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The Woody Plant Seed Manual - University of Rhode Island

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1<br />

effects (Schmidtling 1983). Most <strong>of</strong> the attention has been<br />

on nitrogen and phosphorus, but many trials used complete<br />

fertilizers. Current practice is to base fertilization levels on<br />

soil analyses <strong>of</strong> individual orchards. Typical fertilization<br />

prescriptions for seed orchards <strong>of</strong> southern pines have been<br />

annual application <strong>of</strong> about 400 kg/ha <strong>of</strong> nitrogen, 80 kg/ha<br />

<strong>of</strong> potassium, 40 kg/ha <strong>of</strong> phosphorus, and 50 kg/ha <strong>of</strong> magnesium<br />

(Zobel and Talbert 1984).<br />

Another treatment widely used is manipulation <strong>of</strong> soil<br />

moisture levels. Irrigation <strong>of</strong> seed orchards in conjunction<br />

with fertilization is one practice, and in most cases the<br />

response is positive. Moisture stress has also been used,<br />

although this sort <strong>of</strong> treatment is difficult to apply in the<br />

field. In seed orchards, moisture stress has been created by<br />

root pruning the orchard trees to temporarily disrupt moisture<br />

uptake. <strong>The</strong> effect <strong>of</strong> moisture stress may be through its<br />

effect on carbon allocation in the tree, although other factors<br />

are sure to be involved. Ebell (1970) found that moisture<br />

stress increased the level <strong>of</strong> amino acids in Douglas-fir trees<br />

just as application <strong>of</strong> nitrate nitrogen did, and that both<br />

induced cone formation. When water was supplied to the<br />

trees, protein synthesis increased but cone formation did<br />

not.<br />

Girdling and other wounding treatments have been popular<br />

as a means <strong>of</strong> increasing production in fruit trees. <strong>The</strong><br />

theory behind these actions was that girdling prevented<br />

translocation <strong>of</strong> carbohydrates to the roots, thus raising the<br />

C to N ratio in the crown, which increased fruit production.<br />

Recent experimental evidence provides weak, if any, support<br />

for this theory, and a good explanation for the wounding<br />

effect is still lacking (Owens and Blake 1985). Despite the<br />

uncertainty, girdling is still used in seed orchards <strong>of</strong><br />

Douglas-fir and other conifers.<br />

Timing <strong>of</strong> wounding treatments seems to be important,<br />

at least in some species. Ebell (1971) girdled Douglas-fir<br />

trees at weekly intervals from April to mid-July. <strong>The</strong> optimal<br />

time <strong>of</strong> treatment was about 1 month before the vegetative<br />

buds burst. Many other studies <strong>of</strong> this nature have not<br />

controlled time <strong>of</strong> treatment as well, and timing effects cannot<br />

be determined (Owens and Blake 1985).<br />

Thinning <strong>of</strong> seed stands is another commonly used practice<br />

to increase flowering. Thinning brings about increased<br />

light intensity to the crowns (see previous section) and less<br />

competition for moisture and nutrients. As one might<br />

expect, there is a delay before treatments are usually effective,<br />

ranging from 1 to 4 years (Allen 1953; Owens and<br />

Blake 1985). Flower and fruit production increases attributed<br />

to thinning have been documented for black walnut<br />

(Juglans nigra L.) (Ponder 1979), hoop-pine (Araucaria<br />

8 • <strong>Woody</strong> <strong>Plant</strong> <strong>Seed</strong> <strong>Manual</strong><br />

cunninghamia Sweet) (Florence and McWilliam 1956),<br />

loblolly pine (Bilan 1960; Allen and Trousdell 1961), longleaf<br />

pine (Pinus palustris Mill.) (Allen 1953), and other<br />

conifers. Fertilization at the time <strong>of</strong> thinning enhanced cone<br />

production in Japanese larch (Larix leptolepis (Sieb. &<br />

Zucc.) Gord.) and Japanese red (Pinus densiflora Sieb. &<br />

Zucc.) (Asakawa and Fujita 1966) and ponderosa pines<br />

(Heidmann and others 1979).<br />

Much less is known about stimulation <strong>of</strong> flowering in<br />

tropical and subtropical tree species. Many <strong>of</strong> the same<br />

treatments used on temperate species have been tested in the<br />

tropics also, and, as one would expect, results have not been<br />

consistent. Carbohydrate accumulation and an interruption<br />

<strong>of</strong> vegetative growth <strong>of</strong> the tree are the factors that have<br />

been most frequently associated with increased flower initiation<br />

(Dick 1995).<br />

Structure and Development<br />

Flower primordia are inconspicuous at first and rarely<br />

can be identified without careful microscopic examination <strong>of</strong><br />

the tissues. Initially, there are no external features that serve<br />

to distinguish flower buds from vegetative buds. As flower<br />

buds grow and develop, they become distinguishable from<br />

vegetative buds by their general appearance and location.<br />

Variation among species is significant, but flower buds usually<br />

become wider and longer as they grow and may differ<br />

in color and shape from vegetative buds. In some species,<br />

such as flowering dogwood (Cornus florida L.), flower buds<br />

are distinctive in shape and large enough by late summer<br />

(July to August) for easy identification, thus providing a<br />

preliminary estimate <strong>of</strong> next year’s flower crop.<br />

Flower buds enlarge greatly as the flowering season<br />

nears and conditions become favorable for bud growth.<br />

Individual flowers <strong>of</strong> many species open rapidly once flowering<br />

begins. This is especially true if air temperatures are<br />

unseasonably high. Conversely, colder than normal temperatures<br />

will delay flower opening. Flower opening usually<br />

does not occur simultaneously over an entire inflorescence,<br />

over an entire tree, or even among plants <strong>of</strong> the same species<br />

in a stand, but it may be in progress for many days at any<br />

one location. <strong>The</strong> evolution <strong>of</strong> flowering in this way in wild<br />

populations is a distinct advantage in perpetuation <strong>of</strong> the<br />

species, as short-term events that destroy flowers or prevent<br />

pollination cannot destroy the entire crop.<br />

Flowers <strong>of</strong> woody plants come in many different shapes,<br />

colors, odors, and sizes. <strong>The</strong>y may be minute and inconspicuous,<br />

like the flowers <strong>of</strong> thuja, or they may be large, showy,<br />

and fragrant like the 1-foot wide, white, perfect flowers <strong>of</strong><br />

bigleaf magnolia (Magnolia macrophylla Michx.) (Brown

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