Core Ideas
Soils in the northeastern United States show signs of recovery from acidification between 2001‐2002 and 2014.
Variation in soil chemistry at the landscape and plot‐scales was similar, ...informing study design.
Soils influenced by shallow groundwater were more dynamic than well‐drained soils.
Soils are subject to a variety of stressors including human land use, air pollution and climate change. A challenge for detecting temporal change is disentangling heterogeneity at multiple spatial scales. Forty permanent plots were sampled across the US White Mountain National Forest (WMNF) in 2001 or 2002 and resampled in 2014. Paired t tests detected significant increases in carbon and base cations concentrations and a decrease in Al in the Oa horizon while base cations decreased and Al increased in some mineral horizons. A subset of six plots were intensively resampled in 2015. Pooled variances were calculated using all the six intensively sampled plots from 2014 to 2015. Within‐site variability was comparable to overall variability across the WMNF. When study sites were stratified into hydrologic groups, we found a strong signal in the Oa horizon of increasing carbon and base cation concentrations from 2001–2002 to 2014, suggesting that soils influenced by shallow groundwater contributions from upslope may be more responsive to acidification recovery than soils influenced only by vertical percolation. The initial study design did not consider the role of hydrologic pathways in susceptibility of soils to temporal change and did not include enough plots in each hydrologic group to maximize the power of this stratification approach. However, these results illustrate the potential for hydrologic stratification to improve change detection and interpretation in forest soil monitoring programs. The combined approach to hydrologic stratification and estimating variance components simultaneously at the landscape and within‐plot scales is crucial for calculating sample size needed to detect temporal change.
Case studies from the University of New Hampshire explore all the dimensions of sustainability in campus life, combining frugality and creativity
University communities have the potential to serve as ...models in the development and application of sustainability principles and practices, not only by what they teach and study, but also by how they operate facilities and engage with off-campus partners. With the oldest endowed campus-wide sustainability program in the country, established in 1997, the University of New Hampshire has become a leader in advancing a campus culture of sustainability. The UNH experience provides a unique window into the development of a new and integrated approach to teaching, learning, research, and operations. It is also a valuable guide for other institutions that aim to enhance the quality of campus life while reducing their environmental footprint. The book’s organization along four functional domains (curriculum, operations, research, and engagement) allows faculty, staff, students, and managers to focus on sections of particular relevance to their university roles. Each chapter develops standards of best practices and presents interesting case studies to humanize the larger effort.
Using a dam removal on the Ashuelot River in southern New Hampshire, we test how a sudden, spatially non-uniform increase in river slope alters sediment transport dynamics and riparian sediment ...connectivity. Site conditions were characterized by detailed pre- and post-removal field surveys and high-resolution aerial lidar data, and locations of erosion and deposition were predicted through one-dimensional hydrodynamic modeling. The Homestead Dam was a ~200year old, 4m high, 50m wide crib dam that created a 9.5km long, relatively narrow reservoir. Following removal, an exhumed resistant bed feature of glaciofluvial boulders located 400m upstream and ~2.5m lower than the crest of the dam imposed a new boundary condition in the drained reservoir, acting as a grade control that maintained a backwater effect upstream. During the 15months following removal, non-uniform erosion in the former reservoir totaled ~60,000m3 (equivalent to ~9.3cm when averaged across the reservoir). Net deposition of ~10,700m3 was measured downstream of the dam, indicating most sediment from the reservoir was carried more than 8km downstream beyond the study area. The most pronounced bed erosion occurred where modeled sediment transport increased in the downstream direction, and deposition occurred both within and downstream of the former reservoir where modeled sediment transport decreased in the downstream direction. We thus demonstrate that spatial gradients in sediment transport can be used to predict locations of erosion and deposition on the stream bed. We further observed that bed incision was not a necessary condition for bank erosion in the former reservoir. In this characteristically narrow and shallow reservoir lacking abundant dam-induced sedimentation, the variable resistance of the bed and banks acted as geomorphic constraints. Overall, the response deviated from the common conceptual model of knickpoint erosion and channel widening due to dam removal. With thousands of dams likely to be considered for removal or repair in the coming decades, this study helps to advance predictions of the geomorphic response to dam removal and contributes to a broader understanding of the variability in both style and timing of fluvial responses to disturbances.
