How will ecosystems respond to ongoing and future climate change? Will changes in ecological communities track temperature and precipitation in a linear fashion? Or will abrupt changes occur when particular climatic thresholds are crossed? What sorts of information might help us better anticipate future ecological change?
After a sweltering week of fieldwork, these questions seem quite timely….
Ecosystem state shifts and long-term perspectives. The possibility of rapid transformation of ecosystems in response to climate change is of great concern to ecologists and land managers. Abrupt and dramatic changes in the composition and functioning of ecosystems, commonly referred to as “state-shifts,” have occurred in the past, and may be triggered by climate-induced disturbance events, such as fires or droughts. However, the past century of climate and ecological changes provide only a few examples of such events, and lack long-term perspectives on their effects. Paleoenvironmental records provide long-term perspectives on both climate and ecosystem dynamics, and are therefore well suited for studies of climate-ecosystem relationships.
The Northern Wisconsin Landscape. Northern Wisconsin is a land of lakes…and peat-accumulating wetlands (i.e., peatlands). When the glaciers retreated from this region, they left behind blocks of ice. These ice blocks melted and the depressions that were left on the landscape became the region’s many lakes and wetlands. These basins are commonly referred to as kettles or kettleholes, and are often occupied by lakes or peatlands – and in many cases contain some combination of both. Commonly, kettle lakes are bordered by a peatland that floats on a trapped water layer. When you walk on these peat mats, it feels like you are walking on a waterbed. “Quaking bogs,” “floating bogs,” and “schwingmoors,” are just some of the names used to refer to these unusual ecosystems.
Kettlehole ecosystems and paleoecology. Kettlehole ecosystems provide a range of benefits (i.e., ecosystem services), including habitat for biodiversity, groundwater recharge, sites of carbon storage, and areas for recreation. In addition, these systems have tremendous scientific value, particularly for the study of past ecosystem responses to climate variability, because they leave a detailed record of their own development in the form of peat and lake sediments. By collecting and examining sediment cores from these systems, much can be learned about how they have changed in response to past climate changes.
Recent research has demonstrated that extreme drought and water-level fluctuations probably trigger the initiation and expansion of peatland in kettlehole ecosystems (see related paper), effectively transforming the ecosystem from an open lake to a peatland (with or without a central lake). This transformation likely occurs quite rapidly (i.e., likely within a decade or so) and would be expected to cause abrupt changes in ecological communities and processes. For example, peatland establishment along the margin of lakes leads to changes in lake-water chemistry (e.g., higher acidity, higher dissolved organic carbon) and lakes with and without peat mats are characterized by very different aquatic communities and food-web structure. In fact, lakes with extensive peatland can become too acid to support fish populations. Furthermore, the transformation of kettle lake systems into peat-accumulating wetlands results in dramatic changes in biogeochemical processes, particularly rates of carbon storage.
We are investigating these ideas further by examining the history of peatland development in kettlehole ecosystems along a landscape gradient in northern Wisconsin. Our research aims to provide a long-term perspectives on the role of hydroclimate variability in triggering abrupt and permanent state shifts in ecosystems, as well as to better understand and characterize the immediate and long-term ecological consequences of these events. Such threshold responses are difficult to anticipate, and our results will hopefully provide a foundation for evaluating the relative sensitivity and vulnerability of ecological systems.
“Wading through the slimy ooze
You can drive away your blues
Romping through the swamp”
– Peter Stampel-
Two weeks of fieldwork in late June and early July. Nice work if you can get it? Not really. By the third day, we were almost drained of our life force by wood ticks and deer ticks (the ones that carry Lyme disease). What little blood remained in our bodies nearly boiled in the brutally hot temperatures that followed for the remainder of the work. However, we did manage to make a great deal of progress. The field crew included myself, Alex Ireland (postdoc, Lehigh University), and Michael Clifford (PhD student, Lehigh University).
