PESTS AND DISEASES OF FORESTRY IN NEW ZEALAND
Mycorrhizas and seedlings
MYCORRHIZAS AND SEEDLINGS
From Forest Health News No. 178, October/November 2007
The majority of plants are dependent on a symbiotic relationship
with soil fungi called “mycorrhizas”, which enables an increased
uptake of nutrients and water and may protect the plant from
pathogens. Plant health therefore increases, and the fungus in
turn receives carbon from its host. There are several different
types of mycorrhizal association. The symbiotic type found on
our important plantation trees Pinus radiata and Pseudotsuga
menziesii is called “ectomycorrhiza” (ECM); in this type the
fungus grows around the root tips and forms a visible covering
called a mantle (Figure 1). A PhD study at Ensis, Rotorua,
investigated the ECM communities associated with P. radiata
at Te Ngae Nursery, Rotorua, and in stands of several ages in
Kaingaroa Forest in 2005 and 2006, with a special focus on the
actual fungi colonising root tips. Part of this study investigated
the fate and survival rate of fungal species colonising seedlings
in the nursery in the first years after planting on a clearcut site. It
is known that clearcutting reduces ECM diversity as it removes
the host and hence the carbon source for the fungus. Furthermore
this forestry practice alters the soil conditions, removes the organic
layer through the clearcutting, and turns the site into a harsh
environment for re-establishment. Mycorrhizal colonisation is
imperative for the establishment of P. radiata as the tree is unable
to grow without the fungal symbiosis; after planting it is even more
important to have strong mycorrhizal development to facilitate the
establishment of the seedlings.
Figure 1: Mycorrhizal mantle of Rhizopogon rubescens on
Pinus radiata root tip
Figure 2: Rhizopogon rubescens (left) and Hebeloma sp. (right)
fruiting bodies
The species found in the nursery were Rhizopogon rubescens,
Hebeloma sp. (Figure 2), Tuber sp., and Wilcoxina mikolae.
Seedlings were planted on a clearcut site in Kaingaroa Forest
and ECM on the seedlings were assessed on a monthly basis for a
year. In addition, a 2- and a 7-year-old plantation were assessed.
The study found that all nursery ECM were still present and dominant
during the first year after planting. A few non-nursery ECM species
were found on the root tips after 1 year, but these were of only minor
incidence. The species R. rubescens (Figure 1) was dominant
in the nursery and for the first year after planting. In a study in
Spain on the differential responses of certain fungal species to
environmental factors and their role in the mycorrhization of P.
radiata seedlings (Duñabeitia et al. 2004), R. rubescens was
found to be the most beneficial species for P. radiata seedlings in
terms of colonisation, growth promotion, and tolerance to adverse
environmental conditions. This makes it a desirable species to
have on nursery seedlings. The ECM species composition was
significantly different in the 7-year-old plantation and non-nursery
species dominated. Compared to overseas studies we found this
changeover in species was later and it took longer for new fungal
species to colonise the root tips.
The overall ECM diversity on New Zealand’s plantation species
is low compared to overseas native forests and plantations, and
diversity is even lower in the clearcut environment (Jones et al.
2003). The current study has shown that it takes more than 3 years
after planting for ECM fungi from nearby plantation forests to
establish themselves as successful colonisers on the young trees.
The colonisation by forest ECM was slower than reported in other
studies (Dahlberg & Stenstrom 1991; Menkis et al. 2007). This
finding implies that it is important to have ECM species from the
nursery present on the root tips when the seedlings are planted in
order to bridge the gap until the forest ECM colonise the root tips
and take over the important role of nutrient and water uptake.
Viable inoculum for the nursery fungi is also present in the soil
surrounding the roots and it is recommended that not all of the
nursery soil should be removed from the seedling to ensure the
colonisation of seedling roots by the ECM fungi at planting.
Dahlberg, A.; Stenstrom, E. 1991: Dynamic changes in nursery and
indigenous mycorrhiza of Pinus sylvestris planted out in forest and
clearcuts. Plant and Soil 136: 73–86.
Duñabeitia, M.K.; Hormilla, S.; Garcia-Plaxaola, J.I.; Txarterina, K.;
Arteche, U.; Becerril, J.M. 2004: Differential responses of three fungal
species to environmental factors and their role in the mycorrhization
of Pinus radiata D. Don. Mycorrhiza 14: 11–18.
Jones, M.D.; Durall, D.M.; Cairney, J.W.G. 2003: Ectomycorrhizal fungal
communities in young forest stands regenerating after clearcut
logging. New Phytologist 157: 399–422.
Menkis, A.; Vasiliauskas, R.; Taylor, A.F.; Stenlid, J.; Finlay, R.D.
2007: Afforestation of abandoned farmland with conifer seedlings
inoculated with three ectomycorrhizal fungi — impact of plant
performance and ectomycorrhizal community. Mycorrhiza 17:
337–348
Katrin Walbert
