Term Ecological Research Site Research Articles

Thinning relationships of woody encroachers in a US southwestern shrubland

Woody plant encroachment degrades the economic and environmental potential of drylands by altering processes such as nutrient fluxes, ecohydrology, and ecosystem services. Though past research has investigated the encroachment process, relatively little is known about post-encroachment shrub community ecology. To better quantify dynamics within post-woody encroachment shrub communities, we combined USGS lidar height data and multispectral imagery to estimate shrub density, shrub height, shrub cover, and shrub volume across the Jornada Basin Long Term Ecological Research (JRN-LTER) site in southern New Mexico, USA. Structural estimates were analyzed in search of telltale signs of community competition, specifically, the presence of thinning relationships. Results demonstrated density-dependent thinning relationships in shrub communities of creosote and mesquite, indicating a large role for competition in arid shrub communities even at relatively low shrub densities and cover (∼35% cover). In addition, shrub volume estimates better modeled the expected thinning dynamics of shrub communities than shrub canopy cover measurements. Overall, our results indicate the utility of lidar data in extending two-dimensional descriptions of woody vegetation structure (i.e., woody canopy cover) into the critical third dimension (i.e., woody plant volume), as well as the relative importance of competition and demographic bottlenecks to vegetation structure in drylands.

Exploring the mesoscale connectivity of phytoplankton periodic assemblages' succession in northern Adriatic pelagic habitats

An appropriate model for phytoplankton distribution patterns is critical for understanding biogeochemical cycles and trophic interactions in the oceans and seas. Because phytoplankton dynamics in coastal waters are more complex due to shallow depth and proximity to land, more accurate models applied to the correct spatial and temporal scales are needed. Our study investigates the role of the atmosphere and hydrosphere in pelagic habitat by modelling phytoplankton assemblages at two Long Term Ecological Research sites in the northern Adriatic Sea using niche-forming environmental variables (wind, temperature, salinity, river discharge, rain, and water column stratification). To study the synchronization between the phytoplankton community and these environmental variables at the two LTER sites, we applied current linear and nonlinear numerical methods for ecological modelling. The aim was to use periodic and/or non-periodic properties of the environmental variables to classify the phytoplankton assemblages at one LTER site (Gulf of Trieste - Slovenia) and then predict them at another LTER site 100 km away (Gulf of Venice - Italy). We found that periodicity played a role in the explanatory and predictive power of the environmental variables and that it was more important than non-periodic events in defining the common structure of the two pelagic habitats. The non-linear classification functions of the neural networks further increased the predictive power of these variables. We observed partial synchronization of communities at the mesoscale and differences between the original and predicted assemblages under similar environmental conditions. We conclude that mesoscale connectivity plays an important role in phytoplankton communities in the northern Adriatic. However, the loss of periodicity of niche-forming variables due to more frequent extreme meteorological and hydrological events could loosen these connections and affect the temporal succession of phytoplankton assemblages.

Species Composition and Assessment of Zingiberaceae in the Forest Patches of Mt. Musuan, Bukidnon, Southern Philippines

This study documented 11 species of gingers belonging to nine genera distributed in three tribes and two subfamilies from Mt. Musuan and its vicinity in Bukidnon, Southern Philippines. Data revealed that Zingiberaceae species in these sites constitute ca. 8% of the total number of Philippine Zingiberaceae. Alpinia haenkei C.Presl., A. purpurata (Vieill.) K.Schum., Curcuma zedoaria (Christm.) Roscoe, Etlingera philippinensis (Ridl.) R.M.Sm., Hornstedtia conoidea Ridl., Meistera muricarpa (Elmer) Škorničk. & M.F.Newman, and Zingiber zerumbet (L.) Smith. were recorded in Mt. Musuan, while Amomum dealbatum Roxb., Etlingera elatior (Jack) R.M.Sm., Hedychium coronarium Koenig, Hornstedtia conoidea Ridl., and Wurfbainia elegans (Ridl.) Škorničk. & A.D.Poulsen. were collected in CMU View Deck in Kibulawan. Of the collected species, five of these (3.52% endemicity compared to the total number of Philippine Zingiberaceae) are endemic to the Philippines, while the other six species are introduced. These species are mostly associated with fern species, such Dicranopteris linearis (Burm.f.) Underw. and Lygodium circinnatum (Burm.f.) Sw., and angiosperms Musa textilis and under the shades of dipterocarpaceae species. This paper is the first taxonomic report on Zingiberaceae in Mt. Musuan and vicinity which was not documented in the previous studies conducted at Philippine Long Term Ecological Research (LTER) Sites. Since Mt. Musuan is not a protected area and an open site for ecotourism which is constantly visited by daily hikers, these species might be prone to depletion in their wild habitats. There is an urgent call for in situ conservation efforts that should be done by the concerned authorities of Central Mindanao University to help preserve and protect these ginger species.

