Arctic LTER – Alaska
Arctic Tundra LTER

DETECTION OF REGIONAL WARMING. Long-term observations show a biotic change in the plant communities at Toolik Lake that correlate with the 30-yr-temperature increase in northern Alaska (0.5 degrees per decade). Mosses are reduced, birch and other shrubs increased. If this alteration continues with climate change, then native subsistence hunting will be affected as caribou will avoid birch.

DETRIMENTAL EFFECTS OF ENHANCING FISH PRODUCTION. Stream fertilization resulted in a dominance of mosses after a decade of low-level additions of nutrients. Management strategies for enhancing fish production in streams throughout the world must be modified to prevent dominance mosses.

GREENHOUSE GASES MOVE TO ATMOSPHERE THROUGH AQUATIC PATHWAYS. In the Arctic, much of the dissolved carbon and trace gases (CO2 and CH4) in the soil move into groundwater, streams, and lakes before release to the atmosphere or transport to the ocean. A series of LTER cross-site workshops determined that the importance of surface waters in carbon cycling at landscape-level scales is a common phenomenon throughout the world, and is not confined to arctic or wetland regions.

Bonanza Creek LTER – Alaska

GENERAL WARMING TREND THROUGHOUT ALASKA. 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. Dendrochronological analysis of white spruce also reflects this climatic shift and show that this warming is unprecedented in the past 200 years. Recent warming has caused the active layer and permafrost surface temperatures to increase by 1-2°C, so that annual temperatures at the ground surface and in the upper active layer exceed the freezing point. Permafrost remains stable at some sites only because of the insulative effects of moss and a thick layer of organic soil, but is thawing at other sites creating thermokarst. Permafrost temperatures were relatively stable from the 1950s to the mid-1970s, but have increased in response to the climatic warming that started in 1977.

DROUGHT CONTROLS SPRUCE PRODUCTION. Tree ring-climate correlations confirmed that drought exerts stronger control over production of upland white spruce than does temperature. Against this background of climatic controls over tree growth, we can identify other important stand-level controls over growth, i.e., snow breakage events triggered bark beetle attacks that caused a 3-year reduction in radial growth.

Harvard Forest LTER - Massachusetts
Harvard Forest Website
WARMER TEMPERATURES INCREASE CO₂ UPTAKE. 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.
 

Hubbard Brook LTER – New Hampshire
Hubbard Brook Experimental Forest Website
SULPHATE DEPOSITION DECREASES.Long-term decreases in sulfate concentrations in precipitation and streamwater at the Hubbard Brook Experimental Forest closely correspond to decreases in sulfur dioxide emissions in the eastern U.S. (Driscoll et al. 1998).
MARKED DEPLETION OF LABILE SOIL CALCIUM pools due to strong acid leaching, which has delayed recovery of surface waters from acidic deposition (Bailey et al. 1996; Likens et al. 1996,1998).
CONTROL OF NITRATE LOSSES DEPEND ON TEMPORAL SCALE. Nitrate losses in streamwater at the Hubbard Brook Experimental Forest are regulated over the long-term (decade to century) by cutting history (Pardo et al. 1995) and over the short-term (year to decade) by climatic conditions (Mitchell et al. 1996; Aber and Driscoll 1997).
FOREST PRODUCTIVITY IS ZERO.The northern hardwood forest at the Hubbard Brook Experimental Forest has stopped accumulating living biomass since about 1982 at an age of ca. 65 yr (Likens et al. 1994, 1998).
TRIFLUORACETATE RETAINED IN ECOSYSTEMS.Trifluoroacetate, a by-product of CFC replacement chemicals, is retained in soil organic matter rather than being conservatively transported via drainage water as previously thought. (Richey et al. 1997; Likens et al. 1997).
WHOLE-TREE HARVESTING ALTERS CARBON BUDGETS of the northern hardwood forest due to forest floor losses, decomposition of residual biomass (roots and stumps), retention of organic carbon in the mineral soil, increased soil respiration and increased leaching of dissolved organic carbon (Johnson et al. 1995a).
DECREASES OF ATMOSPHERIC DEPOSITION OF LEAD have resulted in decreases in the lead content of the forest floor, with ultimate accumulation of lead occurring in the mineral soil (Johnson et al. 1995b).
DECLINING BIRD POPULATIONS: Bird populations at Hubbard Brook and the surrounding region have declined since measurements were initiated in 1969 (Holmes and Sherry 1997).
 

