Monthly Archives: March 2016
By Bruce Rottink, Volunteer Nature Guide & Retired Research Forester
Pacific trillium (Trillium ovatum) is the only plant species with its very own festival at Tryon Creek State Natural Area (TCSNA). I suspect there are several reasons for this; First, trilliums have one of the largest flowers in the forest; Second, they are one of the earliest flowers in the forest, coming at a time when we are desperate for any sign of spring (although Indian plum (Oemleria cerasiformis) and the hazels (Corylus sp.) do bloom earlier than trilliums); Third, they grow right on the ground, where they are easily seen (no matter how cute bigleaf maple (Acer macrophyllum) flowers might be, if they’re 60 feet off the ground, who can see them?); Fourth, they’re pretty common, much more so than the dramatic but uncommon (at TCSNA) Columbia lily (Lilium columbianum).
Beyond the pretty flowers, these plants are interesting in a host of other ways.
The Life Cycle of a Trillium
The perennial parts of the trillium, the rhizome and roots, are underground. The above ground parts are only present for the spring, summer, and sometimes autumn. The annual scars on the underground stem (“rhizome”) reveal its age. Trilliums over 50 years old have been found in the western United States. The trilliums probably live longer than this, but very old parts of the stem tend to decay away. Thus, the true maximum age is difficult to determine.
The trillium goes through at least 4 different stages during its life. These stages look different, and are indicated in the chart below (all photos taken at TCSNA). After a trillium seed germinates, the plant will remain in the “cotyledon” stage for one, and only one, whole summer. (This amazes me!) As you can see from the picture with my thumb as a scale, the cotyledon is tiny! Next, the plant will spend one or more summers in the “one-leaf” stage. In this stage the leaf is typically no larger than your thumbnail. Finally, the plant reaches the easily recognizable three leaf stage. Typically, there is at least one year when a 3-leafed plant doesn’t produce flowers. Finally the plant reaches the three leaf stage with a flower.
Note that the plant may spend more than one year in the one-leaf, three-leaf and three-leaf with flower stages. It may also cycle back and forth between some stages, as indicated by arrows in the diagram.
Life Cycle of a Trillium
Diagram by Bruce Rottink, based upon: Ream, Tarn. 1991. Life History and Demography of Trillium ovatum Pursh. (Liliaceae) in Western Montana. Masters thesis, University of Montana.
Note: This symbol means the trillium might stay in this stage for more than one year.
Annual Development of a Flowering Trillium
Over winter, there are normally no above-ground signs of trillium. The first sign of the trillium in the spring is the emergence of a tightly rolled cluster of leaves on a stout stem, as seen below. Occasionally, you will see a trillium stem bent over in an upside down U-shape, and it looks like the stem is struggling to pull the leaves above ground.
Newly emerged trillium plants. On the left, the stem is struggling to pull the leaves above ground; in the right photo, the tight coil of leaves is upright, with a flower peeking out.
Eventually, the leaves unroll, and the flower opens in all its glory, as seen below. For the years 2014 – 2016, I have documented the first trillium flower at TCSNA. They are open enough to see the flower’s sex organs as early as March 2, and as late as March 10.
Recently opened trillium flower with ripe pollen
As the petals age, they start to turn a pinkish color. The intensity of the pinkness is extremely variable. One intensely pink flower is shown below. Based on observations at TCSNA, the petals generally start to turn pink approximately 2 to 3 weeks after they first open. The environmental conditions at any given location probably play a big role for any particular plant. This phase can be even more dramatic than the earlier white phase.
Trillium in pink phase
It’s All About Reproduction
The role of the flower, of course, is to produce seeds. Researchers have discovered that unlike many plants, trilliums do not produce nectar to attract pollinating insects. Apparently trilliums attract insects hungry for their pollen, some of which the insects then unknowingly carry to other trilliums.
At TCSNA, the trilliums don’t have a very high success ratio in producing seeds. In 2013 I took a survey of trilliums along two of TCSNA’s trails. Of 88 trilliums that produced flowers, only 33 produced any seeds. That’s a success ratio of only 37.5%. Not to worry though, studies in other areas have found similarly low success ratios.