An interview with Dr. Emma J. Rosi: On ecosystem effects of pharmaceutical contamination

MIT Science Policy Review spoke with Dr. Emma J. Rosi to better understand how commonly used pharmaceutical and personal care products enter freshwater ecosystems, as well as the challenges this contamination introduces for ecosystem health and policy development. Dr. Emma J. Rosi is a well-established aquatic ecologist with over 20 years of research experience in aquatic ecosystems. She is currently a senior scientist at the Cary Institute of Ecosystem Studies, where she spearheads a research group that is working to understand how pharmaceutical and personal products impact the health of rivers in the context of current wastewater infrastructure and urbanization. She also contributes her expertise to the U.S. Environmental Protection Agency (EPA) as a member of its Science Advisory Board. Moreover, she was the Director of the Baltimore Ecosystem Study (BES), a National Science Foundation Long Term Ecological Research Site. She discusses the challenges and opportunities that could potentially face policymakers working towards preserving the health of aquatic ecosystems in urban environments.

Weekly high-resolution multi-spectral and thermal uncrewed-aerial-system mapping of an alpine catchment during summer snowmelt, Niwot Ridge, Colorado

Abstract. Alpine ecosystems are experiencing rapid change as a result of warming temperatures and changes in the quantity, timing and phase of precipitation. This in turn impacts patterns and processes of ecohydrologic connectivity, vegetation productivity and water provision to downstream regions. The fine-scale heterogeneous nature of these environments makes them challenging areas to measure with traditional instrumentation and spatiotemporally coarse satellite imagery. This paper describes the data collection, processing, accuracy assessment and availability of a series of approximately weekly-interval uncrewed-aerial-system (UAS) surveys flown over the Niwot Ridge Long Term Ecological Research site during the 2017 summer-snowmelt season. Visible, near-infrared and thermal-infrared imagery was collected. This unique series of 5–25 cm resolution multi-spectral and thermal orthomosaics provides a unique snapshot of seasonal transitions in a high alpine catchment. Weekly radiometrically calibrated normalised difference vegetation index maps can be used to track vegetation health at the pixel scale through time. Thermal imagery can be used to map the movement of snowmelt across and within the near sub-surface as well as identify locations where groundwater is discharging to the surface. A 10 cm resolution digital surface model and dense point cloud (146 points m−2) are also provided for topographic analysis of the snow-free surface. These datasets augment ongoing data collection within this heavily studied and important alpine site; they are made publicly available to facilitate wider use by the research community. Datasets and related metadata can be accessed through the Environmental Data Initiative Data Portal, https://doi.org/10.6073/pasta/dadd5c2e4a65c781c2371643f7ff9dc4 (Wigmore, 2022a), https://doi.org/10.6073/pasta/073a5a67ddba08ba3a24fe85c5154da7 (Wigmore, 2022c), https://doi.org/10.6073/pasta/a4f57c82ad274aa2640e0a79649290ca (Wigmore and Niwot Ridge LTER, 2021a), https://doi.org/10.6073/pasta/444a7923deebc4b660436e76ffa3130c (Wigmore and Niwot Ridge LTER, 2021b), https://doi.org/10.6073/pasta/1289b3b41a46284d2a1c42f1b08b3807 (Wigmore and Niwot Ridge LTER, 2022a), https://doi.org/10.6073/pasta/70518d55a8d6ec95f04f2d8a0920b7b8 (Wigmore and Niwot Ridge LTER, 2022b). A summary of the available datasets can be found in the data availability section below.

