Students in general ecology (EES-152) resurveyed a portion of the Lehigh Experimental Forest, to assess changes in tree growth, mortality, and recruitment since 2013. No new trees greater than 1.4 m high were documented, and both growth and mortality varied considerably among species. Over 500 trees were measured, and the plot above shows data for the dominant trees (those with >15 individuals included in the survey).
We will use these data as a springboard for discussion of processes controlling forest dynamics, and will examine some of these issues in greater depth during our discussions and future lab activities.
For now, students should answer the following questions:
1. The dbh measurements were converted into estimates of area, assuming that each tree was a perfect circle in cross-section. Why do you think basal area was used to compare growth among the different species? Why was this expressed as the average change in basal area per tree, as opposed to the total change in basal area for all individuals of the species? What factors might have caused the observed differences in radial growth among species?
2. What does the pattern of mortality and recruitment suggest about the future of the Lehigh Experimental Forest? What factors might have caused the differences in mortality among species during these two years? What factors might be contributing to the lack of new tree recruitment in the forest?
3. Which species had both very high mortality and very low growth during this time period? Do some research on current threats to this particular species, and summarize your research in a short paragraph.
An opportunity for experiential learning
In 1967 Lehigh University Professor Francis Trembley convinced the university to stop mowing a small area of the campus. Professor Trembley named the area the “Tangled Bank,” and it became a place where students could observe nature right outside the classroom. At one point in time there was even a “Tangled Bank” sign on the slope. One year after the mowing stopped, Professor Trembley had the foresight to encourage an undergraduate student to collect and identify all plant species growing on the Tangled Bank, and these collections were archived in the Lehigh Herbarium. Read the full story of the Tangled Bank here (including some great recollections of Lehigh alumni in the comments).
In the fall of 2013, the Lehigh University ecology class (EES-152) completed a botanical survey of the “Tangled Bank” to document what species occupy the site today. The primary objective of this project was for the students to use their data in conjunction with the 1968 plant collections to assess how plant characteristics, such as functional and life history traits, change from early to mid succession. In addition to learning about plant traits and succession, the project allowed students to learn how to apply some commonly used statistical tests to assess differences between groups. They also compared their tree data to a similar dataset collected earlier in the semester from the Lehigh University Experimental forest, a forest that has undergone secondary succession for approximately twice as long as the Tangled Bank. The goal of this comparison was to assess how tree density and biomass change with succession.
Functional traits and succession
What are functional traits? Functional traits are characteristics of species that strongly influence performance, and are therefore fundamental to survival and reproduction. They can be physiological, morphological, or represent reproductive strategies (the latter are oftentimes referred to as life history traits). Plant functional traits include things like photosynthetic pathway (C3, C4, CAM), growth rate, shade tolerance, number of seeds, size of seeds, growth form, life span, seed-dispersal method, and seed viability. Functional diversity (i.e., the total diversity of these traits represented by an ecological community) is increasingly used as an important measure of biodiversity.
What functional traits might be expected to change with ecological succession? Shade tolerance, growth form (herb versus tree), and lifespan might be a few of the more obvious traits that would be expected to change, as plants of later succession include many long-lived trees that compete for light. However, changes in other traits with succession may be less obvious. For example, how might you expect seed viability (i.e., the length of time a seed can survive in the soil before germination) to change with succession?
The Tangled Bank was divided into nine plots, with 3 or 4 students responsible for a complete botanical inventory of each plot. The students identified and estimated the abundance of all plant species, and collected voucher specimens for archival in the Lehigh Herbarium. For trees, diameter at breast height (dbh) was measured and used to calculate total basal area of the forest and the average basal area per tree. The total density of trees (per 1000 m2) was also calculated. Each group submitted an excel spreadsheet with their inventory results and dbh measurements as the first deliverable for the project.
Data compilation and research on plant traits
The data from each group were combined into a class dataset. Students developed lists of the dominant species that grew on the bank in 1968 and 2013 (Table 1, above). Each student was then assigned two plant species (one from 1968 and one from 2013), and required to gather information about the attributes of these species, particularly with respect to life history and functional traits. The objective was to compile as much quantitative and semi-quantiative information on the traits of these species as possible.
The students could use any source of information for this research, as long as they documented their sources; but, we anticipated that they would rely heavily on some of the publicly available databases that have been developed to describe plant characteristics. Unfortunately the shutdown of the US government prevented access to several of these resources, and limited the number of traits that we could examine. However, the students made the best of the situation and found as much information as possible using a variety of sources. They submitted their research in the form of an excel spreadsheet (a template was provided to help standardize the data collection), and this was the second deliverable for the project. Information on sixteen characteristics were found for most species, so we focused our analyses on these (Table 2).
