Biology of seed setting in conifers

The quality of seed, as well as seed production, are both directly and indirectly

of importance for the existence of mankind and also of a considerable

number of animal organisms. With regard to agricultural and garden

plants the question of seed from the food production point of view, becomes

of vital interest on a global scale. The quality and quantity of seed production

on forest trees is an important link in the same chain of interests. For many

people the forest, its raw products and the manufacture of its industrial products

means employment, security and a higher standard of living.

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Apart from price levels, extent of forest area and management of forestry,

the value of the forest land will depend upon our ability to bring into

being in different parts of our country new forests of the best possible utility

or cultivation value, suitable density and composition of species.

The knowledge, gained through seed research and reforestation studies,

of the differing qualitative properties of the natural seed within certain

extreme climatic regions, for instance, in Northern Europe, has helped to

increase understanding in Scandinavia of the genetic and physiologic quality

of forest-tree seed.

Flowering and Seed Setting in Conifers

Afforestation is intimately bound up with the seed question from both

the qualitative and quantitative points of view, and this irrespective of

whether it is a matter of natural or artificial regeneration (through sowing

or planting). The quality of the seed is in its turn dependent upon the genetic

constitution of the seed, and upon the modifying effect of the milieu

upon seed formation and seed maturity. The forced production of seed is

of current importance for tree species with a small seed production (e. g.

Picea abies and Pinus silvestris) and especially in regions with a severe

climate, where seed production is low and the physiological quality of the

seed in most cases poor. In extreme highland country in Central Europe

and in highland areas (as a rule more than 300 m. above sea level) e. g. in

northern Europe the temperature during the vegetative period is often a

striking minimum factor for seed maturity, seed production and for

growth, conditions of reproduction and tree limit.

Other external factors seem more or less to affect the setting and

development of the flower buds, the flowering, seed formation and seed

maturity etc., such as the light conditions, rainfall, wind and nutrition, the

age of the tree, the density and the height above sea level. Except for the

disseminating of the pollen, however, the importance of the wind for the

flowering, seed maturity and reproduction should for northern Europe

probably be restricted to extreme and exposed altitudes. Especially at low

temperatures the wind strengthens the effect of temperature markedly. Of

the external factors, which in the above mentioned biological connections

have been made the object of observations and studies, the temperature

probably with every justification has been generally considered the most

important. The effect of climate upon flowering, seed formation and seed

maturation is generally divided between:

1) year for bud setting;

2) flowering year;

3) year for seed maturation.

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For Norway spruce and other tree species of importance for our forestry,

with the exception of pine, the flowering year and the year for seed

maturation coincide.

Seed Production of Forest Tree s

The capacity of a tree for seed bearing and the factors which influence

it: — The biology of reproduction by trees is still imperfectly understood,

but certain more obvious controls may be distinguished, such as the following:

1. Age. A minimum degree of maturity is essential. The minimum age

for seed-bearing varies enormously with different species, and within the

same species depending upon vigor and growing space.

The minimum age at which viable seed may be produced is of great

importance in silviculture. Trees which reproduce exclusively or chiefly by

seed must be produced in ample quantity if natural reproduction is to be

expected. The statement is frequently made that trees need not be grown to

large sizes in the future, since boards can be fabricated from pulp made

from small sizes, and plastics and other chemical products will replace

lumber. Such assumptions frequently overlook the biological requirements

of the trees as well as the other indirect benefits to mankind from older

trees. However, much may be done to induce early seed-bearing of many

rapidly growing species. Early seed-bearing is also of utmost importance

in tree breeding experiments.

The maximum age should also be considered; this may at times effect

the natural reproduction of very overmature stands. Maturity, not age or

size, is the important factor.

2. Vigor. A number of conditions and factors may be included in this

general term, such as size of crown, assimilative area in proportion to volume

of wood in the bole and branches, exposure to sun, moisture relations,

external injuries, damage by insects and fungi and the like. It is difficult to

separate these and point to anyone as a controlling factor in seed production.

In so-called "crop years" even trees in poor thrift may bear seed, in

"off years" only those most favorably situated or blessed with ample reserves.

3. Environmental Conditions. Many climatic and soil factors influence

seed bearing, but most demonstrations have been made with other plants

than forest trees. It is considered that since the ash constituents of seed are

large, soils low in К and other elements produce poor seed crops. Infertile

soils low in N are unfavorable for the same reason.

4. Adaptability to site: Origin of seed. Exotics and trees raised from

seed collected in a place differing greatly in climate from that of the grow 128

ing site may exhibit their poor adaptation to their new home by relatively

poor seed production in quality and quantity. Ability to reproduce and perpetuate

itself is a fairly good criterion of adaptation of a species. A large

percentage of empty seed, imperfectly developed seed, slow germination,

and incomplete fertilization indicate a certain lack of conformity with the

environment. Trees growing near the limit of their range, where they are

advancing or losing ground, also exhibit. these tendencies in their seed

production.

5. Differencies between species in seed production. It is a commonplace

that some species not only bear frequently, but profusely, with huge

crops on every tree, while others bear rarely and sparingly. These inherent

capacities for seed production may largely determine the so-called "aggressiveness"

of trees in colonizing new ground; such areas are invariably

active seed-bearers. One cannot fail noting that such trees (birches, poplars,

cherries, pines, etc.) are usually exposed to full light, and have ample

growing space. The more shade-enduring trees (beeches, maples, firs,

spruces, hemlocks) are normally less abundant seeders. These tendencies,

of course, are frequently reversed by environment.

The quantities of seed produced by different trees vary so widely both

within the same, and between different species that most data mean little.

If individuals of different species of the same age and size could be grown

side by side, their production would still be scarcely comparable because

of the varying age at which each attains maximum bearing. Furthermore,

great differences in productivity between individuals of the same species,

comparable age and growing conditions have been demonstrated.