Buds: A Bridge to the Future
By Bruce Rottink, Volunteer Nature Guide and Retired Research Forester
Winter is a tough time for the woody plants at Tryon Creek State Natural Area (TCSNA). The air gets both colder and, when the temperature dips below freezing, much drier. Most of the plants stop growing, and some shed their leaves. However, the plants have to be prepared for the next growing season. To prepare, they form buds as a “bridge” to the future. By September the buds are a conspicuous feature of woody plants at TCSNA .
A woody plant’s bud might merely look like a hard little blob on a branch, like this bud of a European hazel (Corylus avellana) growing near TCSNA’s main parking lot.
But the buds of TCSNA’s woody plants are actually quite interesting.
So what exactly is a “bud”?
At the tip of each branch is a small cluster of active cells called the apical meristem. At some point in the spring-summer-fall (it varies with different species), the apical meristem starts differentiating and forming a bud consisting of a variety of structures. These structures can be bud scales, leaves or flowers. These tiny structures rest over the winter, and come spring, they start growing. Even the tough-looking bud scales elongate a bit in the spring.
Below is a picture of a bigleaf maple (Acer macrophyllum) bud which is just starting to open. The different parts of the bud are labeled. The “scale to leaf” transition components have very, very tiny leaf parts at the tip of the scale, you’ll have to look closely.
Last spring I picked another newly opened bud of bigleaf maple which was slightly more advanced than the one pictured above. I took it apart to more clearly show the different components. In the picture below, the parts from the base of the bud are at the left hand side, and the other structures are arrayed in order, right up to the flower, which was at the tip. For completeness, at the base of the floral stem are two tiny meristems (not visible here) that will create next year’s buds.
Since the maples produce structures in pairs, one on each side of the stem, there are always an even number of scales and leaves. The flower is an exception to this rule. The “scale to leaf” transition phase is the most interesting. The leaves and flowers have a perfectly round “stem” connecting them to the branch of the plant. The “scale to leaf transition” structures are dwarf leaves supported by a flattened “stem” that resembles the bud scales in shape. These structures clearly demonstrate the plant’s flexibility when it comes to producing different parts. It’s not a clear “one thing or another” decision. (Note: Not all maple buds have these part-scale/part-leaf structures.)
Now it starts to get really interesting!
Okay that’s the basic pattern, but with dozens of different species of woody plants growing at TCSNA, we’ve got lots of variations in buds.
The first type of “weird bud” is the naked bud. This means a bud that has no bud scales. Our native cascara buckthorn (Frangula purshiana) produces naked buds as seen below. The leaves are fully exposed to the winter environment, but are very tough, and slightly hairy. If you want to see a cascara, go to Beaver Bridge. The cascara is about 5 feet upstream from the bridge on the side of the creek furthest from the Nature Center.
Flower Buds, Leaf Buds and Both of them!
As you saw with the bigleaf maple, some buds contain both leaves and flowers, but some contain only leaves and some contain only flowers.
Oftentimes you can tell if the bud contains flowers even before the buds open. In the picture below are two buds of Indian plum (Oemleria cerasiformis) collected from the same branch. They are just starting to expand in the spring. The big fat bud with the rounded end contains both leaves and flowers, while the skinny one contains only leaves.
Indian plum also teaches us that the term “bud break” is ambiguous at best. Below is a picture of a newly opened Indian plum “bud” containing both leaves and flowers. The young leaves including their tiny veins are clearly visible. The flowers are still contained with their own separate “buds.” So, with the Indian plum we have a bud within a bud.
The Indian plum plants are either male or female, and with rare exceptions, will have only functional male OR female flowers on a single plant.
Keeping it all together
In contrast, some plants have both male buds and female buds on the same plant. Red alder (Alnus rubra) is a good example. The alder tree has buds that only contain leaves, other buds that only contain female flowers, and other buds that only contain male flowers. The photo below shows the three different kinds of overwintering alder buds.
How else can the buds be different?
