The Social Lives of Trees: Part 1
By Emma Ewert, graduate student in the Institute’s 15th cohort.
I have always found trees comforting and familiar. Playing in the dense woods surrounding my childhood home, they were the walls of my forts or the home of woodland fairies. Living in the Salish Sea, my childhood trees were the stately Douglas-firs, scrappy Shore Pines, somber Grand Firs, and the beautiful, queenly Madronas. Before starting the graduate program at NCI, I took this connection for granted. I knew forests, I could sense the differences in species and size that came with different climates and succession stages. However, for the most part, I was more interested in getting through the forests to the high places where I could REALLY see something.
As I learned more about trees and forests, I became more aware of each and every tree. Each new species I learned to see, each new forest I became familiar with added to this growing interest. I started to notice peculiar patterns. Cedars and Western Hemlocks grew tightly along the sides of Douglas-firs, taking advantage of their shade, and creating odd pairs of intertwined trees. Depending on the climate, the same species of tree could look like a gnarled shrub or grow tall and straight, reaching hundreds of feet into the air. I wanted to know why this happened, to get a sense for how and why trees chose to grow in these weird ways.
And then we began to see fires. The sight of so many trees burned and broken made me feel sick to my stomach, especially when we entered older burn areas still covered in ash a year later. I remember finding a Douglas-fir snag. Its bark was black but still intact; deeply ridged and furrowed as if it was still alive. But when I reached out to touch it, the black bark crumbled away into dust. I quickly learned that the lifelessness of these areas was often misleading. In some, living trees still stood, in others snowbrush and willow had begun to grow, even weeks after a fire had passed through. This new life felt special, but I couldn’t understand how a plant could survive in a place that seemed to offer so little.
The reason these trees survive, and the reason most trees can survive, is because they collaborate and communicate with the world around them. Relationships are so important to trees that their root structures are actually hybrid structures of roots and fungi called mycorrhiza. These structures are so inseparable that without mycorrhizal partnerships, most trees would quickly die. These networks connect trees to nutrient sources, water and even to other trees. Under every forest, the networks of hyphae that make up the fungal half of the mycorrhizal relationship surround and enter roots, linking trees up to an acre apart. End on end, these hyphae could cover thousands of miles, making them some of the biggest lifeforms on earth.
Trees exist in a social, interdependent system. The bacteria and fungi they partner with are able to adapt and evolve quickly in response to changes. This allows the larger, slowly growing trees to respond immediately to fluctuations in their environment. Without these partnerships their size and immobility would prevent them from responding quickly, and they would not be able to survive or adapt. This partnership is so important that scientists think that mycorrhizal networks emerged over 400 million years ago, and were a significant reason land-based plants were able to evolve. To truly understand trees, we need to understand the fungi that they live with.
These partnerships are complex, and affect many different processes within the tree. Therefore, before I dig into the mycorrhizal structures and how they work in later posts, I want to look at the everyday processes that keep a tree alive.
How A Tree Lives
Trees are improbable organisms. From our old growth forests, sandy shores or frozen alpine areas, trees have adapted to fill almost every ecological niche. Once they take root, they live their entire life in the same place, and they are unable to move or adapt quickly to any sudden changes on their own. Even so, they shoot out of crevices in cliffs and canyons, grow at the edges of lakes and rivers, in the middle of deserts, and survive snowstorms, rainstorms and howling wind.
Much of this ability to survive comes from the extensive partnerships and relationships trees maintain, but trees are also incredibly effective in their own right. Their ability to create, recruit and transport water, nutrients and photosynthates in the poorest soils and the most marginal environments is truly staggering. The trunk is the the main transportation highway in this system:
In the image above, you can see the major components of the trunk. While thin and easy to overlook, cambium is the driving force in trunk and tree growth. Cambium is considered part of the bark, but it is more accurate to think of it as stem-cell tissue. This stem-cell tissue is responsible for maintaining and renewing the two main transport systems in a tree: the xylem cells inside the tree, also known as the sapwood, and the phloem cells that make up the inner bark of the tree.
