FOR 406 Silviculture : Primary Growth and Its Coordination
Meristems—any tissue that is capable of cell division, at one time or another—the growing points of a tree. They perpetuate themselves & alternate between periods of active division & periods of quiescence or dormancy
Primary growth Þ increase in length (extension) & reproductive growth
Secondary growth Þ always follows primary; increase in girth or thickness
To understand growth one must consider both structure (anatomy & morphology) and function (physiology)
1. Primary growth - derived from apical meristems in shoots and roots
• Gives rise to new buds, leaders and twigs, leaves, flowers, roots
A. Primary Growth of the Shoot Apex
Shoot apex consists of bud scales, embryonic internode, leaf primordia, axillary bud primordia, apical meristem (some buds also have flower promordia), protovascular tissue
• Most buds form in the growing season prior to their extension; most primordia fully formed by the end of the summer, although some winter activity in certain species
• Not all buds break the following spring - some suppressed, some winter killed; flower buds especially susceptible (e.g. flowering dogwood)
• In north temperate species cold treatment required for bud break; the more northern the species, the more cold required
¨ Long and short shoots: two morphologically distinct shoots produced by many trees, e.g. pines, larches, ginkgo, poplars, birches, apple
Short shoots (dwarf shoots or spur shoots)
• Produced from axillary buds; little internode elongation during a growing season
• Increase with age & depth in canopy
Long shoots—elongate normally
Ratio of long shoots to short shoots genetically determined & species specific; e.g. in pine a special case; virtually all foliage on short shoots & they do not grow after the first year
¨ Patterns of primary (shoot) growth in trees
Even under a "uniform" environment (e.g. a greenhouse) or in the tropics, 80% of woody species show some kind of intermittent growth
• In temperate zone all shoot growth intermittent (interrupted by dormant season)
• Involves cessation of activity in the apical meristem and, in most cases, setting a resting
bud (dormancy)
• Some environmental factor (day length, water stress, temperature or a combination)
causes a shift in hormone/inhibitor ratios
• Resumption of growth occurs upon onset of favorable conditions; usually requires a cold
treatment
Determinate or fixed growth
• A single growth flush, followed by bud set
• e.g. hard maple, ash, hickory, most northern conifers
• All leaf primordia preformed in the bud; size of bud correlated with amount of growth
• Shoot growth period short & rate of growth rapid
• Short shoots are always determinate
• Lammas shoots = late season bud flush in some determinate species, e.g. jack pine, usually because of high rainfall
Multiple flushing
• Several flushes of growth over a growing season, with a resting bud set between flushes, • e.g. southern pines, oaks, most subtropical & tropical trees, seedlings of some
determinate species
Each succeeding flush shorter; number of flushes depends on environment
Indeterminate or free growth
• Sustained flush of growth until late season, with true terminal bud formed
• e.g. poplars, aspens, tulip poplar, sweet gum, soft maples, ginkgo, larches, eucalyptus
• Produces morphologically distinct heterophyllous shoots
- early leaves (preformed in the bud)
- late leaves (initiated by the apex during the season)
• Indeterminate habit declines with age
• Cessation of free growth determined by day length and temperature
Sympodial
• Long growth flush followed by shoot tip abortion; no true terminal bud forms
• Only hardwoods,e.g. elms, basswood, birches, willows, sycamore, black & honey locust
• Shoots usually zig-zag
• Death of shoot tip due to buildup of inhibitors (ABA) or frost
Above classification neat but sometimes difficult to apply in nature, e.g. lammas growth vs. multiple flushing (oaks)
Environmental conditions can sometime modify growth patterns
¨ Growth and tree form
Form of trees determined by differential elongation of buds and branches, the strength of the wood formed, and the angle of branching
Excurrent - main stem or leader outgrows lateral branches producing cone- or oval-shaped crowns with a pronounced central stem; branching very flat
• e.g. most conifers, aspen, yellow poplar, sweet gum, pin oak
Decurrent or deliquescent - branches grow as fast as or faster than the leader; central stem disappears from repeated forking to produce a large spreading crown; branches angled
• e.g. western junipers, pinyon pines, most oaks, maples, hickories, elms, etc.