A failed, brown seed capsule, with no seeds. A success: This capsule contains seeds
Even with seed successfully set in a capsule, the trillium isn’t home free yet! Trillium seed capsules contain a lot of energy, and hungry animals are always interested in eating more energy.
The result can be tragic for the trillium, as shown in the photo below, where the seed capsule has been clipped off, and presumably eaten, leaving just the stump of a pedicel (“seed capsule stalk”). Deer have been reported to eat trillium seed capsules, and other animals might as also. While this photo is not proof that something ate the reproductive structure, this is what trilliums look like when the flower or capsule has been eaten. A mid-summer survey last year of 99 trilliums at TCSNA that were not growing near a trail showed that 17% of those which had produced a flower had lost either the flower, or the seed capsule, to some animal. Their off-trail location suggests to me that the flower or seed capsules were not picked by humans.
Stump of the pedicel that held up the missing seed capsule
So we’ve got seed, now what?
By mid-summer the seeds are ripe and the capsules split open, releasing the seeds, as seen below. However, trillium seeds don’t have wings with which they can fly away in the breeze. Instead, each brown, round seed is attached to a blob of soft yellow stuff. Scientists call this blob an elaiosome.
Ripe trillium seed capsule which has split open to reveal the mature seeds, plus a photo of a single seed with elaiosome inset in upper right corner.
The elaiosome contains high levels of fatty acid. For humans, fatty foods are a no-no, but for forest critters, high fat foods are a high-energy treasure. Some arthropods, like ants and the harvestman (pictured below) are attracted to these elaiosomes. [Geek alert! This harvestman is NOT a spider, although there is an actual spider also called a harvestman!]
Harvestman coming to get lunch and spread the seed
The harvestman or ants grab a seed, take it to a safe place and chow down on the elaiosome. They have no interest in the seed itself, and discard it after their feast, which is most often at some distance from the mother plant.
The End is Near!
Having once produced seed, and stored enough solar energy as food for next year, the leaves have done their job. The leaves and stem die slowly and only the underground part of the plant overwinters. While many of the trillium leaves vanish in late summer, a few hang on until late fall, like the example shown below.
Rapidly dying trillium leaves on November 19, 2014
The trilliums of TCSNA bring us great pleasure with their spring beauty, and serve as a herald of abundant and diverse forest delights in the growing season ahead.
By Bruce Rottink, Volunteer Nature Guide and Retired Research Forester
NOTE: This Naturalist Note is the result of a curious reader asking me a question about a previous Naturalist Note. If you have any suggestions for, or questions about, a Naturalist Note, let me know, and if possible, I’ll cover your question in an upcoming Naturalist Note. Warning: Based on the nature of the question, and the time of year, the response might not be fast. – Bruce Rottink
As you walk through the forest at Tryon Creek State Natural Area (TCSNA) you see trees with lots of branches, some dead and some alive. Each branch has a life cycle, and knowing that cycle will help you understand what you see in the forest. Let’s follow the life cycle of a Douglas-fir (Pseudotsuga menziesii) branch. There are minor variations between tree species on how their branches grow and develop, but Douglas-fir is typical of many trees.
In the beginning…
All branches begin life as a bud. The buds start out very tiny, and complete their development in the first growing season. In Douglas-fir, by autumn every needle that will appear the following year is already present as a small mound of tissue inside the bud. Most of the needles have been removed from the twig pictured below so the buds (the brown pointy things) are more visible.
Buds on a Douglas-fir branch
The bud “rests” all winter, with no obvious growth. However, the cool winter temperatures from approximately 32° to 40° F (“chilling”) facilitate chemical changes in the bud which allow it to start growing in the spring. Temperatures below freezing are not effective in the chilling process. If there is not enough cool weather in the winter, the bud will be very slow to start growing come spring.
