General ecology (EES-152) students have finished resurveying a portion of the Lehigh Experimental Forest, with the goal of assessing changes in tree growth, mortality, and recruitment since 2013. A total of 690 trees were measured from across the forest, representing more than a 1/4 of all trees. In the three years since 2013, 70 of these 690 trees have died and only three new trees have established in the study area. Data for the dominant tree species are shown in the plot below.
We will use these data to discuss the processes controlling forest dynamics as the semester progresses. However, for now, students should answer the following questions:
- 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 rates among the different species? Why was this expressed as the average change in basal area per tree? What factors might have caused the observed differences in radial growth among species?
- 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?
- Assuming the rates of total tree recruitment and mortality are representative of future years, when will there be no trees left in this forest? In 2013, there were ~2000 trees in the forest. Show your work and describe how you arrived at your estimate. Do you think it is likely that the trees will really be gone by this time? Why or why not?
- 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.
Today the Pymatuning wetlands spent the entire day in the lab. Our first day without any fieldwork since the course began. However, we made up for it by doing a bit of time travel…
We examined the core we collected from Titus Bog yesterday. We subsampled the sediment and peat, sieved the samples to isolate plant macrofossils (i.e., seeds, leaves, needles, etc.), and identified and tallied the microfossils to determine how the vegetation of the wetland has changed over the past 8000 or 9000 years. The students determined that the site was occupied by a shallow lake prior to the establishment of the modern peatland, with submerged and floating leaved aquatic plants like Najas (water nymph), Nuphar (spatterdock), and Nymphaea (water lily) growing in the deeper portions of the littoral zone. Emergents like Cladium (sawgrass), Rhynchospora (beaked sedge), and other sedges likely occupied the lake margin along with small amounts of Sphagnum moss. The area abruptly became a floating peatland about 350 years ago, when Sphagnum became dominant. The upland vegetation around the site contained Tsuga canadensis (hemlock), Pinus strobus (white pine), and Betula alleghaniensis (yellow birch) for much of the record. Most of the species in the paleoecological record have been observed at the wetlands we have visited during the past two weeks of the course; in fact, quite a few are the “must-know” list.
Our age estimates for the record are tentative and come from a broader study of peatland development at the site by Ireland and Booth (2011). We will discuss our paleoecological record in class tomorrow, along with the Ireland and Booth study, emphasizing the implications for understanding long-term wetland development and hydroseral succession.
After spending considerable time in marshes and swamps over the past two weeks, the Pymatuning wetlanders spent much of today in a bog (well, as they all know it is technically a poor fen). We drove to Titus Bog, located about an hour northeast of the Pymatuning Laboratory of Ecology, and Tim Lyons of the Botanical Society of Western Pennsylvania accompanied us into the bog. The moat swamp surrounding the bog was fun to cross, as the water levels were quite high from the recent rain, and several students were delighted to have the opportunity to get a little water into their waders again.
Peatland ecosystems are quite unique. They leave a detailed record of their own development through time, recording past changes in plant communities, hydrology, and other environmental conditions within the stratigraphy of their waterlogged peat. To examine the paleoecological history of Titus Bog, we collected a peat core capturing most of the upper 9 meters. The students did a great job collecting the core, and tomorrow we will carefully examine the peat and sediments under the microscope to reconstruct how the present-day wetland came to be. We will use the record of past vegetation change as a springboard for a broader discussion of wetland development.
After collecting the peat core, we hiked around the surface of the floating peat mat, where we saw many typical bog plant species including several orchids, cranberries (Vaccinium oxycoccus), leatherleaf (Chamaedaphne calyculata), podgrass (Scheuchzeria palustris), bog bean (Menyanthes trifoliata), and of course lots of Sphagnum moss. We were lucky enough to be on the bog during the brief window that the bog copper (Lycaena epixanthe) was active and mating. These small butterflies occur exclusively in these acidic peatland habitats, where cranberries serve as the host plant.
After we finished our exploration of Titus Bog, we went on a short hike to a very small peatland that has a nice population of purple pitcher plants (Sarracenia purpurea). The students have now seen all of their “must-know” plant species in the field.
We have completed two weeks of wetland ecology at Pymatuning Lab of Ecology… only one more week to go. The Pymatuning wetlands spent the morning discussing freshwater marshes, swamps, and riparian wetlands. We examined vegetation dynamics, food web structure, and biogeochemistry of each of these wetland types, paying particular attention to similarities and differences. Our discussion of the vegetation dynamics of freshwater marshes highlighted the importance of seed banks, climate variability, and herbivores like muskrats (Ondatra zibethicus) in controlling interannual-to-multidecadal scale ecological changes in these systems. The topic turned out to be quite appropriate for today, given the results from our camera traps in the afternoon. We then went over the midterm exam, spending a considerable amount of time working through the details of how nitrogen cycling occurs in the context of the aerobic and anaerobic layers of wetlands. The students all promised that they would study the details of the nitrogen cycle, and other biogeochemical cycles in wetlands, if these will reappear on the final exam. I won’t disappoint.