Blown Away: Extreme Wind Events Impact Grass Mortality

Understanding extremes in ecological change is critical given the anticipated increasing frequency of such events. This is especially true when extreme events trigger transitions to alternative stable states, as in drylands that have seen conversion from grass- to shrub-dominated states. We examined the responses of a Chihuahuan Desert community after record winds in 2019 killed grasses and reduced greenness. We argue that wind has two faces like Janus, aptly the Roman god of transitions, because it participates in change as both a forcing and a feedback and that, similarly, other types of extremes can alter the character of ecological interactions. Photo 1. Record-breaking wind in southern New Mexico in the spring of 2019 had major impacts on grass at the Jornada Long Term Ecological Research site in Las Cruces, New Mexico. The exposed roots of two Sporobolus spp. grasses are a result of these storms. Photo credit: G.S. Okin. Photo 2. In some cases, grasses, like the one on the left in this image, were able to green up following rain in the early summer. Other grasses appear to have been killed by the root exposure and sandblasting that occurred during the extreme wind events in the spring of 2019. Photo credit: G.S. Okin. Photo 3. UCLA undergraduate, Sarah Payne (shown here indicating the extent of root exposure on a native bunchgrass), conducted an NSF-supported summer research project that resulted in a publication of an article in Ecology. Photo credit: G.S. Okin. These photographs illustrate the article “The two faces of Janus: Processes can be both exogenous forcings and endogenous feedbacks with wind as a case study” by Sarah A. R. Payne, Gregory S. Okin, Abinash Bhattachan, and Michael R. Fischella published in Ecology. https://doi.org/10.1002/ecy.3998

Soil carbon availability decouples net nitrogen mineralization and net nitrification across United States Long Term Ecological Research sites

Soil carbon availability decouples net nitrogen mineralization and net nitrification across United States Long Term Ecological Research sites

Effects of preservatives in pitfall traps for collecting arthropods: A comparison of ethylene glycol and five alternative preservatives

Pitfall traps using ethylene glycol as a preservative are widely used to investigate arthropods in forests. However, recently, pitfall traps have been reported to be damaged by wild vertebrates because of the sweetness of ethylene glycol. Therefore, this study compared ethylene glycol with five potential alternative preservatives for the selection of an alternative preservative for use in pitfall traps. Field experiments were conducted at the Namhae Long Term Ecological Research (LTER) site in the southernmost part of South Korea. The five alternative preservatives included salt solution with bleach, which is considered to repel wild animals because of its smell; formalin solution with bleach; salt solution without bleach; formalin solution without bleach; and alcohol solution with a small amount of added glycerin. The number of arthropods collected differed with the type of preservative used; the composition of arthropod assemblages was also different. Among the two alternative preservative solutions expected to repel wild animals, formalin+bleach, and salt water+bleach, the latter was found to be better for arthropod collection.

Dryland soil mycobiome response to long‐term precipitation variability depends on host type

Abstract Climate change is projected to cause shifts in precipitation regimes globally, leading to intensified periods of precipitation and droughts. Most studies that have explored the influence of changing precipitation regimes on ecosystems have focused on changes in mean annual precipitation, rather than the variance around the mean. Soil fungi are ubiquitous organisms that drive ecosystem processes, but it is unknown how they respond to long‐term increased interannual precipitation variability. Here, we investigated the influence of long‐term increased precipitation variability and host type on soil fungal diversity and community composition in a dryland ecosystem. We collected 300 soil samples from two time points and different host type substrate types at a long‐term precipitation variability experiment at the Jornada Long Term Ecological Research site. Next, we used amplicon sequencing to characterize soil fungal communities. Soil fungal alpha diversity and community composition were strongly affected by host type and sampling year, and increased precipitation variability caused a modest, statistically insignificant, decrease in soil fungal evenness. Furthermore, results from our structural equational model showed that the decrease in grass‐associated soil fungal richness was likely an indirect result of host decline in response to increased precipitation variability. Synthesis. Our work demonstrates effects of increase in interannual precipitation variability on soil fungi, and that plant hosts play a key role in mediating soil fungal responses.

