Producing tree seed and propagation
Increased interest in a wide range of native
and exotic tree species provides Australian farmers with the
opportunity to grow seed commercially. The market covers genetically
improved seed (resulting from long-term breeding trials) to
selections of local indigenous varieties. Breeding programs
for commercially valuable tree species involve incorporating
selected wild trees into seed orchards and testing their progeny.
The lead times and costs involved in seed breeding programs
mean that improved seed is currently only available for a
small number of commercial species. For other species, the
choice is usually between seed collected from the wild and
seed collected from plantations. Wild seed collected from
a particular location can be marketed as being of that provenance.
With many organisations and farmers wishing to establish locally
indigenous trees and shrubs there are sometimes very strong
markets for locally indigenous seed. Farmers with native forest
on their property may be able to take advantage of this.
Where buyers are interested in seed that exhibits fast growth
or other desirable traits they may be prepared to pay more
for seed of highly regarded provenance or improved seed
from seed orchards or well managed plantations. (Seed orchards
are simply specially designed and managed plantations grown
for seed production only.)
Common methods of enhancing the quality of any plantation
for seed production include:
encouraging out-breeding by incorporating a range of
promising provenances or seedlots. Outbreeding alone can add
as much as 10% to the growth rate of some tree species.
selective culling for desirable tree characteristics
such as vigour or form so that all the retained trees exhibit
the desirable characteristics.
isolating the plantation away from any native forest
or plantation containing the same species or any others that
may be able to interbreed.
Intensive selection and culling on the basis of growth characteristics
is normal plantation practice. By eliminating the poor individuals
from the stand the seed produced by the remaining trees is
expected to be of higher quality. For frost sensitive species,
such as Spotted Gum (Corymbia maculata), or where there are
other environmental factors that lead to mortality, farmers
may find that they are able to use natural selection to cull
their stands. If sufficient individuals survive, the seed
from these may produce trees that are more environmentally
tolerant of adverse conditions. The only way to demonstrate
that a particular seedlot or selection is superior is by undertaking
a progeny trial. This involves a direct comparison of the
growth and development of seedlings different sources.
Specially tending trees to reduce seed harvesting costs might
also be an option. Widely spaced trees develop a larger canopy
and, commonly, more seed. The use of pollinators, such as
bees, can increase seed set in flowering species. Even tall
native trees can be pushed or bent over to allow seed harvesting
from ground level. Using horticultural hormones that induce
flowering has increased seed production in native trees.
When collecting seed, farmers must be aware of the risks of
inbreeding and hybridisation. A small genetic base in the
original seed lot could increase inbreeding and dramatically
affect seed viability and vigour. Because many eucalypt species
can interbreed, seed collected from mixed species planting,
or from plantations located near native forests or domestic
gardens, may not be strictly true to species.
There are opportunities to sell propagation material from
species that can be grafted or struck from root or stem cuttings.
Many exotic timber treesincluding poplars, pines and
cricket bat willowsare commonly grown from
cuttings. Recent research has found that some of Australias
promising native cabinet timber speciessuch as Blackwood
(Acacia melanoxylon)can also be propagated from root
cuttings. This type of vegetative propagation produces genetic
clones. Mass-producing well formed trees in this way can greatly
reduce production costs (due to pruning, for example, which
can increase production costs in species with high genetic
diversity). However, developing farm forests using clones
can also increase the risk of disease, so it is common to
use several unrelated clones in a plantation.
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