•We examine physical controls on channel response to dam removal.•Channel response reflects spatial gradients and thresholds in sediment transport (Qs).•Channel response deviates from common conceptual models of dam removal.•Qs gradients are often overlooked, but important in predicting erosion and deposition.
Globally, phytoplankton abundance is increasing in lakes as a result of climate change and land‐use change. The relative importance of climate and land‐use drivers has been examined primarily for ...mesotrophic and eutrophic lakes. However, oligotrophic lakes show different sensitivity to climate and land‐use drivers than mesotrophic and eutrophic lakes, necessitating further exploration of the relative contribution of the two drivers of change to increased phytoplankton abundance. Here, we investigated how air temperature (a driver related to climate change) and nutrient load (a driver related to land‐use and climate change) interact to alter water quality in oligotrophic Lake Sunapee, New Hampshire, USA. We used long‐term data and the one‐dimensional hydrodynamic General Lake Model (GLM) coupled with Aquatic EcoDyanmics (AED) modules to simulate water quality. Over the 31‐year simulation, summer median chlorophyll‐a concentration was positively associated with summer air temperature, whereas annual maximum chlorophyll‐a concentration was positively associated with the previous 3 years of external phosphorus load. Scenario testing demonstrated a 2°C increase in air temperature significantly increased summer median chlorophyll‐a concentration, but not annual maximum chlorophyll‐a concentration. For both maximum and median chlorophyll‐a concentration, doubling external nutrient loads of total nitrogen and total phosphorus at the same time, or doubling phosphorus alone, resulted in a significant increase. This study highlights the importance of aligning lake measurements with the ecosystem metrics of interest, as maximum chlorophyll‐a concentration may be more uniquely sensitive to nutrient load and that typical summer chlorophyll‐a concentration may increase due to warming alone.
Plain Language Summary
Clear water lakes are experiencing more frequent water quality problems due to land development and climate change. However, it is challenging to identify how land development and climate change interact to alter water quality because their effects are complex and occurring at the same time. We used three decades of observational data combined with a lake ecosystem simulation model to explore the role of land development and climate change on water quality. Our water quality indicator of focus was phytoplankton, which are small photosynthesizing organisms in the water, often referred to as “algae.” We found that the effects of land use and climate depend on if we look at yearly maximum or average phytoplankton concentrations. Average phytoplankton concentrations during the summer (representing typical summer conditions) increase with either warmer air temperatures or higher nutrient pollution. However, annual maximum phytoplankton concentration (representing phytoplankton “blooms”) only increases with higher nutrient pollution. Typical summer phytoplankton concentrations will likely increase with warmer air temperatures due to climate change alone and increase even further when combined with higher nutrient pollution. To maintain clear water lakes, nutrient pollution should be reduced even more than previously thought to compensate for increasing phytoplankton in a warmer climate.
