You may have heard of embolisms in the context of humans – this is when an artery is blocked by something like a blood clot or a bubble of airref. A similar process can happen with trees and their xylem (water carrying) vessels.

As outlined in trees’ water system, water enters the tree’s root cells and is pulled in a continuous stream up through the xylem by the negative pressure created by leaf transpiration in the canopy. The xylem is actually a “continuous water column that extends from the leaf to the roots”.ref

If a bubble of air gets into the xylem, this breaks the water transport process and stops water from below the bubble being lifted any further. This stops water from reaching any parts of the tree dependent on the xylem cells which have been affected. Embolisms in trees are also known as ‘cavitation’. This process is even audible and apparently explains half of the sound heard from drying wood.ref

Thomas explains this really well in chapter 3 of his book in the section ‘Air in the system’. He says embolisms can occur from water in the xylem being under too much tension (ie. the tree becoming too tall or too high a rate of water being transpired), from xylem damage, or from freezing.

Embolisms are relevant to bonsai enthusiasts for two main reasons. Firstly they explain why a tree may die if it isn’t watered. When in leaf (for deciduous trees) and all the time for evergreens, transpiration will occur as water evaporates from the leaves through their stomata. If transpiration happens faster than the tree can replace water through its roots, embolisms can occur. If too many embolisms occur, the tree might not have enough routes for water to reach the cells, or enough volume of water to meet their needs.

The second reason why embolisms are interesting from a bonsai perspective is that they also explain the two very different xylem structures which can be observed between trees and which have implications for their growth behaviour.

All conifers and some angiosperms called ‘diffuse porous’ trees, add to their xylem network each year, and have many smaller, narrower, interconnected xylem vessels which don’t allow large air bubbles to form. These trees have active xylem in multiple rings (Thomas says that conifers can have 30-40 years worth of rings still actively transporting water up the tree). In cross-section they have small ‘pores’ and a denser consistency.

The alternative approach is taken by ring porous trees, which regrow their xylem vessels every year, and only ever have a single ring of much larger, longer super-pipes of active xylem transporting water. These trees can grow much quicker because their water transport is more efficient in the right conditions (ie. not freezing). But they can’t break bud until they’ve constructed the new year’s xylem. This explains why some species such as oak leaf out relatively late – they have to spend time at the beginning of the season regrowing their xylem ring. An evergreen tree cannot be completely ring-porous, because it would not be able to supply water to its leaves through the winter.

Like everything in nature there are trees which are combination of the two as well. There are some nice microscope images of the xylem vessels of different types of wood here. The difference in size between coniferous species (whose xylem cells are called tracheids) and angiosperm species (whose xylem cells are called vessels) is show in the table belowref:

As can be seen, conifers have tracheids which individually don’t get much longer than 2-3mm or wider that 10-12 μm (note that the table uses a logarithmic scale). Ring porous species on the other hand have vessels in the 1-12cm range in terms of length, and 20-80 μm wide.

The implication for this from a bonsai point of view is that even though our trees are small, for those species which are ring porous trees (mostly deciduous angiosperms) embolisms are perhaps even more of a risk. The scale at which we are working means there are only a small number of vessels available for water transport – a 20cm high tree might have single vessels all the way from root to crown. To mitigate this risk we should ensure these trees are well watered especially when it is hot or windy, and that they have the energy and nutrients to regrow their xylem every season.

I’m still trying to find a definitive source with a list of which species are ring porous and which are diffuse porous, but here’s what I’ve found so far (Ennos, Thomas, and some reference articles ref):

  • All conifers are diffuse porous, as they have tracheids (short & narrow) providing their water transport. Embolisms in these tracheids dissolve naturally due to their small size.
  • Diffuse porous angiosperms have narrower vessels – they are still vulnerable to embolism but less than ring porous trees, and have adapted mechanisms to resolve them, such as pumping water (sap) up from the roots to refill the vessels in spring (Ennos): poplars, beech, birch, maple,
  • Ring porous angiosperms have wide, fast flowing vessels and routinely get embolisms every winter, requiring a new set of vessels to be grown every year: oak, ash, black locust, catalpa, chestnut, hickory, mulberry,