A Guide to Forest Health – A Tree-Physiology Approach
by Alan C. Page, Ph.D. Green Diamond Systems Belchertown, Massachusetts 01007
All trees have at least two carbohydrate allocation mechanisms that determine how the structural strength of the tree is constructed. These physiological systems are basic to successful management of a timber crop. This discussion summarizes how two of these mechanisms work and the implications for forest site quality evaluation and maintenance. The possible connections between the physiology of individual trees and stand management are reviewed and areas of additional research are suggested for further exploration. This paper also presents a preliminary discussion of how the inter-relation of site factors and tree physiology may be affected by the introduction of inoculated biochar to forest soils. It is hoped that these observations may lead to improved understanding of the need for early intervention, improved appreciation for what may happen from different types of thinning, and better application of practices to influence the outcome from management of this and other species. This is a concept paper rather than a prescriptive standard, more definitive research is needed to flesh out where these potential opportunities exist and how to recognize them in practice.
Timber growing is the only natural, long term, self collecting, organic substrate that is completely sustainable as long as we care for how it is gathered and regenerated. It is too important to be relegated to those situations that are convenient for modern consumer lifestyles!
Today management of liquid assets can be easily ranked against many other alternatives. Future management of a timber crop may be judged against these alternatives as well. It has been demonstrated that rapid individual tree growth need not significantly diminish volume growth, and is currently necessary for successful economic performance. The return from management of a forest stand is controlled by:
1) the choices that are made regarding tree quality and stand density,
2) the timing of interventions to remove portions of the stand, and
3) the responses of the trees that remain following the intervention.
Future returns will be modified by payments for many of the normal services that forests do and by enhancements to local site quality such as:
1) payments for CO2 recovery, elimination or reduction of methane release and permanent carbon sequestration,
2) energy and co-product capture for local use or refinement,
3) water cleanup via biochar applications,
4) site enhancement with biochar from local forests through:
1. improved function of the beneficial soil microbial population
2. displacement of pathogenic microrganisms
3. detoxification of organic compounds
4. increased water holding capacity,and reduction of water loss
5. improved soil aeration,
6. enhanced ion exchange capability, resulting in increased nutrient retention
7. resulting higher stand density and volume production.
Forest stands may occupy a wide variety of sites. Individual tree performance is limited by site conditions, location within the stand, and the individual tree's ability to respond to resulting conditions. The physiology of white pine has been more thoroughly studied than most other tree species. The insights gathered from white pine are used here to suggest a general means of understanding what is happening or may be able to happen on a given site. It is quite likely that other species will display a similar set of systems that are available for tuning given the resources to make the appropriate adjustments in stand density or site quality.
1. Photosynthesis: Sugar, available for the main stem and below, flows from foliated branches where an excess of sugar exists. These branches provide this sugar to the phloem of the main stem for further distribution. Efficient sugar production requires soil water available above the wilting point, light, carbon dioxide, and other factors needed for photosynthesis in non-limiting supply. Tree growth stops without freely available soil water.
2.Tree Condition: Tree condition affects the ability of the tree to respond to the provision of increased space. Released trees that are overly tall for their stem strength and root system capability to supply water to the crown will shutdown prematurely. The ability of the foliated area of the crown to produce sufficient sugar for maintenance of crown extension, stem strength, and root system growth depends on the continued supply water from the roots. Trees with any major system imbalance will have difficulty responding to the space provided by thinning.
3. Soil Aeration / Root System Condition: Soil aeration is needed for root metabolism of supplied sugar. Root system vigor is generally inversely proportional to soil water holding capacity and directly related to the rate of water movement in the soil. Maintenance of root system vigor may become a problem in those sites where the water saturation reduces soil aeration.
Eastern United States white pine stands on well-drained sites regularly face a midsummer drought. Most stands on upland soils that have not been thinned for five years will exhaust the accumulated stock of water from winter snow melt and spring rains before the summer is one third over. Where this is the case, thinning must be done regularly to redistribute the available water to a decreasing number of enlarging individuals if they are to grow at an economically effective rate.
There are five major consumers of energy within a tree. These energy utilizers are 1) the annual rebuilding of the new foliage and branches, and repair of any damaged part of the tree, 2) annual root growth, 3) the sporadic production of reproductive structures, 4) the height growth mechanism, and 5) the diameter growth system. Managers may affect all systems by their choice of timing and rigor when treating pine stands. However, the latter two systems present some interesting opportunities for managers to understand what is really going on inside the tree, and thereby affect the economic outcome of their activities.