Key Points
Simulated annual maximum and summer median lake chlorophyll‐a are positively associated with external phosphorus load and air temperature, respectively
A 2°C rise in simulated air temperature significantly increased summer median, but not annual maximum, chlorophyll‐a
Annual maximum chlorophyll‐a may be more informative than median summer chlorophyll‐a to assess nutrient load effects on water quality
This study examines the impact of variation in root‐zone soil moisture (RZSM), a key component of the Earth's hydrologic cycle and climate system, on regional carbon fluxes across seven North ...American ecosystems. P‐band synthetic aperture radar‐derived RZSM estimates were incorporated into the ecosystem demography (ED2) terrestrial biosphere model through a model‐data blending approach. Analysis shows that the model qualitatively captures inter‐daily and seasonal variability of observed RZSM at seven flux tower sites (r = 0.59 ± 0.26 and r = 0.70 ± 0.22 for 0–10 and 10–40 cm of soil layers, respectively; P < 0.001). Incorporating the remotely sensed RSZM estimates increases the accuracy (root‐mean‐square deviations decrease from 0.10 ± 0.07 and 0.09 ± 0.06 m3·m−3 to 0.08 ± 0.05 and 0.07 ± 0.03 m3 ·m−3 for 0–10 and 10–40 cm of soil layers, respectively) of the model's RZSM predictions. The regional carbon fluxes predicted by the native and RZSM‐constrained model were used to quantify sensitivities of gross primary productivity, autotrophic respiration (Ra), heterotrophic respiration (Rh), and net ecosystem exchange to variation in RZSM. Gross primary productivity exhibited the largest sensitivity (6.6 ± 10.7 kg·cm−2·year·θ−1) followed by Ra (2.9 ± 7.3 kg·cm−2·year−1·θ−1), Rh (2.6 ± 3.1 kg·cm−2·year−1·θ−1), and net ecosystem exchange (−1.7 ± 7.8 kg·cm−2·year−1·θ−1). Analysis shows that these carbon flux sensitivities varied considerably across regions, reflecting influences of canopy structure, soil properties, and the ecophysiological properties of different plant functional types. This study highlights (1) the importance of improved terrestrial biosphere model predictions of RZSM to improve predictions of terrestrial carbon fluxes, (2) a need for improved pedotransfer functions, and (3) improved understanding of how soil characteristics, climate, and vegetation composition interact to govern the responses of different ecosystems to changing hydrological conditions.
Key Points
The sensitivity of ecosystem carbon fluxes to root‐zone soil moisture varies markedly across regions and vegetation types in North America
Ecosystems with highest carbon flux moisture sensitivities are eastern deciduous forests, western conifer forests, and dryland grasslands
Findings highlight the importance of improving terrestrial biosphere model predictions of regional soil moisture dynamics
Upland headwater catchments, such as those in the Appalachian Mountain region, are typified by coarse textured soils, flashy hydrologic response, and low baseflow of streams, suggesting well drained ...soils and minimal groundwater storage. Model formulations of soil genesis, nutrient cycling, critical loads and rainfall/runoff response are typically based on vertical percolation, development of soil horizons parallel to the land surface, mineral weathering inputs limited to the rooting zone and drainage from lumped catchment reservoirs (e.g., the subsoil) as the dominant source of stream flow. However, detailed study of the hydrologic reference catchment at Hubbard Brook Experimental Forest, NH, USA shows striking spatial patterns of soil development that reflect the influence of transient water tables within the solum in nearly all landscape positions. Shallow bedrock and variably low hydraulic conductivity in the subsoil promote lateral flow and development of soil horizons along hillslope flowpaths rather than in vertical profiles. We distinguished several morphologic units based on the presence of diagnostic horizons indicative of differing patterns of podzolization and carbon storage. The distribution of soils appears to be highly dependent on local drainability and frequency and duration of transient saturation within the solum. As such, monitoring of hydropedologic groups and transient water table fluctuations may prove to be a sentinel for the effects of climate change on spatial distribution of soils and retention/release of solutes from upland catchments.
•Podzolization was at a hillslope scale with lateral translocation by groundwater.•Water table developed in the solum, despite coarse textures and steep slopes.•Models need to account for translocation across pedon boundaries.
