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Injury from fire and winter storms in Maine set off changes in tree metabolism. Photographs are by the author and Kenneth R. Dudzik, Northern Research Station, USDA Forest Service.

By Kevin T. Smith, Ph.D.

Loss assessments and decisionsEvaluation of tree injury oftenbegins with a loss assessment. Forwinter storm injury, percent crown

loss or branch breakage is often estimated.For injury from fire or some mechanicalsource to the lower trunk, the height andwidth of the killed vascular cambium andresulting scar are often measured. Bothcrown breakage and stem wounds providethe opportunity for infection and over timecan result in wood decay. Procedures to measure, estimate, and

report those losses might be simple orcomplex and may fit into a flowchart orrecipe book. But should these measure-ments or estimates be the basis for treeretention or removal?

Forestry legacyLoss evaluation was developed to meet

the needs of production forestry, such asestimates of mortality of standing trees orthe volume of wood considered as cull orunusable. In the mid-1900s, estimation ofthe volume of discoloration and decay inliving trees received much attentionbecause the economic value of sawtimber

depended on the harvested yield of cleartimber free of defects such as stain, decay,and excessive changes in grain orientation.Defects were essentially any characteristicthat reduced the yield of clear timber.Burls, knots, wound-wood ribs, and miner-al stain were defects and sources of“degrade” with respect to log value. Thisfocus on quantitative loss is understand-able with respect to yield of clear timberfor wood products.How much of this applies to the

arborist? Rather than the forest stand, anarborist is usually concerned about thebenefits and liabilities provided by smallergroups or individual trees. Some of thecharacteristics of form that reduce thevalue of a tree for sawtimber can enhancevalue and interest for the urban, communi-ty, or residential landscape.The need for modern arborists concerned

with the living tree can move beyond eval-uations based on what is lost, to value whatremains and what is being added to the treeafter injury. After all, tree survival and theability to survive depend more on what ispresent than what was lost.

Metabolism, metabolites, and pathwaysOne path to go beyond loss evaluation is

to recognize the context of how tree metab-olism changes in response to injury.Metabolism is the set of chemical reactionsthat capture and release energy and buildtree cells and their contents. Metabolismoccurs along well-defined, stepwise path-ways that modify chemical compounds(metabolites) and pass them along, oftenaccompanied with the transfer of energy.Each step in metabolism requires one ormore protein catalysts (enzymes) and mayrequire small amounts of essential mineralelements. Distinct metabolic pathwaysintersect, with the intermediate or endproducts of one pathway being the startingmaterials for others. The emphasis of onepathway or another results from the under-lying genetic program of the tree asinfluenced by the external environment. The dominant metabolic pathway for

energy capture is photosynthesis.Photosynthesis uses solar energy to splitoff the oxygen from water and the carbonfrom carbon dioxide and to precisely com-bine them to form glucose sugar. Thechemical bonds in glucose store some ofthe solar energy captured by photosynthe-sis. In budgeting terms, photosynthesis isthe only source of income for trees.Glucose is a raw material for amino acids

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that form protein or that are diverted toform lignin for structure, nucleic acids forthe genetic program, and essentially allother biomolecules. Glucosecan be converted into othersugars, linked into chains orpolymers such as starch forenergy storage as well as forthe complex structural carbo-hydrates for cell walls. Newcells are produced by special-ized tissues capable of celldivision (meristems) at buds,shoot tips, and the “new cellgenerator” (vascular cambi-um) located between theoutermost sapwood and theinner bark. Several linked pathways

release energy from glucose toyield carbon dioxide and waterin aerobic respiration. Normalaerobic respiration spends thatincome to cover energy andstructural expenses to feed andmaintain living cells and fornormal growth. The basic processes of ener-

gy capture by photosynthesis,growth, and energy-yielding

respiration were the first physiologicalprocesses in plants to be intensively stud-ied and are collectively termed primary

metabolism. The biosynthesis of othercompounds such as non-photosyntheticpigments and the natural preservatives indurable heartwood were viewed as “sec-ondary metabolism,” although thematerials were essential for trees as weknow them. Even more objectionable isthe concept that secondary metabolites inheartwood or bark are “waste” com-pounds. There are no waste products ormetabolic dead ends with trees and othergreen plants. Plants shed parts, such asfoliage and branches to the outside andheartwood to the inside, when their costsexceed their benefits, but there is no“waste.” Animals do produce waste prod-ucts that they cannot recycle, but not so forplants.Perhaps a more useful approach is to

replace the term “secondary” with “stress”metabolism. Stress occurs in plants operat-ing at or near the operational limits of thetree system. Mechanical injury or breakageis a common cause of metabolic stress.Injury to living sapwood initiates a cascadeof metabolic changes. These changes occurin extant sapwood, in the differentiation ofnew wood from the vascular cambium, andthe production of new apical or tip or api-cal meristems and associated shoots.

Coppiced cottonwood in New Mexico uses stump sprouting to full advantage in the landscape.

The stability of this giant sequoia in California is related more to the thick

woundwood ribs that frame the scar rather than interior decay from this old

fire injury.