Data analysis and results
Students then performed chi-squared and t-tests using excel, for categorical and continuous data respectively, to assess differences in functional traits of the plant communities in 1968 and 2013 (Table 2). The results of these statistical tests were submitted by each student as the third deliverable of the project.
The students found that plants of early succession generally had smaller seeds, longer seed viability in seedbanks, lower age of first flowering, shorter lifespans, smaller maximum height, and smaller average leaf area (Table 2, Figures 1 & 2). The mid-successional plant community had a greater percentage of species with seed dispersal via mammals and birds (Table 2, Figure 1). Early successional species also tended to be less tolerant of shade and were likely more readily eaten by vertebrate herbivores than those of mid succession (Table 2, Figure 1). Plants of early succession also tended to produce more seeds per plant and have faster growth rates, although differences in these two traits did not meet our threshold for statistical significance (p<0.05). Also, no significant differences were found between N-fixation capacity, frost tolerance, fire tolerance, and drought tolerance of plants in the two communities.
Connecting functional traits to Grime’s life history classification
Various life-history classification methods have been developed by ecologists to facilitate thinking about how species are adapted to environmental conditions. J.P. Grime proposed one such classification scheme for plants, which focused on adaptations to the amount of disturbance (i.e., processes that destroy biomass) and stress (i.e., external constraints that limit the rate of production) in the environment. Under conditions of frequent disturbance, ruderal (i.e., “weedy”) species tend to be favored. Under conditions of high stress, plants tolerant of environmental extremes (e.g., cacti, carnivorous plants) tend be favored. Under conditions of low stress and infrequent disturbance, competitive species tend to be favored.
As an example of how species during succession may fit into Grime’s classification scheme, the functional trait data from the Tangled Bank was used to develop indices related to competition, stress, and disturbance. Traits that would likely be selected for in these different environments were grouped and each species was given a composite score for each of the axes shown in Figure 3 based on the total standardized score of the grouped traits. The figure highlights the shift from ruderal species to better competitor species that has taken place between 1968 and 2013 on the Tangled Bank (Figure 3).
Changes in tree density and biomass with succession
Tree data from the Tangled Bank was compared with data that the students collected earlier in the semester from the Lehigh Experimental Forest. Tree density (trees/1000m2) was higher on the Tangled Bank than the Lehigh Experimental Forest, and the average size (basal area) of trees was larger in the Lehigh Experimental Forest (Figure 4A, 4B). Total basal area of trees, which takes into account both density and size, was greater on the Tangled Bank (Figure 4C).
For the students, deliverable #4. Due 8 November. Complete the following tasks/questions:
1) In less than one page (single-spaced), summarize the differences in functional traits of plant communities in early and mid succession on the Tangled Bank. Your summary should include descriptions of why the observed differences likely occur (i.e., processes).
2) What other functional traits, not included in our analyses, might be expected to differ between 1968 and 2013?
3) For many of the traits we were only able to obtain categorical values, and the number of categories we defined varied among traits. For example, dispersal mode had six defined categories whereas nitrogen fixation capacity only had two. Do you think that the number of categories and how we defined them affected our statistical results and interpretation? Using the original excel spreadsheet, create new, broader categories for a trait of your choice by merging data into a smaller number of categories. Does reducing the number of categories impact the chi-squared test result? Does it change the interpretation?
4) What are the assumptions of a t-test? Did our data violate any of these assumptions?
5) Describe the position of the 1968 and 2013 species in the Grime’s life history diagram (Figure 3). What traits do you think were used to develop the species scores along each axis? In other words, which particular traits would likely be most related to each axis? Some traits could be favored under more than one environmental condition (i.e., high stress, high competition, high disturbance). Justify your answers.
6) Where would the species of the Lehigh Experimental Forest likely be place on Figure 3?
7) Assuming that the differences in successional age are solely responsible for the differences in the density, size of trees, and total tree basal area of the Tangled Bank and the Lehigh Experimental Forest, what specific processes likely resulted in these differences?
8) Your answer to #7 assumed that the only difference between the two sites was age, a common approach often referred to as space-for-time substitution. However, what other factors might contribute to the differences between the two sites?
9) Using the data in Table 3 (below), calculate the rate of change of tree density and average tree basal area in a) early succession (first 45 years) and in b) mid-succession (45 to 98 years). Explain what processes likely contribute to these different rates of change.
