Linking topographic, hydrologic, and bioegeochemical change in human dominated landscapes

Abstract

<p>To satisfy a growing population, much of Earth’s surface has been designed to suit humanity’s needs. Although these ecosystem designs have improved human welfare, they have also produced significant negative environmental impacts, which applied ecology as a field has attempted to address and solve. Many of the failures in applied ecology to achieve this goal of reducing neg- ative environmental impacts are design failures, not failures in the science. Here, we review (a) how humans have designed much of Earth’s surface, (b) the history of design ideas in ecology and the philosophical and practical critiques of these ideas, (c) design as a conceptual process, (d) how changing approaches and goals in subfields of applied ecology reflect changes and failures in design, and (e) why it is important not only for ecologists to en- courage design fields to incorporate ecology into their practice but also for design to be more thoroughly incorporated into ours.</p><p>One of the most heavily altered and designed ecosystems in the world is the mountaintop mines of Central Appalachia. Mountaintop mining is the most common form of coal mining in the Central Appalachian ecoregion. Previous estimates suggest that active, reclaimed, or abandoned mountaintop mines cover ∼7% of Central Appalachia. While this is double the areal extent of development in the ecoregion (estimated to occupy <3% of the land area), the impacts are far more extensive than areal estimates alone can convey as the impacts of mines extend 10s to 100s of meters below the current land surface. Here, we provide the first estimates for the total volumetric and topographic disturbance associated with mining in an 11 500 km2 region of southern West Virginia. We find that the cutting of ridges and filling of valleys has lowered the median slope of mined landscapes in the region by nearly 10 degrees while increasing their average elevation by 3 m as a result of expansive valley filling. We estimate that in southern West Virginia, more than 6.4km3 of bedrock has been broken apart and deposited into 1544 headwater valley fills. We used NPDES monitoring datatsets available for 91 of these valley fills to explore whether fill characteristics could explain variation in the pH or selenium concentrations reported for streams draining these fills. We found that the volume of overburden in individual valley fills correlates with stream pH and selenium concentration, and suggest that a three-dimensional assessment of mountaintop mining impacts is necessary to predict both the severity and the longevity of the resulting environmental impacts. </p><p>Chemical weathering of bedrock is the ultimate source of solutes for all ecosystems, a geologic sink of C, and controls the rate at which mountains dissolve into the sea. Human activities bring large volumes of bedrock to the surface and enhance global weathering rates. Here, we show watersheds impacted by mountaintop mining for coal have among the highest rates of chemical weathering ever reported. Mined watersheds deliver nearly 9,000 kg ha-1 y-1 of dissolved ions downstream. This translates into a chemical weathering rate ~ 330 mm ky-1, which is 55-times higher than background total (chemical and physical) weathering. These exceptionally high dissolution rates result from the production of sulfuric acid by pyrite oxidation. As this strong acid rapidly weathers surrounding carbonate materials, it not only releases large amounts of dissolved solutes, it also liberates 10-50 g of rock-derived C m-2 yr-1. This shifts mined watersheds from net geologic carbon sinks to net geologic carbon sources, further adding to the carbon costs from burning coal and deforesting these landscapes.</p><p>The impact from mining will likely last decades for some aspects of recovery and centuries to millennia for others. To examine the paired forest, hydrologic, and biogeochemical changes from mining we used a combination of remote sensing and watershed monitoring. We show that forest recovery on mines is at least twice as slow as typical forest recovery from clearcutting, and that mined areas have persistent low canopy height gaps. These vegetative changes are coupled with decreases in runoff ratios as mines age and water moves through flatter, vegetated landscapes. However, the vegetation change is uncoupled from biogeochemical processes, with strong alkaline mine drainage signals persisting for decades, even as vegetation recovers.</p>