Forest and Freshwater Ecosystem Responses to Climate Change and Variability at US LTER Sites

Abstract Forest and freshwater ecosystems are tightly linked and together provide important ecosystem services, but climate change is affecting their species composition, structure, and function. Research at nine US Long Term Ecological Research sites reveals complex interactions and cascading effects of climate change, some of which feed back into the climate system. Air temperature has increased at all sites, and those in the Northeast have become wetter, whereas sites in the Northwest and Alaska have become slightly drier. These changes have altered streamflow and affected ecosystem processes, including primary production, carbon storage, water and nutrient cycling, and community dynamics. At some sites, the direct effects of climate change are the dominant driver altering ecosystems, whereas at other sites indirect effects or disturbances and stressors unrelated to climate change are more important. Long-term studies are critical for understanding the impacts of climate change on forest and freshwater ecosystems.

Woody plant encroachment of grassland and the reversibility of shrub dominance: Erosion, fire, and feedback processes

AbstractMany grass‐dominated ecosystems in dryland regions have experienced increasing woody plant density and abundance during the past century. In many cases, this process has led to land degradation and declines in ecosystem functions. An example is the Chihuahuan Desert in the southwestern United States, which experienced different stages of shrub encroachment in the past 150 years. Among a wide variety of mechanisms to explain the grass–shrub transitions in this dryland system, soil erosion (both wind and water) and fire are particularly well studied. Here, we synthesize recent developments on the drivers and feedback in the process of shrub encroachment in the Chihuahuan Desert through the intercomparison of two Long Term Ecological Research (LTER) sites, namely Jornada and Sevilleta. Experimental and modeling studies support a conceptual framework, which underscores the important roles of erosion and fire in woody plant encroachment. Collectively, research at the Jornada LTER provided complementary, quantitative support to the well‐known fertile‐islands framework. Studies at the Sevilleta LTER expanded the framework, adding fire as a major disturbance to woody plants. Conceptual models derived from the synthesis represent the general understanding of shrub encroachment that emerged from research at these two sites, and can guide management interventions aimed at reducing or mitigating undesirable ecosystem state change in many other drylands worldwide.

The role of communication professionals in fostering a culture of public engagement

ABSTRACT This study initially reports on qualitative interviews (n = 17) with scientists at two Long Term Ecological Research (LTER) sites in the northeastern United States. These interviews suggest the need for greater attention to the role of communication professionals and institutional leadership in fostering high-quality public engagement. The study also reports on a follow-up quantitative survey (n = 68) conducted to better understand the degree to which LTER scientists’ views about communication professionals were meaningfully associated with perceptions about the need for robust engagement funding. The project was initially designed based on the Integrated Behavioral Model to assess how individual LTER scientists’ engagement-related attitudes, normative beliefs, and efficacy beliefs affected their communication activities. However, the combined results highlight the potential value of additional research and theorization aimed at better understanding the factors that might lead to greater cooperation between scientists and organizational communicators.

Spatial and temporal variability of dissolved aluminum and manganese in surface waters of the northern Gulf of Alaska

