Troy D. Hill

Soil warming, carbon–nitrogen interactions, and forest carbon budgets

Soil warming has the potential to alter both soil and plant processes that affect carbon storage in forest ecosystems. We have quantified these effects in a large, long-term (7-y) soil-warming study in a deciduous forest in New England. Soil warming has resulted in carbon losses from the soil and stimulated carbon gains in the woody tissue of trees. The warming-enhanced decay of soil organic matter also released enough additional inorganic nitrogen into the soil solution to support the observed increases in plant carbon storage. Although soil warming has resulted in a cumulative net loss of carbon from a New England forest relative to a control area over the 7-y study, the annual net losses generally decreased over time as plant carbon storage increased. In the seventh year, warming-induced soil carbon losses were almost totally compensated for by plant carbon gains in response to warming. We attribute the plant gains primarily to warming-induced increases in nitrogen availability. This study underscores the importance of incorporating carbon–nitrogen interactions in atmosphere–ocean–land earth system models to accurately simulate land feedbacks to the climate system.

Fertilization Effects on Elevation Change and Belowground Carbon Balance in a Long Island Sound Tidal Marsh

We report the results of a 5-year fertilization experiment in a central Long Island Sound salt marsh, aimed at understanding the impacts of high nutrient loads on marsh elevational processes. Fertilization with nitrogen led to some significant changes in marsh processes, specifically increases in aboveground primary production and in CO2 fluxes from the soil. However, neither nitrogen nor phosphorus fertilization led to elevation loss (relative to controls), reduced soil carbon, or a decrease in belowground primary production, all of which have been proposed as links between elevated nutrient loads and marsh drowning. Our data suggest that high nutrient levels increase gross carbon loss from the sediment, but that this is compensated for by other processes, leading to no net deleterious effect of nutrient loading on carbon storage or on marsh stability with respect to sea level rise.

Suburbanization and drought: A mixed methods vulnerability assessment in rainy Massachusetts

Abstract This paper presents evidence that water restrictions in suburbanizing eastern Massachusetts towns are becoming more common, controlling for climate. We then assess the relationship between these suburban droughts and residential development. Focusing on the suburbs of Boston, seven towns independent of the Boston water supply system were selected to represent differing levels of sprawl-style growth. Water restrictions are becoming more frequent in all of the towns studied, and models demonstrate that restrictions are increasing in duration, independent of climate. Interviews suggest that residential development is playing a central role in this increasing sensitivity to suburban drought, though other factors are also important. Long-term planning and integration of land-use planning and water management emerged as two key paths for attenuating the impacts of development.

Unusually large tsunamis frequent a currently creeping part of the Aleutian megathrust

Current models used to assess earthquake and tsunami hazards are inadequate where creep dominates a subduction megathrust. Here we report geological evidence for large tsunamis, occurring on average every 300–340 years, near the source areas of the 1946 and 1957 Aleutian tsunamis. These areas bookend a postulated seismic gap over 200 km long where modern geodetic measurements indicate that the megathrust is currently creeping. At Sedanka Island, evidence for large tsunamis includes six sand sheets that blanket a lowland facing the Pacific Ocean, rise to 15 m above mean sea level, contain marine diatoms, cap terraces, adjoin evidence for scour, and date from the past 1700 years. The youngest sheet and modern drift logs found as far as 800 m inland and >18 m elevation likely record the 1957 tsunami. Previously unrecognized tsunami sources coexist with a presently creeping megathrust along this part of the Aleutian Subduction Zone.

Relative Sea-Level Trends in New York City During the Past 1500 Years

New York City (NYC) is threatened by 21st-century relative sea-level (RSL) rise because it will experience a trend that exceeds the global mean and has high concentrations of low-lying infrastructure and socioeconomic activity. To provide a long-term context for anticipated trends, we reconstructed RSL change during the past ~1500 years using a core of salt-marsh sediment from Pelham Bay in The Bronx. Foraminifera and bulk-sediment δ13C values were used as sea-level indicators. The history of sediment accumulation was established by radiocarbon dating and recognition of pollution and land-use trends of known age in down-core elemental, isotopic, and pollen profiles. The reconstruction was generated within a Bayesian hierarchical model to accommodate multiple proxies and to provide a unified statistical framework for quantifying uncertainty. We show that RSL in NYC rose by ~1.70 m since ~575 CE (including ~0.38 m since 1850 CE). The rate of RSL rise increased markedly at 1812–1913 CE from ~1.0 to ~2.5 mm/yr, which coincides with other reconstructions along the US Atlantic coast. We investigated the possible influence of tidal-range change in Long Island Sound on our reconstruction using a regional tidal model, and we demonstrate that this effect was likely small. However, future tidal-range change could exacerbate the impacts of RSL rise in communities bordering Long Island Sound. The current rate of RSL rise is the fastest that NYC has experienced for >1500 years, and its ongoing acceleration suggests that projections of 21st-century local RSL rise will be realized.

