Browsing by Subject "Succession"

Rivers interact with their valleys from headwaters to mouth, but nowhere as dynamically as in their floodplains. Rivers deliver water, sediments, and solutes onto the floodplain land surface, and the land in turn supplies solutes, leaves, and woody debris to the channel. These reciprocal exchanges maintain both aquatic and terrestrial biodiversity and productivity. In this dissertation I examine river-floodplain exchanges on the well-studied Nyack Floodplain, a dynamic, gravel-bedded floodplain along the Middle Fork Flathead River in the mountains of northwest Montana. I quantify exchanges at multiple timescales, from moments to centuries, to better understand how connectivity between aquatic and terrestrial habitats shapes their ecology.

I first address connectivity in the context of a long-standing question in ecosystem ecology: What determines the rate of ecosystem development during primary succession? Rivers have an immediate effect on floodplains when scouring floods remove vegetation and nutrients such as nitrogen (N) and leave only barren soils, but they might also affect the ensuing primary succession through the gradual delivery of N and other materials to floodplain soils. I quantify N inputs to successional floodplain forest soils of the Nyack Floodplain and find that sediment deposition by river flood water is the dominant source of N to soils, with lesser contributions from dissolved N in the river, biological N fixation, and atmospheric deposition. I also synthesize published rates of soil N accumulation in floodplain and non-floodplain primary-successional systems around the world, and I find that western floodplains often accumulate soil N faster than non-floodplain primary successional systems. My results collectively point to the importance of riverine N inputs in accelerating ecosystem development during floodplain primary succession.

I next investigate the role of river-floodplain exchanges in shaping the spatial distribution of a suite of soil properties. Even after flood waters have receded, dissolved N, carbon (C), and moisture could be delivered from the river to floodplain soils via belowground water flow. Alternatively, C inputs and N withdrawals by floodplain vegetation might be a dominant influence on soil properties. To test these hypotheses, I excavated and sampled soil pits from the soil surface to the water table (50-270 cm) under forests, meadows, and gravel bars of the Nyack Floodplain. Near-surface soils had C and N pools and N flux rates that varied predictably with vegetation cover, but soil properties below ~50 cm reflected influence by neither vegetation cover nor aquifer delivery. Instead, soil properties at these depths appear to relate to soil texture, which in turn is structured by the river's erosional and depositional activities. This finding suggests the revised hypothesis that soil properties in gravel-bedded alluvial floodplains may depend more on the decadal-scale geomorphic influences of floods than on short-term vertical interactions with floodplain vegetation or aquifer water.

Lastly, I explore the potential sources of organic C to the diverse and active community of aquatic organisms in the floodplain aquifer, where the lack of light prohibits in-situ organic C production by photosynthesis. I quantify floodplain carbon pools and the fluxes of organic carbon connecting the aquifer, river, and overlying forest. Spring flood waters infiltrating the soil are responsible for the largest dissolved carbon flux into the aquifer, while very large floods are essential for the other major C input, the burial of woody carbon in the aquifer. These findings emphasize the importance of a dynamic river hydrograph - in particular, annual floods and extreme annual floods - in delivering organic C to the aquifer community.

Overall, this dissertation draws our attention not just to the current exchanges of C, N, water, and sediment but to the episodic nature of those exchanges. To fully understand floodplain ecosystems, we have to consider not just present-day interactions but also the legacies of past floods and their roles in delivering solutes, eroding forests, depositing sediments, and physically shaping the floodplain environment.

The mycorrhizal mutualism is one of the earliest and most influential of all terrestrial symbioses. As the primary method used by most plants to acquire nutrients from the soil, mycorrhizal fungi help to shape the structure and composition of many ecosystems. Ectomycorrhizal (EM) fungi play an especially significant role because most EM fungi prefer a limited number of host taxa, and EM plant species likewise associate with only a fraction of the available EM fungi. This host preference issue, combined with the high diversity of EM fungi in forest ecosystems, complicates interspecies competition both among fungi and among plants, because these plant and fungal communities interact.