The Northern Gulf of Alaska (NGA) shelf is a productive high-latitude environment where nutrient dynamics are greatly impacted by the seasonal variability in freshwater input. Iron is a key nutrient on the NGA shelf that directly modulates primary production, and freshwater is a major input of iron to the NGA shelf. However, variability in the input of dissolved iron from freshwater sources is obscured by high biological uptake and by the concentration of organic ligands, which bind iron and help maintain it in solution. Riverine inputs of other lithogenic elements such as aluminum and manganese are expected to behave quasi-conservatively on the NGA shelf and could help provide insight into the variability of iron input in this region.Here we present the seasonal (spring, summer, and fall) distribution of dissolved aluminum (dAl) and manganese (dMn) observed during 2018 and 2019 in the NGA region. Data were obtained during the NGA Long Term Ecological Research (LTER) site cruises and include several surface transects from Kayak Island to Kodiak Island, vertical profiles at several locations sparsely distributed throughout the shelf, and surface information from a Copper River plume study. We find that seasonal patterns in the surface concentrations of dMn and dAl mirrored annual glacial melt cycles, with the lowest values observed in spring and higher values in summer and fall. Sharp cross-shelf gradients were observed for both metals particularly in summer and fall. Surface concentrations decreased (by 1–2 orders of magnitude) away from the outflow of the Copper River, the major point source of freshwater within the NGA LTER site. Extremely high concentrations in the Copper River plume (≤1395 nM dAl, ≤128 nM dMn) and strong correlations with salinity (p = 7.9E-14 for dAl, 1.4E-14 for dMn) highlight the quasi-conservative nature of these metals within the plume. Enhanced dAl and dMn concentrations within nepheloid layers in subsurface waters indicate regions where a sedimentary source of iron could also be important. Residence times for dAl and dMn in surface waters over the NGA shelf were estimated to be 31 days (dAl) and 42 days (dMn) on average based on summer and fall data from both years.

Taking climate change into account: Non‐stationarity in climate drivers of ecological response

Abstract Changes in the global climate system are creating increasingly non‐analogue climate conditions with expectations of non‐stationarity among climate drivers. Decoupling among climate drivers complicates the assessment of ecological response to the changing climate as characteristics that could be once treated as a suite of conditions now need to be treated as potentially independent with possible synergistic effects. Ecologists commonly use ordination techniques (often principal component analysis; PCA) on large climate and environmental datasets to reduce a range of variables to a few axes that are uncorrelated with each other and often explain large proportion of the variation in the original data. However, non‐stationarity, with correlations among variables changing over time, can affect this approach. Here, we use a 37‐year climate dataset from the Niwot Ridge Long Term Ecological Research site (Colorado, USA) to present the use of both moving window principal component analysis and moving window correlation analysis to determine non‐stationarity in climate data. Relationships among climate variables and between input variables and PCA axes changed over time; this obscured interpretation of relationships between PCA axes and an ecological response (plant biomass), suggesting that one‐time PCA for environmental variables may lead to inappropriate inferences. Synthesis. Care must be taken in analysing climate–ecological relationships when predictor variables exhibit non‐stationarity. We present a conceptual decision‐making tree to help ecologists consider when to use PCA and extract axis scores or use alternative approaches for incorporating non‐stationarity into subsequent analysis, including testing variables individually to aid in interpretation, break point analyses and averaging PC scores.

Salt Marsh Pond Biogeochemistry Changes Hourly‐to‐Yearly but Does Not Scale With Dimensions or Geospatial Position

AbstractShallow ponds are expanding in many salt marshes with potential impacts on ecosystem functioning. Determining how pond characteristics change over time and scale with physical dimensions and other spatial predictors could facilitate incorporation of ponds into projections of ecosystem change. We evaluated scaling relationships across six differently sized ponds in three regions of the high marshes within the Plum Island Ecosystems‐Long Term Ecological Research site (MA, USA). We further characterized diel fluctuations in surface water chemistry in two ponds to understand short‐term processes that affect emergent properties (e.g., habitat suitability). Primary producers drove oxygen levels to supersaturation during the day, while nighttime respiration resulted in hypoxic to anoxic conditions. Diel swings in oxygen were mirrored by pH and resulted in successive shifts in redox‐sensitive metabolisms, as indicated by nitrate consumption at dusk followed by peaks in ammonium and then sulfide overnight. Abundances of macroalgae and Ruppia maritima correlated with whole‐pond oxygen metabolism rates, but not with surface area (SA), volume (V), or SA:V. Moreover, there were no clear patterns in primary producer abundances, surface water chemistry, or pond metabolism rates across marsh regions supplied by different tidal creeks or that differed in distance to upland borders or creekbanks. Comparisons with data from 2 years prior demonstrate that plant communities and biogeochemical processes are not in steady state. Factors contributing to variability between ponds and years are unclear but likely include infrequent tidal exchange. Temporal and spatial variability and the absence of scaling relationships complicate the integration of high marsh ponds into ecosystem biogeochemical models.

