They are bug-infested wastelands. Wet and soggy places unfit for agricultural crops. Areas that should be made “useful” by drainage. Or at least those were the prevailing attitudes before the value of wetlands became widely recognized. In fact, government policies actively promoted the drainage of wetlands in the 1800s and much of the 1900s, with various incentive-based programs aimed at “reclaiming” swamps and other “overflowed” lands. Public funding was also provided for drainage activities. However, wetlands are now universally recognized as valuable providers of ecosystem services, playing critical roles in water purification, flood control, storm protection, nutrient removal from agricultural runoff, carbon storage, fishery support, and providing habitat for rare plants and animals. Thus, today many conservation agencies are actively working to identify, manage, protect, and restore wetlands. Many of these efforts are focused on the protection of systems that have been little impacted by human activities, or restoring degraded wetlands to a more natural state. Therefore, knowledge of how humans have impacted our remaining wetlands is critical to successful protection and restoration.
Drainage. Ditching. Filling. Extracting peat. These are some of the more obvious activities that damage or destroy these ecosystems. However, human activities also have indirect effects, such as ecological changes brought on by invasive species, changes in the acidity or chemistry of surface waters, changes in water levels due to groundwater use, and climate change. However, for some wetlands, there may be another indirect effect that has not been fully considered. Dust. Microscopic dust. Could something so small really have a big impact? We recently addressed this question by studying a bog in western Pennsylvania.
Dust, deforestation, and bogs
European settlers logged the vast majority of eastern North America about 100 to 150 years ago, substantially altering terrestrial ecosystems and the regional landscape. In many regions, widespread agriculture was established shortly after logging. Collectively, these activities increased soil-dust movement. Mobilization of dust would have occurred as cultivated fields replaced forests, and the treeless landscape would have allowed dust to move greater distances. This dust landed on adjacent ecosystems, including “wastelands” where agriculture was not possible…those soggy, bug-infested wetlands.
Bogs – a unique type of wetland characterized by very low nutrients – would be expected to be more sensitive to dust deposition than other wetland types. Why? Because the organisms that occur in bogs are adapted to low-nutrient availability. Carnivorous plants, which supplement their nutrient uptake by capturing small insects, are commonly found in bogs, and virtually all plants in bogs have some adaptation that allows them to survive in the nutrient-limited environment. Soil dust contains nitrogen, phosphorus, and other elements, so increasing dust deposition might actually fertilize the bog. You might think that this would be a good thing, as more fertilizer on your garden clearly makes for happier plants. However, on a bog it has the potential to change the outcome of competition among plants, alter microbial communities, and change the rates of important processes like decomposition and primary production. Changes in the relative rates of these processes would alter rates of peat accumulation – a fundamental property of these wetland ecosystems. But is the fertilization effect of dust enough to do these things? Can increased dust deposition cause the composition of bog plant communities to change? How about microbial communities? Is the impact of dust fertilization large enough to alter the relative rates of decomposition and primary production? Can enhanced dust deposition fundamentally change the bog ecosystem, including the ecosystem services it provides?
The ecology of dust?
Of course, in most regions of eastern North America, widespread deforestation occurred over a century ago, so to address our questions a retrospective approach was needed. Luckily, bogs preserve a record of past dust deposition and environmental conditions in the form of peat, which gradually accumulates in these environments. The acidic and oxygen-depleted environment within the peat is well-suited for the long-term preservation of plants and other organisms that occurred within and around the bog in the past. We carefully examined the paleoecological record from Titus Bog, a protected wetland in western Pennsylvania, to assess whether dust deposition increased at the time of deforestation, and if so, how this affected the bog. We collected a series of peat cores from the wetland, and used the information contained in these cores to reconstruct how the ecology of the bog has changed over the past several hundred years (Ireland & Booth 2012).
Our results revealed that before European settlement, Titus Bog was a typical acidic bog, dominated by mosses that thrive in low-nutrient conditions. However, a layer of mineral dust marks the onset of big ecological changes in the peat cores. This mineral-rich layer contains pollen from agricultural weeds that expanded rapidly as Europeans converted forests to fields, allowing us to confidently link the increased dust deposition with human land clearance. Measurements of nutrient content of the peat revealed that the dust fertilized the surface of Titus Bog, and the increased nutrient availability led to changes in plant communities. In particular, it allowed woody plants to out-compete the mosses, shifting the relative abundance of these plant groups. As the plant communities were changing in response to increased nutrients, the microscopic organisms living on the wetland surface also changed, indicating that the dust deposition led to changes in multiple trophic levels. These changes in plant and microbial communities were also associated with increases in rates of decomposition, which may have altered the rate that the system performed one important ecosystem service – the sequestration of carbon dioxide from the atmosphere.
Interestingly, the wetland that exists today is fundamentally different from the one that was present just a few hundred years ago, although the reestablishment of a thin forest buffer may be helping the system slowly recover. Our results highlight the importance of forest buffers, particularly upwind of bog environments, in the management of these systems. Things that are small and easy to overlook, like dust, can have big impacts. In the case bogs, successfully protection really may need to consider what is blowin’ in the wind.
-rkb & awi-