Submergence and Herbivory as Divergent Causes of Marsh Loss in Long Island Sound

Tidal marsh degradation has been attributed to a number of different causes, but few studies have examined multiple potential factors at the same sites. Differentiating the diverse drivers of marsh loss is critical to prescribing successful interventions for conservation and restoration of this important habitat. We evaluated two hypotheses for vegetation loss at two marshes in Long Island Sound (LIS): (1) marsh submergence, caused by an imbalance between sea-level rise and marsh accretion, and (2) defoliation associated with herbivory by the purple marsh crab, Sesarma reticulatum. At our western LIS site, we found no evidence of herbivory: crabs were scarce, and crab-exclusion cages provided no benefit. We attribute degradation at that site to submergence, a conclusion supported by topographic and hydrologic data showing that loss of vegetation occurred only in wetter parts of the marsh. In contrast, at our central LIS site, our observations were consistent with herbivory as a driving force: There were substantial populations of Sesarma, crab-exclusion cages allowed plants to thrive, and vegetation loss took place across a variety of elevations. We also analyzed soil conditions at both sites, in order to determine the signatures of different degradation processes and assess the potential for restoration. At the submergence site, unvegetated soils exhibited high bulk density, low organic content, and low soil strength, posing significant biogeochemical challenges to re-colonization by vegetation. At the herbivory site, unvegetated soils had a characteristic “riddled-peat” appearance, resulting from expansion and erosion of Sesarma burrow networks. The high redox potential and organic content of those soils suggested that revegetation at the herbivory site would be likely if Sesarma populations could be controlled before erosion leads to elevation loss.

Effects of seasonality and environmental gradients on Spartina alterniflora allometry and primary production

Abstract Predictions of how salt marsh primary production and carbon storage will respond to environmental change can be improved through detailed datasets documenting responses to real‐world environmental variation. To address a shortage of detailed studies of natural variation, we examined drivers of Spartina alterniflora stem allometry and productivity in seven marshes across three regions in southern Louisiana. Live‐stem allometry varied spatially and seasonally, generally with short stems weighing more (and tall stems weighing less) in the summer and fall, differences that persist even after correcting for flowering. Strong predictive relationships exist between allometry parameters representing emergent stem mass and mass accumulation rates, suggesting that S. alterniflora populations navigate a trade‐off between larger mass at emergence and faster rates of biomass accumulation. Aboveground production and belowground production were calculated using five and four approaches, respectively. End‐of‐season aboveground biomass was a poor proxy for increment‐based production measures. Aboveground production (Smalley) ranged from 390 to 3,350 g m−2 year−1 across all marshes and years. Belowground production (max–min) was on average three times higher than aboveground; total production ranged from 1,400 to 8,500 g m−2 year−1. Above‐ and belowground production were both positively correlated with dissolved nutrient concentrations and negatively correlated to salinity. Synthesis: Interannual variation in water quality is sufficient to drive above‐ and belowground productivity. The positive relationship between nutrients and belowground production indicates that inputs of nutrients and freshwater may increase salt marsh carbon storage and ecosystem resilience to sea level rise.

Nitrogen and carbon concentrations and stable isotope ratios: Data from a 15N tracer study in short-form Spartina alterniflora and Distichlis spicata

We present four datasets that provide information on primary production, nitrogen (N) uptake and allocation in two salt marsh grasses, short-form Spartina alterniflora and Distichlis spicata. These four datasets were generated during a month-long stable isotope (15N) tracer study described in the companion manuscript (Hill et al., 2018). They include an allometry dataset containing mass and height data for individual plants harvested from Colt State Park, Bristol, Rhode Island and used to nondestructively estimate plant masses. A second dataset contains weekly stem height measurements collected over the course of the 15N tracer study. Also included are high resolution data from 49 vegetated compartments (leaves, stems, fine/coarse roots, rhizomes) and bulk sediment depth intervals, reporting the mass, carbon and N concentrations, and stable isotope ratios measured following the harvest of cores over time. Additionally, we provide a complementary dataset with estimates of microbial removal from potential and ambient denitrification enzyme assays. These data, along with source code used in their analysis, are compiled in the NitrogenUptake2016 R package available from the Comprehensive R Archive Network.