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A CLINAL HODEL OF TREE IMPROVEMENT
Roy R. Silen
·
U.S. Forest Service Pacific N.W. Research Station 3200 Jefferson Way Corvallis, OR
97331
U.S.A. The words UNIFORM and CLINAL are used here to describe
contrasting tree improvement models. As with agricultural crops, an
assumption for both models is that improved seed should genetically match
the effective growing season length of the planting site within. 10 days to
assure full biomass productivity.
The present worldwide tree improvement
model is termed UNIFORM because it delineates a zone of growing season or
other climatic uniformity from which parent trees are chosen.
Cuttings or
seedling families from them are arranged randomly in seed orchards to
produce a single seed mixture suitable for planting anywhere in the zone.
Since growing seasons change by only about 10 days per degree of latitude
in the gentle topography of eastern United States or western Europe, an
area of over 2-million acres can often be found to meet this criterion in
The summe r
a one-degree by one-degree square of latitude and longitude.
rains and high humidities permit growth through all the summer months.
The same criterion is difficult to meet in the complex
mountainous Pacific Northwest.
Each 400-foot rise in elevation shortens
growing seasons about 10 days (from Hopkins' Law).
Summer drought
regularly truncates growing seasons. Wilting point commonly occurs by
late July, but can occur as early as mid-June in rain shadows of
mountains.
Snow packs on north exposures delay onset of spring growth.
Frosts can occur any month in valleys that drain cold air from mountains.
Uniform zones of the above criterion become ridiculously small in such
environments forcing most western tree improvement programs to accept
larger growing-season variation than 10 days in most commercial breeding
zones.
The CLINAL model is proposed in order to meet the 10-day
The key to the model
growing-season matchup of seed with planting site.
is the concept of producing many seed mixes in the same orchard.
The
simplest example would be for one aspect on a uniformly sloping mountain­
side.
Parent trees could be placed in the orchard in essentially eleva­
tional bands and seed could be collected from the band that matched the
planting-site elevation on the mountainside.
For complex mountainous
topography more typical of the West, an elegant solution is a maplike
placement of clones or families in orchards to match their corresponding
Such orchards have been established at
coordinate location in the forest.
the J.E. Schroeder orchard complex in the Williamette Valley owned by the
State of Oregon.
With abundant tested parents, essentially local seed of
correct growing season length and desired ga:ins could be provided for each
planting site.
But even with limited parent numbers in a multiple
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arrangement of an orchard,
seed collection can start with such selected
Any additional seed needs would come from the band of
local parentage.
adjacent parents in the orchard having equivalent growing season length.
Growing season maps are inadequate in the Pacific Northwest due
to poor weather station coverage.
Fortunately, the relationship between
inherent growth rate and the effective growing season appears strong.
Maps of inherent growth rate are being prepared for 111.Jch of the Douglas­
fir region from progeny data of commercial tree improvement programs that
now test more than 25,000 parent trees.
These maps appear to sample the
long-term climatic variation better than is possible with present weather
Where sample cold or drought gradients are documented, the
records.
matchup appears essentially templatelike on several maps.
The computations of genetic gains in growth rate are more
complex for the CLINAL than the UNIFORM model.
For each specific seed mix
the parental population size is limited to the parents in the orchard that
contribute to it.
1987.
In:
T.. J.B.,
Morgenstern,
eds.
E. K.;
Boyle,
Proceedings of the 21st
meeting of the Canadian Tree Improve­
ment Association:
Part 2: Symposium on
tree improvement-progressing together;
1987 August 17-21;
ON:
Truro,
NS. Ottawa,
Canadian Forestry Service:
181-182.
Reproduced by USDA Forest Service For Official Use