Students in ecology (EES-152) at Lehigh University share pictures of our field activities via Twitter. Below are some highlights from the first few field labs, which were focused on tree identification and forest inventory methods. The students resurvey experimental forest plots original established in 1915….
“It is interesting to contemplate a tangled bank, clothed with many plants of many kinds, with birds singing on the bushes, with various insects flitting about, and with worms crawling through the damp earth, and to reflect that these elaborately constructed forms, so different from each other, and dependent upon each other in so complex a manner, have all been produced by laws acting around us. . .” – Charles Darwin
Where is the tangled bank?
Snippets of what little I could recall of the above quote from Charles Darwin bounced around my head as I read labels on some herbarium specimens.
My task was to reorganize this collection of carefully flattened and labeled dried plants, to make it more useful as a teaching resource.
According to the labels on these particular specimens, they were collected from the “Tangled Bank.” No other location information was given. Where was this place? What was its significance? The labels indicated that they were all collected by a person named David J. Mazsa. Maybe I could track this person down…
Finding David Mazsa
Google is amazing. Within a minute of searching, I found a person by the name of David Mazsa with a degree from Lehigh Unversity. Perhaps he was the collector? I sent him an email and received a prompt response….
“You are in luck- I am the guy you are looking for.
“Tangled Bank” is the sloped area right behind Williams Hall. Dr. Trembley had an idea to allow that mowed and manicured area to proceed through natural succession and become a “tangled bank” (I think the term is from Charles Darwin). Dr. Trembley convinced Lehigh to stop mowing and allow the area to begin to revert to it’s more natural state. In his view this would be so much more attractive and diverse than the mowed lawn. I honestly do not remember the exact chronology but my best guess is that they stopped mowing some time in 1967 and my samples were from the following year but I am not sure. Dr. Trembley found some money for me to work on the bank and take samples to record changes in the plant species distributions. I am amazed to learn that I left a mark! I graduated from Lehigh in 1969 and although I visited Dr. Trembley once or twice afterwards, I don’t know how long the project lasted or what additional data was collected.
Dr. Trembley was one of the people responsible for my career path. I was born and raised in Bethlehem and he wrote a lot in the local newspapers. He was one of the reasons I chose Lehigh. He was a combination of an old time naturalist who loved the diversity of living things and was eternally curious about how they interacted. Walking on a field trip with him was like having a window to the world of nature. We hung on his every word and loved his stories. But he was also politically astute and he was in the middle of Environmentalism of the 60s and 70s.
He taught Ecology with a set of note cards that were at least thirty years old. When we asked him if it was boring teaching the same stuff over and over again, he noted that the Ecology material was much the same, but the students changed every year and his interaction with them made it different every year. I have just finished my 40th year as a high school teacher and now I understand how right he was about the difference in students from year to year.
I also remember that our grade in Ecology was determined by a semester exam and that the question was “Describe life in, on, and around inland waters.” Although that was the only science course at Lehigh that I didn’t ace (I got a B I think), That course and Dr. Trembley are certainly one of the big reasons I have spent much of my life teaching and working toward making the world a more sustainable place.
I hope I helped identify the location for you. Thanks for bringing back some very fond memories.
So the “tangled bank” is the slope behind Williams Hall!
Of course! This is the only non-manicured place on the Asa Packer campus. And before the Department of Earth & Environmental Science moved from Williams Hall into the STEPS building, my office used to face it. For five years I looked directly out my window at the “tangled bank.” And I agree with Trembley, it is more beautiful and diverse than a mowed lawn. An aerial photograph from 1971 shows that much of the slope was still characterized by low-growing vegetation four-years after they stopped mowing.
After David’s email, I also found mention of the “Tangled Bank” in a book on the history of education at Lehigh University (Yates 1992, page 238):
“The biologist Fran Trembley was speaking and writing on the dangers of technology long before the subject became popular with the general public. In 1951 he had his title changed to professor of ecology. Students flocked to his classes. In order that they might have a nearby spot in which to study ecosystems, he asked that the slope above Williams Hall be left in a wild state – a tangled bank, he called it, using a term coined by Darwin. The administration complied with the request and put up a sign, “Tangle Bank,” to identify the place.”
