Trees are fascinating organisms that adapt to a wide variety of factors in their environment to ensure their continued survival. One of their more interesting adaptations is the ability to right themselves (or the attempt to do so) if the tree begins to develop a lean. This is achieved through the development of reaction wood, which is a response by the tree to develop additional wood, either on the side of the lean or opposite the lean, in an attempt to straighten its form.
There are two types of reaction wood:
1) Tension Wood
2) Compression Wood
Tension wood forms on the upper side of the lean and is produced in angiosperms, which comprise most of the trees we see around our parks and roadsides. Compression wood on the other hand, forms on the underside of the lean and is produced in gymnosperms, which are mainly non-flowering plants such as conifers.
Reaction wood works by counteracting the pull of gravity on a tree's structure by either pulling (tension wood) or pushing the trunk upwards (compression wood). Tension wood has a high proportion of cellulose, up to 60% of its overall mass, which aids its ability to exert a pulling tension on the trunk. Compression wood on the other hand has a high proportion of lignin, up to 40%. Lignin is resistant to compression, which gives the wood the ability to push the stem upwards as the tree grows.
Whilst often associated with the trunks and sometimes roots of trees, did you know this reaction is found in the branches as well? As branches grow longer and larger, they become heavier and may bend over time. If left unattended by the tree, it may result in the eventual snapping of the branch. Reaction wood’s purpose in this case is to right the branch back to its original intended angle by pushing or pulling the branch as the branch grows in size.
Overall, by maintaining the stability of the trunk, roots or branches, reaction wood helps trees resist mechanical forces such as heavy winds, which would otherwise snap or uproot the tree. As arborists, understanding and spotting the presence of reaction wood in trees is an important part of our overall risk assessment of trees and tree care regimes. Depending on the tree species, degree of lean and presence of a target, we may conduct pruning works to lighten the load on the side of the lean, further reducing the tree’s chance of failure.
So, the next time you stroll through our forests or admire a tree's majestic branches, remember that beneath the bark lies a complex world of adaptation and resilience, thanks to the incredible phenomenon of reaction wood.
● Encyclopedia of Forest Sciences, accessed at:
● Journal of Wood Science, accessed at:
● Major Differences, accessed at:
● Seattle Arborist, accessed at: