With over 40 years of continuous data collection across many biomes, the Long Term Ecological Research (LTER) Network is a rich source of information for testing big-picture concepts about how ecosystems work. Luckily, the Network also brings together a group of scientists with creative ideas about how to wring new insights from diverse data sources.
The LTER synthesis working group process is designed to capitalize on the experiments, contextual knowledge, data, and creativity of the LTER Network. By funding small groups of scientists from inside and outside the Network to work intensely together on a synthesis project, the process encourages the ecological community to use existing data to probe novel theories, test generality, and search for gaps in our understanding.
The active Synthesis Working Groups are listed below.
For more about synthesis at the LTER, including our proposal process, see the synthesis homepage or follow the quick links below.
Current Working Groups
Ecosystem transitions: increased variability and regime shifts
Human impacts on ecosystems can result in persistent compositional shifts that are difficult to reverse even after relaxation from perturbations. Considerable debate remains on whether these observed shifts in ecosystems are due to the existence of tipping points and systems with alternative attractors, or whether observed shifts in ecosystems represent communities in alternative trajectories that will eventually reach a common stable point. In addition to human perturbations, ecosystems are also experiencing other transient dynamics, such as increased climate variability, which could promote or prevent state shifts. Using cross-site synthesis of LTER experiments that have simulated human perturbations or climate variability, this synthesis effort will test whether and which observed compositional shifts are a result of critical transitions or transient dynamics. Researchers will use these data to develop and inform theory that will improve predictions on the magnitude and frequency of perturbations and climate variability needed to promote or prevent lasting shifts in ecosystem composition.
Fire and Aridland Streams
Quantifying interactive effects of fire and precipitation regimes on catchment biogeochemistry of aridlandsIncreases in the frequency, extent, and severity of wildfires could have long-lasting and wide-ranging effects on hydrology and biogeochemistry of catchments, with consequences for ecosystem services including provision of drinking water. In aridlands, effects of fire will depend on interactions with the precipitation regime, which is also undergoing long-term change toward longer and more severe droughts and more extreme events. This SPARC synthesis group accelerates the ongoing efforts of the Collaborative for Arid Stream Synthesis (CRASS), which has applied data synthesis and time series modeling to test a conceptual model describing interactive effects of fire and precipitation on catchment biogeochemistry. The group has begun synthesis of long-term stream chemistry and discharge records (i.e., LTER, LTREB, USGS, CZO, NEON) for aridlands of the western U.S., where water supplies are particularly vulnerable to changing quality and quantity. Preliminary statistical analyses have quantified interactive effects of fire and precipitation on the timing, severity, and duration of water quality impairment. SPARC funds support an in-person meeting including refining and testing the conceptual model and completion of a manuscript. Ultimately, the group's goal is to reduce uncertainty in predicting changes to watershed processes and water quality following wildfire in the Anthropocene.
From poles to tropics: A multi-biome synthesis investigating the controls on river Si exports
Riverine exports of silicon (Si) directly influence global carbon (C) cycling through the growth of diatoms, ubiquitous autotrophs in marine and freshwater systems, which account for ~25% of global primary production. Rivers play essential roles in processing and supplying the Si necessary for diatom growth, but we have limited knowledge of the controls on river Si exports, especially how they vary across biomes. Prior work has shown conflicting importance of various drivers, such as lithology, riverine productivity, and terrestrial vegetation in controlling river Si exports. Capturing a baseline understanding of how these factors influence Si exports across biomes is essential for understanding freshwater and marine C cycles, especially during this period of rapid climatic warming. This synthesis will answer three specific research questions related to the roles of 1) terrestrial vegetation, 2) river productivity and 3) climate warming in controlling river Si exports across biomes. Our proposed sites span the globe (e.g., Antarctic, tropical, temperate, boreal, alpine, Arctic systems), and present a unique cross-network opportunity to connect LTER-based research with that of the Critical Zone Observatory and USGS. Together, we will create the first data-driven predictive framework of how riverine Si exports will respond to global change.
Identifying environmental drivers of plant reproduction across LTER sites
- assess how generalizable temporal patterns of mast seeding are across species and disparate locations;
- test how environmental drivers and past performance influence mast seeding along a continuum from non-masting (i.e., low temporal variability) to strongly masting (i.e., high temporal variability) species; and
- compare statistical approaches for finding environmental drivers for plant reproduction.
Marine consumer nutrient dynamics
Consumer-mediated nutrient dynamics of marine ecosystems under the wake of global change
Increases in the frequency and severity of disturbance events as a result of global change are altering population and community dynamics of marine animals. Given that animals are key recyclers of nutrients in many ecosystems, these ecological impacts may have consequences for ecosystem function. Consumer-mediated nutrient dynamics (CND) are an integral part of biogeochemical cycles, but to-date long-term studies are lacking. Without long-term data across large spatial scales, it is difficult to predict how ecosystems will respond to disturbances. The synthesis group plans to estimate CND over broad spatiotemporal scales by integrating empirical models of consumer nutrient excretion and egestion with time-series of consumer populations across ten marine and coastal LTER sites. They will address two main objectives:
- characterizing spatiotemporal patterns in the magnitude and variability of CND and;
- evaluating the resilience of CND to variable disturbance events.