We then went to Pymatuning Creek Marsh to collect the shallow wells that we installed last week to record water-level fluctuations within different vegetation zones. We also collected the camera traps. Clearly white-tailed deer occasionally use the marsh, but the students were particularly pleased that they captured video of a muskrat. There was clear evidence of them in the marsh, as there often is marsh environments; however, they tend to be active at night or around dusk so they are not often seen.
The video is embedded below.
And a raccoon….
The Pymatuning wetlanders discussed salt marshes this morning, with a focus on plant adaptations to salinity, causes of plant community zonation, food-web structure, energy flow, and sulfur biogeochemistry. And of course, some discussion of when herbivory goes awry, with guest appearances by lesser snow geese, Littoraria snails, and nutria.
We then sampled macroinvertebrate communities at two more wetlands. We almost got the van stuck in the mud, but with 7 wetlanders pushing the van we made it. After sampling we had lunch and looked over the ice cream selection at the local ice cream place in Linesville. It seemed like a good day for ice cream to me, but the students decided that they would rather spend the time identifying macroinvertebrates in the lab. The entire afternoon was devoted to macroinvertebrate data collection.
The Pymatuning wetlanders started off the morning by exploring the wetlands along the edge of Pymatuning Lake via canoe. We examined the hydrophobic leaves of American Lotus (Nelumbo lutea) and compared it to the white water lily (Nymphaea odorata). We also added a number of submerged aquatic plants to our “must-know” plant list, including bladderwort (Utricularia sp.), hornwort (Ceratophyllum demersum), waterweed (Elodea canadensis), and two invasive plants, curly leaved pondweed (Potamogeton crispus )and Eurasian milfoil (Myriophyllum spicatum). We also found all three common floating plants growing in association: lesser duckweed (Lemna minor), greater duckweed (Spirodela polyrhiza), and the smallest flowering plant in the world, waterfall (Wolffia sp.). Botanizing was a pleasant and fun way to start the day.
We spend a short time in the classroom finishing up our discussion of wetland biogeochemistry, by taking a close look at phosphorus cycling in wetlands. We then discussed intrinsic and environmental controls on decomposition and production, and the implications for nutrient cycling. To reinforce their newly acquired knowledge of wetland biogeochemistry, the students will complete an assignment based on a laboratory experiment we conducted as part of a semester-long course in wetland ecology at Lehigh University (“smelling your way down the redox ladder”).
We then went to the spillway for lunch, where the students were amazed and disgusted at the density of bread-fed carp. The continual influx of bread is breathtaking. And not just bread, I have watched the carp eat a tray of cupcakes, waffles, and sandwiches. And this is all in the few minutes that I have spent there each year. It is truly unbelievable that this sort of thing is happening in 2015, given our knowledge of the ecological problems that it creates, particularly with respect to phosphorus loading. Our discussion of phosphorus limitation in the morning was on our minds as we watched the carp. Dr. Andy Turner from Clarion University has a nice description of the craziness on his blog. Linesville must make a tremendous amount of money selling moldy bread to tourists. This year I noticed that they are even selling shirts. “Carpe feed’m”….really?…..
After lunch we headed to Hartstown Swamp, where we carefully avoided poison sumac (Toxicodendron vernix) while we explored the edge of the swamp and added a number of swamp plants to our “must-know” list. None of the students actually made the mistake of collecting poison sumac, although there were a few moments of of panic when collections of green ash (Fraxinus pennsylvanica) came out of bags back in the lab. I did nothing to promote these brief moments of panic of course 🙂 As week one comes to an end, we have identified nearly 50 wetlands plants. I’m looking forward to next week.
After a breakfast of energy-rich waffles, the Pymatuning wetlanders slowly descended the rungs of the redox ladder into the world of wetland biogeochemistry. The microbes rule this world, and we examined the ways they make living by examining nitrogen, iron, manganese, sulfur, and carbon cycling in wetlands. Electron acceptors, photosynthesis, oxidation, reduction, aerobic respiration, diffusion, mineralization, nitrification, denitrification, sulfur bacteria, photosynthetic sulfur bacteria, redox potential, ferric iron, ferrous iron, nitrogen fixation, sulfer-reducing bacteria, extended glycolysis, heterotrophs, chemoautotrophs, facultative anaerobes, obligate anaerobes, methanogenic bacteria, cation exchange capacity, and other trophic-genic-ifications until our brains were full and it was time to cool off in the marsh.
We spent much of the afternoon at Pymatuning Creek Marsh, where the students established transects along the moisture gradient from the edge of the wetland to the interior, and quantified the distribution of vegetation, water-table depth, and pH. While the students collected data I had a little time to quietly explore the marsh a bit, and I took a few pictures…
It was hotter than yesterday and the deer flies (Chrysops sp.) were relentless. Much blood was lost. But we obtained the necessary data and managed to collect a few more plant specimens. This group of students has a fantastic attitude and they are all quite a lot of fun. We returned to the lab to press plants and sort out the unknown plant species that they encountered along the transects.
Tomorrow we will explore the lacustrine wetlands of Pymatuning reservoir, and visit a swamp and some shallow water environments to round out our “must-know” plant list for the first week of class.
And a few more students are now contributing to the twitter feed: #PLEwetlands