What holds up a tree?

As a tree specialist I am occasionally asked ‘what holds up a tree?’.  I’ve discussed this with engineers and no-one can really explain how trees can so effectively hold up their tonnes of trunk and branches, high above ground level and while buffeted by some pretty fierce winds.

Seeing a tall, massive tree is an awesome experience, especially if it is growing on some difficult terrain.  Then the tree appears to cling tenaciously to life, over-riding the constraints of its situation.

Close to the base of the trunk the thick roots act like anchors, embedded in the soil.  These roots taper quickly to become long, thin cables that can efficiently transport water back to the trunk and up to the leaves.  They also add stability to the anchoring roots as though they are guy lines supporting a tent or flag-pole.  But these anchors and guy lines are dynamic and growing.  The tree adds wood and strength wherever the stresses are greatest.  Stresses at the base of the trunk are a major area receiving added strength every year.

There’s still a lot we don’t know about roots.  Recent research has found that we need to revise some of the things thought we knew about them.  For instance the root system is not a mirror-image of the above ground parts.  Roots usually only reach a depth of around 1m to 2m and most are found within 600mm of the soil surface.  This is where the roots can get the oxygen, water and nutrients they need.

To overcome the stresses put on the base of the tree by the great weight of the trunk and branches the root system needs to be extensive.  If it can’t grow downward it must grow outward.  But the roots aren’t like ‘sniffer dogs’, with some inner sense to guide them to sources of water.  Rather they are opportunistic, growing more strongly where conditions are favourable and dying out where they are not.  This explains why you can never be sure where roots are until you dig them up.  This may change in the near future as ground penetrating radar is being developed to map root systems.

Roots are found in greater profusion at the edge of the canopy of the trees.  Here the roots are well positioned to utilise the rain that washes off the trees.  But this is not the limit of the extent of the root system.  Expect them to grow outwards for at least the same distance as the height of a tree, often much farther.

Roots are amazing: they hold the tree up both physically and by sustaining its health.  When we encounter tree roots in the garden we shouldn’t damage or cut them without thinking carefully about the effects this might have on the tree.