The height growth system determines how long it will take for a seedling to develop into a forest tree. Height growth is the last visible growth activity to stop before a tree dies. Thus a minimal amount of energy reserves is always allocated to extension of the terminal shoot. Height growth is controlled by limiting the very small amounts of energy that a tree invests in late season bud development. (A white pine bud has all the cells that may elongate into the next shoot preformed.) The next season's weather and the stand structure influence the outcome from the previous year bud set. New buds are again set before hardening off and dormancy. Motion of the leader may curtail investment of sugar in upper growing points. The amount of sugar invested in terminal buds affects the number of preformed cells that make up the portion of the bud that will become a shoot next year.
Tree diameter growth determines how long an existing tree must be held to be merchantable. Diameter growth is a seasonally continuous process that has no preset limitations. Diameter increase is limited by the ability of cambial cells to divide. Adverse temperature, water conditions, and lack of stretching within the cambial zone may limit the frequency of cell division and duration of the active period. It has been shown by Wilson1 that cambial cell initials and undifferentiated xylem daughter cells may divide more frequently if the cells rapidly reach sufficient length for transverse division to occur. In the case of the cambial daughter cells this rate of elongation is especially important because the division process takes a lot of energy. This energy in the form of sugar must migrate from the phloem through the newest cambial daughter xylem cells to the site of the older enlarging xylem cells. As the path for transport of sugar is populated by new cambial cell xylem divisions, the availability of large amounts of sugar to the older cells decreases. Swaying of the stem through the growing season has been shown to facilitate elongation of potential dividing cells in areas of deformation if resources are available. This deformation is normally highest where the tree is anchored to something stable - usually the ground. Tree guying experiments have shown that the point of highest diameter growth can be moved to the region immediately above and below the guying.
White pine is both tolerant of shade and sufficiently genetically diverse for some individuals to escape the competition of closely spaced neighbors by augmenting normal height growth when needed. The emergent character of white pine is accomplished by an adjustment of the energy allocated to leader bud development. This mechanism is very conservative because the ramifications of greater height are so profound in all systems of a tree. The priority given to height growth depends on the situation around and within the individual tree. If a tree is located in a protected situation and thus is vulnerable to overstory crown closure more height growth may be advantageous. Overtopped trees are the first to die in periods of drought. A tree in a more solitary, exposed position is vulnerable to wind-caused stem breakage. Greater than normal height growth in exposed situations necessitates increased amounts of structural support throughout the tree. Thus each new increment of height growth can subject a tree to conditions that may either injure the tree, or require increases in resources to be spent at many points within the tree. Marquis2 reported that red oak saplings in open stands have much slower height growth than those in the normal competitive situation for young red oak. In green house trials it has been demonstrated that seedlings whose terminals were shaken for thirty seconds per day reduced terminal growth by fifty percent compared to unshaken controls. This author has observed individual pine saplings in small openings (up to one half an acre in central Massachusetts so that they are still protected from greater wind stress) produce temporarily increased height growth. The normal height increment, one-half to two feet per year, may be increased to as much as five times that found in dense pole stands, up to four feet per year, for three to five years.
Regular motion of the terminal during the growing season may reduce allocation of sugar to terminal elongation. But trees in still situations with adequate water, light, and nutrients will grow at full site height potential (unless this mechanism has been genetically selected against in local strains i.e. tall thin trees have been selected against by snow loads etc.) Higher site quality has generally been measured by the greater height attained by dominant trees within a specified time. Given the complexity of the height growth mechanism, one should not be surprised if anomalies are found when applying such a general "rule of thumb" as the site index parameter.
Creating small openings in an overstory does not greatly increase wind movement around saplings. Increased availability of light and water for sugar production also results from the removal of competition. Openings of 1/3rd to 1/2 acre should be sufficient to reduce overstory competition and cause enhanced height growth (in advanced regeneration with 50% or more stem length of live crown.) The overstory that is left to regulate the ground level wind speed should be significantly taller (twenty to forty feet) than the trees expected to grow taller at an accelerated rate.
