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.
Back in June an alligator was found in the Lehigh Canal. Apparently it wasn’t the first one found in the broader Lehigh Valley.
But perhaps just as surprising are a couple of potentially new plant arrivals. Or are they just summer visitors? Last week I noticed sizable populations of two aquatic plant species, water lettuce (Pistia stratiotes) and water hyacinth (Eichhornia crassipes), in the canal at Sand Island in Bethlehem. Both of these species float unattached on the water surface, like the more common duckweeds, and they often grow in dense mats that make fishing and boating difficult, crowd out other plant species, and alter water chemistry and light penetration. To my knowledge, neither species is confirmed to occur naturalized in Pennsylvania but it is not uncommon to see them cultivated in backyard ponds (USDA Plants: water hyacinth, water lettuce).
The populations of water lettuce and water hyacinth in the Lehigh Canal consist of scattered colonies extending from about the Hill-to-Hill Bridge (Route 378) east past the New Street Bridge (Fahy Bridge), to about the point where the Sand Island Trail meets the towpath (D&L Trail). The total distance is about a half mile. The water lettuce appears to cover a slightly greater distance than the water hyacinth, and the plants are generally smaller in height as you head east (downstream) from the Main Street Bridge.
Water lettuce and water hyacinth are tropical or subtropical in origin. The two species have dramatically expanded their range in warmer regions in recent years, where they have cause considerable ecological and recreational impacts. However, given that both species are sensitive to freezing temperatures, they have not not been regarded as major threats in the Northeast. However, some uncertainty about this assumption has emerged in the last several years. For example, a few years ago populations were found in the lower Great Lakes (Adebayo et al. 2010), and resurveys found both species in three subsequent years (Maclsaac et al. 2016), raising concerns about the potential for the establishment of persistent populations in more northerly locations. Although freezing typically kills individuals of both species they can produce seeds that survive cold temperatures; in fact, water lettuce seeds can still be viable after a few weeks in solid ice (Pieterse et al. 1981). Maclsaac et al. (2016) suggested that the two species likely persist in the lower Great Lakes due to annual reintroductions by humans (both species are sold for ponds/aquariums), but also noted that at least in the case of water hyacinth, seasonal regeneration from viable seeds may be occurring.
For background, the Lehigh Canal was built in 1827 to transport anthracite coal from the upper Lehigh Valley, and it remained in operation until the early 1940s. Heavy transportation and industrial activity along the canal and river corridor, as well the development of the surrounding Allentown-Bethlehem-Easton region led to numerous environmental problems, including pollution, habitat degradation, the spread of invasive species, and eutrophication of the canal. However, the towpath along the canal is now a natural-area corridor and the old towpath is a great place to bike, run, hike, fish, bird, and observe nature from within the urban and suburban matrix of the Lehigh Valley. Near Sand Island in Bethlehem, the canal itself gets pretty green by mid-summer, as the slow-moving water warms and algae proliferate. Invasive eurasian water milfoil (Myriophyllum spicatum) and curly-leaf pondweed (Potamogeton crispus) are common submerged plants within the canal, and provide a favorable substrate for filamentous algae. The habitat is ideal for water lettuce and water hyacinth, except for the fact that it freezes in the winter.
Have these species been in the canal in previous summers? Are these populations persisting, or did this expansion occur just this year? Perhaps the two species came into the canal with the pet alligator :) Although this was the first time I noticed the two floating species, I don’t frequent this particular area of the towpath often. Will they reemerge next summer? Are they producing viable seed? Lots of questions, and certainly something to watch. The observations have been submitted to iMap Invasives, a database of invasive species.
Of course, floating plants are also very good at moving. Maybe not as fast as an alligator, but fast enough for me to watch a cluster of water lettuce floating down the canal. Perhaps on its way to Easton?
Adebayo, A., E. Briski, O. Kalaci, M. Hernandez, S. Ghabooli, B. Beric, F. Chan, A. Zhan, E. Fifield, T. Leadley, and H. MacIsaac. 2011. Water hyacinth (Eichhornia crassipes) and water lettuce (Pistia stratiotes) in the Great Lakes: playing with fire? Aquatic Invasions 6: 91-96. DOI 10.3391/ai.2011.6.1.11.
MacIsaac, H.J., A.P. Eyraud, B. Beric, and S. Ghabooli. 2016. Can tropical macrophytes establish in the Laurentian Great Lakes? Hydrobiologia 767: 165-174. doi:10.1007/s10750-015-2491-y
Pieterse, A. H., L. Delange, and L. Verhagen. 1981. A study on certain aspects of seed germination and growth of Pistia stratiotes L., Acta Botanica Neerlandica 30: 47–57. doi:10.1111/j.1438-8677.1981.tb00386.x