A Tree of Two Worlds- On the Novelty of Pine’s Shade Intolerance

Pine (Pinus spp.) trees fascinate me. Somewhere along their hundreds of million-year evolutionary history they decided to radically change their growth strategy away from their fellow coniferous, needle-leaf, trees. Pine trees instead adopted approach that was surprising similar to the new kids on the block, a new type of tree that was dramatically changing life on earth, the deciduous, broad-leaf, trees.

Coniferous trees (spruce, fir, hemlock) take a slow and steady approach to growth. While deciduous trees (oaks, maples, ash) live fast and die young. They grow in big bursts and don’t make it anywhere near the ages ancient coniferous trees can. Conifers leaves/needles last multiple years and are incredible efficient in water usage. Deciduous leaves typically only last less than a year, and their broad, flat surfaces use a large amount of water. Coniferous trees focus on growing tall with a long trunk and small side branches. Forming a triangular shape that helps them obtain immortality. Deciduous trees chase the sun, forming large branches that stick out every direction and angle.

Now it’s worth noting here that there are exceptions for every rule when it comes to plants. Its easy to find trees that behave differently to above generalizations (especially in diverse, intricate forests like the Amazon), but typically my generalizations above are correct. Having all these exceptions makes it hard to talk about trees. It’s one of the biggest things I’ve struggled with when blogging. How to generalize without misleading. How to find patterns and conclusions when there are always exceptions. It’s a wee bit frustrating.

However, there’s a rule about pine trees that I haven’t found an exception to yet. Pine trees seem to be the only conifer genus that has no shade tolerant species. This doesn’t sound like much, but stick with me. As I mentioned earlier, conifers take the slow and steady approach to growth. This allows conifers to become “climax species’”, which means they are the final form of species that end up dominating a forest. Slow growth + water efficient leaves is a combination that allows conifer seedlings to survive under the dense shade of other trees in a forest, biding their time till they can take over. Deciduous trees prefer open areas and grow best under full sun. Typically. deciduous trees establish a forest and then coniferous trees take it over.

Pines don’t grow in shade though. They need full sun and open areas like deciduous trees do. Why? Why do pines, which have needles and are coniferous trees, grow like deciduous trees do?

This is a puzzle that’s been bugging me. It’s a solution science can’t really provide a nice packaged answer for because there are so many variables. So, here’s my attempt at it! In which I make a bunch of generalizations! But I believe if you look at it as a whole provides a pretty good answer.

We can start by looking at some physical characteristics that pine trees have that make them different from other conifers. An easy one is the length and ‘bunching’ of pine needles. Other conifer needles are short and spread out, often still remaining far back on the older sections of the branches, while pine tree needles are only found on the end of branches. Having smaller, spread out needles allows conifers to catch more sun, while the pine trees bunched long needles block the sun from other needles in the bundle, meaning pines need more sun to ensure every needle gets some.

White Pine needles (left) vs Eastern Hemlock needles (right)

A good example of this is the pine tree and hemlock on the block right near my home. During rainfall, the sidewalk area under the hemlock remains dry due to the density of needles throughout the tree, especially the ones far back on branches close to the trunk. The sidewalk area under the pine gets very wet. This is due the lack of needles in the inner area of the tree. This is also a good explanation why hemlock is a climax species! No rain getting through also means no sun. Not much can grow under a hemlock except for the most shade tolerant plants.

Another physical characteristic that shows why pines need sun is that size the of their buds. Pines typically have bigger buds than other coniferous trees. Deciduous trees also have big buds because they grow in big bursts during the spring. Inside buds are early preformed leaves and flowers, which helps with quick growth because they don’t need to be made from scratch. To form these large buds and ensure they actually grow and are not wasted, lots of energy is needed. That energy of course, comes from the sun, so deciduous and pine trees need more of it than coniferous trees.

Having these large buds also gives pines the ability to preform a neat deciduous tree trick that conifers can’t do. Pines and deciduous trees have dormant buds in their trunk, called epicormic buds. These buds can start growing whenever they are needed and turn into branches when the tree needs more production. A widely used logging technique in ancient times, called pollarding, was to consistently cut tree branches off back to the base of the trunk. When they would resprout via epicormic buds, you would have a constant supply of long poles of wood which were extremely handy in building homes, tools, and weapons.

Another difference we can notice between pines and conifers is that pine cones are woodier and denser than other conifer cones. Other conifer cones use thinner, scale-like wood in their cones. Since pines have woodier cones, they need more energy from the sun to grow the wood for these cones. This is like how deciduous nut trees (oak, walnut) often need more sun energy than other deciduous trees because they are packing so many carbohydrates into their nuts.

Woody Red Pine cones (left) and Scaly White Spruce cones (right)

So, a pine tree’s leaves, leaf orientation, buds, and cones all show why pine trees need more sun than other conifers. There are a few other things worth mentioning as well. Typically, pines grow thicker bark and have more resin (a sticky substance that hardens quickly which trees use to cover wounds) than other conifers, they, once again, need more sun energy to build these.

Pine’s ability to grow quickly and in open areas, while still growing straight and tall, is why they are a favorite for lumber and forestry plantations. Their thick bark, woody cones, and resin production, also makes them among one of the first tree species that pops up after a forest fire (or survives the fire). A number of pine trees have cones that actually won’t open and release seeds until after a forest fire has happened. The resin and thick wood in the cones prevent the seeds from getting burnt. Also releasing them after the fire ensures they will have access to full sun.

You’d think all of these characteristics of pine, which has the immortality of conifers (the oldest trees in the world are the Bristlecone Pines) and their water-use efficiency (Pines are one of the few trees that can grow in sand, where water drains extremely quickly) plus the deciduous tree tricks of quick growth and dominance of open areas after forest fires, would make pines an extremely important species in the post-climate change world.

However, unfortunately pines strategy of being a coniferous tree growing like a deciduous one, isn’t actually that helpful to the planet as climate change get worse. There are three reasons for this. The first is that pines are actually maybe the most invasive tree in the world due to their ability to take over open areas and because exotic pines are often planted and escape forestry plantations. Non-native pines taking over open areas stops native species from growing in those areas, ultimately reducing the biodiversity of an area because of the damage to all the native animals reliant on native trees.

The second is that there’s evidence lately that forest fires are becoming to intense and frequent for even pine trees to grow back. All the nutrients in the soil get burnt up by these fires and the land becomes too dry that even pine trees can’t grow.

The third is the threat of bark beetles, which capitalize on pine trees weakened from drought and forest fires. Pines often produce dense forests of mostly pine trees because they can outcompete other tree species. Pine forests are usually less tree-diverse than other forests which can turn pine forests into a bit of an all-you-can-eat buffet for bark beetles. British Columbia lost an area of about 18 million hectares, almost twice the size Iceland, to the mountain pine beetle from the 90s to 2000s. Worse bark beetle outbreaks are expected in the future.