This is a revised proposal based on positive feedback of the group's 2018 submission and input from colleagues during a cross-site workshop at the 2022 LTER ASM. With funds from LTER LNO, this diverse working group will synthesize LTER data to improve understanding of CND over broad spatiotemporal scales under the wake of global change.
Pelagic community structure
Interannual variability and long term change in pelagic community structure across a latitudinal gradient
Recent synthesis has shown both similarities and differences in how pelagic marine ecosystems have been influenced by cyclic and long term changes in the marine environment. The pelagic community structure synthesis group uses comparative data to test a series of conceptual models describing how communities respond to stochastic and long-term change along the latitudinal gradient represented by the four participating LTER sites. Their multipronged team approach employs two major lines of enquiry:
- examining whether patterns & processes discovered in the California Current Ecosystem (CCE) apply to other pelagic sites, and
- exploring whether recently proposed global pelagic community responses apply to the LTER sites, including how such responses are modulated by season and how they may have changed over decadal time frames.
Producers, Consumers and Disturbance
Response of Primary Producers and Primary Consumers to Environmental Change: from small-scale disturbances to seasonal and long-term changes
This LTER SPARC Synthesis Working Group brings together LTER researchers interested in understanding how disturbances and environmental change across timescales are altering the production and transfer of organic matter from primary producers to herbivores. All ecosystems are subject to temporal variations in biological production and consumption over broad time scales (from diel to decadal) in response to changes in the environment. Understanding the flow of C and energy from primary producers to their consumers provides essential information about ecosystem properties and functions. Both terrestrial and aquatic ecologists assess ecosystem primary production and the amount of autotrophic C transferred to higher trophic levels, irrespective of how challenging it is to assess these transfer rates. The study of primary producers and consumer interactions is essential to fully understand and predict the ecosystems response to anticipated increased disturbances and environmental changes driven by anthropogenic activities. The group's goals include synthesizing the current status and identifying research needs to establish a mechanistic and predictive understanding of the trophic interactions from primary producers to their consumers.
Selection across scales
Selection across scales—merging evolutionary biology and community ecology to understand trait shifts in response to environmental change
Selection acts on traits at both the community level, determining community assembly, and at the population level, determining the outcome of evolution. Selection at both scales combines with phenotypic plasticity to cause shifts in community-level mean trait values (average species trait values weighted by their relative abundance) in response to environmental change. If selection is typically concordant and populations and communities respond in the same direction, then responses to selection within species will amplify shifts in community mean trait values. In contrast, if selection at population and community scales are not correlated or occur in opposite directions, shifts in community mean trait values will be lower than expected based on shifts in species abundances. Plasticity will influence community-level trait values in similar ways: when plastic shifts parallel/oppose selection, community mean trait value changes will be amplified/reduced. The Selection across Scales synthesis group combines the expansive community composition data from LTER experiments with approaches, ideas, and datasets from evolutionary biology to investigate whether plasticity, selection at the population scale, and selection at the community scale are concordant or discordant. The group's framework and findings will help predict long-term shifts in the community-level mean trait values that determine ecosystem functions.
Soil phosphorus control of carbon and nitrogen
Do actively cycling C and N pools depend ultimately on soil P supply? A cross-biome synthesis
In terrestrial systems the nitrogen cycle is more open than the phosphorus cycle. New N accumulates by biological N fixation and atmospheric deposition, and is readily lost from the system when N is in excess of biological demand. In contrast, available P is supplied from more slowly cycling soil pools already present in the system. Thus, long term rates of ecosystem N accumulation may be constrained by the rate at which available P is provided from stocks of slowly cycling P. In a related workshop at the LTER All Scientists' Meeting (ASM), participants found soil N to be positively correlated with both total and slowly available soil P within each of nine long-term research sites across North America, including six LTER sites. The SPARC project includes ASM workshop participants as well as additional members. The objectives of this synthesis are to:
- produce a paper synthesizing the dependency of soil N accumulation on slowly cycling P within sites, based on the results of the ASM workshop,
- compile a comprehensive database for soil P stocks across terrestrial LTER sites including information on the availability of related datasets on productivity and nutrient cycling, and
- scope additional synthesis papers and potential activities.
The Flux Gradient Project
The Flux Gradient Project: Understanding the methane sink-source capacity of natural ecosystems
While biogenic CH4 emissions are thought to be of a similar magnitude to anthropogenic emissions, biogenic emissions remain the most uncertain source of the global CH4 budget. The vast areas with relatively small uptake and emission rates have been largely understudied but could contribute significantly to regional and global budgets. Upland ecosystems can exhibit unexpectedly large annual CH4 fluxes and should not be excluded from observation networks. Yet, current eddy covariance towers measuring CH4 fluxes are biased toward wetlands, and other areas where we expect to observe large fluxes. To improve our understanding of biogenic fluxes, the Flux Gradient Project will utilize infrastructure from the National Ecological Observatory Network (NEON) at co-located LTER-NEON sites to calculate CH4 fluxes. In addition to the fluxes at co-located sites, we will also utilize CH4 fluxes from LTER, Ameriflux and Fluxnet sites. We hypothesize that upland ecosystems will fluctuate from being a sink to a source depending on moisture conditions. Quantifying the CH4 budget of natural ecosystems is important for assessing realistic pathways to mitigate climate change, because uncertainties in the magnitude, size, and location of sources and sinks are currently limiting budget development.
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