White pine trees increase stem diameter growth below the base of the live crown in two ways. One involves stem flexing. The other results from increased water availability to the tree. Stem flexing causes an increase in the division rate of the cambial initials and daughter xylem cells in areas of the stem that stretch the most. Increased water availability may cause the period of diameter growth to be extended. It is possible for diameter growth to continue until temperature and day length cause the shut down of all systems. The details of this system have been described by Wilson1, Hunt & Mader3, and by Jacobs4. These factors frequently occur together when young stands are thinned. There are three characteristics of the released tree that are important.
1)Individual trees must have sufficient crown to immediately respond to the increased water available from the removal of competitors.
2)The stems of released trees must be short enough that the roots receive sufficient sugar to grow into areas of free water.
3)The stand must be open enough to allow the available wind to bend the stems of the remaining trees throughout their length. This bending must be sufficient to signal the diameter growth mechanism that the tree should be strengthened or it will have no affect.
Repeated flexing of the main stem during the growing season may allow increased cambial and daughter xylem cell divisions as long as conditions remain favorable for sugar production in the crown. During the growing season a tree allocates available sugar passing down the phloem to dividing cells in the zone of bending as long as water and light are available to the leaves, and sufficient sugar is translocated past all other potential users and delivered to the roots. Normal xylem cell division is enhanced particularly on the side of the stem that is stretched the most as the tree sways in the wind.
Trees in stands that are resistant to winds are likely to support one another enough to suppress stem diameter growth. The soils occupied by dense stands frequently reach the wilting point by midsummer and all further growth shuts down until conditions change.
Mechanisms of carbohydrate allocation to elongation and diameter growth can be viewed as potentially mutually exclusive switches within a given tree. A tree that is bent enough to stimulate the cambium and adjacent daughter cells to divide at an accelerated rate may also have the height growth reduced to less than one-half of the site potential by the shaking of the terminal bud. Whenever conditions are excellent for sugar production all mechanisms may operate at full site potential.
forest Site Determinants
Below ground factors are influenced by all that follow, but deal primarily with two functions: water supply vs aeration, and nutrient availability. Both of these areas involve rooting depth and soil particle size. Past historic loading of the soil surface and chemical cementation can accomplish the same thing – packing pore space so completely that a layer near the surface prohibits the downward drainage of surface water and / or penetration of the same layer by roots. It has been shown that inclusion of appropriately inoculated and nutrient populated biochar in moderate quantities (3 to 50tons per acre) can materially change the site conditions by modifying the structure of the soil layers where the biochar is incorporated. These factors will be dealt with in detail later and more study will show how particular soil types react to such additions. Species specific factors can also be critical to the performance of a tree or group of trees. These include root grafting between trees of the same species and production of compounds that inhibit the growth of other species.
Above ground conditions include those mentioned in the climatic arena but are also modified on a micro-scale by the influence of neighboring trees and stands as well as topographic situations. Obviously the topographic factors are less likely to change than either the presence or absence of neighbors or the thermal or wind screen nature of adjacent stands. There are many parts to these interactions and while general interactions will be discussed the details will be left to the manager to decipher.
Eventually climate controls what is possible. The most fecund sites will be ruined for native species if the climate shifts enough in any direction. Possible shifts include: incidence of violent storms, less frequent rain, excessive wind, shifts of thermal maxima or arrival of the last frost free day by several weeks, lack of synchronization of temperature and day length patterns, and many more. It appears that we are in for several of these kinds of shifts in the near future. If this happens much of what is written here will still apply but to different species than were present a few years ago.
The following matrices are offered as a conceptual categorization system to describe the opportunities that may be faced in the economic culture of trees. The range of site conditions is presented in graphical format. The purposes of management activities are presented in Figure 1. Suggested stand density for effective management over the variety of sites that white pine may occupy is presented in Figure 2. The numbers and list of management problems and opportunities are suggestions only, and are supplied as examples rather than as a fully researched set of limits. Considerable study needs to be done to confirm and quantify both the categories and the limits. It is suggested, in fact, that this is more appropriate as a conceptual framework for a practicing forester to organize his own experience than as something that ought ever be published as a fixed set of guidelines.
NOTE: Thinning is nearly always needed for diameter growth response on sites with little or no wind. This is due to the lack of stimulation of the cambium when there is little bending of the stem. Whereas frequent thinning is needed on dry sites to redistribute scarce stored water. On these dry sites, without thinning trees run out of water in late May or June and all growth stops. The only place where frequent thinning is not called for is in that unique block of sites where sufficient water is available for all growth functions, photosynthesis occurs at optimal levels, and frequent strong breezes provide the stimulation for rapid diameter growth.
