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Sorry if this is a naive question, but it occurred to me as I was walking across campus today; How does a tree decide which limb is allocated the most resources, and thus grows the largest?

My first thought is the first branch to deviate from the base receives the most energy investment, and so on, but I don't have the time (or will) to actually test that. Does it have more to do with efficacy of the limb for photosynthesis? Or is it largely due to chance/unknown variables?

Thanks in advance, I feel this is something I learned back in botany and just forgot completely.

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There is Milton’s Law of resource availability and allocation.

It states that branches act as if they were different individuals, and they grow relative to the availability of their own resources.

The cells that grow are called meristems, there are different kinds of meristems, cambium ones that grow the trink, apical meristems that grow at the top,

Apical dominance is phenomenon where one meristem prevents or inhibits the growth of other meristems. As a result, the plant will have one clearly defined main trunk. For example, in trees, the tip of the main trunk bears the dominant meristem. Therefore, the tip of the trunk grows rapidly and is not shadowed by branches.

If the dominant meristem is cut off, one or more branch tips will assume dominance. The branch will start growing faster and the new growth will be vertical.

Over the years, the branch may begin to look more and more like an extension of the main trunk. Often several branches will exhibit this behaviour after the removal of apical meristem, leading to a bushy growth.

The mechanism of apical dominance is based on the plant hormone auxin. It is produced in the apical meristem and transported towards the roots in the cambium.

If apical dominance is complete, it prevents any branches from forming as long as the apical meristem is active. If the dominance is incomplete, side branches will develop.

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