Tree Architecture and Growth

Transcription

1 Tree Architecture and Growth Conceptually, a tree is a tower supporting many small solar collectors. The objective is to get lots of solar collectors (leaves) in the air with the least cost in photosynthate / carbon. The design must be strong enough to last decades or even centuries. Over-designing wastes materials needed in other parts of the structure. Unlike towers, trees are dynamic and grow, but growth comes only from materials trees themselves produce. Forest Stand Dynamics Winter 2003 Stand development patterns emerge because trees have innate growth habits and respond with characteristic growth patterns to physical surroundings (site, weather, disturbances) and to other plants competing for the same growing space. The shape of individual trees and whole stands is a key to past history. Trees add new growth to old structures that developed under different conditions. Tree shape is primarily determined by primary growth, allowing new growth to respond to recent or current environmental conditions. Primary and Secondary Growth Primary growth- growth from a bud, root tip, or other apical meristem Secondary growth- growth from cambial or lateral meristems Meristems are localized areas that act as cell factories and cause elongation, thickening, and branching - producing new cells by division; these cells enlarge and differentiate into the many kinds of tissues that make up a tree. Apical meristems are found at the tips of each growing shoot and root and are responsible for elongation and producing the cells that form new meristems. Lateral meristems are located in the cambium between the wood and the bark; these meristemsproduce cells that thicken roots, stems, and branches and cells that increase the size of the lateral meristem. Primary and Secondary Growth Primary and Secondary Growth Roots and shoots grow additively- new cells are added that make them longer or thicker; total size is the sum of these additions. Growth in TOTAL number of roots and shoots is multiplicative because the rate is determined by the number of parent shoots multiplied by the branching rate per shoot. Branches are not formed randomly; branching pattern - phyllotaxy - arises from the apical meristems and can be alternate (oak, hickory, cottonwood), opposite (maple, ash, dogwood), or whorled (eastern white pine, Douglas-fir, true firs). Tree form arises from annual repeating patterns of branching, elongation, thickening, and death. Differences in form arise from different process RATES, not different processes. A sequence of cells, tissues, or organs at increasing distance from the meristems that formed them represents a developmental sequence in time. A record of cambial activity is stored in the wood and a record of annual elongations is found in the shoot system. It is possible to work back in time to determine the development of the tree over time (stem analysis, dendrochronology). Ice storms, insects, diseases, management, etc. affect form; trees react by trying to restore original growth patterns, but evidence of past events often remains. 1

2 Annual rings are a key to past climate and disturbances. This tree established in 1685 and was only an inch in diameter when it survived its first fire. Fire scars from this and nearby trees show fires occurred at intervals from 7-14 years until about In 1994, after nearly 100 years of fire exclusion, this and all nearby trees were killed in the Tyee Fire on the Wenatchee NF. Pistol Butt is caused by soil creep on steep slopes or by snow damage when tree was a seedling or sapling. Gravity Because of gravity (aided and abetted by wind, rain, and snow) aerial parts of trees tend to bend down. One solution is to orient the main trunk with respect to gravity. This is termed geotropism. Most conifers are strongly geotropic. The shoot goes up, the root goes down. Strength When shoots go up very far, they face the problem of developing sufficient strength to avoid breakage from wind or their own weight Anchoring Solutions - thicken roots near the base of the stem to make them stronger; develop a deep taproot Solution 1 - develop wood - strong walled cells composed initially of cellulose, then lignin Solution 2 - thicken stems and branches to make them stronger 2