One of the most important ways that buds are different is that some species have determinate, buds and other species have indeterminate buds. Overwintering determinate buds contain all the organs (like leaves, needles, flowers, whatever) that will appear the following year. Indeterminate buds hold only a few of the organs that may appear next year.
A good example of a determinate bud is our Douglas-fir (Pseudotsuga menziesii). In the winter, all of the needles that will grow out of the Douglas-fir bud the next year are already present in primitive, miniature form call “primordia.” Pictured below is a Douglas-fir bud which I collected in late August and stripped off all its scales. Each little needle primordium will turn into an actual needle early next spring. Two of the primordia are indicated with black arrows. All of the needles destined for the 2016 branch are represented by a little bump of tissue. This entire green structure is approximately 2 mm (1/12”) in diameter. For plants with determinate buds, it is easy to see why the environment of one year is so important in influencing the growth of the plant in the following year.
In contrast, TCSNA’s black cottonwood (Populus balsamifera var. trichocarpa) has an indeterminate bud. For cottonwood this typically means that there are three or four relatively big pre-formed leaves that overwinter in the bud. Come spring, these leaves will expand very quickly, and start producing sugar for the plant. If the weather conditions are good, the apical meristem will create another leaf from scratch, and when that is done, the tree might produce a couple of more leaves, etc. This is why for indeterminate plants, shoot growth in any one year is profoundly affected by the environment in the current year, not the previous year!
Below is a whole cottonwood bud, and the same bud with the scales removed. In the second photo you can easily see two preformed leaves, two more are hidden on the backside of the bud.
The photo below shows a single, preformed cottonwood leaf in the bud. The light streak is the main vein which will go down the center of the mature leaf.
The photo below is of a black cottonwood shoot not too long after it emerged from the bud. You can see the 3 large leaves which were preformed, and the fourth, smaller, leaf which formed after the bud broke.
For the woody plants at TCSNA, the overwintering buds ARE the future. It is amazing that the fate of something as large as a tree rests within the tiny buds which bridge the gap between the growing seasons. All things considered, buds deserve a lot more attention than they receive.
Is This an Early Spring?
By Bruce Rottink, Volunteer Nature Guide and Retired Research Forester
If I’ve heard it once, I’ve heard it a hundred times in the last couple of months – “This is really an early spring!” By “early spring” most folks are referring to a year when leaves and flowers appear earlier than “normal.” The conversation quickly moves to “How early?” and this is where some data would be handy. Welcome to phenology!
What in the world is “phenology?”
Phenology primarily deals with the relationship between climate and the timing of biological phenomena like flowering in plants or bird migration. The starting point of phenology is creating long-term records of when certain biological events occur. Using these records, one can then make statements about “early springs,” “late autumns,” and other periodic climate-related events.
I started keeping phenological records for Tryon Creek State Natural Area (TCSNA) in mid-spring of 2013. Since then I’ve made observations every 7 to 10 days on about 70 specific, individually tagged plants.
Cut to the chase. Is this an “early spring?”
Since the Tryon Creek Phenology study is new, the question I’m really answering is “Was 2015 an early spring compared to 2014?” After studying the records for the study plants, I found there are really three answers.
Answer #1: Yes, it was an early spring!
Based on 8 Indian plum (Oemleria cerasiformis) plants that produced flowers both years, in 2014, the average date for the opening of the first flower was March 10th. The average date for opening of the first flower in 2015 was February 18th, a difference of 20 days! For the flowering of Indian plum, 2015 was an “early spring” for sure.
Vegetative bud break in red elderberry (Sambucus racemosa var. racemosa) was also earlier this year. Based on 3 plants, the average date of leaf emergence in 2014 was February 23rd, while this year it was February 14th, a difference of 9 days.
Finally, four Pacific waterleaf (Hydrophyllum tenuipes) plants produced visible flower buds on March 29, 2014, but this year, they all produced flower buds 11 days earlier, on March 18th.
Answer #2: It was a normal spring!
