The cambium – or more precisely the vascular cambium – is a layer of cells underneath the outer and inner bark and outside the wood of a tree. It’s officially defined as a ‘meristem’ – that is, a region of cells capable of division and growthref. You may recall the ‘shoot apical meristem’ in the post about How Trees Grow – this is the part of the shoot which is actively dividing and creating new cells at the tip of the shoot (known as primary growth). The vascular cambium does something similar – it divides to create the vascular system – a layer of xylem cells on one side and a layer of phloem cells on the other. The vascular cambium is where part of the secondary thickening of a tree takes place, as the xylem layers become the wood of the tree, and the phloem layers become the inner bark (the outer bark has another meristerm – more here). Always in between there is a single-cell thickref layer of vascular cambium. See below for an image of the cambial zone, phloem and xylem cells.
Two types of cells exist in the cambium – vertically elongated ‘fusiform initials’ and horizontally oriented ‘ray cell initials’. The fusiform initials produce xylem and phloem cells and the ray cell initials produce rays (which cut across the tree connecting xylem and phloem).ref The ray cells can create gum and resin channels, which can also be activated when the cambium is wounded. You can see a resin duct cavity in the image above, as well as a ray.
Like a lot of growth in a plant, the activity of the cambium meristem involves plant growth regulatorsref. Auxin levels peak in the middle of the cambial zone, where cells are dividing, cytokinin peaks in the developing phloem cells and gibberellin peaks in the developing xylem cells. A study in Populus found that increased local biosynthesis of cytokinin led to increased trunk biomass and radial size (width).ref ‘Local’ biosynthesis in this case meant a tree which had been transgenically modified to produce more cytokinin.
The vascular cambium slows down or stops completely during winter in temperate zones, this depends on the tree’s phenology. More detailed information about the vascular cambium can be found in this book.
From a bonsai point of view the main takeaway is that the vascular cambium tissue underneath the bark is critical for your tree’s growth so avoid damaging it.