursery sites, and
plots established within stands of trees aged from one year to the end
of rotation. Once this study is complete, information on beneficial
mycorrhizal species may enable inoculations that will enhance tree
growth.
Katrin Walbert is a PhD
student from Germany, currently working at Ensis Forest Biosecurity and
Protection in Rotorua. The mycorrhizae research at Ensis is being
conducted using FRST funding, and part of the funding is being used to
support Katrin’s PhD project with Lincoln University.
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