Branch shedding in trees

I have previously spoken about branch shedding (at times also known as cladoptosis) over on my thread on Arbtalk – see here, here, and here. The purpose of this post is to try and draw together everything I have researched on branch shedding, so to provide for a more centralised reference point for those seeking to learn more about the process. I highly recommend that anyone looking to learn more about branch shedding checks out Alex Shigo’s A New Tree Biology and T. T. Kozlowski’s Shedding of Plant Parts. Whilst both were written over 30 years ago, the material is very detailed and, in the case of the latter book, heavily referenced. Things have advanced since then somewhat, but these publications offer crucial context and ‘set the scene’, so to speak.

Natural branch shedding – what is it?

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 however, only the species of Coniferales (conifers) and Gnetales are able to ‘practice’ cladoptosis – to expand, only 2 of the 9 genera of Pinaceae (pines) possess such an ability). In these species, Millington & Chaney (1973) outline two distinct mechanisms by which a branch may be shed: (1) physiological processes (cladoptosis), and (2) an interaction of biotic and mechanical agents (‘self-cleaning’ or ‘natural pruning’).

The physiological mechanism is much akin to how leaf abscission (shedding – during autumn for deciduous trees) operates. Separation 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 (cork-like tissues). 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 instead 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 significant competition. These dead branches are colonised by fungal saprophytes (fungi that colonise upon deadwood) 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).

Turning attention towards cladoptosis, the process, as elucidated to above, involves the senescence (physical deterioration) and subsequent death of a branch through the re-allocation of energy to other parts of the tree (Kozlowski et al., 1991). This is achieved by the tree ‘identifying’ a branch which it needs to lose (as it is not sustainable to retain it – usually due to poor light exposure) and thus begins the process of abscission, which sees the tree grow a layer of specialised tissue where the parent branch (or stem) meets the branch, which cuts off the vascular supply to the branch (Thomas, 2000). This tissue is corky, full of antimicrobial substances, and sits above the ‘wound’ – the branch is shed beyond this point, allowing for this ‘protection’ zone to remain (Bhat et al., 1986). This ‘wound’ is then occluded during the following growing seasons by a callus that adopts a circular shape (Kozlowski et al., 1991; Shigo, 1986; Watson, 2006), and once fully occluded there may be a distinct gall-like shape (a raised ‘bump’) that remains (Rust & Roloff, 2002). Ethylene is one of the plant hormones responsible for this process, as it encourages the re-allocation of resources away from shaded areas of the crown (Karban, 2015).

In certain species, such as oak, the process may be staggered (Shigo, 1986). The tree may first create a protection zone out at a distance from the branch junction, which leads to failure at that point. A long stub is then left, which can then be shed further down the line, by the same processes. Shigo (1986) also details that conifers behave slightly differently to broadleaved trees. Whilst a conifer branch is alive, resin is impregnated into the core wood of the branch junction (that can at times propagate out into the branch itself). As the branch begins to die and the shedding process begins, the tree seals off the small area surrounding the branch base to resist progress of potential pathogens. Once the branch has died, it will break where the resin core ends.

On this oak, we can see two dead laterals that are in the process of being shed. Notice the ‘snub’ at the base of either dead branch.

It is also important to note that shedding may occur when epicormic sprouts are no longer required by the tree. Usually a response to very heavy pruning (particularly at the wrong time of year – when leaves are not present or are not fully formed) due to the depletion of energy reserves that must be regained (sprouting initiates in areas where energy levels are low), the spouts may over time become unnecessary due to shading and due to the regaining of energy reserves. Usually this shedding occurs during the third year following sprouting (Shigo, 1991).

Branch shedding is ultimately a deliberate act by trees (Thomas, 2000). Certain species will be more prone to cladoptosis than others, even when of the same genus – dwarf or infertile (or both) cultivars are less likely to shed branches, due to their lower energy demand, when compared to forest species (Shigo, 1991); as are amenity trees less likely to shed lower branches due to the lack of competition for light (Shigo, 1986). However, the intricate array of drivers behind the branch-shedding process are not fully understood, though as trees evolved in groups within forests it is of little wonder why cladoptosis does occur – one simply has to look at a forest to see that branches lower on the trunk do not exist, as the intense shading makes their retention less than worthwhile (Shigo, 1986).

Another example of a dead branch being shed on oak. Once shed, there will be the typical ‘bump’ on the stem, typically indicative of a branch once being attached at that point.

