After a winter of relative dormancy while much of the natural world slumbers, our senses kick into high gear as the warmer months progress. The explosion of life and activity means there are many amazing sights, sounds and smells for us to take in. But while most species grab our attention via one or perhaps two of those senses, the scots pine manages to tantalise all three.
The sight
We all know what a splendid sight old scots pines are, with their scaly red bark and sprawling asymmetrical crowns, and we love them for it. Their gnarly, deep green beauty is justification alone for being chosen as Scotland’s national tree in 2014. But that’s not the sight I’m talking about. No, I’m talking about the sight of pale yellow dust. Pollen.
Its arrival is hard to miss, and typically manifests itself in three conspicuous ways.
Firstly, after it’s rained, many puddles have a gooey yellow film on the water’s surface. On first glance it looks like oil, especially after the wind has pushed and spun the water around, painting the pollen into lines and swirls. Puddles become abstract works of art, and wouldn’t look out of place in a Jackson Pollock exhibition.
More usually though, long after those puddles have drained away, you’re left with conspicuous yellow ‘high tide’ marks across the landscape. It’s as though someone has taken a thick piece of chalk and drawn horizontal contour lines all over the place.
Secondly, during a prolonged dry spell with no rain to wash particulates away, you’ll find everything coated in a fine (and sometimes not-so-fine) dusting of pale yellow. It gets everywhere.
And thirdly, when a dry spell coincides with gusty winds, you will see great big clouds of pollen wafting skyward above trees and forests. This can be rather stressful for anyone managing woodlands, and especially for anyone managing both visitors AND woodlands because it does, on first glance, look very much like smoke.
In most Mays/Junes at Mar Lodge Estate, as I’ve been walking out of Glen Derry or Glen Dee on fire patrol, I’ve caught sight of this ‘smoke’. My heart misses a beat and I can feel the adrenaline surging, a mixture of dread and action. It is with considerable relief, therefore, that I realise that it’s pollen, not smoke, and the panic subsides.
Scots pine pollen comes from the male flowers. Yep, flowers! I’m not sure how many of us have ever considered that pines have flowers in the conventional sense, but they do.
Scots pine is a ‘monoecious’ species of tree. Meaning ‘single house’, moneocious refers to the fact that each individual specimen of the species has both male and female reproductive parts. It’s commonplace in the tree world, and includes familiar trees like birch, alder, larch and oak.
Some other plant species are ‘dioecious’ (‘two houses’) where a given individual of the species has either male or female reproductive parts, but not both. Examples of dioecious trees are aspen, juniper and holly.
Pollen is the medium through which plants fertilise other plants of the same species. The pollination process is the same for both monoecious and dioecious plants – pollen from the male reproductive parts needs to reach the female reproductive parts – it’s just that a monoecious plant can pollinate itself and others, whereas a dioecious plant can only pollinate plants other than itself.
There are advantages and disadvantages to both methods. Being able to reproduce without needing another individual of your species is advantageous if there are none nearby, however this isn’t a great long-term strategy for species survival.
Studies into the viability of scots pine seeds finds that while infertile seeds are present in all types of fertilisation (including dioecious cross-fertilisation with other individuals), they are much more likely when a pine has self-fertilised. So, while self-fertilisation is a handy back-up strategy in an emergency, it can also lead to long term decline without the input of fresh genetic material from other populations.
Plants do have strategies to avoid self-fertilisation, however. And in the scots pine’s case, the male and female flowers are generally kept some distance apart on different branches or on different areas of the tree.
The male flower of the scots pine is called the microstrobilus, and it grows at the base of the new season’s shoots. It is pale yellow and cone-like in shape, a few centimetres in height, and is essentially a catkin made of a number of flowers. Each flower has anthers, on which the pollen grains can be found.
These flower ‘cones’ are strikingly obvious this time of year, more so than the mature seed-bearing cones, as they dot the scots pines with pale yellow blobs (reddish brown later in the season) from top to bottom. There is, however, considerable variation from one tree to another. Some trees will be absolutely covered, while others won’t have any at all.
The female flower is called the megastrobilus, and it grows at the tips of the new season’s shoots. It is smaller and less obvious than the male flower, and you need to look much closer to find it. It’s a small, reddish-pink flower, and looks much like the more obvious purple flowers you might have seen on larch trees. This flower contains the ovules, which sit waiting for pollen from male flowers to reach them.