3 Growth Reduced Photosynthetic Efficiency Growth creates need for more photosynthate - which requires more leaves Solution - branch to produce more leaf-bearing axes, giving an exponential increase in leaf number Adding additional leaves creates the tendency for some leaves to shade others, reducing photosynthetic efficiency Solution - develop a branching pattern and leaf arrangement that minimizes overlapping Monolayer on outside of tree crown (good in low-light or at low latitudes where sun is overhead) Multi-layer distribution throughout crown (good at higher light intensities or at high latitudes where sunlight comes from the side) Shaded Branches Water Loss During Photosynthesis As the tree grows, branches on the lower and inner parts of the tree become shaded and produce less photosynthate. Solution - Kill off branches that are not self-sustaining and concentrate leaves at the edges of the crown unless the crown is sparse and the tree is growing in full sunlight Increasing the number of leaves increases water loss through stomata when CO 2 is taken up during photosynthesis Solution - Proportionally increase the number of fine roots to take up water. Maintain feedback mechanisms between roots and leaves so they remain in balance (basis of pipestemcarbon balance models) As the tree gets larger, more water and nutrients need to move up and there is more photosynthate to transport down. Solution - Continually create more transport cells (the function of the cambium which creates the cells that differentiate into xylem (to move water and nutrients up the tree) and phloem (to move photosynthate down)) Age and Injury As the tree gets older, the probability of serious injury increases. Solution - Produce replacement shoots or roots that grow in the same direction as the broken one and replaces its function a) express tiny shoots/roots that have been stored for emergencies (short lag time) b) grow new shoots/roots (number and orientation not predetermined so can adapt to specific injury) c) change orientation of existing branches (lag time decreased as new growth not required) 3

4 Design Problems of Trees and Some Solutions And They re Still Working On This One or 120 m appears to be the maximum tree height possible internal water stress increases with tree height increased height requires increased thickening at the base, adding weight and causing structural problems Shoot Development Growth of the terminal shoot controls the tree s architecture by controlling branch growth crown shape stem growth Shoot growth entails the development of primordia from the apical meristem plus active shoot expansion. Shoot Development Primordia are small precursors of stems or leaves formed within the apical dome before and during active shoot elongation. The top of the apical meristemis a dome. Bulges form on the dome flanks and expand by cell division to form leaf primordia. As primordia develop, the dome expands upward, adding younger primordia within and above older ones (think of an artichoke.). Older primordia expand during shoot elongation to form leaves or needles. Shoot Development Shoot Development Most woody plants have woody bud scales that protect primordia during adverse seasons. A few species (witch hazel) have naked buds. Budbreak signals the period of active shoot elongation. During shoot elongation the primordia expand rapidly, develop stems, leaves, and lateral primary meristems (generally shoot or flower buds). Elongation is triggered by environmental conditions such as accumulated temperature (degree days, blossom degree days), day length, moisture, or combinations of these. Cessation of elongation can be triggered by drought or photoperiod. Many tropical species have periods of active elongation and dormancy, generally triggered by moisture availability. 4

5 Root Development Root growth can occur throughout the year under favorable conditions. Root growth usually peaks in early spring prior to shoot elongation and in late autumn after shoot elongation and cambial activity cease (this is why early spring and autumn are good times to plant landscape trees). Different tree species are predisposed to different root growth patterns, but soil conditions exert a strong modifying force on root growth. Tree Growth Patterns There are about generalized growth patterns for woody plants. Five growth patterns are common to temperate forest tree species in the northern hemisphere: Preform or fixed growth Neoform or sustained growth Recurrent growth Terminal florescence Aborted tip (sympodial/zigzag) growth Preform or Fixed Growth Preform or Fixed Growth A pattern of bud development, dormancy, and activation found in many trees that survive periods of environmental extremes by going dormant. New bud scales are formed from leaf primordia at tip of new shoot before or during active elongation. Apical meristemdevelops new leaf primordia PRIOR to next year s active growing season. Primordia reflect conditions of year they are formed, NOT the year they are expressed. At budbreak, the preformed primordia elongate, with expansion limited by the number of primordial cells formed the previous year. Examples of trees with the preformgrowth pattern include oaks, true firs, Douglas-fir, hickories, spruces, ashes, and some pines. Trees with preformgrowth generally form distinctive annual rings (ring porous) because of differences between early and late wood. Lammas Growth A variant of pre-form growth where a second flush of growth occurs in mid to late summer from buds that would ordinarily flush the following spring. This generally occurs when growing conditions have been unusually favorable. Lateral buds as well as terminal buds can develop as lammas growth. When a lateral bud flushes it can take over the terminal position, leading to a crooked stem. 5

