The shedding of lateral twigs and branches is a frequently-observed phenomenon of woody plants, with many species of angiosperm and gymnosperm having the capacity to shed such laterals (among gymnosperms for instance, only Coniferales and Gnetales species are able to ‘practice’ cladoptosis – to expand, only 2 of the 9 genera of Pinaceae possess such an ability). Data available at the time of this book being written outlined two distinct mechanisms by which a branch may be shed:
(1) physiological processes (cladoptosis)
(2) an interaction of biotic and mechanical agents (‘self-cleaning’ or ‘natural pruning’)
The physiological mechanism is much akin to how leaf abscission operates. Seperation of the branch from the adjoining structure occurs along well-defined ‘cleavage zones’ and is preceded by both the weakening of tissues local to the region and formation of a periderm. This process is known as cladoptosis, and typically only operates successfully for small branches and twigs – large branches may also be shed from the bole, however (of course, more infrequently).
Interactions of biotic and mechanical agents, on the other hand, can be dubbed otherwise as ‘self-cleaning’ or ‘natural pruning’. Particularly in dense stands (woodlands), branches lower on the bole often will die as a consequence of significant shading brought about by high levels of competition. These dead branches are colonised by fungal saprophytes and insects, which in time will decay, weaken, and eventually facilitate failure of the branch in loading conditions (rain, wind, snow, animal activity, or otherwise).
So what influences or drives cladoptosis / branch shedding?
Branch and twig abscission will occur as a result of an array of physiological and environmental factors: low vigour, water supply, age, and unique site factors. However, the relationship between these four main drivers is poorly understood (or was, at the time of this source being written).
Typically, branches that abscise are weak and lack vigour. For illustrative purposes, insect or fungal infections may trigger a decline in vigour, in turn initiating cladoptosis. In Populus serotina, for example, twigs that arise from small buds and make poor growth are usually shed come autumn (fall). Further, if a branch produces a serious abundance of flowers year-on-year, internodal distances progressively reduce, photosynthesis of the branch is lessened, and the decline in ‘carbohydrate budget’ of the branch eventually is compromised – the branch is then abscised. In support of such a claim, Quercus alba have been observed to shed twigs with less distance between nodes and retain only the twigs with greater internodal spacing.
Cladoptosis will also vary significantly with age. When young, Quercus alba will very rarely – if at all – shed any twigs. However, come maturity, twigs abscise frequently. The retention of leaves throughout winter on young specimens is throught to be a driver behind the lack of abscission. Cupressaceae species will also shed commonly in maturity, though not so before. This trend is bucked however by Castilla elastica (Panaman Rubber Tree), which sheds twigs frequently when young, though by maturity has developed branches that need not be shed.
Relating to summer branch drop, water deficits will also initiate shedding. In very dry summers in Ohio for example, many angiosperms were observed to drop branches before 15th July – branches of the trees continued to abscise until – and even partially into – autumn. Ephedra sp. will for example shed branches as a defence mechanism against water stress – as will Araucaria araucana, when on thin, dry, sandy soils.
Source: Millington, W. & Chaney, W. (1973) Shedding of Shoots and Branches. In Kozlowski, T. (ed.) Shedding of Plant Parts. USA: Academic Press.