Pollen is released by the tree all the time during flowering, but it’s on those warm, dry and breezy days that it gets dispersed en-masse. You get a sense of how readily it leaves the male flowers if you simply tap one of them with your finger. You can see the pollen rising up, like a thin smoke.
Scots pine pollen is large enough to see individual grains with the naked eye, but not so large that you can make out the finer details. When they land on a surface they look like tiny dots, but under magnification they are shaped more like fortune cookies. They have two air bags to help the grain get airborne. Actually, they look a bit like one of those Mickey Mouse hats.
It’s an incredible sight, therefore, when a gust of wind surges through a dense pine woodland and wafts the pollen aloft. Given how small the grains are, there’s a mind-boggling amount of pollen in those clouds. One study from the early 20th Century reckoned that just one scots pine flower produces in the region of 150,000 pollen grains!
Surprisingly, despite it seeming to be fall or waft everywhere, it appears that most scots pine pollen has a local, rather than a far-reaching impact.
Scandinavian studies have found that scots pines there are mostly fertilised by pollen from close by. Nevertheless, when there’s so much pollen, it’s to be expected that some will be caught in winds and be transported a long distance. Hundreds of kilometres, in fact.
This raises interesting questions about the ability of scots pines in one location to fertilise others many hundreds of kilometres away.
Estimating the fertilisation rates in a patch of scots pines by non-local pollen is difficult, but 5-10% of pollen in a Swedish study of a scots pine plantation was thought to be non-local (several hundred kilometres distance) in origin. In a similar Spanish study, it was 5%.
But is the pollen even viable after that distance? Well, a study in 1963 found that 75% of pollen that had travelled 70km across the sea from Germany to Sweden was still viable.
One wonders, therefore, whether scots pines better adapted to hotter, drier climes in southerly latitudes, might be sending their pollen north to fertilise more northerly pines? In an age where northern regions are warming up quickly with climate change, might fertilisation by pollen from drier, warmer-adapted locations, help northern populations prepare for the warmer, drier climate that is undoubtedly coming their way?
Genuinely, I’ve no idea. But after reading all these studies, it did get me pondering these imponderables.
Anyway, regardless of where the pollen comes from, after pollination and fertilisation the female flower slowly hardens into a small, green cone. It then takes two years for it to mature into the larger, brown, seed-bearing cone familiar to us all.
The seeds are obviously very important to the tree, so they’re well protected. The cone is composed of overlapping woody scales, each of which can house two seeds. This means that there could be as many as 100 seeds in a single cone, but a couple of dozen seeds in total is more typical.
The scales protect the seeds from the elements, but also from being eaten. They’re difficult to get into unless you’ve got a) sharp teeth and dexterity of a red squirrel or b) an especially useful crossed-bill to prise the scales apart.
However, under the right conditions, cones readily open-up with little persuasion, which brings us to our next scots pine sense.
The sound
On those blue-sky summer days, with low humidity and a hot sun blazing down, any walk through a pine woodland is likely to accompanied by a continuous popping sound from up above. This is the sound of pine cones opening up, and for readers of a certain age it sounds like the snap, crackle and pop of milk being poured onto Rice Krispies.
I already knew that temperature played a part in the process, but wasn’t sure to what extent, so I went online to try and find out more. In so doing, I stumbled upon fascinating research by German scientists, published just a couple of years ago, where they set out to explain the mechanics behind the opening of scots pine cones.
If you’ve ever picked a pine cone or branch up, or even simply leaned on a scots pine trunk, you’re probably familiar with the sticky resin the tree contains. It’s found all over the tree, but is also inside the cones, and the study suggests that resin acts as a latch on the scales.
They found that on hot, sunny days with low humidity, the cone scales quickly lose moisture. As a result, they want to curl outwards, but they’re prevented from doing so by the resin, which acts as a glue.
However, when the cone heats up in the sun, its surface can be as much as 20C warmer than the surrounding air. At 45-50C the resin in the scales starts to melt, relaxing from a solid state into a viscous liquid. This releases the latch and then, with explosive elastic energy, the scales open suddenly with an audible crack.
In controlled experiments, all of the scales on a pine cone were observed opening all at once in at least one case, but typically the cones’ scales opened over a number of separate movements.
