Katherine B. Lininger

Pleistocene Megafaunal Collapse, Novel Plant Communities, and Enhanced Fire Regimes in North America

Demise of the Megafauna Approximately 10,000 years ago, the Pleistocene-Holocene deglaciation in North America produced widespread biotic and environmental change, including extinctions of megafauna, reorganization of plant communities, and increased wildfire. The causal links and sequences of these changes remain unclear. Gill et al. (p. 1100; see the Perspective by Johnson) unravel these connections in an analysis of pollen, charcoal, and the dung fungus Sporormiella from the sediments of Appleman Lake, Indiana. The decline in Pleistocene megafaunal population densities (inferred from fungal spore abundances) preceded both the formation of the lateglacial plant communities and a shift to an enhanced fire regime, thus contradicting hypotheses that invoke habitat change or extraterrestrial impact to explain the megafaunal extinction. The data suggest that population collapse and functional extinction of the megafauna preceded their final extinction by several thousand years. The decline in Pleistocene megafauna led to the formation of novel plant communities and increased fire. Although the North American megafaunal extinctions and the formation of novel plant communities are well-known features of the last deglaciation, the causal relationships between these phenomena are unclear. Using the dung fungus Sporormiella and other paleoecological proxies from Appleman Lake, Indiana, and several New York sites, we established that the megafaunal decline closely preceded enhanced fire regimes and the development of plant communities that have no modern analogs. The loss of keystone megaherbivores may thus have altered ecosystem structure and function by the release of palatable hardwoods from herbivory pressure and by fuel accumulation. Megafaunal populations collapsed from 14,800 to 13,700 years ago, well before the final extinctions and during the Bølling-Allerød warm period. Human impacts remain plausible, but the decline predates Younger Dryas cooling and the extraterrestrial impact event proposed to have occurred 12,900 years ago.

Floodplain downed wood volumes: a comparison across three biomes

Downed large wood (LW) in floodplains provides habitat and nutrients for diverse organisms, influences hydraulics and sedimentation during overbank flows, and affects channel form and lateral migration. Very few studies, however, have quantified LW volumes in floodplains that are unaltered by human disturbance. We compare LW volumes in relatively unaltered floodplains of semiarid boreal lowland, subtropical lowland, and semiarid temperate mountain rivers in the United States. Average volumes of downed LW are 42.3 m3ha-1, 50.4 m3ha-1, and 116.3 m3ha-1 in the semiarid boreal, subtropical, and semiarid temperate sites, respectively. Observed patterns support the hypothesis that the largest downed LW volumes occur in the semiarid temperate mountain sites, which is likely linked to a combination of moderate-to-high net primary productivity, temperature-limited decomposition rates, and resulting slow wood turnover time. Floodplain LW volumes differ among vegetation types within the semiarid boreal and semiarid temperate mountain regions, reflecting differences in species composition. Lateral channel migration and flooding influence vegetation communities in the semiarid boreal sites, which in turn influences floodplain LW loads. Other forms of disturbance such as fires, insect infestations, and blowdowns can increase LW volumes in the semiarid boreal and semiarid temperate mountain sites, where rates of wood decay are relatively slow compared to the subtropical lowland sites. Although sediment is the largest floodplain carbon reservoir, floodplain LW stores substantial amounts of organic carbon and can influence floodplain sediment storage. In our study sites, floodplain LW volumes are lower than those in adjacent channels, but are higher than those in upland (i.e., non-floodplain) forests. Given the important ecological and physical effects of floodplain LW, efforts to add LW to river corridors as part of restoration activities, and the need to quantify carbon stocks within river corridors, we urge others to quantify floodplain and instream LW volumes in diverse environments. This article is protected by copyright. All rights reserved.

River beads as a conceptual framework for building carbon storage and resilience to extreme climate events into river management

River beads refer to retention zones within a river network that typically occur within wider, lower gradient segments of the river valley. In lowland, floodplain rivers that have been channelized and leveed, beads can also be segments of the river in which engineering has not reduced lateral channel mobility and channel-floodplain connectivity. Decades of channel engineering and flow regulation have reduced the spatial heterogeneity and associated ecosystem functions of beads occurring throughout river networks from headwaters to large, lowland rivers. We discuss the processes that create and maintain spatial heterogeneity within river beads, including examples of beads along mountain streams of the Southern Rockies in which large wood and beaver dams are primary drivers of heterogeneity. We illustrate how spatial heterogeneity of channels and floodplains within beads facilitates storage of organic carbon; retention of water, solutes, sediment, and particulate organic matter; nutrient uptake; biomass and biodiversity; and resilience to disturbance. We conclude by discussing the implications of river beads for understanding solute and particulate organic matter dynamics within river networks and the implications for river management. We also highlight gaps in current understanding of river form and function related to river beads. River beads provide an example of how geomorphic understanding of river corridor form and process can be used to restore retention and resilience within human-altered river networks.

Geomorphic Controls on Floodplain Soil Organic Carbon in the Yukon Flats, Interior Alaska, From Reach to River Basin Scales

Floodplains accumulate and store organic carbon (OC) and release OC to rivers, but studies of floodplain soil OC come from small rivers or small spatial extents on larger rivers in temperate latitudes. Warming climate is causing substantial change in geomorphic process and OC fluxes in high latitude rivers. We investigate geomorphic controls on floodplain soil OC concentrations in active-layer mineral sediment in the Yukon Flats, interior Alaska. We characterize OC along the Yukon River and four tributaries in relation to geomorphic controls at the river basin, segment, and reach scales. Average OC concentration within floodplain soil is 2.8% (median 5 2.2%). Statistical analyses indicate that OC varies among river basins, among planform types along a river depending on the geomorphic unit, and among geomorphic units. OC decreases with sample depth, suggesting that most OC accumulates via autochthonous inputs from floodplain vegetation. Floodplain and river characteristics, such as grain size, soil moisture, planform, migration rate, and riverine DOC concentrations, likely influence differences among rivers. Grain size, soil moisture, and age of surface likely influence differences among geomorphic units. Mean OC concentrations vary more among geomorphic units (wetlands 5 5.1% versus bars 5 2.0%) than among study rivers (Dall River 5 3.8% versus Teedrinjik River 5 2.3%), suggesting that reach-scale geomorphic processes more strongly control the spatial distribution of OC than basin-scale processes. Investigating differences at the basin and reach scale is necessary to accurately assess the amount and distribution of floodplain soil OC, as well as the geomorphic controls on OC. Plain Language Summary Rivers transport organic carbon (OC) from the landscape to the ocean, but that carbon is deposited along the way in floodplains and remains there for varying lengths of time. River processes also create bare sediment surfaces on which carbon can accumulate, and that carbon can then be eroded by the river and transported downstream. Assessing the physical controls on floodplain soil carbon is important for understanding how carbon is transported from the landscape to the ocean and for determining the spatial pattern of carbon on the landscape. Soil carbon is particularly important in arctic and boreal regions, where climate change is modifying permafrost (perennially frozen soil) and releasing previously frozen carbon to the atmosphere. The hydrology and the amount of nutrients delivered to the Arctic Ocean by rivers are also affected by climate change, and floodplains are mediators of water and sediment fluxes. We look at OC concentrations between different floodplains and between different geomorphic (physical) environments in the Yukon Flats region in interior Alaska, an area with discontinuous permafrost. Our results indicate that OC varies among river basins and among geomorphic environments, highlighting the need to assess OC on different scales.