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CompartmentalizationTo supply adequate moisture to the liv-

ing network of cell contents (the symplast),water is under tension or negative pressureas it is pulled from the roots to the foliage.When living sapwood is injured, the col-umn of liquid water in sapwood vesselsand tracheids snaps as air bubbles areintroduced into the wood system. Treesresist the spread of air bubbles by pluggingthe water-conducting xylem with stressmetabolites such as resin or corky water-proofing. In many species, livingparenchyma cells are sacrificed to formtyloses as their cell membranes are forcedthrough pores in wood cells and plugwater-conducting cells. This essentially immediate process of

plugging is quickly followed by shiftsfrom primary to stress metabolism in liv-ing sapwood to produce additionalwaterproofing and anti-microbial bound-aries. These shifts are part ofcompartmentalization, the system of pre-formed and induced boundaries that resistthe spread of dysfunction and infection.These boundaries are not perfect.Compartmentalization is just one survival

process. Other strategies for trees includethe ability to sprout from an establishedroot system or to produce large amounts ofviable seed. Some tree species invest moremetabolic energy into compartmentaliza-tion, other species less so.

Wound wood and closure Shifts in metabolism after wounding can

cause dedicated cells to lose their special-ization (dedifferentiate), divide, and form amass or pad of thin-walled undifferentiatedcells (callus). Callus most frequently formsat the wounded edge of the existing vascu-lar cambium and sometimes from livingray cells at the wound surface. Within aperiod of weeks, a new vascular cambiumcan become organized within the callus.This new vascular cambium tends tobecome continuous with the surviving vas-cular cambium at the edge of the wound. Whether from new or surviving vascular

cambium, xylem cell production isenhanced, resulting in wide annual incre-ments of dense and strong wound wood.The width of wound wood rings may has-ten wound closure, reducing theavailability of the injured wound surface to

Three years after ice injury in New Hampshire, the spread of discolored wood and loss of healthy sapwood is resisted by

reaction zones (closed/black arrows) and a barrier zone (open arrows).

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infection. Perhaps more important is thatthe vigorous, localized ribs of wound woodstrengthen the wounded stem by more uni-formly distributing the mechanical stresscaused by the wound. If considered at all inproduction forestry, wound wood was con-sidered cull and a loss of value. Thearborist can use wound-wood productionas a marker of vigor and vitality as well asfor structural support that can be greaterthan that lost from decay of the inner core.

Stem sproutsIn addition to wound-wood formation,

metabolic shifts due to stem wounding caninduce formation of new buds or meris-tematic points that can germinate alongwith latent buds. These new sprouts orshoots can bear new foliage in subsequentyears to replace that lost from earlierbreakage of branches. In addition to thegeneral contribution to photosynthesis,new sprouts can provide sugar to fuellocalized growth and biosynthesis.Traditional forestry discouraged sprouting

as the associated knots over time reducedthe yield of clear lumber. Arborists arerightly concerned that stem sprouts initial-ly have weak points of attachment to thestem, and clients may object to profusesprouting. The metabolic tradeoff is thatsprouting is the key to long-term survivalof trees that had broken branches andcrown loss.In part, the practice of arboriculture

came out of production forestry. Given thedifferences in objectives, arborists do notneed to be bound by the requirements oftimber production. Compartmentalizationboundaries, wound wood, and sproutingcan be positive indicators of value ratherthan a source of excessive concern.Estimates of crown and stem volume losscan be useful. But the resilience of the treeto restore lost function also needs to be

31TREE CARE INDUSTRY – AUGUST 2015Circle 7 on RS Card or visit www.tcia.org/Publications

About 10 years after ice storm injury of this sugar maple in New Hampshire, the loss of functional sapwood can be traced

to the old broken top.

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considered. Indeed, each tree is both vul-nerable to injury and competent to recoverand thrive based on the limits of treemetabolism.

Additional resources:Luley, C. J. 2015. Biology and assess-

ment of callus and woundwood. ArboristNews 24(2):12-21. Available from:

http://www.urbanforestryllc.com/publica-tions.asp. Mattheck, C., Bethge, K., Weber, K.

2015. The Body Language of Trees:Encylopedia of Visual Tree Assessment.Karlsruhe Institute of Technology:Karlsruhe, Germany. 548 p.Shortle, W.C., Dudzik, K.R. 2012. Wood

decay in living and dead trees: a pictorial

overview. USDA For. Serv. Gen. TechRep. NRS-97. 26 p. Available at:http://www.nrs.fs.fed.us/pubs/40899.Shortle, W.C., Smith, K.T., Dudzik, K.R.

2014. Tree survival 15 years after the icestorm of January 1998. USDA For. Serv.Res. Pap. NRS-25. 4 p. Available at:http://www.nrs.fs.fed.us/pubs/45483.Smith, K.T. 2015. Compartmentalization,

resource allocation, and wood quality.Current Forestry Reports 1: 8-15. Availableat: http://www.nrs.fs.fed.us/pubs/47727.Smith, K.T. 2013. Compartmentalization

of pathogens in fire-injured trees.Proceedings of the Landscape DiseaseSymposium, University of CaliforniaVentura County Cooperative Extension.Available at: www.nrs.fs.fed.us/pubs/43329.

Kevin T. Smith, Ph.D., is supervisoryplant physiologist with the USDA ForestService, Northern Research Station, inDurham, New Hampshire. This article isbased on his talk on the same subject atTCI EXPO 2014 in Hartford, Connecticut.An audio recording of that presentationcan be accessed by going to this page inthis issue of TCI Magazine online attcia.org, under Publications, and clickinghere.

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Three years after severe ice storm injury in New

Hampshire, ash and birch trees survive by sprouting and

rebuilding new crowns.