Key Findings

2004 mesquite dunelands without grasses and 2008 mesquite dunelands with perennial grasses.
Credit: Jornada LTER

The shift from grassland to shrubland is not the only alternative state for desert vegetation. Jornada Basin LTER research has documented: (a) shifts from desertified shrublands back towards native grassland, (b) shifts between different shrubland types, and (c) shifts from grasslands or shrublands to novel ecosystems dominated by non-native annual or perennial grasses. State changes depend on wind and water movement patterns, spatial variation in soil and vegetation type, and triggers such as multiple years of precipitation at levels above or below long-term average.

Connectivity modifying structures (ConMods) trap and retain wind-transported seeds and organic material, reducing connectivity and facilitating local recruitment.
Credit: Jornada LTER

Locations that are functionally connected in the landscape experience greater materials and energy transfer, which ultimately influences spatial and temporal vegetation dynamics in desert landscapes. In pilot studies, small connectivity modifying structures (ConMods) increased grasses and forbs relative to areas without ConMods.

Researchers conduct fieldwork in the Chihuahuan Desert of New Mexico.
Credit: ASU

Using long term observations and a water balance approach, JRN LTER researchers determined that small watersheds on piedmont slopes are large contributors to groundwater recharge on the Jornada Basin. This was one of the first studies to quantify groundwater recharge in arid region first-order watersheds.

Late fall switchgrass harvest at the GLBRC / KBS LTER biofuels scale-up site; Photo Credit: J.E.Doll, Michigan State University
Credit: J.E. Doll- Michigan State University

Research from KBS LTER reveals not only how changes in cropping practices improve ecosystem service flows, but also the economic value of those flows. Paired studies of farmers and consumers track farmer willingness to provide changed practices along with consumer willingness to pay for ecosystem services that come from those changed practices, such as climate mitigation, water quality regulation, and natural pest control.

Kangaroo Rat
Credit: John Anderson

Desert rodent biomass depends on an interaction between shrub cover and precipitation – more rodent biomass is associated with grasslands following droughts and with shrublands following wet years. This pattern can be largely explained by the irruption of folivores (which prefer shrubbier vegetation) during wet years and suggests that rodent population dynamics are likely to change following climatic shifts.

Organ Mountains-Desert Peaks National Monument
Credit: Curtis Monger

Researchers at JRN LTER are incorporating machine learning into complex dataset exploration. The data exploration interface is capable of suggesting potential analytical approaches to new users based on interactions with previous users.

During a decadal cooling period, productivity and hydrological connectivity synchronously decreased among terrestrial and aquatic ecosystems. As summer air temperatures and solar radiation stabilized in the following decade, the ecosystem moved back toward pre-cooling period conditions but in an asynchronous manner. This was due in part to the fact that the end of the cooling period was punctuated by the highest glacial melt summer on record.

Credit: Amy Chiuchiolo / MCM LTER

Soil invertebrate communities in long term monitoring plots are responding to long term and seasonal changes in temperature and water availability, with key taxa exhibiting distinct responses. These changes are favoring rarer hydrophilic taxa, while the dominant species, an endemic free-living nematode which prefers cold dry soils, is declining in monitoring and experimentally manipulated plots.

Credit: MCM LTER

Photosynthetic and mixotrophic eukaryotes are the dominant primary producers in the stratified water columns of the dry valleys lakes. Nutrient amendment experiments have shown that growth of chlorophytes, an obligate photosynthetic phytoplankton group, are stimulated by the addition of nitrogen or phosphorus in Lake Fryxell and nitrogen in Lake Bonney. Conversely, when communities are transplanted to the high light environment of open water moats, chlorophyte abundance and photosynthetic activity declined significantly. These results indicate that climate-related changes have conflicting impacts on phytoplankton communities (increased nutrient input versus lake level rise/expanding moats).

Credit: Aslan Wright-Stow Hilke Giles, Antarctica NZ Pictorial Collection: K081C 08/09

Record melt and thaw events over the past decade have increased the physical connectivity of the McMurdo Dry Valleys ecosystem. Researchers at MCM LTER have tested hypotheses that focus on responses, such as increased biogeochemical cycling and changes in biodiversity. These studies suggest that landscape morphology is changing as permafrost thaws, and that biological communities are indeed responding to altered climatic conditions (e.g., high and low flow controls on stream benthic mat abundance).

Credit: Tracey Jones, Antarctica NZ Pictorial Collection: K024C 07/08

In the austral summer, the shallow margins of ice-covered lakes melt, forming moats around the permanent ice covers of the lakes. Waters here interact with streams, soils, and the atmosphere (unlike those under the permanent ice). Recent study of these moats has uncovered these as the locations of the highest biomass per unit area in the dry valleys landscape.

Credit: MCR LTER

Researchers at MCR LTER have been at the forefront of evaluating how OA will affect the structure and function of future reefs. The ecosystem engineers that structure coral reefs – stony corals and calcified algae – are uniquely threatened by low seawater pH. Using time series data to determine experimental conditions, researchers have tested for coral susceptibility to low seawater pH, the dependence of these responses on space, time, and functionality scales, and the implications for future coral reefs.

Credit: MCR LTER

The diverse coral community on Moorea’s outer reefs has repeatedly shown a remarkable ability to recover rapidly following massive disturbances. In the last decade, a predator outbreak and cyclone devastated coral across the seascape, yet recovery was more rapid than has been observed anywhere in the world. Moorea Coral Reef LTER researchers have gained critical insights into the processes, connectivities, and feedbacks governing coral reef resilience. This has provided the basis for general management strategies to help restore and strengthen coral reef community resilience.

