Is the size of a conifer tree's root system proportional to the size of the tree?
By the same token will limiting tree size also have an effect on root system size?
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Yes and yes - but 'proportional to the size of the tree' is somewhat meaningless really. The type of soil and the conditions such as oxygen levels in the ground, obstructions in the ground, availability of water sources, all contribute to how far and wide a tree's roots may spread, and this is difficult to predict. In general, you can expect most of the roots to be in the top 3 feet of soil, with a possible spread up to 3 times the height of the tree.
Many pines, for instance, produce a tap root initially, with lateral root growth out from that - the tap root may continue to function, or it may stop growing and further laterals may spread out a foot or two down, so you've almost got two branches of root system in a sort of double tier. Planted on a steep gradient, many long roots will form on the higher side, to anchor the plant better, and these tend to run near the surface, or in the top foot or so. If there is more water available one side than the other, other roots will head for the water source, so there may be many more fibrous roots in that area. These are some examples of why root spread and depth is, to a large extent, unpredictable.
Restricting or reducing topgrowth causes less root growth or some root dieback respectively, but even so, its still next to impossible to know how much root material there will be and how far it'll spread - its all dependent on local conditions, in particular, below the top of the soil.