Looking at the date of bud break of 10 Indian plum plants, a different story emerges. In 2014 the average date when leaf tissue extended beyond the bud scales (as seen in the photo below) was February 4th. This year, that date was February 1st, just three days earlier. This is approximately the amount of experimental error one might expect in this study. In other words, this difference might be real or it might be a consequence of the length of time between measurements.
The particular vine maples (Acer circinatum) I monitored, rarely produced flowers, so I had to rely solely on the date of first bud break. I checked the date when the leaf tissue extended just 2 to 3 mm (about 2/10ths of an inch) beyond the red bud scales. In the picture below is a vine maple bud that is at this stage. The leaf tissue at this point has a whitish, hairy appearance.
On average, the six plants that were examined in both 2014 and 2015 reached this stage on March 23rd in 2014, and March 21st in 2015. This is a difference of just 2 days. In this study, that is within experimental error of being identical. So at least for vine maple, there was no difference in time of bud break between the two years.
Answer #3: It was a late spring!
Some of the plants seemed to be a little later this year than last. For example, based on 4 salmonberry (Rubus spectabilis) bushes, in 2014 the leaves first extended beyond the bud scales on February 11th. This year, they were a week later, with the leaves not extending beyond the bud scales until an average of February 18th.
Early, Normal & Late? What’s the deal?
For the vegetative and flower buds of native plants in temperate regions to open up (“break”) and begin growing, two things need to happen. First they need to experience cold (termed “chilling”), and second, they need to experience warmth (termed “forcing”). In that order! Why do the plants do this? While the buds of most plants are pretty tough, the young shoots are relatively delicate. So the plant faces two challenges in the timing of bud break.
Challenge #1: The plant doesn’t want the buds to break too early and start growing just before a big freeze hits. The plant “knows” that even after a short warm spell in December, it could turn fatally cold in January.
Challenge #2: The plant doesn’t want to start growing too late in the year, like mid-July! Starting growth too late means it would miss out on some really nice growing weather in late spring and early summer.
These challenges have been met by plants developing a requirement for a certain number of hours of chilling and then forcing. The definition of “chilling” depends upon which study you read, but generally temperatures in the range of approximately 32 to 50° F count as “chilling.”
“Forcing” is the amount of warmth a plant receives prior to bud break. Typically, any temperature over 50° F counts as forcing. Anytime the temperature is over 50° for an entire hour, the plant is credited with 1 forcing-hour.
The diagram below shows the basic relationship between “chilling,” “forcing,” and bud break. The black curve between the pink and green areas represents the combined amount of chilling and forcing that are needed before a plant can start to grow. The general rule is that the more chilling a plant gets, the less forcing it needs to bud break. Different species of plants have chilling/forcing curves with different shapes. Give yourself bonus points if you noticed that when a temperate region plant receives no chilling, there can never be enough forcing to cause the buds to break!
The “Cold Winter” Model
The diagram below represents a plant after a fairly cold winter. The blue line at the bottom of the chart shows how much chilling this example plant received. The red vertical line shows how much “forcing” it requires to start growing. In this diagram, the winter has been fairly cold. Therefore very little forcing is needed to allow bud break.
The “Warm Winter” Model
This diagram represents what happens in a fairly warm winter. Once again, the blue line represents the amount of chilling a plant received, and the red line represents the amount of forcing the plant needs for bud break. By comparing the two diagrams, you can see that with less chilling, the plant needs more forcing to achieve bud break.
How is this relevant to the question, “Is this an early spring?”
The differences observed in my study may have come about in several ways. One attractive explanation is this: A “warm winter” easily satisfies the chilling needs of those plants with a low chilling requirement, but not those with a high chilling requirement. Once it starts to warm up in the late winter, the plants with a low chilling requirement would only need a small amount of forcing prior to bud break. Plants with high chilling requirements would not have had all the chilling they really “wanted” and thus would have needed extra-large amounts of forcing prior to bud break. Thus these plants would have delayed bud break and blooming.
So the real answer is that 2015 compared to 2014 was an early spring, a normal spring and a late spring, depending upon what plants you were watching.
