Eastern shade-tolerant conifers

Abies balsamea (L.) Mill. - balsam fir

Image 1. Two seedlings emerging after germination in a moss bed: seed coats are still present on the elongating cotyledons, late June.

Image 2. The top of the leader of a small tree in the seedling stage of development, mid-May: The terminal bud has a whorl of subterminal buds around it; lateral buds occur in axils of certain leaves lower down the leader.

Image 3. A tree in the seedling stage of development, growing in dense shade beneath an overstory of older trees and appearing as though it is about 7 years old, the annual nodes being evident where whorls or part-whorls of branches originate on the main axis: however, the bent manner of earlier growth low on the stem where it is in the mat of sphagnum moss suggests that the seedling is much older - see No. 4.

Image 4. The tree of No. 3 after it had been carefully pulled from its position in the moss bed: it is obviously considerably older than 7 years, probably more like 40 years - there are many relatively fresh roots originating in the main stem above branches (note the long branch with a leafy branched tip shown to the left), and many older ones on main-stem tissue below; the position of the hypocotyl that would signify the position where the germinant originated is along the portion of the stem shown at the bottom of the picture. This form of development is typical of balsam fir growing in moist, shaded areas: young stems are beaten down, overgrown by moss, or covered by litterfall, the original roots have difficulty in functioning and their places are progressively taken by new roots developed from stem tissues further along the structure. The stem can always be determined as such by the pith in its centre: roots have no pith at their centres. The capacity for balsam fir (and some other very shade-tolerant species) to maintain its place in poor growing conditions by such rooting along the stem gives to the species its great tenacity for life, and its long-term (decades of) maintenance in the seedling stage of development. If conditions improve, such small "old" trees are fully capable of developing into normal-looking saplings, and subsequently, mature trees, 20 to 50 years older than they appear.

Image 5. Schematic diagram showing how trees may maintain themselves for many years (decades) in the seedling stage of development under heavy shade in moist situations.

Image 6. The upper portion of a vigorously growing sapling, in June, as new shoots begin to elongate: whorls of branches are evident at annual nodes, and interwhorl branches are evident laterally along the stems between annual nodes.

Image 7. The upper crown of a pole-size tree in August, after annual elongation has been completed:the pattern of whorl and interwhorl branching is clear, both down the main stem and along the branches.

Image 8. Mid-sized trees showing typical crown form.

Image 9. Mature trees, about 80-years-old from release (release from an extended period of suppressed growth in the seedling stage): note the presence of erect seed cones along branches at the tops of the crowns in this September shot.

Image 10. A tree about 140 years of age from release: this tree survived earlier spruce budworm attacks that killed most of the other trees of similar age and permitted release of understory trees that grew up around such trees.

Image 11. An old mature road-side tree, probably about 150 years of age: the tufted appearance at the top of the crown results from a large crop of seed cones, and the drooping of the older branches in the lower crown results from their long retention as extending structures on this long open-grown, old tree (such features are commonly associated with Picea mariana (Mill.) B.S.P., but can occur in Abies balsamea).

Image 12. Lower stems in an 80-year-old (from release) stand.

Image 13. Bark of an 80-year-old (from release) tree.

Image 14. Diagram of seed-cone and pollen-cone zones in medium and heavy cone-producing years: distribution of cones is shown among whorl and interwhorl branches and upon a 3-year-old and a 5-year-old whorl branch, the zones overlap, but upon different kinds of shoots (shoots in different positions) in the heavy cone year, but not in the medium cone year.

Image 15. Pollen-cone buds starting to form in axils of most leaves along the undersides of shoots, late June.

Image 16. Fully developed pollen-cone buds in September, in axils of leaves along the undersides and along the sides of shoots.

Image 17. A preformed pollen cone revealed by removal of the bud scales of its bud, October: latent buds are evident in axils of removed leaves (green scars) above, and to the right of the exposed, downward-oriented pollen cone.

Image 18. Pollen cones expanding, having burst through their buds, mid-May.

Image 19. Pollen cones from 14 trees, in mid-May, showing the range of colour among pollen cones: these colours were consistent for these 14 trees over a period of 12 years.

Image 20. Pollen cones just past the pollen-shedding stage, mid- to late May.

Image 21. Pollen cones after pollen shedding, early June: note how the axes that support the cones have grown to carry the cones clear of their bud scales so as to assist in dispersal of the pollen.

Image 22. Pollen grains of Abies balsamea.

Image 23. View, from above, of a reasonably vigorous shoot from the female zone of the crown, showing, at the end of June, a bud in the axil of a leaf on the shoot's upper surface, about one-third of the way along the shoot, that will differentiate as a seed-cone bud: other similar buds occur in axils of leaves at about 70 and 85 % of the distance along the shoot (compare No. 24).

Image 24. The same shoot as in No. 23, in late September, showing how the three seed-cone buds have developed to their overwintering state, and are larger than the terminal and subterminal buds partially visible at the end of the shoot.

Image 25. A preformed, overwintered seed cone seated on the axial portion around which the bud scales (that have been dissected away) were situated: the cone's surface is made up of spirally arranged bracts (late March).

