Scale leaves are a curious form of conifer leaves which cover up the stem in interlocking patterns. I believe they are called ‘scale’ because they look a bit like fish scales in the way they overlap but I have not found an authoritative source which confirms this. Scale leaves appear primarily in the Cupressaceae family – including junipers, various cypresses, Arborvitae/Thuja, redwoods & Callitris, as well as in the Podocarpaceae family including Dacrydium and Acmopyle. Sometimes the scale leaf form is the mature foliage, while the juvenile foliage takes a needle form (see my post on conifer needle leaves).
Some examples of scale foliage leaflets are below:
You’ll note that I called them ‘leaflets’. The actual leaves are the individual scales that you see in the images, which all combine to create a larger leaflet which is actually a short shoot. The leaves are wrapped around and connected to the stem of the shoot underneath.
Scale leaves are usually in opposite pairs, and depending on the species can have main or ‘facial’ scales and lateral scales with slightly different anatomy. Below is a scanning electron microscope image of Thuja occidentalis leaves which demonstrates these two scale types.
Probably the most distinctive attribute of scale-leaved species is the leaf pattern. Each species has a distinct cross-sectional profile, with different leaf shapes and configurations, these are what ultimately create the three-dimensional shape of the leaflet.
Great work was done on this by some Iranian researchers, who created the following cross-section drawings which I have matched to images of the species in each drawing. These show how the scale leaves attach to each stem, the positions of vascular bundles, resin ducts (large holes) and stomata (which are mostly present in the grooves indicated by ‘S’).
As can be seen in the drawings, scale leaves are very simple, usually with a single vascular bundle (other than Juniperus excelsa above which has none), palisade and spongy mesophyll cells for photosynthesis and the darker transfusion cells which move water to and from the stem.ref
In terms of their performance, there actually isn’t a lot of information out there comparing scale leaves with needle or flat leaves. One study found that Thuja leaves were about on par with pine needles in terms of photosynthesisref and another found that juvenile needle leaves of Juniperus sabina outperformed its scale leavesref. Many studies seem to conflate needle and scale conifer leaves and talk about both of them having strong performance in high sun, low humidity situations. There must be some benefit, because quite a few species ‘graduate’ to scale foliage as they age, but I haven’t found any research explaining what that benefit may be.
One weird and wonderful variation of the scale leaf is the unusual ‘axial’ leaf of the coast redwood. Most people associate the coast redwood (Sequoia sempervirens) with a flat-leaved leaflet as shown on the left – and in fact this type of leaf makes up 95% of the leaf surface area of these trees. But 5% is made up of the axial-leaved ‘twiglet’ on the right. These leaves are optimised to absorb water, having much less waxy coating than the flat leaves and contribute up to 30% of the water requirement of the tree. Which can be high, given the size of a coast redwood!
So what are the bonsai implications?
Since scale leaves are associated with older trees, they are preferred for bonsai. If your tree is still in its juvenile phase, you need to let it grow as it’s believed that the trigger for changing phase to mature foliage is the number of meristem cell divisions.ref Pruning the leader on these trees will keep them permanently in a juvenile state, so let the tree grow until it develops mature foliage, then you will need to use all branches & foliage *after* this point to style your tree.
Also remember that scale leaves usually appear on short shoots, which abscise (fall off) as a unit. But don’t worry because usually there will be a bud waiting at the base of the short shoot to replace the one which fell.
Finally if you look at how scale leaves connect to the stem, I believe that the technique of pinching leaves (and stem) off instead of pruning with secateurs or scissors would leave less dead material on the tree. Cutting straight through a stem is always going to sever one or more scale leaves and cause them to die and go brown. The pinching technique is when you use your fingertips, and pull the stem gently so it breaks at a natural breaking point between leaves.