Nature is always simple, but sometimes it hides that simplicity under layers and layers of seeming complexity. Much of science is dedicated to burrowing through the complexity to get to the simplicity. At one meeting for scientists Dr. John Gordon, former head of the Oregon State University Forestry School, welcomed us by saying,
“You’re coming here confused, and when you leave, you’ll still be confused, but you’ll be confused at a higher level.”
Sometimes that’s the most we can hope for!
Seeds: Then and Now
By Bruce Rottink, Volunteer Nature Guide and Retired Research Forester
Almost every plant at Tryon Creek State Natural Area (TCSNA) started from a seed. Back “Then”, in the mid-1800s, Socrates Hotchkiss Tryon staked his land claim at the mouth of Tryon Creek. “Then” logging started in the nearby forests. From that time until “Now” we have basically the same kinds of seeds at TCSNA.
So what has changed?
Well, the first thing is the abundance of seeds. The plant’s most important activity on a day-to-day basis is making sugar using just sunshine, water and carbon dioxide from the air. This sugar is in essence the “money” of the plant. Just like you have to budget your money, the plant has to decide where to invest its resources too. And, just like you, the plant only has so much “money.”
The plant’s options are to invest in roots, shoots, leaves or seeds. The roots are pretty important since they absorb the water and minerals the plant needs to stay alive. The leaves are important, because they make the sugar the plant lives on. And the stems are pretty important because they are what holds the leaves up in the air so they can catch some decent sunlight.
But what about the seeds?
Nature’s only measure of success is whether or not an organism leaves offspring. So the plant better invest in seeds. Well, maybe. If it is one of the few plants at TCSNA that dies at the end of each season, like our jewelweed (Impatiens capensis) it really needs to put some priority on seeds, or the family line is finished! On the other hand, if it is a perennial plant like Douglas-fir (Pseudotsuga menziesii) that might live 500 years, it doesn’t need to produce seed every year, but it can’t put it off forever.
Scientists have found that when sugar is in short supply, perennial plants will invest their sugar in leaves, stems and roots before they will invest in seeds.
Why would sugar be in short supply?
There are many reasons why a plant’s sugar supply would be low. For many plants here at TCSNA the most important issue is the amount of light the plant gets. With too little light the plant makes less sugar. With less sugar, the plant invests it in leaves, stems and roots, but not seeds. For example, you’ve probably seen salal (Gaultheria shallon) plants (see photo below) in the forest at TCSNA.
There is one patch of salal growing under a dense canopy of trees that I’ve been watching for 4 years. Struggling along in heavy shade, not once has it flowered, much less produced fruit and seeds! In contrast, the salal growing in my yard in the full sun produces abundant delicious berries every year.
The dominant trees always have lots of sunlight, but for the other plants, light is an important contrast between “Then” and “Now.” From a plant’s perspective, logging can be either good, or bad! For the straight, tall trees, it’s bad news because they are going to be cut down. But, for many (but not all) of the smaller plants and bushes like thimbleberry (Rubus parviflorus) struggling to stay alive under those big nasty, light-hogging Douglas-fir, logging means “Happy Days are Here Again!” (Assuming of course that the plant doesn’t get ripped out of the ground when a log is dragged over it.) Post-logging, the surviving plants are basking in the full sun and its leaves are pumping out sugar like crazy! With lots of sugar, there’s enough to invest in leaves, stems, roots, and lots of seeds! It was also a great time for the few trees the loggers left behind because the trees were too small, growing in inaccessible areas, or were in some way defective.
Lots of seeds back “then,” what else changed?
Logging is also an excellent way to churn up the ground, exposing mineral soil. That’s a good thing, because in the undisturbed forest, much of the ground is covered with “litter.” No, not litter like candy wrappers and dog-poop bags, but the litter of fallen leaves, small twigs, old cone scales, and such, as shown in the photo below.
Scientists know that litter can form a barrier to the successful germination and establishment of new plants, particularly for species with small seeds. In addition, this litter layer, also known as “duff”, has a tendency to dry out quite quickly, depriving new seeds and seedlings of optimum moisture conditions. For these smaller-seeded species, there is nothing like exposed mineral soil to provide the perfect place to start a new life.