Year Two – The Bud Bursts
After the appropriate chilling, warmer temperatures in the spring cause the bud to burst open. Both the branch and the needles then elongate to their final mature length that summer. The bud at the tip of the branch (the terminal bud) will grow the longest, and extend the branch more or less straight out from the end. The buds further back from the tip (the lateral buds) will create side branches at roughly a 45 to 75 degree angle to the main branch.
This cycle of bud formation and the bursting of the bud to form a new branch is repeated year after year, and eventually, the branch of a typical Douglas-fir will look something like the one pictured below, which has been stripped of needles. In the picture below, the branches have been painted to indicate which year they elongated:
Pink = elongated in 2015
Green = elongated in 2014
Yellow = elongated in 2013
Blue = elongated in 2012 (only part of 2012 growth is included in this picture)
Branch of Douglas-fir stripped of needles.
The Beginning of the End
As seen in the picture above, the green branches labeled as “dying branch” did not elongate in 2015, hence they don’t have a pink segment at the end. These branches still had needles in 2015 as Douglas-fir needles tend to persist 5 or 6 years. At this point these branches are being heavily shaded by the branches above it on the tree, and thus receive minimal sunlight for photosynthesis. Sometime in the next several years these dying branches will lose the last of their needles, and the twig will be completely dead.
Getting Rid of Dead Branches
The process for getting rid of dead branches is dramatically different than the falling of leaves. First of all, the leaves have a layer of cells (the abscission layer) at the base of the leaf. These cells weaken at the appropriate time so they are no longer able to hold the leaf onto the plant. Branches have no such abscission layer. Let’s take a look at how the branch is actually attached to the trunk of the tree. Below is a typical cross section of a tree trunk. Note how I have cut through the trunk vertically.
Note the edge of the vertical cut through this trunk
The photo below shows the surface of the vertical cut through the center of the trunk of a Douglas-fir. I cut the branch off about 1-1/2 inches from the trunk.
Vertical cut through a young Douglas-fir
The brown pith is relatively “soft” tissue that is formed at the center of every branch the very first year that branch elongates. The important thing to notice here is that the branch is very well integrated into the stem of the tree. There is no obvious place where the branch can easily separate from the tree trunk.
With no predetermined place for the dead or dying branch to separate from the main stem, it just hangs on after the branch dies. These branches will persist until they are knocked off by some other branch during a windstorm, or decay to the extent that they can no longer hold up their own weight. The Douglas-fir, growing near Old Main Trail pictured below has a large number of dead branches.
Douglas-fir with lots of dead branches
On the other hand, some trees have trunks which have very few dead branches, such as this tree pictured below also along Old Main Trail.
Douglas-fir with very few dead branches
What Causes This Difference?
Within a given species, the presence of very few dead oftentimes will indicate the tree grew up with lots of competition. With lots of competition, the lower branches die while they are still fairly small. If a tree has very little competition, it will keep its lower branches a long time, and they will be very large when they finally die. Small branches decay and fall off the trunk more rapidly than large branches.
How Might Branch Retention Affect Me?
Dead branches hanging onto the trees represent a possible entry point for tree decay. Additionally, if you are harvesting trees for lumber, the branches are what create “knots.” If the branch was alive at the time a particular section of the tree trunk was growing, the branch wood is tightly integrated into the board in what is called a “tight knot.” An example of a tight knot is seen below.
Tight knot which is integrated into the board
Loose knots form when a branch which is already dead is still attached to the tree. The dead branch is surrounded by the newly developing wood in the tree trunk. Loose knots can seriously impact the strength of a board, depending upon their size. In the split piece of wood pictured below, note how the grain of the wood curves around a now-missing branch. The resulting loose knot has already fallen out.
A “loose knot” has fallen out of this piece of wood
When lumber is cut from trees with loose knots, these knots tend to fall out, creating inferior boards like the one seen below.
Looking through where a loose knot has fallen out of the board
The whole story
Like everything else in the forest at TCSNA, branches have a life cycle: First as the engine that drives the growth of the tree, then as a dead branch hanging on the tree, and finally as dead material on the ground for recycling. Next time you are walking through the park, take a look at the trees and their branches. Notice which parts of the branches’ life cycle you can see.