Natural branch shedding – why does it occur?

Millington & Chaney (1973) state that 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.

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 hybrid black poplar, 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, inter-nodal distances (the distance along the shoot between buds) progressively reduce, photosynthesis of the branch is thus impacted, and the decline in the ‘carbohydrate budget’ of the branch eventually leads it to become compromised – the branch is then abscised. In support of such a claim, Quercus alba (white oak) have been observed to shed twigs with less distance between nodes and retain only the twigs with greater inter-nodal 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 thought to be a driver behind the lack of abscission. Cupressaceae (cypress) 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 (a phenomenon by where branches may drop, without warning, during very dry periods), water deficits will also initiate branch 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 (Schaffner, 1902). Ephedra spp. (ephedras) will, for example, shed branches as a defence mechanism against water stress – as will Araucaria araucana (monkey puzzle), when on thin, dry, and sandy soils. Exactly whether summer branch drop is ‘intentional’ remains open to debate, however.

This image, which was taken on 17th July 1901, shows a Populus deltoides that has shed numerous branches during summer. The author (Schaffner, 1902) notes that this phenomenon, dubbed as ‘self-pruning’ (where foliage and branches were shed prematurely), occurred throughout the summer – all the way up until leaf abscission during autumn (when the remaining leaves were shed).

Natural branch shedding – what are the benefits?

Trees will naturally shed their branches so that their crown is not clogged by a profuse amount of branches. This process typically sees lower branches shed as higher branches occlude the light from the lower ones – this trait is particularly common with Pinus spp. (pines) and other excurrent species (Shigo, 1986; Shigo, 1991; Thomas, 2000). Essentially, if a branch is not producing enough carbohydrates (through photosynthesis) to maintain its own ‘mass’, then it will likely be shed – retaining the branch is counter-productive to the energy system of the tree, which seeks to retain efficiency at all times (Karban, 2015; Rust & Roloff, 2002; Thomas, 2000). This can be seen in trees of any age.

Cladoptosis also ensures that the tree does not have an unnecessary wind sail area. By retaining only the branches necessary for efficient energy production, unnecessary branches do not increase the wind sail of the tree; such an increase in wind sail would increase risk of windthrow, and may in fact require additional root growth to accommodate for increased wind sail (Thomas, 2000), which in itself requires more energy.

Cladoptosis also ensures that organisms located within the tree’s dead and dying branching structure do not pose any more of a threat to the tree than is ‘necessary’ in the long term. A tree can always re-grow branches (and roots), so cladoptosis is also beneficial in a defensive aspect (Shigo, 1986). Further, where species such as oak have been pruned and epicormic growth (twiggy growth along the branches) has ensued, the shedding of the very small twigs through ‘cladoptosis’ is very frequent in the years following – the twigs are ‘pinched off’ by the branch as it lays down its annual growth ring.

For species such as willow and poplar, the shedding of branches can even be a way of propagation. As willows and poplars are commonly found along water courses, one of their propagation techniques is to shed branches via cladoptosis, then having these shed branches travel downstream and then potentially take root when washed-up (Thomas, 2000).

Sources:

Bhat, K., Surendran, T., & Swarupanandan, K. (1986) Anatomy of branch abscission in Lagerstroemia microcarpa Wight. New Phytologist. 103 (1). p177-183.

Karban, R. (2015) Plant Sensing & Communication. USA: University of Chicago Press.

Kozlowski, T., Kramer, P., & Pallardy, S. (1991). The Physiological Ecology of Woody Plants. UK: Academic Press.

Millington, W. & Chaney, W. (1973) Shedding of Shoots and Branches. In Kozlowski, T. (ed.) Shedding of Plant Parts. USA: Academic Press.

Rust, S. & Roloff, A. (2002) Reduced photosynthesis in old oak (Quercus robur): the impact of crown and hydraulic architecture. Tree Physiology. 22 (8). p597-601.

Schaffner, J. H. (1902) The self-pruning of woody plants. Ohio Nature. 2 (1). p171-174.

Shigo, A. (1986) A New Tree Biology. USA: Shigo and Trees Associates.

Shigo, A. (1991) Modern Arboriculture. USA: Shigo and Trees Associates.

Thomas, P. (2000) Trees: Their Natural History. UK: Cambridge University Press.

Watson, B. (2006) Trees – Their Use, Management, Cultivation, and Biology. India: The Crowood Press.