The major purpose for categorization of forest site is to facilitate the rapid identification of the economically appropriate number of trees per acre at any stage of stand development. Given the establishment of the general category of the site, it remains only for the upper residual basal area levels to be tried and the performance of the stand to be observed for one to three years. If the identification of site and tree condition was made correctly, diameter and height growth response of the stand should follow an expected path of development. Where tree condition is well matched to the treatments, tree growth rates should reach and sustain the maximum expected from the site. If the stand is in any of the still sites, and regular, three to five year treatments are not possible, diameter growth rates will decline sharply within ten years. In such stands the dominant-tree annual radial growth will move rapidly back to the normal rate of ten or more rings per inch as the overstory fills in the holes in the canopy made by the thinning. There are several pitfalls to watch for in appraising what and why performance was or was not obtained:
1) Tall stands in situations of poor water availability or poor soil aeration are very vulnerable to damage from over thinning. Even small openings in tall dense stands may cause all parts of edge trees to demand more resources than can be supplied. Tall trees with small crowns given even moderate stem bending may be difficult to get either to respond or to survive after heavy thinning. This is due to the extensive requirements for sugar by all parts of the stem and a limited ability of the roots to supply the water needed to sustain sugar production. The only situations in which it is appropriate to expect response from tall trees are those in which root health is assured and there is water freely available throughout the growing season.
2) Wetter sites may present a very delicate balance between individual tree-root water extraction ability and root growth needed to follow the water table as it drops through the summer. Water filled void space in poorly drained soils can lower soil aeration to the point that roots die back to a level with adequate aeration. It is not uncommon for larger trees to act as pumps in such sites. These large trees continually remove large amounts of water thereby keeping soil aeration at an acceptable level for growth of all occupants of the site. Overstory thinning has to be done carefully in order not to damage roots too much or to take out too many of the major water removers. If the thinning has been too heavy, there will be occasional wind thrown trees occurring within five years of the thinning. Growth rates may quickly drop too less than half of the before thinning value. Comparable site productivity may not occur again until a new stand emerges with the ability to maintain adequate soil aeration.
This leads into the need for enlightened management decision making to continue to do the two things that have always been needed to avoid both the midsummer drought and the wind shadows that come from both tall trees and topographic effects. Over time decisions based on experience with biochar’s effects and the above specific characters of a site should be well worth employing.
Significant research into the action of biochar in soils has been done within the last ten years by J. Lehmann at Cornell University. His group has shown that the increased cation exchange sites in soils having biochar incorporation come from charged areas on the carbon of the biochar. Dr. Ogawa of the Kansai Power Environmental group has studied the incorporation of biochar into soil by micro-organisms for much longer because the use of char in soils has been a significant part of Japanese agricultural practice for generations.
There is little question that the incorporation of micro-organisms in biochar offers protected sites for beneficial bacteria to form commensual communities that are free from predator destruction and a site for deposition of additional carbon based materials (glomulin) that act as a glue to hold soil particles to the biochar granules. The biochar is also colonized by hyphae of soil mychorrizal fungi which move moisture and nutrients between these sites and plant roots.
These facts have been known in detail for soils without biochar and are described in publications by the USDA and Soil Science Society of America. The addition of biochar to soils acts to enhance the ability of the soil micro-biota to do their job and is likely to have many of these predictable outcomes:
1.Improvement of soil porosity for air and water – biochar has a large surface to volume ratio that comes from the cellular structure of the plant material that was charred,
2.Soil carbon having a much longer residence time in the form of biochar carbon than is possible for more reactive soil carbon from humus,
3.Greater soil structural strength – this strength develops from the stable micro-biotic connections made between soil particles and biochar through the glues that are secreted by beneficial soil bacteria and fungi that reside in the biochar,
4.Maintenance of a productive relationship between plant roots and beneficial microbes – damaging soil organisms can overwhelm plant roots when the actions of these pathogenic organisms can not be effectively countered by weak responses from unaided or damaged plant roots – beneficial soil micro-organisms can quickly replace or substitute for an extensive plant root system thereby allowing the plant to concentrate energy that would have been used in plant root growth in other areas of the plant,
5.Increased ability of plant roots to gather water and nutrients during periods of stress – the same principle is at work here as in 4 above,
6.Greater volumes of available soil water because of the increased void space from the enhanced soil structure and the strong affinity that biochar has for the soil solution in general.