Image 26. A seed-cone bud bursting, in mid-May: the tips of emerging bracts are visible beyond the extending inner bud scales at the tip; leaves have been removed from the supporting stem to provide a clear view (see Nos. 27 and 28).

Image 27. The seed cone of No. 26, three days later: the whole structure has enlarged greatly, and bracts have expanded and their awns are beginning to be reflexed such that pollen could be sifted down among them and into the inner parts of the cone where ovules have developed on the upper surfaces of the still-small ovuliferous scales; this is the start of the receptive stage of development (see No. 28).

Image 28. The seed cone of No. 27, ten days later (late May): bracts are fully reflexed, or spread outward, and above some of them, at their bases, the dark tips of growing ovuliferous scales are visible; the further growth of the ovuliferous scales (or cone scales) will completely seal the cone and prevent entry of any materials; this is the late receptive stage.

Image 29. The upper part of the seed-cone (or female) zone of the crown with seed cones in the late receptive stage of development: note the presence of seed cones on both the whorl and interwhorl branches, but on each of which they occur only on shoots that elongated the previous year (late May).

Image 30. A longitudinal section through a seed cone at the receptive stage in mid- to late May: bracts, ovuliferous scales and some ovules are shown as structures that are carried on either side of (all around) the axis, in which a central pith, vascular tissues, and cortex are visible. It can be seen that the bracts are reflexed somewhat, and that there is considerable space between successive bracts into which pollen could find its way. The ovuliferous scales have extended not much more than 40% of the lengths of their respective subtending bracts, and ovules show as swellings on their adaxial (or upper) surfaces. Because two ovules are borne on the inner adaxial surface of each ovuliferous scale, but towards the scale edges where the ovules extend over the scale edges and their integuments are directed downwards in a flared, tubular manner, the ovules are seen on portions detached from their surrounds, such as in the 4th main structure down from the top on the right, and the 13th main structure down from the top on the left. In these two cases, the egg cell of the ovule shows as a relatively large lightly stained or clear "O" surrounded by darkly stained tissues, the lower part of which is the nucellus of the ovule clearly exposed to the air between the flared extended walls of the integument that form the micropyle: it is to these flared walls of the integument that pollen grains attach and accumulate during pollination.

Image 31. A seed cone in early June, after the receptive stage: the tips of ovuliferous scales (cone scales) are growing rapidly and all but those at the base will overgrow the bracts.

Image 32. Seed cones in early to mid-June with all but the awns of their bracts overgrown by their cone scales: note the exudation of resin at the tips of the cones, and that new shoot elongation is now well under way (see also Nos. 33, 34 and 36 to 39).

Image 33. The same seed cones as in No. 32, in late June, with all bract awns except those near the cones' bases overgrown by the cone scales: the new shoots are now close to being fully elongated (see also Nos. 34 and 36 to 39).

Image 34. The same seed cones as in Nos. 32 and 33, at full size in mid-July (see also Nos. 36 to 39).

Image 35. The upper (adaxial) surface of a cone scale taken from the central region of a seed cone at the end of June when seed-cone growth was nearly completed: two ovules/seeds (this is about the time of fertilization and hence the beginning of seed formation) are situated at the base of the scale, and each is covered by a wing-base, the rest of the wing extends over (and originated from) the upper surface of much of the scale.

Image 36. The same seed cones as in Nos. 32 to 34, in mid-August, as scale drying and "browning" is occurring (the browning starts first in the hairs of the scales) (see also Nos. 37 to 39).

Image 37. The same seed cones as in Nos. 32 to 34, and 36, at the end of August/beginning of September when scales have dried and are separating: tips of purplish seed wings can be seen where some scales have spread apart (see also Nos. 38 and 39).

Image 38. The same seed cones as in Nos. 32 to 34, 36 and 39, in mid-September after wind action has begun to carry scales with their bracts, and seeds away from the erect cone axes: note how the resin at the cone tips tends to hold apical scales together longer (see also No. 39).

Image 39. The same seed cones as in Nos. 32 to 34, and 36 to 38 at the end of September when most seed-bearing scales have been separated and their seeds dispersed: the axes remain on the trees for decades.

Image 40. Some relationships among the seed cones produced on a single tree: seed-cone size is expressed in terms of numbers of scales (or bracts) as shown by the numerals, and varies in distinctive patterns down the crown and down individual branches.

Image 41. Seeds of Abies balsamea: those above the scale bar, which is in millimetres, have wings still attached; those below the scale bar have the flattened portion of the wing broken off (the normal way to remove wings - or really part-wings - in this species, when seeds are to be stored or sown mechanically). Seeds are shown with upper and lower surfaces exposed: those with lower surfaces exposed show how the base of the wing embraces much of the underside of the seed, and some with upper surfaces exposed show where resin vesicles exist between wing-base and seed - vesicles that would be broken to release their resin if the wing-base was pried away from the seed.

Image 42. A 7-year-old plantation of Abies balsamea grown and shaped by shearing for Christmas trees, July.

Image 43. A 7-year-old plantation of Abies balsamea grown and shaped by shearing for Christmas trees, July.

Image 44. Seed cones on a 7-year-old, plantation-grown Abies balsamea Christmas tree, July.

Information provided by:
Dr. G.R. Powell
Faculty of Forestry and Environmental Management at UNB