Despite recent attempts at documenting mycorrhizal fungi in the context of ecological succession, many questions remain about the underlying causal relationships among EM fungi, soil conditions, and plant community assembly. The succession of mycorrhizal fungi often mirrors the succession of plants, and ectomycorrhizal (EM) community composition may affect the outcome of competition among trees during succession. In a pine-oak seral system, we tested the ability of Pinus taeda and Quercus alba seedlings to associate with EM fungi when planted under both conspecific and heterospecific adults. We found that EM communities under pine and oak canopy were distinct regardless of seedling identity, indicating that the fungal associations of adult trees determine which EM species are available in the soil. In addition, pine seedlings planted under oak canopy showed decreased mycorrhization and growth compared to those planted under pine canopy, while oak seedlings showed no negative effects of heterospecific planting. This impaired ability of pine seedlings to associate with the EM community established under oaks may deter pine recruitment and facilitate the late-seral replacement of pines with oaks.

While EM fungal communities correlate with the dominant species of host tree, soil properties do as well, making it difficult to establish causality among these three variables. Soil was collected from oak- and pine-dominated stands and dried to kill off mature mycelium, leaving only the spore bank as a source of inoculum for pine and oak seedlings. EM root tips were collected for molecular identification of fungal species based on ITS barcoding, and soil samples from field and laboratory conditions were analyzed for fungal diversity using 454 sequencing. We found a reduced influence of canopy type and a more pronounced influence of seedling identity when compared to the EM communities on seedlings planted in the field, suggesting that adult trees do alter the availability of fungi by directly promoting the growth of their preferred EM associates.

The availability of EM fungi can also affect seedlings at the interface between EM- an AM-dominated forest. We tested the hypothesis that seedlings of Dicymbe corymbosa which recruit outside of monodominant stands have limited access to EM symbionts compared with those which recruit inside D. corymbosa stands. EM root tips and rhizosphere soil were collected from seedlings along two transects inside monodominant stands and three transects in the transition zone into mixed forest dominated by AM associates. Seedlings inside monodominant stands yielded both a greater quantity of mycorrhized root tips and a higher diversity of EM species than transition zone seedlings. Of the fungal families commonly found on adult roots, the Boletaceae were notably underrepresented on all seedlings. In the transition zones, high-throughput sequencing of soil also detected a decrease in EM diversity with distance from the parent tree.

Seedlings of D. corymbosa may benefit from recruiting within monodominant stands by tapping into common mycorrhizal networks (CMNs) to acquire low-cost nitrogen and, potentially, photosynthates produced by conspecific adults. Leaves of stand adults, stand seedlings, and mixed-forest seedlings were collected for stable isotope analysis to track the transfer of nitrogen and carbon through CMNs. The δ13C and δ15N results contradicted each other, suggesting that more complicated interactions may be playing out among adults, seedlings, and fungi.

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.

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.

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.

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.

Pastoral succession is a necessary topic for non-denominational churches. According to the Barna research group, clergy are aging, candidate pools are shrinking and the North American Church as a whole is rapidly approaching a mass pastoral succession. One of the primary issues, however, is that there are not many models that are readily available for leaders to follow to transition well, meaning there is no plan in place before the actual transition occurs. Although transition may be difficult, it is in fact inevitable since one leader cannot stay in position forever. One of the more pressing issues facing our congregations is not the ability to address the what, but the failure to implement the how and when. The objective of this research is to convey the need for succession specifically in independent churches, encourage fellow pastors to think “with” the biblical narratives that highlight leadership transitions and consider what happens when these stories are read in light of contemporary questions about pastoral leadership and transitions. Finally, the goal is to help leaders and their congregations to see transition as an intentional, ongoing process instead of a one-time event and to provide the necessary tools to begin implementing the process of transition. The key ingredients of a healthy pastoral transition involve locating someone chosen by God and affirmed by the predecessor, who earns the trust of the congregation and leads with confidence.