7.Strong retention of soil anions and cations,
8.Detoxification of organic molecules because of long residence time for compounds held on charged sites on boichar
These factors explain the rapid recovery of pine trees in Japan shown by Dr. Ogawa. The effective application of pre-inoculated biochar around the base of a tree will greatly improve the availability and rate of uptake of water and nutrients.
The “sweet spot” shown in Figure 1 where there is no thinning needed and in Figure 2 where the stand density may reach over 400 square feet of basal area per acre is likely to be expanded by biochar for the same reasons. This highly productive area of forest site characteristics comes from a steady supply of soil water all summer long and adequate soil aeration so that the tree roots can do their job effectively. This is only part of the story as is explained in the text because the tree may not be stimulated to use the excess sugar created unless the stem is deformed by wind.
The identification of site potential and the maintenance of stand utilization of the site's properties at the upper limits of the site potential may be much more complicated than generally imagined. Fifty year old white pine stands have been recorded having over four hundred square feet of basal area per acre, average stand height of one hundred plus feet tall, and many trees over twenty inches in diameter. These stands generally have had little or no management activity involved in the attainment of that performance. It appears that the few-recorded stands in this condition quickly developed the appropriate spacing for that site. These stands had no evidence of recent or pending mortality.
The management opportunities and the flexibility of timing of activities and equipment used vary with the site characteristics. The drier sands present many opportunities for: - frequent thinnings, - the ability to carry multiple canopies, and - the relative insensitivity to which trees are left as crop trees. Wetter and windy sites may require much more care in how and when the management activity is conducted.
There appear to be three options for thinning of wet sites: 1) wait for the new stand to develop naturally after the present residual stand falls apart, 2) start the stand over again by liquidating the rest of the overstory, or 3) trenching to provide the required drainage. Fertilization may help with foliage production and development of the water removal power of the dominant trees, but if applied too late it may just aggravate the worsening water supply problems within a tree that has root system problems. Fertilizing a site with root rot already started may simply accelerate the rate of decline by increasing the nitrogen supply available to the fungi.
On windy, well drained, but moist sites where excessive stand density could result in a midsummer drought, early adjustment of stand density may limit this problem. Such optimal stand adjustment may occur naturally as follows: Tall closely spaced trees reduce the crown depth by mechanical abrasion. Removal of horizontal branch buds as the trees brush against one another kills those branches that are nearly marginal sugar producers. The shortened crowns retain only those branches that produce the most sugar per unit of water consumed. NOTE: (vertical branches near the top of the tree require much less mass to support given loads than do horizontal branches.) Thus branches that approach the horizontal are quickly eliminated in thick stands. The continuous supply of in-flowing water allows photosynthesis to occur all season long in the branches of highest efficiency.
On wet, less windy, sites it may be necessary to identify those trees that can become major water users early and make sure that the side branch removal described above does not occur. This deep crown development will assure high rates of water use from these dominant individuals. Frequent thinnings may be needed to accomplish this crown maintenance in fast growing situations.
Hilly sites may develop wind gaps. Areas that appear to have been thinned sufficiently and are stocked with well-balanced trees may still not respond because the diameter growth mechanism may not be turned on. Understanding what is enough in such complicated terrain will remain challenging.
These last statements serve only to indicate how complicated the prediction of outcomes from "stand production maintenance" may become. There are other things happening beyond the stand border on wet, still, and super sites. Correctly anticipating them is part of the art of silviculture. We must be humble in judgment of our knowledge, and be prepared to accept the worst - especially from activity in tall stands on wet sites. It may turn out that management in tall stands may come in two forms either very infrequent activity consisting only of harvesting or very frequent activity that attempts to maintain individual tree vigor at high levels by whatever means are necessary, and that nothing in between is economically or physiologically possible.
Economic management of white pine also requires the removal of branches at appropriate times to minimize the production of black knots, but that is beyond the scope of this paper.
1.Wilson, Brayton F., 1970, "The Growing Tree", University of Massachusetts Press, Amherst, Massachusetts
2.Marquis, David, ,
3.Hunt, Fred & Mader, Donald, , Ph. D. Thesis UMASS