Highlights of Recent LTER Research

This brief list of highlights from recent LTER research is organized into the following categories:
Biodiversity studies
Regional-scale studies
Disturbance and Response
Socioeconomic Research

BIODIVERSITY STUDIES

Andrews Experimental Forest LTER
A recently completed PhD thesis concerning exotic plant species confirms (1) roads are major corridors for invasion, (2) disturbance frequency dramatically fosters invasion, (3) exotic species can become an unseen part of the biodiversity of a site in its seedbank, with the potential to become important components of early succession where they’ve not occurred before, and (4) fragmentation of heretofore undisturbed landscapes (via road construction and logging) can dramatically increase the latter process.
The Andrews Forest herbarium is being used by a State of Oregon task force, which is addressing statewide biodiversity patterns of plant species. Information from our carefully vouchered specimens is being merged with that from the Oregon State University herbarium collections to form a core database of distribution and relative abundance of species and their varieties. A similar project for the state of Washington has also used a subset of the specimens. Both projects are working closely with USGS/NBS.
A Master's thesis has explored the landscape patterns of distribution and abundance of reptiles and amphibians in aquatic and riparian habitats over a 40,000 acre (16,000 ha) watershed. Correlations between life history attributes and the geomorphology of the watershed are proving quite strong for many species. The next phase of this project will be to (1) design and test efficiency of different population monitoring methods, and (2) develop and test habitat-association models for selected species.

Konza Prairie Natural History Area LTER
Scott Collins (NSF and KNZ) published a paper in Science on the role that grazing can play in maintaining biodiversity in grasslands. Collins, S.L., A. K. Knapp, J.M. Briggs, J.M. Blair and E.M. Steinauer. 1998. Modulation of diversity by grazing and mowing in native tallgrass prairie. Science 280: 745-747.

North Temperate Lakes LTER
Species structures of fish communities in small lakes of northern Wisconsin and Finland are better predicted from extinction factors than from the degree of isolation. This results because reinvasion occurs many years to centuries after an extinction event in isolated lakes and the stamp of the extinction is left for observation.
Analysis of long-term Canadian data indicates that zooplankton species richness estimated from 12 years of annual data is about twice that from a single year of sampling. Species turnover, even with intense sampling, is high and mostly related to sampling error.
Long-term data reveal timelags in effects of invaders on lake communities. In Sparkling Lake Cisco went extinct 16 years after smelt invasion; in Trout Lake the loss in aquatic plant diversity was delayed as long as 20 years after rusty crayfish invasion. The loss of plant diversity appears reversible as crayfish exclosures resulted in plant regrowth.
Two previously undescribed species of aquatic fungi have been found in the LTER study lakes. These species occur on stems and leaves of rooted aquatic plants.
A spatially explicit dispersal model of exotic smelt showed that isolated lakes are a refuge for native species. With present rates of human transfer of smelt, 1,000 simulation years were required for smelt establishment in the isolated lakes.

REGIONAL SCALE STUDIES (more information on LTER Regionalization Studies)
H.J. Andrews Experimental Forest
Regional Carbon Dynamics
LTER, other NSF and NASA funds have been used to develop an analysis system to examine carbon sequestration dynamics for the last 20 years in the Pacific Northwest. The system currently integrates remote sensing, ecological modeling and GIS. Economic analysis is now being added so that monetary as well as ecological limits to the system can be examined. A key finding is that this region is a significant temperate zone source of carbon to the atmosphere [overview document Cohen et al. 1996. BioScience 46(11):836-844]

Bonanza Creek LTER
Extension of LTER results to the regional scale
We have parameterized the models LINKAGES, FORCYTE-10, and CENTURY based partially on data collected in the BNZ LTER project. The CENTURY model effectively simulated the observed results of C and N fertilization, providing a basis for modeling biogeochemical cycling at BNZ. Sensitivity analyses with these models highlighted the importance of: (1) root dynamics in C-budget models; (2) the effect of the vegetation canopy (specifically trees and moss) on soil temperature regime; (3) the difficulty of extrapolating processes from intensive sites to the North American boreal forest; and (4) the importance of precipitation in predicting future forest productivity in global change analysis. In addition we have worked with the EROS data center to develop maps of climatic and ecosystem parameters that will be essential for modeling in LTER3 (Table 1.3).

