So we know what roots achieve for a tree, but how are they structured? To start with tree roots are either woody or non-woody. Woody roots have undergone secondary thickening and are long-lived, like the trunk and branches, and provide the structural framework for the tree.ref
The ‘root collar’ is the area on the tree’s trunk where the roots join the main stem, and where there is typically a root flare (the root collar is still part of the trunk though, which is why it shouldn’t be buried in soil).ref At the base of the root collar, there are usually five or more primary structural roots that “descend obliquely into the soil before becoming horizontal within a short distance of the trunk” and these taper rapidly within 1-2m of the trunk.ref These are known as lateral roots since they grow in a lateral (horizontal) direction.
In his book ‘Trees, Their natural history’, Thomas says that trees develop a root plate, which is wide and shallow (vs the commonly held view of a root ball, which is only applicable to certain trees). Having a wide root plate helps trees achieve two of their main goals – to support and strengthen the tree against wind & weather, and to access waster and nutrients which are concentrated in the top layer of soil.
According to Thomas, root systems are more variable than shoot systems because the underground environment is more variable than aboveground. When roots encounter an obstacle underground, they fork, and as they fork and expand underground the main lateral roots can fuse into each other. This creates a criss-crossing of roots, which provides greater structural strength than if the roots were not connected. Roots can also connect to other trees’ roots (and even detect if they are ‘kin’ or not).
Structural lateral roots can develop into buttress roots, which have been found to provide tension strength in high-wind situationsref – as a little girl growing up in Australia the best fun could be had climbing over the huge roots of the Moreton Bay Figs (Ficus macrophylla).
In addition to lateral roots, most bonsai enthusiasts will have encountered the dreaded tap root. A tap root is the root generated by a new seedling (Thomas), which grows downwards and becomes a thick structural root. The tap root can become dominant in the root system and be a total pain for bonsai – it often generates its own lateral roots, creating a second root plate and makes it hard to get the tree into a bonsai pot. But luckily according to Thomas and others (and personal experience) the tap root isn’t necessary and can be removed. This is always best done sooner rather than later so that energy is not diverted to its growth vs the roots you do want to keep.
As well as tap roots, other structural roots trees create include sinker roots which go deeper into the soil (often to find water), can set up a secondary root plate, and also grow back upwards to create a ‘root cage’ (Thomas).
Susan Day et alref say “although structural roots comprise most of the root biomass, they account for a small percentage of total root length and root surface area.” The remainder of the root surface area is comprised of fine roots, which are the main mechanism for the tree to extract water and nutrients from the soil. Connecting the main structural roots to the fine roots are a network of tapering roots which branch off the structural roots.
A study of nine North American tree species found that in eight species roots <0.5 mm in diameter accounted for >75% of the total number and length of roots assessed.ref Thomas quotes a study on Douglas fir estimating that 95% of the total root length comes from roots <1mm and about half less than 0.5mm.
As noted above the fine roots are non-woody and don’t undergo secondary thickening – this means they die and are replaced by new roots. It’s quite hard to measure this and there is differing information about fine root lifespan, but the above study found the average fine root lifespan to range from an average of 153 to 359 days. This is also expressed as a ‘fine root turnover rate’ and based on this data table fine roots of gymnosperms turn over more slowly than angiosperms (some Pinus species 20% per year vs beech 100% per year).
The fine roots are concentrated in the top part of the root plate, where most of the nutrients and water are located (20-30cm of soil, and the leaf litter & humus if present). Like the stems aboveground, the roots are constantly developing and growing, with new root tips being created by the root apical meristem (RAM) (this is described below). How the root goes about absorbing water and nutrients from the soil is covered in this post: How roots absorb water & nutrients.
These fine roots are what we are trying to encourage in bonsai as they enable the tree to extract the most water and nutrients from their environment, while still fitting into a small pot. What we want in the fine roots is lots of branching and ramification – just like aboveground – read more about encouraging this in ramification of Roots (lateral root development).
The below diagram shows the ratios of leaf, stem and root biomass to total tree mass for a data set including 3700 ‘woody’ plants (ie. trees!)
As you’ll notice, the larger the tree gets, the more the stem (trunk) represents of the total biomass. However the ratio of roots to total biomass stays within a range from 16% to 40%. By comparison the ratio of leaf mass has a much wider range all the way from 60% down to 2%. So there is a certain baseline amount of root biomass needed to maintain a tree.
This mass is mainly made up of the structural roots, as although the fine roots comprise the vast majority of the root surface area, they are very light in comparison to the woody roots.
So bonsai nerds, what to make of all this? Key info is the fact that fine roots die and regrow on a regular basis – and – kill that tap root! Help your tree be more stable by encouraging a root plate of connected structural roots, and you won’t need a deep root ball or a tap root. Nebari and root mass should be around 20% of the mass of the tree for an old tree look.