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Local climatic drivers of changes in phenology at a boreal-temperate ecotone in eastern North America
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Ecosystems in biogeographical transition zones, or ecotones, tend to be highly sensitive to climate and can provide early indications of future change. To evaluate recent climatic changes and their impacts in a boreal-temperate ecotone in eastern North America, we analyzed ice phenology records (1975–2007) for five lakes in the Adirondack Mountains of northern New York State. We observed rapidly decreasing trends of up to 21 days less ice cover, mostly due to later freeze-up and partially due to earlier break-up. To evaluate the local drivers of these lake ice changes, we modeled ice phenology based on local climate data, derived climatic predictors from the models, and evaluated trends in those predictors to determine which were responsible for observed changes in lake ice. November and Decem- ber temperature and snow depth consistently predicted ice-in, and recent trends of warming and decreasing snow during these months were consistent with later ice formation. March and April temperature and snow depth consistently predicted ice-out, but the absence of trends in snow depth during these months, despite concurrent warming, resulted in much weaker trends for ice-out. Recent rates of warming in the Adirondacks are among the highest regionally, although with a different seasonality of changes (early winter > late winter) that is consistent with other lake ice records in the surrounding area. Projected future declines in snow cover could create positive feedbacks and accelerate current rates of ice loss due to warming. Climate sensitivity was greatest for the larger lakes in our study, including Wolf Lake, considered one of the most ecologically intact ‘wilderness lakes’ in eastern North America. Our study provides further evidence of climate sensitivity of the boreal-temperate ecotone of eastern North America and points to emergent conservation challenges posed by climate change in legally protected yet vulnerable landscapes like the Adirondack Park.
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The River Discontinuum: Applying Beaver Modifications to Baseline Conditions for Restoration of Forested Headwaters
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Billions of dollars are being spent in the United States to restore rivers to a desired, yet often unknown, reference condition. In lieu of a known reference, practitioners typically assume the paradigm of a connected watercourse. Geological and ecological processes, however, create patchy and discontinuous fluvial systems. One of these processes, dam building by North American beavers (Castor canadensis), generated discontinuities throughout precolonial river systems of northern North America. Under modern conditions, beaver dams create dynamic sequences of ponds and wet meadows among free-flowing segments. One beaver impoundment alone can exceed 1000 meters along the river, flood the valley laterally, and fundamentally alter biogeochemical cycles and ecological structures. In this article, we use hierarchical patch dynamics to investigate beaver-mediated discontinuity across spatial and temporal scales. We then use this conceptual model to generate testable hypotheses addressing channel geomorphology, natural flow regime, water quality, and biota, given the importance of these factors in river restoration.
Keywords: fluvial geomorphology, hierarchical patch dynamics, stream ecology, river continuum concept, river restoration
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Migrating Like a Herd of Cats: Climate Change and Emerging Forests in British Columbia
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We combine climate tolerances of tree species with probable changes in insect, disease, fire, and other abiotic factors to describe probable changes in distribution of tree species in British Columbia. Predicting changes in forests confronts three major sources of uncertainty: predicting weather and climate, predicting tree species’ responses, and predicting changes in factors modifying the trees’ responses (e.g., pathogens, insects, and fire). Challenges in predicting weather exist because climate projection models differ and downscaling climate is difficult, particularly where weather stations are sparse. Challenges in predicting responses of individual tree species to climate are a result of species competing under a climate regime that we have not seen before and they may not have experienced before. This challenge is aggravated by the differential response of pathogens and insects, as well as by the effects of changes in fire frequency. We first examine the responses of in- dividual species, then we consider implications for broad regional forests. Despite the uncertainty, some trends are more likely than others. We present estimates of the relative species composition of future forests in British Columbia.
KEYWORDS: climate change; insects; new regional forests; pathogens; moisture stress; tree migration
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Key role of symbiotic dinitrogen fixation in tropical forest secondary succession
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Forests contribute a significant portion of the land carbon sink, but their ability to sequester CO2 may be constrained by nitrogen 1–6, a major plant-limiting nutrient. Many tropical forests possess tree species capable of fixing atmospheric dinitrogen (N2)7, but it is unclear whether this functional group can supply the nitrogen needed as forests recover from disturbance or previous land use1, or expand in response to rising CO2 (refs 6, 8). Here we identify a powerful feedback mechanism in which N2 fixation can overcome ecosystem- scale deficiencies in nitrogen that emerge during periods of rapid biomass accumulation in tropical forests. Over a 300-year chronose- quence in Panama, N2-fixing tree species accumulated carbon up to nine times faster per individual than their non-fixing neighbours (greatest difference in youngest forests), and showed species-specific differences in the amount and timing of fixation. As a result of fast growth and high fixation, fixers provided a large fraction of the nitrogen needed to support net forest growth (50,000 kg carbon per hectare) in the first 12 years. A key element of ecosystem functional diversity was ensured by the presence of different N2-fixing tree species across the entire forest age sequence. These findings show that symbiotic N2 fixation can have a central role in nitrogen cycling during tropical forest stand development, with potentially important implications for the ability of tropical forests to sequester CO2.
