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Sunday, 15 September 2019

How does a tree branch resist gravity?

We look at tree branches without perhaps realising how they manage to support such massive weights.

It seems that Conifers and Broadleaves have evolved different solutions to the same problem but both of them use what is known as "reaction growth".

Reaction Wood forms as a response to gravity and has different cell structure to normal wood, giving it extra strength, whilst laying down much wider rings which give extra bulk. The resultant cross section is often elliptical, particularly in conifers (as seen in photo below).

Broadleaved trees produce Tension Wood on the upper side of a branch, pulling the branch upwards.

Whereas Conifers produce Compression Wood on the underside of a branch, pushing the branch upwards.

Tension Wood has more cellulose and contains a specialised gelatinous layer in the cells, which is rubbery in texture.
Compression wood produces much more lignin and this altered, stronger growth has been utilised by hunters in the American Arctic to make bow staves from trees that were otherwise unsuitable.

                   Conifer with large bulge of compressive growth below the sideways trunks

                              Ash tree showing tension wood on the upper side of this huge branch

                            Conifer adapting with compression wood pushing against gravity

Conifer with unusual growth pattern

   Conifer with lost branch showing the wider rings (compression wood) formed on the underside


  1. Fascinating Philip. I've noticed that in beech hedges after trimming, some shoots bend upwards to fill the space created above them by the gardener.

  2. Thank you for this - I will look at trees with added attention to detail now.