North Temperate Lakes
Regional analyses of lake ecosystems
Changes in lake ice phenology are being used as an indicator of global climate change and variability. Duration of ice cover has decreased in the last 150 years in lakes throughout the Northern Hemisphere. El Niño influences and interdecadal climatic shifts are not synchronous around the hemisphere.
The hydrologic position of a lake in the landscape is a concept, similar to the stream continuum concept, that explains many systematic differences among adjacent lakes in northern Wisconsin. Many chemical, physical, and biological (including human development) features and processes vary systematically with the position of the lake in the landscape. Comparison with other lake districts in the northern hemisphere suggests this is a recurring pattern of many, but not all, lake districts.
Chemical responses of lakes to a late 1980s drought throughout the upper midwest depended on the lakes' relative hydrologic settings. Lakes low in the landscape tended to respond in more predictable ways than lakes higher in the landscape
Coherence in the temporal behavior of lakes is providing a tool for making regional predictions of lake behavior in a lake district and region and for understanding how external climate and internal drivers control the behavior of individual lakes through time.

Bonanza Creek Experimental Forest
Climate
Climate research describing (1) the regional patterns of climate for Alaska, (2) the patterns of seasonal and interannual variability in climate at BNZ, and (3) the changes in microclimate caused by topography and vegetation succession, summarized climate data for the entire state and used a krieging routine to produce maps of monthly temperature, precipitation and climate zones for Alaska (Hammond and Yarie 1996). Contours of mean annual temperature show a general northward movement of the 0°C isolines from the 1960s to the 1980s, indicating a warming trend throughout Alaska.
Paleoecology Studies
Paleoecological studies have collected water and sediment samples on a transect of 50 lakes spanning a latitudinal gradient from 60-70° N to establish correlations between lake (e.g., depth, water temperature) and catchment (e.g., vegetation) properties. Preliminary data indicate latitudinal variation in diatom assemblages (I. Gregory-Eaves, pers. comm.). Aquatic pollen assemblages reflect lake depth (Edwards et al. submitted). At a treeline lake, the neoglacial (ca 3500-present) stable isotope, pollen, and macrofossil records indicate productivity changes coinciding with probable climatic fluctuations; the treeline, however, appears to have remained relatively stable. Age structure of trees and fire scars at latitudinal treeline indicate that a recent expansion of forest into tundra is related to an (yet unspecified) interaction with fire. Other research indicates regional expansion of white spruce ca 8500 yr BP is correlated with a major increase in effective moisture, consistent with LTER dendroecological data indicating strong moisture control over white spruce growth. Analyses of high-resolution Holocene pollen records at two lakes show fluctuations in the relative abundance of spruce and hardwoods at 200-500-yr intervals, which likely reflect non-climatic physical or biotic disturbances (Bigelow 1997).

Coweeta Hydrologic Laboratory
I. Land Use History and Socioeconomic Accomplishments
We have assembled a 50-year land use history of our 70,000 km2 regionalization study area based on aerial photographs (1950s) and satellite data (1970s and 1990s).
Sediment sampling from bogs in the region has been used to study the land use history over the last few millennia showing that both fire and vegetation history are extremely variable across the region.
Newly developed socioeconomic models indicate that the relationship between market factors, site physical variables, and land use choices have shifted substantially since the 1950s.
These same models have been used to predict future land-use across the region and identify sites particularly susceptible to heavy impacts and disturbance.

Shortgrass Steppe
In the past three years, our scientists have studied the impacts of landuse management, and have found that cultivation not only has strong impacts on ecosystem structure and function, but has a strong forcing effect on regional climate. Regional landuse patterns appear to have a larger impact on regional climate than does the signal from global greenhouse-induced climate change.

DISTURBANCE STUDIES
Harvard Forest
An eight-year study of the exchange of CO2 between Harvard Forest and the atmosphere revealed the following: The forest was observed to sequester an average of 2.1 tonnes carbon per ha each year, reflecting regrowth after disturbance in the early 19th and 20th centuries. Interannual variations (±50%) in CO2 uptake were observed in response to variations in climate: warmer temperatures correlated with more net uptake of CO2, opposite to expectations from simple ecosystem models.
The long-term nature of this study is the most important factor in the results. LTER provided the site, a wealth of valuable background data , interdisciplinary colleagues and the leverage to get other agencies' funding.
The results are relevant to a range of policy issues that did not exist when the scientific investigation started (e.g., Kyoto). The work led to initiation of two networks of similar sites, one in the US and one in Europe.