Credit: MCR LTER

Reefs worldwide have abruptly and increasingly shifted from coral to seaweed dominated communities. Experiments at MCR LTER revealed that a large disturbance can cause a coral reef to flip to seaweeds indefinitely. Experiments and models showed that multiple stable states (e.g. corals or seaweeds) can continue to thrive under the same levels of herbivory. They also discovered that the seaweed state is stabilized by the development of structural and chemical defenses that reduce the palatability of mature (but not juvenile) algae.

Credit: Russell Schmitt

The powerhouse mutualism between the coral animal and its symbiotic dinoflagellate algae is the backbone of coral reef ecosystems. Moorea Coral Reef LTER research has produced counter-intuitive results, specifically that flexibility with respect to symbionts does not automatically make corals resilient – a finding that has had profound implications for understanding the susceptibility of coral colonies to stress. Similarly, MCR LTER researchers have shown that major feedbacks involving other microbes affect coral health, particularly bacteria key to the dynamic nutrient cycling on reefs.

Credit: NTL LTER

Long term records show declining ice duration, lake warming, and increased variability in decadal lake level cycles. However, the magnitude of these physical changes, and their ecological consequences, differ substantially among lakes, including differences in warming rates, shifts in fish populations, and fluctuations in water clarity.

Credit: NTL LTER

Terrestrial organic carbon (C) entering lakes can be stored, sent to the atmosphere as CO2, or passed downstream. Long term measurements of hydrology and C were used to understand and model the fate of terrestrial C in lakes. In Wisconsin’s 6,400 km2 Northern Highland Lake District (NHLD), the fraction of organic C converted to CO2 varied substantially among lakes due to hydrology. Nonetheless, lakes accounted for about 40% of C storage, although they represent only 13% of the region’s area.

Credit: NTL LTER

Regime shifts are large, persistent, and often abrupt changes in ecosystem structure and function that may be difficult to reverse. Through long term whole ecosystem experiments and measurements, NTL LTER researchers have described regime shifts involving lake eutrophication and food web structure, and have used these case studies to develop conceptual and mechanistic models. These models are used to anticipate ecosystem shifts and evaluate the utility of management actions to prevent them.

Credit: NTL LTER

Long term pre-invasion records provide an essential baseline for understanding invasive species effects, which can have profound consequences for ecosystems and society. In a key example, the spiny water flea invaded Lake Mendota, leading to massive declines in water quality and a loss in ecosystem services valued at $140 million.

Credit: Erika Zambello

Although bacteria play a central role in processes affecting lake water quality, the taxa participating in these activities are largely undescribed. To address this knowledge gap, NTL LTER researchers have generated the largest freshwater microbial genome collection to date. These studies reveal a paradoxical pattern of large differences in community structure over time and among lakes, paired with the presence of specific taxa that are always present everywhere (the core lake microbiome), and communities that are surprisingly resilient to disturbance.

Credit: Russ Hopcroft

The coastal Gulf of Alaska water column is becoming progressively more stratified — the entire water column is warming, but more rapidly at the surface than near the seafloor, while near surface waters are becoming fresher. This is due to multiple factors including the air-sea heat flux, ocean heat flux convergences, the stabilizing influence of runoff, the destabilizing effects of cooling and vertical mixing, and the wind driven cross-shelf buoyancy flux. Stratification impacts the water column mixing in winter that helps reset the shelf for the next season’s biological production. Therefore, the concentration and composition of the phyto- and zooplankton community shows direct and indirect connections to the thermal conditions of the Gulf of Alaska shelf. These far reaching implications for upper trophic levels could only be detected using a high quality, multi-decade dataset.

Credit: Ana Aguilar-Islas

Reductions in primary producer average cell size and biomass followed the 2015-2016 warming, as did corresponding reductions at higher trophic levels. In addition, southern zooplankton species of smaller body size invaded the region and anomalous increases in gelatinous zooplankton population were observed. These changes, which could represent a window into the future of the Northern Gulf of Alaska, were associated with widespread seabird mortality, reduced forage fish abundance, and range shifts or reductions in commercially important groundfish species. The anomalous nature of the warm period was only evident in the context of long term observations establishing the typical subarctic character of this pelagic ecosystem.

Credit: NGA LTER

Although glacial input leads to high iron concentrations during summer and fall within the narrow Alaska Coastal Current, the ratio of iron to nitrate over the Northern Gulf of Alaska shelf in spring can be low enough to lead to nutritional stress in diatoms. This mismatch in essential nutrients likely affects phytoplankton community evolution during the spring bloom.

Credit: Seth Danielson

Modeling at NGA LTER investigates how the complex interplay between the strongly seasonal freshwater discharge at the coast and offshore eddies controls horizontal gradients of limiting nutrients (nitrate and iron). This work builds on previous modeling studies in the region that utilized Seward Line observations to improve the accuracy of simulated physical and biogeochemical fields. Moreover, previous models quantified the importance of eddy induced entrainment of shelf iron to offshore primary production, which extends to phyto- and zooplankton community structure.

Credit: PAL LTER

Shifts in sea ice are affecting the WAP ecosystem and biogeochemistry. Despite dramatic shifts in Antarctic food webs, the number of the keystone krill species (Euphausia superba) has not changed significantly over the PAL LTER study area. However, researchers have observed reduced juvenile recruitment following positive anomalies of the Southern Annular Mode. North of PAL LTER, E. superba population centers in the southwest Atlantic sector have been contracting southward for the past 90 years.