Buds and Tree Growth II FRST 200 UBC

Epicormic Buds

A tree produces a large number of lateral buds over its lifetime as it grows but many lateral buds may remain suppressed (dormant) and not flush for several years. These tiny buds, called epicormic buds, were once axillary meristems and generally do not contain leaf primordia. - Epicormic buds on the stem (or branch) accomplish a small amount of growth each year just to keep pace with the increase in diameter. - Although they may grow a tiny bit each year, epicormic buds remain buried in the furrowed bark until fully released. Because they are embedded in the stem and under the bark, its hard and not obvious to see the,.

Spur Shoots

Another name for short shoots is spur shoots and they are common in many tree species, such as larches, cottonwoods, true cedars, ginkgo, birches and apples. - Terminal buds may also produce short shoots, but this is not common. Under this scenario, these may switch back and forth, year to year depending on the conditions.

Epicormic Shoots

Epicormic buds may be suddenly released and grow into lateral shoots on the stem These are called *epicormic shoots* or *water sprouts*. - Production of epicormic shoots varies with species and is more common in angiosperms than in gymnosperms. Oaks, for instance, tend to have lots of epicormic shoots. - The release of epicormic buds and stimulation to create new lateral branches is often responding to sudden exposure to light or warmth (ex: after thinning or pruning) and following injury. - Quite common on urban or bent trees - The growth of epicormic buds and shoots is generally undesirable in forestry as this causes knots and reduces wood quality.

Shade growth response

For many species, the response is to grow up "out" of the shade to avoid the poorer light quality. - Stems become taller and leaves get larger in area. In existing plant structures, this is primarily through cell elongation. - Branching is also suppressed to ensure that carbon and nutrient resources are dedicated to creating height growth. This is a typical response that happens in shade intolerant species Of course, a number of forest understory plants have adapted to the altered red:far-red light ratio and don't have this kind of response. These are called shade-tolerant species. In forest stand management, closer planting of trees can induce this shade avoidance response and create tall, thin, straight stems with fewer branches (therefore, fewer knots). - It has been noted that young trees growing side-by-side will grow at the same rate. - This is not because they are necessarily from the same genetic background, but because plants are very sensitive to the red:far-red light ratio - so much so, that they sense an altered ratio of light coming off of a neighbouring plant - Again, this happens because a canopy is absorbing red wavelengths and reflecting far-red wavelengths. - The reflected far-red wavelengths alter the light ratio of a neighbouring plant and this is sensed as a signal of impending competition

Height Pattern

Height growth of a plant over its lifetime usually follows a sigmoidal pattern: slowly at first, then fast, then slow again. - This same pattern holds for a single season of growth by the leader (the main shoot at the tip of the tree) - The rate of leader growth is controlled by the terminal apex (terminal apical meristem)

Growth rate

In determinate species, growth of the leader is usually rapid and completed relatively early each season. Ex: rapid period of shoot growth in Douglas-fir, shoot elongation rates may be as high as 2-3cm per day during the early part of the growing season. Overall of around 70cm-100cm, lammas 1-5cm In indeterminate species, growth of the leader is generally slower but extends over a much longer growing season(stopping with a summer environmental signal, such as a critical photoperiod cue).

Determinate Tree Species

In determinate temperate trees, the number of leaves and stem internodes (i.e. the phytomers) are determined during bud formation, which happens in the preceding year. - However, internode extension (and therefore total stem height) is strongly affected by environmental conditions of the current year. This means that for determinate species, overall growth in any one year is determined both by environmental conditions of the previous year and the current growing season. - Bud scale primordia (for next winter's bud) are initiated early in the season. This is the first thing the apical meristem does and it often takes place in late May. About mid-June, the apical meristem switches over to the production of leaf primordia. - Most of the time, these newly formed buds will not break and will wait until next spring to grow. - However, if late summer conditions are favourable for growth, these new buds may flush (i.e., lammas growth). Ex: Douglas-firs will undergo lammas growth if growing conditions in mid-late summer are good. - The meristem will then form another bud - this one will not break until the following spring. If no lammas occurs, the buds will become dormant by September and will be unable to break.