Luquillo Experimental Forest
Recently analyzed data from the latest census (1995-96) of the Hurricane Recovery Plot (Jill Thompson, Nick Brokaw, and Jess Zimmerman) indicate that Hurricane Hugo (1989) did not have a major impact on the community composition of forest in the Luquillo Experimental Forest. While 9% of trees died as a result of the hurricane, death was most common in a handful of species and the remaining species showed little impact. Whatever losses occurred were balanced to a degree by recruitment. The major exception was the pioneer tree Cecropia schreberiana which increased in abundance nine-fold after the hurricane. We identified C. schreberiana as a pivotal species in post-hurricane recovery of the forest in our 1994 proposal. Its premier role in the recovery of the plot supports this hypothesis. All in all, these data support the idea that forests of Puerto Rico exhibit striking resilience to hurricane disturbance Fred Scatena and other LUQ researchers are monitoring the Mamayes River for effects of water abstraction on stream organisms. Fish and freshwater shrimp must migrate from headwaters to estuaries and back as part of their life cycle. Dams obstruct these migrations and water abstraction, at low flows, can completely obliterate downstream migration of juveniles and damage estuaries below by removing all incoming freshwater. Following pressure to change their ways, local water authorities changed the design of a new water filtration plant to take groundwater adjacent to the stream without building a dam and to maintain minimum stream flows. LUQ researchers are cooperating to determine which of these new methods lessen impacts on stream organisms and estuaries.

Mcmurdo Dry Valleys
The simple food chains in MCM soils appear to be strongly influenced by human disturbance. Soil warming and increased moisture and carbon availability decrease the abundance of the omnivore-predator species, increase the abundance of a microbivorous species (Freckman and Virginia 1997) and alter soil respiration (CO2 efflux).

Bonanza Creek Experimental Forest
Disturbance regime
We have used permanent plots to demonstrate the impacts of disturbance on the structure, composition, establishment, and mortality of forests and to use historical records and tree-ring chronologies to extend our observations back in time (Juday and Marler 1997). These data become the basis of models that were initially largely conceptual (Van Cleve et al. 1991) and which now operate only at large temporal and spatial scales (Starfield and Chapin 1996).
Fire is the major disturbance in interior Alaska. Vegetation distribution and fire scar analysis at BNZ suggest a fire return interval of 70 to 110 years. Maps of lightning strikes are a good predictor of fire frequency, but the area burned is influenced more strongly by climate and vegetation. Logging is extensive only near transportation corridors (the Pacific Ocean and interior road networks) but has become an issue of public concern. The heavy sediment load of the glacier-fed Tanana River supports an aggrading system resulting in a dynamic equilibrium between active erosion and silt bar formation. We are developing a 200-yr chronology of river height through correlations of (1) river height at LTER sites along the Tanana River with that at USGS gauging stations (1962 - present), (2) river height at gauging stations with ring width of white spruce, and (3) a 200-yr ring-width chronology for the floodplain.
Insect outbreaks are extensive only in more continental regions of southern Alaska, where climate is relatively warm. Here they have eliminated spruce forests over broad areas, leading to extensive areas of grasslands. High population levels of spruce budworm were first observed in BNZ in 1989. Repeated defoliation in 1991 and 1992 caused top-kill in trees and mortality in seedlings and saplings. Outbreaks of bark beetles also caused significant mortality in 1993. We are extending these records back in time based on distinctive signatures in tree rings.
We are just beginning to explore the role of pathogens in boreal forest. Barley yellow dwarf virus infects Calamagrostis canadensis at relatively high rates (~30%; Malmstrom, unpublished). This grass strongly competes with spruce seedlings during establishment and could change the successional trajectory from forest to grassland following disturbance. We are initiating a study on the role of the virus as a regulator of successional trajectory.

SOCIOECONOMIC RESEARCH
Central Arizona - Phoenix
The urban fringe project tracks the spatial distribution of the expanding urban fringe between 1990 and 1997. A clear interpretation of data analyzed thus far is that every location is changing in metropolitan Phoenix. A donut spatial structure results. While housing loss occurs in central locations, net residential densities increase at the urban fringe.

Coweeta Hydrologic Laboratory
Land Use History and Socioeconomic Accomplishments
We have assembled a 50-year land use history of our 70,000 km2 regionalization study area based on aerial photographs (1950s) and satellite data (1970s and 1990s).
Sediment sampling from bogs in the region has been used to study the land use history over the last few millennia showing that both fire and vegetation history are extremely variable across the region.
Newly developed socioeconomic models indicate that the relationship between market factors, site physical variables, and land use choices have shifted substantially since the 1950s.
These same models have been used to predict future land-use across the region and identify sites particularly susceptible to heavy impacts and disturbance.