Monthly Archives: March 2015
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.
The fruits of the plants in TCSNA ripen at different times of the year. Cottonwoods shed seed in late May, Douglas-firs in July and grand firs in late August or September. On your next hike at Tryon Creek State Natural Area, keep a look out for the seeds and fruits. You’ll be amazed at the inventiveness of our plants.
You never know what you might spot along the trails at Tryon Creek State Natural Area!
The refreshing rains brought an unlikely wildlife sighting to Tryon Creek this weekend, a river otter! Have you ever spotted one in the park before?
“To be fortunate enough to observe the otters as they frolic in the bounding, rushing water, or in a placid, mountain lake is to behold the epitome of aquatic grace and beauty among mammals” – Chris Maser, Mammals of the Pacific Northwest.
It was indeed a rare treat enjoyed by all who spotted it.
Want to learn more about this “otterly” cute critter?
Check out some fun facts here!
Squirrel vs. Tree: Food Fight in the Forest
By Bruce Rottink, Volunteer Nature Guide and Retired Research Forester
Many people visit Tryon Creek State Natural Area (TCSNA) to renew themselves amidst the forest’s tranquility. Tranquility? In the forest? Surely you jest! On an almost hourly basis new life starts as eggs of everything from birds to mosquitoes hatch and seeds germinate, while other lives end as owls catch mice and bugs fly into spider webs.
One of the conflicts at TCSNA revolves around our native Douglas squirrel (Tamiasciurus douglasii) and our stately Douglas-fir (Pseudotsuga menziesii) trees.
The battle is over the seeds of the Douglas-fir. The seeds are small, but filled with concentrated food energy. That stored energy can be used to either nourish a squirrel, or grow a new tree. A mature green cone, and the seeds from one medium-sized Douglas-fir cone are pictured below (note: the cone and seed pictures are not to the same scale).
The squirrel’s perspective!
For the squirrel, the Douglas-fir cones with their many seeds are a convenient package of food. Sort of like take-out pizza in a box. The squirrel can clip a single cone off the tree, and quickly snip off all the cone scales to get the seeds. That’s a lot more efficient than hunting around for the individual little seeds one by one.
Evidence of the squirrel’s love of Douglas-fir seeds can be seen all over the forest in the mini-messes (technically they’re called “middens”) the squirrels leave behind. The squirrels are very skilled at clipping the scales off the cone to get the seeds. The “after lunch” photo below shows the bumpy central axis of a Douglas-fir cone (red arrow), and the cone scales the squirrel left behind.
The tree’s strategy!
The Douglas-fir tree doesn’t produce seeds just to ensure that countless generations of sassy little squirrels can frolic through the forest. The tree’s plan is that once the seeds get ripe, the cones open up and release the seeds. The seeds fall to the ground and “bingo!” a new bunch of Douglas-fir trees. To the tree, the picture below is success! A cone has evaded being harvested by squirrels and has dropped its seeds. Once the cones release the ripe seeds, the seeds are pretty much safe from the squirrels.
So how do cones work anyway?
The mechanism that opens the cone is based strictly on hydraulic pressure. When the cone gets ripe and dries out, it opens to release the seeds.
Below is a longitudinal section through a ripe, closed, Douglas-fir cone. You can easily see the big bend in the light-colored stalk (red arrow) connecting the cone scale to the cone’s central axis (blue arrow). Just by good luck, I sliced through the edge of a seed (the white body indicated by the yellow arrow.)
Here is the same cone after it has dried and is completely open. Note the difference in the shape of the cone scale’s stalk.
The secret behind the cone opening is that in the cone scales there are two different layers of cells. The inner layers of cells are tough and don’t expand when they get wet or contract as they dry out. However, the outer layers are made of a different kind of cell that does expand when it is wet and contracts when it gets dry. To see how these layers work together to open and close the cone, click on the video below.
In fact the cones can open and close many times, even after they are dead because of this hydraulic mechanism. The following three pictures are of the same cone. The first picture was taken after the cone had been stored in my garage for 18 months. It was dead and dry. The second picture is after it was soaked in water for 2 days. The third picture is when it was subsequently dried out for another 2 days.
Thus, the squirrel has only a short time when it can harvest the seed-laden cones. (This re-closing also explains why when you pick up a “closed” cone on the trail on a rainy mid-winter day and tear it apart looking for seeds, you probably won’t find any!)
The squirrels strike back!
So let’s see; the squirrels would like to have cones full of seeds available all year. But shortly after the seeds mature, the cones dry out and shed their seeds. What to do, what to do? Hmmmm! We’ve got to keep those cones from drying out!
Ah-ha! The squirrels figured it out! Cut the green mature cones off the tree and bury them in the moist ground to keep the cones damp. That way the cones will stay closed and ready for the squirrels all year round! Sounds good, but does it really work?
To answer this question, late last summer I gathered some closed, green Douglas-fir cones. I put half in a wire mesh pouch on top of the ground in my backyard, as pictured below. I dug an approximately 2” deep hole and buried the other cones. (“Yeah, I know! I gotta get a life!”).
The cones in a cage opened within a week. On February 15 this year, I dug up the cones I’d buried, and washed them off. They were still closed, as you see below. Score one for the squirrels! I dried out these cones, and ripped one apart to prove the seeds were still there. You can see the results below.
We tend to think of squirrels burying cones to hide them. Sort of like sticking them in the back of a closet where no one else can see. Well, that’s partially true. But in fact, it’s more like putting them in the refrigerator where they will be hidden, and preserved in a way that keeps the “handy package of seeds” intact.
The Trees’ Revenge
So the squirrels figured out how to steal the tree’s cones and bury them for future meals, did they? Too bad for the trees! But maybe the trees got the last laugh. Squirrels may have figured out the “bury the cones” strategy, but their memories aren’t perfect. Estimates of the percentage of cached cones the squirrels actually re-find varies from 10 to 25%. So maybe the tree wins after all, by turning the squirrels into little tree planters!
How about you?
With nearby grocery stores, most modern humans don’t spend a lot of time thinking about storing and preserving food, like the squirrels do. When I was a kid (Oh, here we go!) in Minnesota we grew lots of carrots in the garden. Come fall we’d dig them up, and cut off the tops. Then I’d dig a pit about 2 feet deep (my Dad said digging the pit would help me build “character”) toss in the carrots, pile on as many dead tree leaves as possible, and top it off with about 4 inches of dirt. It kept the carrots crisp and tasty well into the winter.
Think about the foods you eat, and how they are preserved. Is anything dried? Salted? Stored in liquid? Refrigerated? In spite of our technology, maybe we’re not so different from the squirrels after all!