Reaction wood

Anatomy of Angiosperms: Reaction wood

1. 1
Reaction wood is formed as a
response by the tree to a
triggering event such as tipping
from the vertical.
It is also known to regulate the
orientation or angle of branches
relative to the main stem
Reaction Wood
(Haygreen & Bowyer)

2. Reaction wood
Reactionwoodinawoodyplantiswoodthatformsinplaceofnormalwoodas
aresponsetogravity,wherethecambialcellsareorientedotherthanvertically.
Itistypicallyfoundonbranchesandleaningstems.Itisanexampleof
mechanicalacclimationintrees.
Progressivebendingandcrackingwouldoccurinpartsofthetreeundergoing
predominantlytensileorcompressivestresseswereitnotforthelocalised
productionofreactionwood,whichdiffersfromordinarywoodinitsmechanical
properties.Reactionwoodmaybelaiddowninwiderthannormalannual
increments,sothatthecrosssectionisoftenasymmetricorelliptical.The
structureofcellsandvesselsisalsodifferent,resultinginadditionalstrength.
Theeffectofreactionwoodistohelpmaintaintheangleofthebentorleaning
partbyresistingfurtherdownwardbendingorfailure.

3. Reaction wood
Therearetwodifferenttypesofreactionwood,whichrepresenttwodifferent
approachestothesameproblembywoodyplants:
Inmostangiospermsreactionwoodiscalledtensionwood.Tensionwood
formsonthesideofthepartoftheplantthatisundertension,pullingittowards
theaffectingforce(upwards,inthecaseofabranch).Ithasahigherproportion
ofcellulosethannormalwood.Tensionwoodmayhaveashighas60%
cellulose.
Ingymnospermsandamborellaitiscalledcompressionwood.Compression
woodformsonthesideoftheplantthatisundercompression,thereby
lengthening/straighteningthebend.Compressionwoodhasahigherproportion
ofligninthannormalwood.Compressionwoodhasonlyabout30%cellulose
comparedto42%innormalsoftwood.Itslignincontentcanbeashighas40%

4. •Thecontrollingfactorbehindreactionwoodappearsto
bethehormoneauxin,althoughtheexactmechanism
isnotclear.Inaleaningstem,thenormalflowofauxin
downthetreeisdisplacedbygravityandit
accumulatesonthelowerside.
•Theformationofreactionwoodmayactinconjunction
withothercorrectiveoradaptivemechanismsin
woodyplants,suchasthigomorphism(adaptive
responsetoflexure)andgravitropism(thecorrection
of,ratherthanthesupportof,lean)andtheauxin-
controlledbalanceofgrowthratesandgrowth
directionbetweenstemsandbranches.Theterm
‘adaptivegrowth'thereforeincludes,butisnot
synonymouswith,theformationofreactionwood.

5. •Asarule,reactionwoodisundesirableinanystructuralapplication,
primarilyasitsmechanicalpropertiesaredifferentfromnormal
wood:italterstheuniformstructuralpropertiesoftimber.Reaction
woodcantwist,cuporwarpdramaticallyduringmachining.This
movementcanoccurduringthemillingprocess,makingit
occasionallydangeroustoperformcertainoperationswithout
appropriatesafetycontrolsinplace.
•Forinstance,rippingapieceofreactionwoodonatablesawwithout
asplitterorrivingknifeinstalledcanleadtokickbackofthestock.
Reactionwoodalsorespondstomoisturedifferentlyfromnormal
wood.
•Traditionally,compressionwooddoeshavenicheapplications.For
instance,huntersinnorthEurasiaandtheAmericanArcticwere
knowntoharvestcompressionwoodforbowstaves,becausethe
increaseddensityandcompressionstrengthofthiswoodenabled
themtomakefunctionalweaponsoutoftreespeciesthatwould
otherwisebeunsuitableforthispurpose,duetotheirlowstrength
andlowdensity.

6. 6
The terminology used to
describe reaction wood
formed in softwoods and
hardwoods comes from the
stresses normally present in
those locations.
However, those stresses
themselves are NOT
responsible for the formation
of reaction wood.
(Haygreen & Bowyer)

7. 7
Compression wood –macroscopic appearance
Compression wood
(Hoadley)

8. 8
Light microscope Scanning electron
microscope
Transmission electron
microscope
Compression wood –microscopic appearance

9. 9
Ultrastructure of longitudinal tracheids
(Josza)

10. 10
Normal wood Compression wood Compression wood
Compression wood –microscopic appearance

11. 11
Compression wood –characteristics and properties
Anatomy
•Wider growth rings
•More latewood
•Shorter longitudinal tracheids
•Rounded cells with intercellular spaces
•Helical striations (following S2Ө)
Ultrastructure
•Larger S2Ө
•S3 absent
•New S1L layer
Chemistry
•More lignin
•Less cellulose
•Hemicelluloses differ
Properties
•Higher wood density
•Compression strength ↑
•All other strengths ↓
•Brittle failure
•Greater longitudinal shrinkage
•Lower pulp yields
*

12. 12
Tension wood –macroscopic appearance
Tension
wood
(Hoadley)

13. 13Aspen normal woodAspen tension wood
G-layer
Tension wood –microscopic appearance
(Hoadley)

14. 14
Microfibril orientation
Tension wood –appearance of G layer

15. 15
Tension wood –characteristics and properties
Anatomy
•Fibers affected not vessel elements
•Gelatinous fibers (G-layer)
Ultrastructure
•SG -after S3
-replaces S3
-replaces S2 + S3
-replaces some of S1 + S2 + S3
•Microfibrils less closely packed
•Low Өin G-layer
•Higher S1Ө
Chemistry
•More cellulose
•Less lignin
•Hemicelluloses differ
Properties
•Higher wood density
•Compression strength ↓
•Seasoning defects
•Higher pulp yields
•Poor workability (“fuzzy” grain)
*

16. Location of reaction wood formation
16(Haygreen & Bowyer)
Compression wood
formation in horizontally
oriented stem.
Compression wood is
found on underside of
stem –not on side of
stem under
compression.

17. Location of reaction wood formation
17
(Haygreen & Bowyer)
Reaction wood formation in
growing looped stem.
Compression wood is found
consistently on underside of
stem and tension wood is
found consistently on upper
side of stem (regardless of
the nature of the stresses
experienced in those
locations).
a. Softwood
b. Hardwood