Indeterminate Tree Species

In trees of indeterminate growth, there are some leaf primordia (preformed leaves) already present within the bud. - In many species, there are only a few of these leaf primordia (ex: 3-5 in poplars) although some species can have numerous leaf primordia (ex: 15-20 in western hemlock). - Regardless of the number of preformed leaves, the apical meristem will have the capability of producing additional phytomers (i.e., new leaves called neoformed leaves and corresponding stem internodes). - Neoformed leaves are generally produced throughout the growing season Environmental conditions during the growing season will affect the extension of internodes and the rate of neoformed leaf production. Unlike determinate species, this means that overall growth in any one year is primarily determined by the environmental conditions of the current year. - The apical meristem generally forms bud scales in mid-late summer following the appropriate environmental cues. - Bud scales are followed by the production of leaf primordia that will be present within the overwintering bud. - Like determinate species, lammas growth can occur if growing conditions in late summer are good. - The apical meristem will then form another bud - this one will not break until the following spring. If no lammas occurs, however, the buds are completely formed and become fully dormant by late autumn. - 100-150 centimetres of growth in total

Long Shoots vs Short Shoots

Long shoots are lateral branches that may have indeterminate growth, while short shoots are lateral branches that have determinate growth. - Short shoots generally flush leaves in the spring and then produce a bud (ex: they accomplish no more leaves and stem growth is suppressed). - In indeterminate species, short shoots (which have determinate growth) always maintain the capability to become indeterminate. - Lateral short shoots can suddenly become a long shoot (i.e., switching to indeterminate growth) in response to some environmental change or plant trauma. - For instance, a short shoot can switch to create a new branch in a different direction, to occupy an opening in the canopy, or if the terminal apex of the branch is lost through damage.

Apical dominance

One of the determining factors for branch growth is hormonal control through apical dominance. - Apical dominance arises from the terminal apex of the plant through *auxin* (a plant hormone involved in growth). - Auxin is produced in high amounts through the mitotic divisions and cell expansion at the shoot apical meristem. - The auxin then travels from tip to base in a plant through the vascular tissues. - This production and downward flow of auxin from the terminal apex acts to suppress or control lateral plant growth. - It does so by preventing the growth of lateral buds, and controlling the elongation and angle of growth of the lateral branches. - If the terminal apex of the leading shoot is removed or damaged, apical dominance is lost and the downward flow of hormones and hormonal suppression stops. - Without the presence of auxin, the lateral buds or branches may begin to grow. [Eventually, one lateral bud or branch will assume dominance and become the new leader, acting to effectively suppress growth below it by producing auxin.]

Phytochrome

Shade response and effects on stem elongation is controlled by phytochrome. - One of the main photosynthetic pigments, chlorophyll a, preferentially absorbs red wavelengths. - There is a high amount of red wavelengths in sunlight, but absorbance by chlorophyll a in leaves means there is a lower amount of red wavelengths underneath a tree canopy. - Sunlight has a ratio of red:far-red wavelengths >1 (i.e., slightly more red than far-red), but this ratio dips and becomes lower as red light is being absorbed by canopies (~ creating a ratio less than 1 underneath the canopy). - The red:far-red light ratio is important to plants because it can activate phytochrome and trigger a *shade growth response*.

Determinate Species

Species with bud contain all the leaf primordia for next year's growth. Ex: spruces, Douglas-fir, and oaks.

Indeterminate Species

Species with bud that contain only a few leaf primordia but also have apical meristem capable of initiating additional (neoformed) leaves during growing season. Ex: poplars, birches, and western hemlock. *It is important to note that all tree species start as indeterminate in the first year or two of growth as seedling. Species with determinate growth will switch in subsequent years.

Growth of Stems

Stem elongation is largely influenced by light quality. - Light has several effects on the development of plants, such as seed germination, bud set, cold hardiness, etc. Stem structure is strongly influenced by light. For example, in low (or complete absence of) light, seedlings become very thin and spindly. - Stem elongation is enhanced and shoot growth is strongly favoured over root development. - There can be similar effects on stem elongation and root/shoot ratios in forest trees, as enhancing stem growth over root growth can contribute to wind-throw along the sides of cut blocks or after partial cutting.

Stump sprouts

Stump sprouts are shoots that arise from epicormic buds near the tops of cut stumps of some tree species. - This is common in angiosperm trees and shrubs but is uncommon in most gymnosperms. Ex: One exception is the coastal redwood where stump sprouts are an important means of vegetative regeneration in natural and managed stands.

Growth period

The period of "fast" growth is often exponential and are called growth periods

Tapering

The stem or trunk of a tree is wider at the bottom than it is at the top. - Essentially means the trunk is an elongated cone shape (being narrower at the top). - Due to light quality and wind, tree growing in the open can taper more rapidly than trees growing within a dense stand.

Growth of branches

There are substantially more lateral buds produced by a plant than terminal buds. - The growth of these lateral buds is what produces branches and the growth pattern is generally the same as terminal buds (following a sigmoidal pattern). - One key difference is that the period of growth for lateral branches is shorter and the amount of growth accomplished is less.