Our work this summer was focused on two sites – Triangle Lake and Long Lake. The first several days were spent measuring basin depths with a probe rod so that maps of the underlying basin morphologies could be developed. We then collected a series of peat cores at the two sites, located along transects that captured the gradients of basin depth. The peat and sediments will be analyzed for plant remains and carbon content later this summer and fall, and used in conjunction with radiocarbon dating to determine when the peat-mat established in different portions of the basins. This information will allow us to reconstruct how and when the peat mat expanded at the two sites. The timing of peat mat expansion will be compared between the two sites, as well as to other sites in the region and to paleoclimate records, to assess the role of climate variability in triggering peat-mat expansion. Last winter we collected sediment cores from the central lakes that occupy the Triangle and Long Basins, and these will be examined to assess how the lake ecosystem responded to bog-mat advances of the past (see related blog post and video here). Ultimately the datasets from the lake and peatland components of the ecosystem will be integrated and used to develop a predictive model of bog mat expansion and impacts, which will be used to anticipate potential future changes in these valuable ecosystems. A short video highlighting some of our activities during the past few weeks is below.
-rkb and awi-
A fun video highlighting some of the fieldwork:
Perhaps Northern Wisconsin is not a typical spring break destination…it certainly doesn’t make any top ten lists. However, for seven people from three universities (Lehigh University, University of Wisconsin, and University of Minnesota) it was our Miami Beach for the past week. We did not do any body surfing or swimming; in fact, we were glad that the abundant lakes in this region were well frozen. We did not do any ice fishing, skating, or skiing either. Nor did we go to sample cheese. We were there to travel back in time.
Specifically, we went to collect sediment cores from several lakes as part of a project aimed at understanding how aquatic and wetland ecosystems have responded to past climate changes. We are working in this region because there are abundant kettle depressions – low areas occupied by lakes and wetlands that were formed many thousands of years ago as glaciers retreated northward following the last major glaciation. These areas provide a range of ecological services, including wetland and aquatic habitat for biodiversity, sites for groundwater recharge, important roles in biogeochemical cycles (e.g., carbon storage), and areas for recreation. Lake chemistry and regional groundwater flow has also been well characterized in this region of northern Wisconsin, because it is part of the US Long Term Ecological Research Network (North Temperate Lakes LTER).
Kettlehole ecosystems are also ideal for the study of past ecological responses to climate variability. They have significant linkages to the broader landscape through groundwater flow, are hydrologically sensitive to changes in precipitation, and most importantly, they leave a detailed record of their own development in the form of peat and lake sediments. We are using the paleoecological record preserved in these ecosystems to determine how they have responded to climate changes of the past 10,000 years – and hopefully use this information to better anticipate how these and other ecological systems may change in the future.
Kettlehole basins are typically occupied by lakes, peatlands, or some combination these two habitat types. Lakes that are bordered by extensive peatland are quite different in water chemistry and biological communities than those that lack adjacent peatland, and systems characterized by peatland can serve as large carbon sinks (i.e., they take carbon out of the atmosphere and store it as peat) and are therefore important when considering regional carbon budgets. Data from other studies indicate that the establishment and expansion of peatland in kettlehole basins is triggered by moisture variability, particularly prolonged drought events, and peatland expansion can occur rapidly – probably within a decade or two. Threshold responses like this are difficult to anticipate or predict. Abrupt peatland expansion in these systems likely leads to rapid changes in carbon accumulation rates and alters lake-water chemistry and lake biota. However, dynamics, rates, and characteristics of these changes are not well understood.
Although this trip was focused on coring lakes, we will return this summer to collect cores from the peatland component of these basins. Below is a homemade video documentary showing the progress we made last week and the kind of work that is involved in the collection of lake sediments. Over 30 meters of lake sediment has now been transported back to the laboratory, where it will be analyzed to obtain information on past environmental changes. As you can see, spring break in Wisconsin was a lot of fun!
Video created using iMovie and a Canon Powershot D10 (waterproof!). Music credits: Winter Wonderland (Sonny Rollins), I walk the line (Johnny Cash), Nice work if you can get it (Thelonious Monk), Cold water (Tom Waits), Broke down engine (Bob Dylan), When the levee breaks (Led Zeppelin), Two little feet (Greg Brown), Nameless Banjo Riff (Pete Seeger), All work and no play (Van Morrison).