I’ll be honest, I find conifer needles to be quite weird. As someone raised in the southern hemisphere, true pine trees were something we had at Christmas and that was about it (this is true because the most common Christmas tree found in Australia is Pinus radiata). But actually, needles really are just plain old leaves. They contain all the same components as ‘normal’ leaves, like an epidermis, photosynthesising mesophyll cells, green chlorophyll pigment, xylem/phloem, stomata and so on. They just happen to have pushed the ‘leaf’ form to the extreme, ending up extremely long, thin and tough.
Within conifers, what are *called* needle leaves are found across most of the families, including Araucariaceae (Norfolk Island pine, Cook pine, Hoop pine), Cupressaceae (juniper, Thuja, Cryptomeria japonica), Pinaceae (pines) and Podocarpaceae (Platycladus).
At this point though I think it’s important to point out that the Pinaceae family is the least similar to all the other conifer families, having diverged from them very early in evolutionary history. Look at the ‘family tree’ of conifers below and you can see that Pinaceae including pine, cedars, larch, fir and spruce, has been evolving separately for the longest of any conifer family. This means that ‘needle leaves’ in Pinaceae are not the same thing at all as needle leaves in other families, and they probably need to be treated as two separate sub-categories.
This is why it can be confusing to understand what people are talking about when they refer to needle leaves in conifers. Needle leaves in Pinaceae are obvious – they are long, thin, spiky, tough, 3-dimensional and in mature foliage form in clusters known as fascicles which are actually short shoots (see my post on shoots for more on these). What you and I would call pine/fir/spruce needles – as per these examples:
Needle leaves in other families are a bit more ambiguous. Sometimes they are referred to as ‘awl-shaped’ or ‘sabre-shaped’ and often they have a needly element but also a scale-leafy element. Needle leaves in these families, such as Juniperus and Cupressus, often mature into scale leaves. Here are a few examples of non-Pinaceae needle leaves:
As is obvious, these are different to Pinaceae needles and it’s not just the leaf shape and configuration – one major difference is that these species usually have a photosynthetic stem which pines definitely do not have. You can sort of see how these leaves could change to become scale leaves by the way they are attached to the stem in alternating pairs – if they just shrink and get closer to the stem you could see a mature scale leaf emerge.
For the sake of the rest of this post, I’m going to focus on Pinaceae needle leaves, since these are persistent needles whereas most of the needle leaves on other families are juvenile (although there are some exceptions).
One of the key attributes of Pinaceae needle leaves is that they have a 3-dimensional profile – usually with a quadrangular, triangular or semicircular cross-section, as shown in the images below (1,2,3 & 5 needle pines):
Anatomically, Pinaceae needles are like other conifer leaves in only having one or two vascular bundles for water & sugar transport. Their stomata are arranged more or less evenly around the needle and appear in lines. But needles are unique in having an unusual type of mesophyll cell (a cell used for photosynthesis). Instead of cylindrical palisade cells lined up under the epidermis like other plants, needles have frilly looking ‘arm palisade parenchyma’ (see below). These have very lignified (woody) cell walls which intrude into the cells and it is this feature which is thought to provide needles with their extreme cold resistance.
Pinaceae needles are actually *the* most frost-hardy leaves of all. Mountain pine needles survive temperatures down to −93°C and can still perform gas exchange (oxygen for cell respiration) even when their needles are frozen.ref They are also highly resistant to herbivory, due to being tough, spiky, full of toxic resins and not very nutritious. And they live anywhere from 2 to 45 years (Bristlecone pine has the record).ref The needle is one tough leaf!
Pine foliage is also heteroblastic – which means it has one type of foliage during its juvenile phase, which lasts 1-3 years, and a different type during its mature phase. Juvenile needles don’t appear on fascicles, instead developing directly on the stem, and their profile is more flattened – although it’s not super noticeable until you know to look.ref Mature foliage develops in fascicles (bundles) and is the familiar three dimensional profile – they fall off as a group when the fascicle falls off. See examples below – Pinus cembroides juvenile on the left and adult on the right of both frames.