Luquillo Experimental Forest LTER – Puerto Rico

HUMAN IMPACTS ON TROPICAL FORESTS OUTLAST NATURAL DISTURBANCES. The effects of deforestation for agricultural use that occurred more than 60 years ago left a distinct "footprint" in terms of forest succession and species composition that has outlasted two major hurricanes since 1932.

McMurdo Dry Valleys LTER – Antarctica

GLOBAL WARMING ALTERS FOOD WEBS. The food-web in the MCM is now thought to be the simplest on Earth, with the majority of the soils comprised of one bacterial feeding nematode species and one omnivore species. Soil warming experiments indicate a change in the dynamics of the soil food web, suggesting that global warming would severely alter OM decomposition and nutrient cycling in the soils

North Temperate Lakes LTER – Wisconsin

DURATION OF LAKE ICE COVER DECREASES. 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.

EXTINCTION BEST PREDICTOR OF BIODIVERSITY. Species structures of fish communities in small lakes of northern Wisconsin and Finland are best 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.

EFFECTS OF EXOTICS SLOW TO APPEAR. Long-term data reveal timelags in effects of invaders on lake communities. In Sparkling Lake cisco went extinct 10 years after smelt invasion; in Trout Lake the loss in aquatic plant diversity was delayed as long as 20 years after rusty crayfish invasion.

RECOVERY AFTER ACIDIFICATION SLOW. After six years of recovery from acidification, the zooplankton community structure in the treatment basin of Little Rock Lake still differs greatly from the reference basin even though chemical recovery has occurred.

GLOBAL ICE PHENOLOGY DATABASE. A global database of ice phenology on 753 Northern Hemisphere lakes and rivers has been implemented for an international group of researchers, the lake ice analysis group.

Palmer Station LTER – Antarctica

WARMING TREND IN THE WESTERN ANTARCTIC PENINSULA. A recent workshop brought together a multidisciplinary group of scientists with a range of records to evaluate the paleohistory of the Western Antarctic Peninsula (WAP). These records Include: historical and modern instrument records, marine sediment cores, ice cores and Evidence from the excavation of abandoned penguin rookeries. All evidence, paleo to Modern, are consistent is showing a warming trend in the WAP in the recent past and especially during the latter half of this century. These combined records provide insight for climate change on multiple time scales, show the Palmer area is particularly sensitive to climate change and provides a new perspective with which to evaluate ecosystem response to this change.

Shortgrass Steppe LTER – Colorado

GRAZING HAS LITTLE EFFECT ON ECOSYSTEM. Over the past 15 years we have dealt with one of the most important issues facing both private and public land managers in the shortgrass steppe region - the effects of livestock grazing on ecosystem structure and function. Our results have indicated that livestock grazing has limited impacts on shortgrass steppe ecosystem structure and function, likely due to a long evolutionary history of grazing by bison and other large herbivores.

LANDUSE AFFECTS REGIONAL CLIMATE. Cultivation not only has strong impacts on ecosystem structure and function, but also 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.

Virginia Coast Reserve LTER – Virginia

STORMS RESHAPE COAST. Use of aerial surveys and Global Positioning System equipment show that winter storms have a major impact all along the coast. Beaches are eroded in excess of 50-m in many locations.

FLOODING ALLOWS MARSHES TO KEEP PACE WITH SEA LEVEL RISE. A study designed to simulate the effects of accelerated sea level rise on organic carbon production and consumption in salt marshes showed little effect of flooding on marsh grass production. The major effect of increased flooding was depression of sediment respiration. Another disturbance that accompanies increased flooding is wrack (dead and decaying marsh grass) deposition on live grass stands. Wrack caused death of the underlying marsh grass with perhaps a short-term increase in decomposition rates; however, over the long term sediment respiration rates were significantly reduced. Depressions in decomposition rates with no accompanying decrease in production may allow accretion in organic-rich marshes to keep pace with increasing sea level.

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