Exotic oryx interact with shrub encroachment in the Chihuahuan Desert

Biotic invasions can interact with ongoing landscape transitions in introduced ranges, acting synergistically to accelerate landscape change. The African oryx (Oryx gazella), a large ungulate native to the Kalahari Desert in southern Africa, was intentionally released into the Chihuahuan Desert of southern New Mexico to provide increased hunting opportunities. Oryx have subsequently dispersed widely, including into the adjacent Jornada Basin Long Term Ecological Research site, which contains native grassland communities imperiled due to shrub encroachment. We examined how landscape use and diel activity of oryx interacts with shrub encroachment at Jornada Basin. We hypothesized that oryx would be more common on unencroached grasslands that provide preferred forage, and that oryx would be most active at dawn and dusk (crepuscular) when ambient temperatures were favorable and predator surveillance was possible. We sampled 24 sites across a broad gradient of shrub encroachment from 2014 to 2018 using camera traps to evaluate our hypotheses. Oryx relative abundances were higher on unencroached grasslands, especially in years of low to moderate precipitation. We found oryx were most active during crepuscular periods, and this behavior was consistent across the shrub encroachment gradient. Our research suggests exotic oryx could contribute to shrub encroachment by focusing their foraging on remaining grasslands, especially during drier years, which may adversely affect livestock production and other ecosystem services.

Shrub encroachment creates a dynamic landscape of fear for desert lagomorphs via multiple pathways

AbstractShrub encroachment is transforming arid and semiarid grasslands worldwide. Such transitions should influence predator–prey interactions because vegetation cover often affects risk perception by prey and contributes to their landscape of fear. We examined how the landscape of fear of two desert lagomorphs (black‐tailed jackrabbit,Lepus californicus; desert cottontail,Sylvilagus audubonii) changes across grassland‐to‐shrubland gradients at Jornada Basin Long Term Ecological Research site in the Chihuahuan Desert of southern New Mexico. We test whether shrub encroachment shapes risk differently for these two lagomorphs because of differences in body size and predator escape tactics. We also examine whether an ecosystem engineer of grasslands (banner‐tailed kangaroo rat,Dipodomys spectabilis) mediates risk perception through the creation of escape refuge and whether trade‐offs exist between shrub encroachment and the local reduction of banner‐tailed kangaroo rats caused by shrub expansion. We measured perceived predation risk with flight initiation distances (FIDs) and then used structural equation modeling to tease apart the hypothesized direct and indirect pathways for how shrub encroachment could affect perceived risk. A total negative effect of shrub cover on FID was supported for jackrabbits and cottontails, suggesting both species perceive shrubbier habitat as safer. Increases in fine‐scale concealment also reduced risk for cottontails, but not jackrabbits, likely because cottontails rely on crypsis to avoid predator detection whereas jackrabbits rely on speed and agility to outrun predators. Perceived risk was reduced when individuals were near kangaroo rat mounds only for cottontails because the smaller species can use banner‐tailed kangaroo rat mounds as refuge. Shrub encroachment greatly reduced the availability of mounds. Thus, a trade‐off exists for cottontails in which shrub encroachment directly reduced perceived risk, but indirectly increased perceived risk through the local extirpation of an ecosystem engineer. Our work illustrates how the expansion of shrub encroachment can create a dynamic landscape of fear for populations of prey species involving direct and indirect pathways contingent on prey body size, escape tactics, and activities of an ecosystem engineer.

