What causes nurse logs?
In Nurse logs in a coastal Oregon forest, Shel Marcuvitz recounts that “trees often do not fall until they have been standing for a time and may have some of the characteristics of the rotten wood before they fall to the ground (4).” In other words, nurse logs are not a simple phenomenon that has a singular cause. Based on my research, they are not even limited to a single type of forest. Shel Marcuvits discuses the forests of the Pacific Northwest in his thesis, and included in it are the results of many other scientists that study this, and related phenomena which will be discussed later, in this same region. In Importance of nurse logs as a substrate for the regeneration of pioneer tree species on the Barro Colorado Island, Panama, Evelyn Sanchez, et. al. reports their findings on the vital role of nurse logs in maintaining biodiversity in the tropical forests of Panama.
These tropical forests have markedly different climate than the forests of the Pacific North West. The Panamanian Forests have a four-month dry season (Sanchez 430) contrasted with the PNW forests in which the dry season is mitigated by the presence of summer fogs (Macuvits 14). Decaying wood is also superior to the soil substrate in the forests of the Western Carpathians, even though it is often not as common to find tree starts growing there (J. Szewczyk 45). Even though the forest is once again different, in this case a temperate forest (37), nurse logs are still present. They even play role in the forests in the Far East, where Fukasawa reports that “small seeded pioneer tree species Clethra barbinervis (Ericales) were found to be preferentially established on rotting fallen logs of the Japanese red pine Pinus desiflora (1).” There is something universal about this act of decaying wood serving, in many cases, as the most desirable places for new trees to grow. These logs are part of the natural lifecycle of the forest.
Why does one find more tree starts on decaying wood than regular soil?
Across all the studies I read, the results are inconclusive. There is always a certain amount of subjectivity involved when one is examining rotting logs on the forest floor to determine things such as their decay class. Marcuvitz concludes that it is much too complex to determine a single factor that makes a rotten piece of wood any better than say, a mound of soil on the forest floor. He does tentatively put forth the idea that a major factor could be that there is less competition for nutrients and sunlight for tree starts on decaying wood than on the forest floor (69). The theory of less competition leading to increased seedling density on nurse logs is echoed by other researchers. Sanchez conveys that these sites not only “decrease competition” but they also “increase light availability”, and “provide refuges from fungal pathogens” (430). This is an example of how complex finding an answer to this question is. It is likely not one of these factors, but some combination of the three which allows these seedlings to have an increased change of survival on nurse logs. In the abstract of effects of wood decomposer fungi on tree seedling establishment on coarse woody debris, Fukasawa writes that a large buildup of debris on the forest floor is one factor that influences why they observe more tree seedlings sprouting on decaying logs, which generally are not host to as thick of a layer of debris, than on the forest floor (1). Similarly, Szewczyk mentions a study by by Harmon and Franklin in 1989 which concluded that in the forests of the western united states a certain species of tree was more often not able to sprout on the forest floor as a direct result of the intense competition, but he also discusses another idea in his paper. He puts forth the idea that it is only small seeds which cannot survive on the forest floor and that large seeds were not likely to be present on nurse logs (45).
How do small seeds get onto nurse logs?
One of the first conclusions of nurse logs in a coastal Oregon rainforest, is that it is not practical to believe that random wind patterns are sufficient to explain why one sees these seeds growing more readily on nurse logs than on the forest floor. Perhaps there is some other natural agent at play? Initially, I had the thought that maybe these smaller seeds, since they are more likely to be present on nurse logs than larger seeds, were carried there by small animals. Alas, I came across the conclusion in Sanchez’s article that “animal-dispersed species also had a significantly greater proportion of their seedlings rooted in soil than did wind-dispersed species (432). It seems that this is not an adequate explanation. This topic is ripe for increased research. Although it is beyond the scope of this project, there are many other factors that could help explain why one more often sees small seeded tree species on decaying logs than on other substrates, such as the presence of special fungi on decaying logs that assist the seedlings in obtaining water, the varying moisture content of decaying bark, or the complicated relationships between specific species of tree and the untold number of microclimates that will influence when a seed is able to germinate. At the moment, I am obliged to agree with the statement that “apparently, there is something about rotten wood that is essential to tree seedling growth (Marcuvits 5).”
How is this related to the artist Buster Simpson?
Besides being exposed to a whole world of science that was before completely unknown to me, I also came across a finding the article importance of nurse logs as a substrate for the regeneration of pioneer tree species on Barro Colorado Island, Panama, the was not given much attention within the article, or in any other research I found, but it is of extreme relevance to my thought process concerning Buster Simpson and his motivation as an environmentally conscious artist in an urban environment. On page 434 of the article I found that “nurse logs…play an especially important role in initiating forest succession after agricultural abandonment.” Knowing this, Simpson’s Host Analog is imbued with deeper meaning. It can be seen as not only a spectacle where one is able to witness the curious property of decaying logs giving life to new trees, but also a metaphor for the idea that humans have had such an impact on the natural environment with urbanization, Portland being the example here, that it will take something as powerful as a nurse log to restore it. It also invokes the idea that should our cities one day become empty, processes such as this will overtake them.
Why aren’t nurse logs used more often to restore areas that have been subject to environmental degradation?
The obvious answer is the forests where these nurse logs already are dependent upon them, and removing them would prove detrimental to the forest. Yet even if there were an abundant number of these logs, I do not think that they would be widely used as restoration tools. We as a society are primarily concerned with efficiency, even in our methods of restoration. The scope of a nurse log’s work, several hundred to one thousand years, is not applicable to the human way of thinking. We do not plan parks by position these decaying logs around an area and allow them to slowly consume it, guaranteeing biodiversity and healthy soil. Instead, we raise trees in nurseries and plant them in places we wish to restore once they have reached a certain height. In this way, it appears to us that we are making an immediate difference in that area. Restoration is something that we need to see for it to be effective in the public’s eyes. With this in mind, one is able to prove the necessity of artist’s such as Buster Simpson.
Another example of the idea that restoration is something that we need to see for it be effective in the public’s eyes.
Tetrapods are large concrete structures with three to four “arms” that protect the coastline against the ceaseless buffeting of the waves (Raunekk 3). They protect the shape of the coast, and thus allow people to appreciate the natural environment safely. At the same time, they are halting the natural process of erosion that the ocean is trying to inflict on its surrounding banks. These structures are useful because they allow people to experience the wildness of the ocean, in marked contrast the urban environments in which they likely dwell, and so from that perspective are useful to the goals of the environmentalist –to get people to appreciate the natural world. This comes at the expense of altering the course of the ocean’s waves and so in reality what is being experienced is a human constructed version of nature.
Where is the balance between appreciating the untamed natural world and altering it in ways that will ultimately benefit it?
This is something I urge the reader to consider. Are we able to admit that even our actions to restore the environment are altering its natural cycles? Does unaltered nature, in any form, still exist anywhere in the world?