Groundwater flow direction within the critical zone of headwater catchments is often assumed to mimic land surface topographic gradients. However, groundwater hydraulic gradients are also influenced ...by subsurface permeability contrasts, which can result in variability in flow direction and magnitude. In this study, we investigated the relationship between shallow groundwater flow direction, surface topography, and the subsurface topography of low permeability units in a headwater catchment at the Hubbard Brook Experimental Forest (HBEF), NH. We continuously monitored shallow groundwater levels in the solum throughout several seasons in a well network (20 wells of 0.18–1.1 m depth) within the upper hillslopes of Watershed 3 of the HBEF. Water levels were also monitored in four deeper wells, screened from 2.4 to 6.9 m depth within glacial drift of the C horizon. We conducted slug tests across the well network to determine the saturated hydraulic conductivity (Ksat) of the materials surrounding each well. Results showed that under higher water table regimes, groundwater flow direction mimics surface topography, but under lower water table regimes, flow direction can deviate as much as 56 degrees from surface topography. Under these lower water table conditions, groundwater flow direction instead followed the topography of the top of the C horizon. The interquartile range of Ksat within the C horizon was two orders of magnitude lower than within the solum. Overall, our results suggest that the land surface topography and the top of the C horizon acted as end members defining the upper and lower bounds of flow direction variability. This suggests that temporal dynamics of groundwater flow direction should be considered when calculating hydrologic fluxes in critical zone and runoff generation studies of headwater catchments that are underlain by glacial drift.
Groundwater levels within the soil zone of a steep, headwater catchment was monitored throughout several seasons. The hydraulic conductivity of soil units was measured, and the relationship between groundwater flow direction, water levels, surface topography, and subsurface topography of low permeability soil units were analysed. Results show that groundwater flow direction follows surface topography during higher water tables and is influenced by the subsurface topography or low‐permeability soil units during lower water tables.
Input of organic matter into stream channels is the primary energy source for headwater ecosystems and ultimately carbon to the oceans and hence is an important component of the global carbon cycle. ...Here, we quantify organic‐rich fine sediment mobilization, transport, and storage in a Strahler fourth‐order stream during individual intermediate‐sized storm events. By combining measurements of fallout radionuclides (FRNs) 7Be and 210Pb and stable water isotopes with a conceptual model of suspended load trapping by channel margins, we find that the channel bed was consistently a source of suspended load to the channel margins. Relative to storage on the channel margins, suspended load export increased through the spring and summer, perhaps related to the in‐channel decomposition of organic debris as indicated by its FRN exposure age and changing bulk δ13C composition. Trapping of suspended load by riparian margins limits sediment transport distances, which, given sufficient discharge to fully suspend the load, is nearly independent of stream discharge for sub‐bankfull discharges. Limited data indicate that the fractional size of the channel margins where trapping occurs decreases with increasing watershed area. Increasing transport length and decreasing fractional margin area with increasing watershed area results in a systematic downstream decoupling of the channel from local terrestrial organic matter exchange. These findings provide a framework for understanding suspended load dynamics in formerly glaciated regions where sediment production and fluxes are generally low and thus the annual input of organic debris is a major component of suspended load budget.
Plain Language Summary
The decomposition of organic‐rich debris (leaves, twigs, etc.) within stream channels serves as an important organic carbon source to stream margins or banks. During moderate storm events, we observed that the channel bed was consistently a source of organic‐rich suspended load that is then trapped by the channel margins. Through spring and summer less of the suspended load is trapped by the margins, increasing the fraction of the suspended load exported. This decreased trapping and increased export may be related to changes in the character of the suspended load due to the in‐channel decomposition of organic debris. Trapping of suspended load by channel margins limits the transport distance of suspended load, systematically decoupling the channel from the channel margins with increasing watershed size. These findings provide a framework for understanding suspended load transport in formerly glaciated regions where the annual input of organic debris is a significant component of suspended load budget.
Key Points
We observe seasonal variations in the ratio of suspended load exported from the watershed versus stored in channel margins
Greater fractional area of deposition along margins of headwaters facilitates trapping of suspended load that limits suspended load export
Increasing transport length with increasing watershed area systematically decouples the channel from terrestrial organic exchange
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