When branch autonomy fails: Milton's Law of resource availability and allocation

The branch autonomy principle states that the critical characteristics of a branch's carbohydrate economy (photosynthesis, respiration, growth, etc.) are largely independent of the tree to which the branch is attached, as long as light is the primary factor limiting photosynthesis and growth. However, this may not be generally true because in the spring, photosynthates are translocated from a tree stem into branches, and the amount of photosynthate available for translocation should be a function of the tree's canopy status. And the correlative inhibition principle states that a branch's priority for allocation of carbon and other resources is controlled not only by its own environment, but also by its position relative to other branches on the same tree. A study of the lower limit of branch growth and survival in trees of different sizes shows that the latter principle is more important: even though dominant trees have more resources to allocate, branches on suppressed trees are able to grow and produce new foliage at solar irradiances where branches on dominant trees die. Thus branches are sufficiently interdependent that a positive carbon budget by itself does not ensure branch survival; branch position relative to other branches on the same tree is also important. Other findings indicate that this result is quite general: regardless of the stress involved, a stressed branch on a tree where all other branches are also stressed does better than a similarly stressed branch on a tree where some branches are relatively unstressed. Although branch autonomy is an important and useful principle, it is not an absolute rule governing branch growth.