There are a few different terms bandied about to describe buds which pop up in unexpected positions on a tree – ‘adventitious’, ‘dormant’, ‘suppressed’ ‘preventitious’ ‘proventitious’. Epicormic growth is actually just growth which forms on old-growth units of the tree – not in the current season’s growth.ref It’s great for bonsai because it helps keep the foliage condensed and well-ramified and allows you to develop branches which are closer to the trunk, to keep the profile compact.
This study proposes standardising on the terms ‘adventitious’ and ‘preventitious’. The key difference between the two types is how they develop – preventitious buds originate exogenously (due to an external trigger) and descend from a shoot apical meristem, while adventitious buds develop endogenously (due to internal triggers) from previously non-meristematic tissue.ref
A key concept here is that of the meristem which we encountered back in How trees grow. A meristem is an area in a plant containing stem cells – cells which can become any other type of cell. Trees maintain four active meristems which are continuously producing new stem cells as well as differentiated cells used to build the plant (the shoot apical meristem, the vascular cambium, the cork cambium and the root apical meristem). These allow the plant to respond to damage by growing new organs. The different types of epicormic buds arise from epicormic meristems, which are traces of meristematic tissue which are not located in the active portions of the above-ground meristems.
Preventitious buds arise from meristematic traces which are triggered to become full buds and sprout at some point in the future, for example if the tree is wounded or damaged and needs to generate more foliage for photosynthesis. In fact these preventitious meristems may come from axillary buds which aren’t activated during their growth season. Particularly in angiosperms, many more buds are generated than are activated in a growth season, and these buds may stay dormant until needed. Not only that, but mature buds approaching bud burst have tiny buds inside them as well, which become next season’s buds if a bud extends. If it doesn’t extend, there are 2 years of dormant bud tissue available for future activation.ref
Adventitious buds happen when a callus or other wound response creates meristematic tissue which connects to vascular growth (eg. the cambium) and establishes a trace similar to preventitious buds. In the future this trace can become a shoot.
This study identified four different strategies for epicormic bud development – external clustering, isolated buds, detached meristem and epicormic strands.
External clustering is where “trees produce relatively small, persistent axillary buds, which develop into epicormic complexes consisting of numerous buds and shoots”. Even though they are not visible, every year they extend with annual growth creating more meristematic tissue and/or leaf primordia (embyonic leaves), and sometimes shoots as well. These bulges on tree trunks are a familiar sight on many trees – such as this Linden tree near my house:
The majority of species which are known to be prolific producers of epicormic shoots fall into the external clustering strategy. I often see Oak trees in Richmond Park with rounded protuberances on their trunks – these are epicormic complexes under the bark.
The isolated bud strategy is the “initial production of larger external epicormic buds, mainly high buds, which are less persistent and less likely to form large clusters.” These buds are buried in the bark, or in the case of gymnosperms a meristematic ‘bud base’ is left in the bark.
The detached meristem strategy is also observed in conifers and involves “the maintenance of minimally developed meristems hidden in leaf axils” which require some trigger (like fire) to become active. These meristems are not connected to the vascular system but can connect later when they create buds. Members of the Araucariaceae family have been found to use this strategy, such as the Hoop and Wollemi pines. Ref1, Ref2
The final strategy is epicormic strands “characterized by the presence of extensive meristematic strands within the bark that are capable of producing a continuous series of ephemeral epicormic buds” – this is observed in Eucalyptus.
One key point is to understand why a tree develops epicormic buds into shoots – and the answer to this is that is a response to stress – stressors can include insect defoliation, fire, frost, wind damage, disease, drought , intense competition, low site quality, bole orientation, vascular embolisms and heavy pruning.ref The bad news is that epicormic branches have a reputation for being weaker and not very long-lived. In his book the Wild Trees Richard Preston references epicormic branches on Coast Redwoods, noting that the people who climb these trees avoid putting weight on epicormic branches since they are liable to shear off the tree.
To work out where epicormic buds might appear on a tree, go back and read Buds, as preventitious buds in particular will develop in places where buds could have formed in previous growth period.