The Woody Plant Seed Manual - University of Rhode Island

1
effects (Schmidtling 1983). Most of the attention has been
on nitrogen and phosphorus, but many trials used complete
fertilizers. Current practice is to base fertilization levels on
soil analyses of individual orchards. Typical fertilization
prescriptions for seed orchards of southern pines have been
annual application of about 400 kg/ha of nitrogen, 80 kg/ha
of potassium, 40 kg/ha of phosphorus, and 50 kg/ha of magnesium
(Zobel and Talbert 1984).
Another treatment widely used is manipulation of soil
moisture levels. Irrigation of seed orchards in conjunction
with fertilization is one practice, and in most cases the
response is positive. Moisture stress has also been used,
although this sort of treatment is difficult to apply in the
field. In seed orchards, moisture stress has been created by
root pruning the orchard trees to temporarily disrupt moisture
uptake. The effect of moisture stress may be through its
effect on carbon allocation in the tree, although other factors
are sure to be involved. Ebell (1970) found that moisture
stress increased the level of amino acids in Douglas-fir trees
just as application of nitrate nitrogen did, and that both
induced cone formation. When water was supplied to the
trees, protein synthesis increased but cone formation did
not.
Girdling and other wounding treatments have been popular
as a means of increasing production in fruit trees. The
theory behind these actions was that girdling prevented
translocation of carbohydrates to the roots, thus raising the
C to N ratio in the crown, which increased fruit production.
Recent experimental evidence provides weak, if any, support
for this theory, and a good explanation for the wounding
effect is still lacking (Owens and Blake 1985). Despite the
uncertainty, girdling is still used in seed orchards of
Douglas-fir and other conifers.
Timing of wounding treatments seems to be important,
at least in some species. Ebell (1971) girdled Douglas-fir
trees at weekly intervals from April to mid-July. The optimal
time of treatment was about 1 month before the vegetative
buds burst. Many other studies of this nature have not
controlled time of treatment as well, and timing effects cannot
be determined (Owens and Blake 1985).
Thinning of seed stands is another commonly used practice
to increase flowering. Thinning brings about increased
light intensity to the crowns (see previous section) and less
competition for moisture and nutrients. As one might
expect, there is a delay before treatments are usually effective,
ranging from 1 to 4 years (Allen 1953; Owens and
Blake 1985). Flower and fruit production increases attributed
to thinning have been documented for black walnut
(Juglans nigra L.) (Ponder 1979), hoop-pine (Araucaria
8 • Woody Plant Seed Manual
cunninghamia Sweet) (Florence and McWilliam 1956),
loblolly pine (Bilan 1960; Allen and Trousdell 1961), longleaf
pine (Pinus palustris Mill.) (Allen 1953), and other
conifers. Fertilization at the time of thinning enhanced cone
production in Japanese larch (Larix leptolepis (Sieb. &
Zucc.) Gord.) and Japanese red (Pinus densiflora Sieb. &
Zucc.) (Asakawa and Fujita 1966) and ponderosa pines
(Heidmann and others 1979).
Much less is known about stimulation of flowering in
tropical and subtropical tree species. Many of the same
treatments used on temperate species have been tested in the
tropics also, and, as one would expect, results have not been
consistent. Carbohydrate accumulation and an interruption
of vegetative growth of the tree are the factors that have
been most frequently associated with increased flower initiation
(Dick 1995).
Structure and Development
Flower primordia are inconspicuous at first and rarely
can be identified without careful microscopic examination of
the tissues. Initially, there are no external features that serve
to distinguish flower buds from vegetative buds. As flower
buds grow and develop, they become distinguishable from
vegetative buds by their general appearance and location.
Variation among species is significant, but flower buds usually
become wider and longer as they grow and may differ
in color and shape from vegetative buds. In some species,
such as flowering dogwood (Cornus florida L.), flower buds
are distinctive in shape and large enough by late summer
(July to August) for easy identification, thus providing a
preliminary estimate of next year’s flower crop.
Flower buds enlarge greatly as the flowering season
nears and conditions become favorable for bud growth.
Individual flowers of many species open rapidly once flowering
begins. This is especially true if air temperatures are
unseasonably high. Conversely, colder than normal temperatures
will delay flower opening. Flower opening usually
does not occur simultaneously over an entire inflorescence,
over an entire tree, or even among plants of the same species
in a stand, but it may be in progress for many days at any
one location. The evolution of flowering in this way in wild
populations is a distinct advantage in perpetuation of the
species, as short-term events that destroy flowers or prevent
pollination cannot destroy the entire crop.
Flowers of woody plants come in many different shapes,
colors, odors, and sizes. They may be minute and inconspicuous,
like the flowers of thuja, or they may be large, showy,
and fragrant like the 1-foot wide, white, perfect flowers of
bigleaf magnolia (Magnolia macrophylla Michx.) (Brown