Today, the “tangled bank” is a small, diverse woodlot, with an overstory that includes tulip poplar (Liriodendron tulipifera), American beech (Fagus grandifolia), black cherry (Prunus serotina), several species of maples (Acer spp.), as well as several other deciduous species. Various shrub species occur throughout, particularly along the margins, and poison ivy (Toxicodendron radicans) and Virginia creeper (Parthenocissus quinquefolia) are common. It is a very different place than it was in 1968. A few more photographs are below (click on one to view as slides).
What can students learn from the tangled bank?
This fall, students in general ecology (EES-152) at Lehigh University, will revisit Trembley’s “tangled bank.” Forty-seven years of ecological succession have passed since the mowing was stopped, and because of David Mazsa’s collections we know many of the pioneer species that grew on slope after just one year.
Students in the ecology class will complete a botanical inventory of the slope, and the data will be used to introduce and explore the topic of secondary succession. Furthermore, the students will use modern databases to research and compare information on the functional traits (e.g., seeds per plant, energy content per seed, growth habit, mode of dispersal) of plants in early succession versus mid-succession (i.e., today). We will report our results on this blog later in the fall semester!
Trembley’s last lesson.
Unfortunately, this will probably be the last lesson taught by Trembley’s “Tangled Bank.” A renovation of Williams Hall is planned to begin later this year, and as part of this effort the area will be relandscaped…removing the “tangle” from the bank.
Perhaps only this ecologist would be saddened by such a thing.
A beautiful spring day at Lehigh University, and many of the trees have sprung-out small leaves. I took a short walk on campus with Michelle Spicer to look for some old trees…and there are plenty of these on Lehigh’s campus. In fact, many of the larger oaks are well over 200 years old. However, we wanted to determine if a few particular trees mentioned in a 1934 article in the Brown & White, shown below, were still present.
The results….two out of three is not bad.
Gnarled old oak on the west-end of Christmas-Saucon Hall.
Yes, now over 300 years old (!), this old giant is still there and doing well. Although I have walked by this white oak (Quercus alba) hundreds of times, I didn’t really take much notice of it. You don’t realize how big it is until you approach it, and the size of its base is a bit concealed by its close proximity to the building.
“Old Man of the Mountain” wild cherry.
Unfortunately, this apparently charismatic and previously well-known tree is no longer around. Wonder when it came down?
Ginkgo tree in the plot of land on the corner of Packer and Brodhead.
Yes, the Ginkgo (Ginkgo biloba) is still there and doing quite well. This area of campus is also one of my favorites, a wonderful park of many large trees – sugar maple, tulip poplar, norway spruce, oaks, and even a Kentucky coffeetree. Many of the trees in this area still have labels (labels are on some trees throughout campus, but quite a few have fallen off).
Addendum, 27 August 2013
Students in EES-152 (Ecology) at Lehigh University were given the 1934 article as part of an introduction to tree identification. They managed to find both the “gnarled oak” and the Ginkgo. Some of the best pictures are below…
The “gnarled oak”
Success! Michelle Spicer (Lehigh MS student) has successfully obtained funding for her forest research using the relatively new method of crowdfunding (see previous post). Admittedly, I had some doubts about the crowdfunding model when I first heard about it, particularly how successful it would be at engaging the public in science research. I also wondered how much of the funding for these projects was typically obtained from the general public versus immediate friends and family of the investigator. However, Michelle’s research topic seemed to be well suited to crowdfunding so I was excited when she decided to give it a try. She ended up being more successful than I would have guessed.
Michelle exceeded her funding goal, and was able to attract contributors from outside of her immediate network of family and friends. Her proposal was funded by 23 individuals, and although 10 of these individuals were family and close friends, this group of people only provided about 20% of the total funds. About 80% of the funds were from people that were outside of her immediate network, and these were nearly all people that she had never personally met; i.e., friends of family, friends of friends, friends of friends of friends, Lehigh University alumni, and people that found her proposal through SciFund, Rockethub, and their networks. The median contribution was $25, although contributions ranged from $10 to $1000. Most surprising to me was that the two largest contributions ($500 and $1000) were from people that she had never met.
It is also worth noting that through this process, Michelle’s SciFund proposal was shared extensively on Facebook, Twitter, and other social media outlets – and therefore a lot of people learned about her project and the Lehigh Experimental Forest. Clearly the process of engaging the public and alumni in this project has only just begun, and Michelle has already had some nice correspondence with some of her funders. Michelle plans to post updates of her research progress on her blog, In the Forgotten Forest, where anyone interested may post comments, suggestions, and questions as her research progresses. A big thanks to all the funders, and to all of the people that “shared”, “liked”, and “tweeted” Michelle’s proposal.