I have become increasingly convinced that this cycle is an entity so well integrated that it seems hardly possible to investigate or survey any of its parts successfully if they are separated from the whole (Sarvas 1974, p. 3).
Each year, a tree makes itself anew by adding modules of primary and secondary growth to the surface of the previous year’s crown, stem, and roots. This deposition of new growth keeps the tree alive, as no tree can persist without renewing its tissues. It also allows the tree to achieve a size and physiological stage of maturity that supports sexual reproduction. Moreover, it replicates the distinctive species phenotype, which results from the interplay of genome, and environment broadly construed.
How then are primary and secondary growth initiated and controlled to yield the uniform product of each year’s growth cycle? What provides the stimulus that drives the growth of apical and lateral meristems when internal and external conditions are conducive to biological activity?
I propose here that the initiation of leaf primordia on shoot apical meristems that sets in motion the annual suite of tree growth activities. This may occur immediately or after a period of dormancy. Once begun, those growth activities proceed to completion, unless prevented by disturbance or resource depletion.
The determinate nature of this synthesis does not exclude familiar sources of variability. Even determinate processes that are predictable in their outcome are influenced by environmental variation, historical events in a tree’s long developmental time frame, and the unique genetic legacies of each taxon. Much of the supporting evidence cited below originated from studies in Pinus because pines are the most studied of trees. Precise applicability to other taxa remains to be determined. The concept of leaf primordium initiation as a driving force for plant growth has been raised with regard to wheat (Triticum aestivum L., Hay and Kemp 1990) but appears to be a novel explanation for a complex woody plant.
Lanner RM 2017. Primordium initiation drives tree growth. Ann. For. Sci. 74: 11. 10.1007/s13595-016-0612-z.