Under pressure: compaction on tree roots

The roots perform very important functions for the tree. The root is described as an organ of the tree typically found below the surface of the soil, the non-leaf, non-node bearing part of the tree’s anatomy.

Perhaps the most obvious purpose of the roots is to anchor and support the trunk and crown of the tree above ground. The layout of roots is often misunderstood and assumed to mirror the canopy, though this is not usually the case with the roots spreading laterally as far as 12x the diameter of the trunk, typically staying in the top 600mm of soil. This provides a wide and firm base for the tree to resist wind forces and the pull of gravity as it moves in the wind.

The roots also perform a vital function in the absorption of water and nutrients. This is started in the fine root hairs at the tips of the roots which are typically located around the dripline of the canopy. Water enters here through osmosis and travels up to the leaves through the inner bark’s xylem (using capillary action). This water then is used in the leaves for the process of photosynthesis.

Damage to tree roots can occur in many ways – from physical damage to chemical damage. Often overlooked is indirect damage to the roots through soil compaction. Roots are particularly susceptible to soil compaction, as the reduced pore space reduces the amount of water and oxygen present in the soil. The restricted pores also mean that growing root tips struggle to advance through the soil and this stimulates the roots to thicken up giving greater pressure to force through compacted areas. It is this type of root growth that pushes up tarmac and concrete.

Compaction can have a far-reaching effect within the soil. A handful of soil is suggested to contain more organisms than the entire human population on earth, and these organisms cannot function in the restricted environment of compact soil where air and moisture is shut out. It is these micro-organisms that keep the soil conducive to healthy root growth, as root tips rely on an open-pore structure to grow the root network and with this expand the canopy. As root growth is entirely opportunistic, growing where it can, areas of compaction will cause the roots to thicken and advance slowly whereas favourable conditions will allow faster root growth. These two scenarios can occur on opposite sides of the tree and lead to a thicker, fuller crown on one side compared to the other.

Compaction can be eased however, and even removed using proprietary systems. This is particularly critical in areas of high traffic, both pedestrian and vehicular for the health of the tree to be continued.