Juvenile and mature needles have different properties with juveniles having 3x the photosynthetic capability of mature needles. This is believed to be able to provide such species with a ‘fast return’ strategy whilst establishing and a ‘slow return stress-resistant’ strategy when older.ref
Surprisingly needle leaves actually have better photosynthetic performance than other conifer leaves – despite that fact that Pinaceae have lost key genes required for photosynthesis in other plants.ref Their improved performance is believed to be because water has to travel further than in non-needle leaves, but there is likely also a genetic factor which hasn’t been discovered yet.ref Needle leaves take a couple of years to reach their full photosynthetic capacity, but once they do, from then on it reduces with age. One study found that for spruce leaves this declines linearly from the 2 year point, reducing to 30% or less of the maximum photosynthetic capacity by the 6 year point.ref
When photosynthesising, trees need to deal with transpiration, which is the evaporation of water from the stomata in their leaves and is the main driver of their water requirements. Needle-leaved trees have a massive advantage in this domain, as many species have wax deposits in their stomata which reduce transpiration. It has been found that wax deposits in Sitka spruce stomata reduce transpiration by two thirds but photosynthesis by only one third.ref Unfortunately these waxes are degraded by pollution, so needle leaved trees can dry out in high pollution areas.ref
Since needle leaves can generate more energy from the light they have, and from a wider range of sun angles, they can survive in poorer light conditions than flattened-leaved species. Along with their cold resistance and ability to minimise transpiration, this is why the boreal forest comprises mainly Pinus, Abies and Picea species, which all have needle leaves.ref
Finally a note on buds and how needle leaves develop on a tree. Leaf buds on mature needle-leaved species form as part of a shoot rather than individually, and these are usually determinate, which means that everything is formed inside the bud. So before bud break the leaf primordia (baby needles) are sitting inside the bud. Below are some cross-sections of long shoot pine buds (which you might know as candles) and a spruce bud (on the right). The brackets show where short shoot buds are located, and within these the baby leaves are waiting to emerge with the shoot.
Contrary to some advice, needles do all their extension in their first year of growth. After this they replace or add to their phloem annually, but not their xylem.ref This means there may be some thickening of needles, but no lengthening after the first year. They may be shorter with increased dryness and poverty of the soil.ref Eventually needles fall off along with their fascicle and the other needles in their group. However, pines have been shown to retain their needles up to twice as long if they have been defoliated (eg. by insects).ref
What’s the impact of this all for the bonsai enthusiast? Firstly your needle-leaved species are going to be tough, they will cope with reduced water, poor soil, wind, rain and freezing temperatures. They definitely do not want to be inside.
Secondly as per conifer leaves in general, needle leaves are not as plastic or regenerative as angiosperms – they are part of a shoot and form in the bud, so there aren’t as many styling options as you find with angiosperms or even flat or scale-leaved conifers. And they have a more relaxed time frame than angiosperms do – needles may stick around for a long time – usually this will be 2-4 years in most species and low-medium elevations but can be a lot higher. So your styling decisions can’t be completely redone on an annual basis – a better approach for needle-leaves is a gradual evolution towards a vision. These trees suit bonsai practitioners with patience and a slow, thoughtful approach.
Bonsai folk like small leaves and in the case of needles, short needles. This is achieved in one of two ways. The first way is that long shoots (candles) are completely broken off early in the growing season. This forces the tree to activate dormant foliage buds at the base of the shoot, which don’t have the time or resources to develop full length needles. I’m not sure whether breaking the part of the candle off (as is also advocated) would also reduce needle size since this retains the active short shoots at the base of the candle to still develop. This practice might have a slightly different effect of creating more short shoots with more needles, so giving denser foliage, rather than shorter needles. The second way to reduce needle size is to starve the tree of water and nutrients, but I’d say manipulating candles would be better for the health of the tree. Obviously to add branches to a tree you need to leave the long shoots in place to develop, as these are what create the long-term framework.
To finish off, I just have to share one of the brilliant images created by Gerhard Vicek who does microscope cross-sections of plants – below is a cross-section of a Cedrus atlantica needle. The beautiful staining he does of his samples makes the different cell types really clear. There are epidermal cells (in red) on the outside, below these the unusual frilly arm palisade cells (in green). Then you have the transfusion cells in a brown & white ring, which move water to the outside of the needle and sugars to the centre. In the centre you can see two vascular bundles with the tiny xylem & phloem cells. Truly his images are art for the bonsai science nerd!