No net insect abundance and diversity declines across US Long Term Ecological Research sites

Recent reports of dramatic declines in insect abundance suggest grave consequences for global ecosystems and human society. Most evidence comes from Europe, however, leaving uncertainty about insect population trends worldwide. We used >5,300 time series for insects and other arthropods, collected over 4-36 years at monitoring sites representing 68 different natural and managed areas, to search for evidence of declines across the United States. Some taxa and sites showed decreases in abundance and diversity while others increased or were unchanged, yielding net abundance and biodiversity trends generally indistinguishable from zero. This lack of overall increase or decline was consistent across arthropod feeding groups and was similar for heavily disturbed versus relatively natural sites. The apparent robustness of US arthropod populations is reassuring. Yet, this result does not diminish the need for continued monitoring and could mask subtler changes in species composition that nonetheless endanger insect-provided ecosystem services.

Effect of offshore waves and vegetation on the sediment budget in the Virginia Coast Reserve (VA)

AbstractThe potential for rapid coastline modification in the face of sea‐level rise or other stressors is alarming, since coasts are often densely populated and support valuable infrastructure. In addition to coastal submergence, nutrient‐related water pollution is a growing concern for coastal wetlands. Previous studies found that the Suspended Sediment Concentration (SSC) of coastal wetlands acts as a first‐order control of their sustainability, but SSC dynamics are poorly understood. Our study focuses on the Virginia Coast Reserve (VCR) Long Term Ecological Research (LTER) site, a shallow multiple tidal inlet system in the USA. We apply numerical modelling (Delft3D‐SWAN) and subsequent analyses to determine SSC dynamics within the VCR. In particular, we consider two important controls on SSC in the system: vegetation (seagrass and salt marsh) and offshore waves. Our results show that vegetation colonies and increased wave energy lengthen water residence time. The reduction in the tidal prism decreases SSC export from the bay via tidal inlets, leading to increased sediment retention in the bay. We found that alongshore currents can enhance lagoon SSC by importing fine sediments from an adjacent inlet along the coastline. Our numerical experiments on vegetation seasonality can improve the understanding of wave climate impact on coastal bay sediment budget. Offshore waves increase sediment export from coastal bays, particularly during winter seasons with low vegetation density. Therefore, our study can help managers and stakeholders to understand how to implement restoration strategies for the VCR. © 2020 John Wiley & Sons, Ltd.

Snowfall Fraction, Cold Content, and Energy Balance Changes Drive Differential Response to Simulated Warming in an Alpine and Subalpine Snowpack

Despite widespread warming in mountain regions, little research to date has explored the physical mechanisms driving the variable response of snowpacks to changes in climate, instead focusing primarily on empirical relationships such as seasonal air temperature or elevation. In this work, we evaluate how differences in snowfall fraction, cold content, and the snowpack energy balance produce simulated changes to snow accumulation and melt at an alpine and subalpine snowpack in the Niwot Ridge Long Term Ecological Research site. For our analysis, we created a 23 y baseline simulation using the SNOWPACK model forced by historical hourly meteorological data from water year 1991 through 2013. We then perturbed hourly air temperature in 0.5°C increments from +0.5°C to +4.0°C above baseline and increased incoming longwave radiation accordingly. For every 1°C of warming, peak snow water equivalent declined by 43.9 mm in the alpine and 54.3 mm in the subalpine, melt onset shifted 6.2 d earlier in the alpine and 8.8 d in the subalpine, the snow season shortened by 10.7 d in the alpine and 16.4 d in the subalpine, and melt rate increased by 0.2 mm d-1 in the alpine while decreasing by 0.4 mm d-1 in the subalpine. We found the alpine snowpack was less sensitive to warming for three primary reasons: 1) Snowfall fraction decreased less rapidly per 1°C of warming than in the subalpine; 2) Cold content still consistently developed throughout the snow season, preventing mid-winter melt events; 3) Changes to snowmelt rate were not significant because increases to the turbulent fluxes balanced decreases in the radiative fluxes with earlier snowmelt onset. Additionally, at 3°C of warming and greater, the subalpine site experienced a fundamental shift where significant melt could occur throughout the entirety of the winter as cold content was no longer large enough to buffer against positive energy fluxes. In some years, the subalpine snowpack became transient with several cycles of accumulation and melt per winter. This tipping point suggests sites with lower cold content—like the subalpine studied here—are likely to be more sensitive to producing increased winter melt as warming continues over the coming decades.