An Ever Changing Forest…
Tryon Creek’s Ever Changing Forest
By Bruce Rottink, Volunteer Nature Guide and Retired Research Forester
From year to year, the array of plants in the forest at Tryon Creek State Natural Area (SNA) seems to change very little. But this apparent stability is an artifact of the human timescale we use to assess the changes in the forest. On a timescale encompassing centuries, the array of plants at Tryon Creek SNA is undergoing constant change.
Three of the main reasons why a forest’s composition changes are:
a) replacement of the native plants by non-native species;
b) devastating disasters, like volcanic eruptions, or massive landslides;
c) normal forest succession.
While the issue of non-native plants is very important, for this note I will focus solely on normal “forest succession.”
What is Forest Succession, anyway?
“Forest succession” is the process by which certain species of plants gradually replace other species of plants. When a landscape is new and barren, such as after a landslide or volcanic eruption, certain plants start to grow in these areas. These are called pioneering plants. These pioneers are adapted to these highly disturbed conditions, and have a huge advantage over non-pioneering plants. However, these pioneering plants slowly and unwittingly change the environment in a way that gradually gives other species an advantage over the pioneers.
How do plants change the environment?
One of the most important ways that these pioneering plants, especially trees, change the environment is by generating lots of shade. The pioneers love full sun, but their very existence reduces the amount of sun that reaches the ground where young plants start. Thus the pioneering plants may only dominate the landscape for a generation or two of plants. Plants that play a part in the forest for only a short while are called “seral” species. [Think of “seral” as being one part of a “series”, as in things that follow one after another.]
At Tryon Creek SNA, and many other parts of western Oregon, the first trees to typically occupy disturbed landscapes are either red alder (Alnus rubra) or Douglas-fir (Pseudotsuga menziesii), depending upon local factors, for example, soil moisture and the proximity of a seed source. Both of these species do best with full sunlight. In forestry terms, they are called “shade intolerant,” or simply “intolerant.” One result of their need for sunlight is that the leaves on the lower branches of the mature trees die off due to lack of sunlight, and for these species it’s a long way up to the first living branch, as seen in the photos below.
To demonstrate this intolerance I took a picture of myself standing among some young Douglas-firs at Tryon Creek SNA, click on the tree to see the results.
Me standing amidst a patch of Tryon Creek’s young Douglas-firs
Can’t see the young Douglas-firs? That’s because there aren’t any at Tryon Creek SNA (with the exception of a few that were planted by humans, most of which died quickly, according to long-time volunteer Phil Hamilton). The young Douglas-fir cannot thrive, and rarely survive long, under the shade of older trees. If you let your mind wander ahead about 500 years, roughly the lifespan of a Douglas-fir, you can probably see where this is going.
So what happens when the pioneers die?
Enter the western redcedar (Thuja plicata) and the western hemlock (Tsuga heterophylla). In contrast to Douglas-fir, both of these species can easily grow in considerable shade, even in the shade of their own parents (there’s a big hint). These species are called “shade tolerant” or simply “tolerant”. There is no better portent of the future than the picture below; a young western redcedar growing at the base of an old Douglas-fir along the Red Fox Trail.
Similarly, there are the many young western hemlocks growing happily under the towering Douglas-fir, like the one below along the Old Main Trail. As you see, because the hemlock is tolerant of shade, unlike the Douglas-fir, it has branches that reach nearly to the ground.
Eventually, barring any major disturbance, the forest at Tryon Creek SNA will forever be dominated by western redcedar and hemlock. The trees of this “forever” forest are called “climax species.”
I recently did a quick count of young conifers less than about 4” in diameter growing within approximately 10 feet of the Old Main Trail between the Nature Center and the junction with the Red Fox Trail. The tally? western redcedar – 17; western hemlock – 6; Douglas-fir – 0; The best that anyone can recall, only a couple of the western redcedar in this stretch of trail were planted by humans.
Do the trees change anything else in the environment?
Producing more shade is the main way that trees drive the succession process, but there are a few other factors at work as well. For example, the Naturalist Note of August 18, 2014 highlighted the important role of red alder in converting the unusable nitrogen in the air, into a form of nitrogen the plants can actually use. This process is called nitrogen fixation. Because the alder can do this, they have a competitive advantage over other species on those disturbed landscapes with low nitrogen. But as the nitrogen-rich alder leaves fall to the ground and decay, their nitrogen is added to the soil. With more nitrogen in the soil, the alders make the site better for non-nitrogen fixing species, and hence lose their competitive advantage.
Another minor factor in forest succession is when some of the pioneer trees die and fall to the ground. The fallen logs, after they have a chance to decay a bit, are the perfect spot for young hemlock and western redcedar to start growing. These logs are called “nurse logs.” Rotten wood acts like a sponge, and soaks up lots of water that the young trees then use. Additionally, the competition on the forest floor can be intense. At Tryon Creek SNA, the Pacific waterleaf (Hydrophyllum tenuipes), for example, can grow in dense beds over a foot tall, and provide stiff competition for a young conifer seedling less than 2 inches tall. By growing on top of the log, the young conifer seedlings can escape a lot of that competition. Below is a prime example of a western hemlock that got started on a nurse log. The log rotted completely, leaving the roots (see red arrow) above ground level.
These nurse logs can help change the forest in a subtle way. There are many examples at Tryon Creek SNA of western hemlock and western redcedar that got started on nurse logs or nurse stumps. However, I’ve never seen a Douglas-fir, either at Tryon Creek, or during my 10 years as a professional forester in this corner of Oregon, which got started on a nurse log/stump. Thus, as the pioneering species die, their remains provide great places for the climax species, and only the climax species, to get started.
Challenge: Can you find this western redcedar along the trail at Tryon Creek SNA?
(*Hint: it’s in the southern end of the park, south of the Red Fox Bridge; get your trail map here)
Exploring the trails of this forest you’ll find signs of what it once was, but now can you imagine what the future will bring?
Today we enjoy the big Douglas-fir/alder forest at Tryon Creek SNA. Thanks to the Oregon Department of Parks and Recreation, and the Friends of Tryon Creek, your great-great-great-great-great…. (okay, you get the idea) … grandchildren will be able to enjoy the amazing hemlock and redcedar forest that they will know as Tryon Creek State Natural Area.
Posted on February 3, 2015, in Plants & Wildlife, Trees, Uncategorized and tagged Douglas-fir, forest, Forest Succession, Native Plants, pioneering plants, Plant reproduction, plants, Red Alder, seedlings, Seeds, Seral, Trees, Tryon Creek State Natural Area, waterleaf, weather, Western hemlock, Western redcedar. Bookmark the permalink. Leave a comment.