Trees all start out as excurrent (or else they'd never become trees); most then revert to deliquescence as they get older (soon in open, much later in closed stand)
• Onset of deliquescence may hastened on a severe site
• Excurrent trees show strong apical control; hormones produced by the leader suppress growth of lateral branches; in decurrent species apical control soon lost or weakens
Weeping nature of some trees and shrubs due to low strength of wood; woods of high strength branches can stick straight out
Shrubs are inherently slow growing and lack apical control or their wood is so weak they can't continue upright growth or they form terminal inflorescences
• A severe site, animal browsing, or insect attack can create shrubs from trees
Adventitious buds
• Arise from parenchyma tissue exterior to the xylem in unusual places - not associated with apical meristems or primary tissues
• Not common in shoots, but all buds produced by roots are adventitious
• Most develop into shoots the same season they are formed - usually stimulated by wounding or top-kill of tree
Suppressed buds
• Axillary (i.e. normal) buds within the bark that do not normally elongate
• Lay down new bud scales & leaf primordia & grow outward as tree grows in diameter; vascular trace goes all the way to the pith;
• Hormones from tree crown normally keep suppressed buds from elongating, but if supply of hormones cut off or declines, epicormic branches are produced from these buds
- Can happen when trees are released from competition
- Will also form below a wound (e.g. pruned branch), canker, or girdle
- Also form on upper side of leaning tree - hormones flow to lower side
• Basal sprouts (stump sprouts) formed in a tree that has been killed suddenly or cut
• A local hormonal imbalance can cause these buds to proliferate, forming a burl
B. Primary Growth of Roots
Distinctly different growth pattern than shoots
• Apex gives rise to tissues in 2 directions; extension growth only a few mm beyond apex
• New roots are not produced from buds
• No lateral organs produced by root apex (root hairs are not lateral roots)
Role of growth hormones poorly understood; levels of IAA & GA that promote shoot growth will suppress root growth
Lateral roots arise in the pericycle in association with xylem poles of the stele; no buds or predictable patterns of emergence along root
¨ Periodicity of root growth
Not nearly so neat & tidy as shoots; roots will grow whenever conditions permit; not all roots growing at once; growth indicated by white roots
In general, peak of root extension activity in the spring and, to a lesser extent, in the fall
• Initiation of root growth in spring can precede, coincide with, or, rarely, follow the initiation of shoot growth
• Usually, roots grow over a much longer period than shoots (esp. determinate species) and can go grow into the fall
• Growth of roots determined by environmental factors (soil temperature, water availability, aeration at the root tips) as well as activity of the shoots
¨ Fine root turnover
Fine roots (<2 mm diameter) can die or become brown (suberized) & then some become woody; but most of them die, usually within the first year or two
Fine roots, are simultaneously being produced and dying (= turnover); production of new roots exceeds death in spring, then gradually changes to the reverse in fall
1/3 to 2/3s of the total yearly food budget (carbohydrates produced by photosynthesis) can be allocated to fine roots; more on harsh sites than good sites
¨ Suberization
Fine-root surface turns brown, cortical & epidermal cells die & slough off, cell walls lignify & become impregnated with suberin (a wax-like substance); prelude to secondary growth of root & development of cambium
Can suberized roots absorb water & nutrients? Much argued, but evidence in favor of a "yes" answer, e.g. most of roots in spring before new root growth are suberized in some trees, but they can absorb water, albeit not as efficiently
¨ Architecture of tree roots
Most (~90%) of fine (feeder) roots within 6 inches of the surface & in the humus layer
Many trees also have tap roots and sinker roots; they are after water, esp. in arid habitats (30-100+ feet down)
Heterogeneous nature of soil influences root morphology, e.g. rocks & stones, hardpans, bedrock, water table, changes in the profile, etc.)
Roots may extend out several times the width of the crown
More standing biomass in the top of the tree than in the roots and this becomes more accentuated with age
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