What kind of seeds are there are Tryon Creek SNA?
The diversity of seeds at TCSNA is amazing. Some of the major differences are how the seeds are dispersed and the size of the seeds. These are very important decisions. The best chance for a young perennial plant is to get away from the mother plant, so it doesn’t have to compete with it. So plants have developed a wide variety of mechanisms for getting their seed spread around. Then too, the plant has to decide on seed size. Putting a lot of resources into a few large seeds would give the resulting seedlings a real leg up when they are first getting established. But on the flip side, if the plant produces very few seeds, and they just happen to land on a rock, or get eaten, too bad! Alternatively, the plant could produce millions of smaller seeds, so that at least some would surely land in favorable sites and avoid being eaten, but then they don’t have much energy with which to get started.
How do seeds get spread around?
At Tryon Creek, seeds mainly are dispersed by either the wind, or by various animals. Two good examples of wind-dispersed seeds are our black cottonwoods (Populus balsamifera ssp. trichocarpa) and our bigleaf maples (Acer macrophyllum). Although both are wind dispersed, they choose radically different ways to do that. The cottonwoods have tiny seeds with each seed sporting a large fluffy mass of fine hairs that completely obscures the actual seed (see the picture below.) This fuzz suspends them in the air as they drift around the forest. The maple has gone with a more traditional idea, having a wing (as you can see in the picture below). This wing arrangement helps the seed drift away from the parent tree.
Other plants use a different type of wing to disperse their seeds, a bird’s wings. All they have to do is produce a nice attractive fruit. The fruit attracts birds that digest the fleshy part of the fruit, and excrete the seeds at some distant location. Two examples of this from TCSNA are the western wahoo (Euonymus occidentalis) and red huckleberry (Vaccinium parvifolium), as seen in the pictures below. (In both cases, the blue lines represent a length of half a centimeter, about two tenths of an inch.)
The third, and most unusual way that plants are dispersed is used by the jewelweed that grows abundantly in the wet areas near the creek. When the jewelweed seed pods are ripe, there is hydraulic tension in the walls of the pod. When the pod is touched, or just naturally dries out, the pod “explodes.” The walls of the pod curl backwards with amazing speed and force. This sends the seeds flying through the air to a new location. Before and after photos of a seed pod are shown below.
How important is seed size?
Seed size varies a lot. The monster seed of TCSNA is the beaked hazel (Corylus cornuta var. californica). There are lots of medium sized seeds like Douglas–fir and western wahoo, while the “tiny” end of the spectrum is represented by red huckleberry. Find them all in the photos below, which are all to the same scale; the blue line represents half a centimeter (about 2 tenths of an inch).
It is interesting to note that there is no clear relationship between fruit size and seed size. Look at the vastly different sizes of the red huckleberry and western wahoo seed above. Then look back earlier in this note at the fruits of these two species. The fruits are nearly identical in size.
Clearly, seed size has little or no relationship to the ultimate size of the plant. Douglas-fir grows phenomenally larger than either beaked hazel or western wahoo, and yet the Douglas-fir has the smaller seed. Scientists in California studying seed size of over 2,500 native plant species have come to an interesting conclusion. First of all, shrubs and trees have seeds adapted to their environments in totally different ways. For shrubs, those species with the largest seeds are best adapted to areas with heavy shade and fierce competition. But for trees, the species with the largest seeds are the ones best adapted to survive on drier sites. The trees of TCSNA follow that rule quite nicely. Both black cottonwood and red alder (Alnus rubra) are commonly found in moist sites, and have relatively small seeds. Douglas-fir, with seeds dramatically larger than either of those two species, are commonly found on somewhat drier sites.
So the bottom line is that compared to the post logging “Then”, we probably “Now” have a lot fewer seeds produced by the shrubs and ground cover plants at TCSNA due to heavier shading. And especially for the smaller-seeded plants, the presence of an undisturbed litter layer is holding back the success of the seeds that are produced.