Xylem cells are the outer tree rings we see when we look at the cross section of a trunk. Tubes carry water and dissolved nutrients (often collected from fungi or bacteria) from the roots to the leaves or needles. As xylem get older, they become clogged by the minerals and tannins they help to transport. This makes them more solid, and as they lose the ability to transport nutrients, new xylem cells are created and the older cells become the heartwood. The heartwood allows the trunk to perform its secondary function as the structural skeleton of the tree. While not transporting water or nutrients, the old xylem cells support the tree as it continues to add new rings and grow larger.
Phloem cells, which grow on the outside edge of the cambium and create the inner bark, have a complementary role. They transport photosynthates from the needles out to other parts of the tree, and to the roots, where carbohydrates play a key role in mycorrhizal partnerships. Similar to the xylem, the inner phloem cells closest to the cambium act as transport, while the external, clogged layers become the bark of the tree, shielding the tree from external threats.
Scientists are only just now beginning to understand how trees regulate and direct these flows. The current theory for how trees draw water up from the roots is that it uses the evaporation of water from its leaves or needles to create a pressure differential that moves water up the tree. In the other direction, we know that trees use active transport to exude photosynthates into the soil, and the combination of gravity and differences in chemical concentration seems to draw liquids through the phloem. Before trees can transport materials, they have to acquire them. Getting water and nutrients from the soil is difficult, and requires the aid of many different partners, but trees are masters of getting energy from the sun through photosynthesis.
The best part of being big and tall in the plant world is that you have access to the most sun. This makes photosynthesis a relatively easy process for trees. Finding light and growing towards it is the main mechanism behind how all plants, trees included, grow and shape themselves. You can sometimes find trees with branches that have grown straight out from the trunk only to turn 90 degrees when they reached an open area. Trees are also extremely good about spacing their branches and leaves so that each leaf and each branch gets the most sunlight possible. If you ever sit underneath a maple tree, or examine the cone shape of a conifer, this engineering ability is readily apparent.
Once they are structurally able to maximize their light intake, trees take drastically different structural approaches to maximizing photosynthesis itself. Deciduous trees grow leaves every year, which takes more energy, but their leaves are broader, thinner and can photosynthesize more efficiently. They don’t throw away all their hard work when they lose their leave however. Deciduous trees, before loosing their leaves, actually pull all the chlorophyll proteins back into their trunk and store them, stopping the end of the leaf with cork to prevent any extra water from evaporating. Because the chlorophyll that is removed is what makes leaves green, this energy saving process is what gives us the beautiful reds, yellows and oranges of fall leaves.
In contrast, evergreen trees choose to keep their “leaves” and photosynthesis year round. In the Pacific Northwest, evergreen trees are generally our conifers, which all have needles. These needles are thicker, darker, and less efficient at photosynthesis. However, their abundance and ability to take advantage of winter sunlight allows them to survive. Even though evergreen trees don’t lose their leaves every year, it is estimated that Douglas-firs, and probably other trees, lose about a third of their needles every year, and that needles survive for about 2 to 3 years.
The needles and leaves of a tree use the energy and carbon from photosynthesis to create simple carbohydrates and other compounds, called photosynthates. Their growth structures and huge number of leaves or needles allow them to be powerhouses of photosynthesis, producing way more than they need for themselves. These surplus photosynthates are what allow trees to create reciprocal relationships with other organisms, and with each other. The photosynthates get transported down the tree to the roots by the phloem, and out into the soil, recruiting fungi and bacteria by providing them with a source of food.
Root systems in trees are as varied as branching systems. Many of our conifers send down tap roots that access deep sources of water and help anchor the trees as they grow to incredible heights. Others, like the Quaking Aspen, grow long trailing rhizomes (root-like appendages) just underneath the soil that can take root and create new groves of aspen clones in the right conditions. Tree roots are also where we encounter a tree’s most important partner: mycorrhizal fungi, which will be explored next time.
For more information on trees, two great books are:
The Tree by Colin Tudge – http://www.indiebound.org/book/9780307395399
Northwest Trees by Stephen Arno – http://www.indiebound.org/book/9781594850417