6 Neoform (Sustained )Growth Neoform (Sustained) Growth Not all primordia develop prior to active shoot elongation. A bud containing a small number of primordia is set near the end of the growing season. The following year, these primordia develop during shoot elongation, with new primordia continuing to develop and shoots elongating as long as conditions are favorable for growth. Amount of growth mostly depends on environmental conditions during the CURRENT year. Shoot expansion is generally not as rapid as for species with preformed growth but often continues for a longer time. Species with neoform growth are generally more sitesensitive than species with preformed growth. Species having neoformgrowth include sweetgum, hemlock, red alder, yellow poplar, and red maple. Neoform (Sustained) Growth Diffuse Porous v Ring Porous Wood Trees with neoformgrowth produce less definitive growth rings (diffuse porous) because there is little difference between wood formed early in the season and wood formed late in the growing season. Diffuse porous red maple Ring porous sassafras Recurrent Growth Recurrent Growth Typical of southern pines; superficially resembles preformed growth. An inner bud is formed at the top of an enclosed shoot prior to the opening of the outer bud. A preformed shoot with another inner bud is telescoped inside the topmost inner bud. Several preformed shoots and inner bud combinations can be telescoped together. At budbreak, each bud and shoot breaks and elongates in sequence, but often before the previous shoot has fully elongated. Vigorous trees produce more growth flushes than weaker trees. There are no annual whorls of branches; whorls occur at the ends of the telescoping shoots and not at the end of an entire year s growth. Trees with this growth pattern often have false rings in the annual xylem, making aging difficult. 6

7 Terminal Florescence Aborted Tip Growth The terminal shoot develops into a flower. The flower exerts control over lower branches, causing them to grow more laterally, resulting in a series of forking branches. Plants with this growth form include horsechestnut, sumac, devil s club and many tropical trees. Also referred to as sympodial or zigzag growth. Trees with this pattern abort their terminal shoots, leaving the lateral branches to resume growth. This pattern is distinctive, but can derive from either preformor neoformgrowth patterns. Each leaf acts as a continuation of the stem, and the distal part of the stem then grows at an angle. Trees often die back during unfavorable summers, then resume growth the following year. Aborted Tip Growth More on Growth Trees with this growth pattern include birches, elm, redbud, and sugarberry. Very young trees of many species exhibit neoform growth, even those that as adults have preformgrowth patterns. Foxtail - develops when the terminal shoot grows without producing any lateral branches; this often occurs with temperate climate pines grown in the tropics, perhaps as a result of there not being enough stimulus for the terminal shoot to cease growth or produce lateral buds and branches. Apical dominance is the tendency for CURRENT year lateral buds to remain dormant as the shoot expands. Auxins (growth hormones) in the terminal bud flow downward, inhibiting growth of proximal (current year) lateral buds. Auxin concentrations weaken as they flow downward, so PREVIOUS year s dormant lateral buds flush and lateral branches grow vigorously. Species with strong apical DOMINANCE (the ability to keep current year lateral buds dormant) have weak apical CONTROL (they are unable to control the flushing and growth of prior year lateral buds) The result is a tree with long, heavy, lateral branches (oaks) 7

8 When there is weak apical control, lateral branches can grow upward and act as leaders, leading to a decurrent or rounded growth form. Older trees often revert to a decurrent growth form. A tree with weak apical DOMINANCE is NOT able to completely suppress the growth of current year lateral buds, so they flush. Auxins from these buds also move downward and and RESTRICT growth on previous year s flushed buds. ALL the branches remain rather short since auxin from the terminal bud keeps current year lateral branches short and auxin from each years lateral branches keep prior years lateral branches short. The result is a columnar tree with short lateral branches (Fir) Apical control refers to the physiological condition governing the excurrent (single stem, pyramidal) or decurrent (branchy) pattern of growth. A tree with strong apical control maintains a single stem, while one with weak apical control has many branches and a rounded crown. The term apical control will be encountered MUCH more frequently than the term apical dominance. Apical dominance determines whether or not a bud will expand; apical control determines how much the bud will grow once it does expand. Strong apical dominance = weak apical control = branchy tree Weak apical dominance = strong apical control = columnar tree PREFORMED SPECIES DO NOT NECESSARILY HAVE STRONG APICAL DOMINANCE. The textbook uses the term epinastic control when referring to the concept of apical control. There is no such thing as epinastic control! Epinasty refers to greater growth on the upper side of a branch, causing it to bend downwards. Trees growing in shade lose APICAL control: their lateral branches grow longer relative to their terminal leader. 8