The experiment subsequently observed how newly-opened pine cones would later close and open again, depending on the environmental conditions, but they would always do so smoothly and quietly. The explosive ‘crack’ only occurred the first time a pine cone had opened
The researchers had originally assumed that the initial crack would be key to seed dispersal, somehow catapulting the seeds out of the cones with explosive force, but this was never observed under the controlled conditions. Seeds always remained in situ regardless of how fast the cone opened.
Bottom left: a pair of seed ‘wings’ housed between the scales.
Bottom right: pine seeds with and without wings.
The study makes for an easy and fascinating read, but I’m left not knowing why scots pine cones have that violent initial opening if it doesn’t actually play a role in ejecting the seeds? The research paper mentions that the cones were securely mounted so they could be filmed, but it’s not clear how they were positioned. On an actual tree, cones face downwards so that seeds can fall out when the cone is open, and so now I’m wondering if perhaps the experiment didn’t replicate that aspect, because SURELY the violent cracking of the cones has served some evolutionary purpose for seed dispersal?
Ugh, maybe I’m overthinking it. But regardless of the reason for it, the force and speed of the pine cone opening does, apparently, make the scots pine one of the fastest moving plants on the planet!
Cones will close when the humidity increases, for that is indicative of wet weather. Rain is bad for scots pine seed dispersal because the seeds risk being washed out of the sky too close to the tree. Cones therefore close up to avoid wasting their seeds.
The seed itself is attached to a single wing. When it falls from the cone it spins through autorotation, keeping the seed in the air as long as possible. This is important, as scots pine likes the sunlight and tends not to grow below an existing canopy. It needs to get far away from its parent, into the sunlight.
Seeds can fly almost 200m from the tree, but it’s more typical to fly just 10 to 50m. This is why, in places where natural regeneration of woodland is being encouraged, you’re likely to see a single old granny pine surrounded by a close skirt of youngsters.
Intriguingly, it has been suggested that this method of seed dispersal is a throwback to the Carboniferous period, some 300 million years ago, when there weren’t as many animal-related methods of seed dispersal available.
The smell
The third and final sense is smell, because on warm days, as you walk through a pine woodland, there’s a beautifully sweet piney aroma hanging in the air.
The smell comes from the pine resin within the tree. It is exposed to the outside world via the cones, but also via open cores and holes where the tree has been damaged, often by wood-boring insects.
Resin contains volatile organic compounds called terpenes, which readily evaporate in warm weather. The non-volatile components of resin, which cannot easily evaporate, crystalise and harden when they reach the exterior, which seals the wound and protects the tree. But the terpenes easily lost through evaporation rise, disperse, and fill the forest with that sweet aroma. The hotter it is, the more evaporation takes place, and the stronger the smell.
And oh man, it smells amazing!
When I get that first whiff of pine on a warm day, I have to physically stop and take it in. Quite literally, in fact, because it’s always a looooooong inhalation through the nose.
I always find it weirdly calming.
Now, that’s partly because when I pause and stand still, and take a deep breath, it creates a moment of mindfulness for sure, which always feels good. But the long, deep inhalation will also, by its nature, bring more oxygen into the blood, boosting levels of that happy chemical, serotonin. That’s simply the outdoors working its natural magic.
But even beyond that, the pine aroma does feel as though it adds something more to the mix when you’re exposed to it. Something fresh. Something soothing. Something relaxing.
Most notably among the terpenes are ‘pinene’ and ‘limonene’, volatile organic compounds that have been found to have positive impacts upon human health.
Through inhalation they have an anti-inflammatory effect, helping to relax and open up our airways, but once absorbed within our bodies they also have wider-reaching effects.
They’re known antioxidants, are antimicrobial, and have even been linked to decreases in tumour growth in mice. They’ve also been shown to positively impact the brain, improving memory and cognitive performance.
Of course, while all those medical benefits undoubtedly have merit and are all very interesting, they’re also rather distant and nebulous when you’re simply out & about to have a nice walk. So, let’s instead focus on the simple fact that scots pine on a hot sunny day is just a really nice smell. One that makes you feel good, which isn’t surprising given it’s been proven to lower blood pressure, reduce heart rate, and reduce levels of the stress hormone, cortisol.
My only caveat here, as with the pollen clouds, is that the smell does raise my anxiety levels a tad when I’m at work, because a strong smell of pine on the air is often a tell-tale sign that the landscape is ready to combust.
But in that situation I need only pause to look around, take a long deep inhalation, and remind myself…… where better to help manage anxiety and lower your stress levels, than a beautiful, popping, aromatic pine woodland?