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Predator-induced reduction of freshwater carbon dioxide emissions
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Predators can influence the exchange of carbon dioxide between ecosystems and the atmosphere by altering ecosys- tem processes such as decomposition and primary production, according to food web theory1,2. Empirical knowledge of such an effect in freshwater systems is limited, but it has been suggested that predators in odd-numbered food chains sup- press freshwater carbon dioxide emissions, and predators in even-numbered food chains enhance emissions2,3. Here, we report experiments in three-tier food chains in experimental ponds, streams and bromeliads in Canada and Costa Rica in the presence or absence of fish (Gasterosteus aculeatus) and invertebrate (Hesperoperla pacifica and Mecistogaster mod- esta) predators. We monitored carbon dioxide fluxes along with prey and primary producer biomass. We found substan- tially reduced carbon dioxide emissions in the presence of predators in all systems, despite differences in predator type, hydrology, climatic region, ecological zone and level of in situ primary production. We also observed lower amounts of prey biomass and higher amounts of algal and detrital biomass in the presence of predators. We conclude that predators have the potential to markedly influence carbon dioxide dynamics in freshwater systems.
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Natural and Beneficial Floodplain Functions: Floodplain Management— More than Flood Loss Reduction
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This is a position paper prepared by the Association of State Floodplain Managers, (ASFPM), a non-profit professional organization dedicated to reducing flood losses and protecting floodplain functions and resources in the United States.
Background
With the passage of the National Environmental Policy Act over three decades ago, the United States established a foundation for protecting the environment amidst human development. In Section 101 of the Act, Congress declared that
. . . it is the continuing policy of the Federal Government, in cooperation with State and local governments, and other concerned public and private organizations, to use all practicable means and measures, including financial and technical assistance, in a manner calculated to foster and promote the general welfare, to create and maintain conditions under which man and nature can exist in productive harmony, and fulfill the social, economic, and other requirements of present and future generations of Americans.
However, the reality is that we seldom achieve this “productive harmony” with regard to our rivers, streams, wetlands, and coastal lowlands. As we move into the new century, we face hard choices about our riverine and coastal floodplains1. Relatively unfettered economic development, with only a token allowance made for floodplain functions and resources, cannot continue as the status quo. Instead, we need to strike a balance between development and the benefits that would be realized if we were to protect the natural functions of floodplains and coastal areas.
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Modeling sediment accumulation in North American playa wetlands in response to climate change, 1940–2100
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Playa wetlands on the west-central Great Plains of North America are vulnerable to sediment infilling from upland agriculture, putting at risk several important ecosystem services as well as essential habitats and food resources of diverse wetland-dependent biota. Climate predictions for this semi-arid area indicate reduced precipitation which may alter rates of erosion, runoff, and sedimentation of playas. We forecasted erosion rates, sediment depths, and resultant playa wetland depths across the west-central Great Plains and exam- ined the relative roles of land use context and projected changes in precipitation in the sedimentation process. We estimated erosion with the Revised Universal Soil Loss Equation (RUSLE) using historic values and downscaled precipitation predictions from three general circulation models and three emissions scenarios. We calibrated RUSLE results using field sediment measurements. RUSLE is appealing for regional scale modeling because it uses climate forecasts with monthly resolution and other widely available values including soil texture, slope and land use. Sediment accumulation rates will continue near historic levels through 2070 and will be sufficient to cause most playas (if not already filled) to fill with sediment within the next 100 years in the absence of mitigation. Land use surrounding the playa, whether grassland or tilled cropland, is more influential in sediment accumulation than climate-driven precipitation change.
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Model Projections of an Imminent Transition to a More Arid Climate in Southwestern North America
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How anthropogenic climate change will impact hydroclimate in the arid regions of Southwestern North America has implications for the allocation of water resources and the course of regional development. Here we show that there is a broad consensus amongst climate models that this region will dry significantly in the 21st century and that the transition to a more arid climate should already be underway. If these models are correct, the levels of aridity of the recent multiyear drought, or the Dust Bowl and 1950s droughts, will, within the coming years to decades, become the new climatology of the American Southwest.
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Late Pleistocene California droughts during deglaciation and Arctic warming
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Recent studies document the synchronous nature of shifts in North Atlantic regional climate, the intensity of the East Asian monsoon, and productivity and precipitation in the Cariaco Basin during the last glacial and deglacial period. Yet questions remain as to what climate mechanisms influenced continental regions far removed from the North Atlantic and beyond the direct influence of the inter-tropical convergence zone. Here, we present U-series calibrated stable isotopic and trace element time series for a speleothem from Moaning Cave on the western slope of the central Sierra Nevada, California that documents changes in precipitation that are approximately coeval with Greenland temperature changes for the period 16.5 to 8.8 ka. From 16.5 to 10.6 ka, the Moaning Cave stalagmite proxies record drier and possibly warmer conditions, signified by elevated à18O, à13C, [Mg], [Sr], and [Ba] and more radiogenic 87Sr/86Sr, during Northern Hemisphere warm periods (Bølling, early and late Allerød) and wetter and possibly colder conditions during Northern Hemisphere cool periods (Older Dryas, Inter-Allerød Cold Period, and Younger Dryas). Moaning Cave stable isotope records indicate that wet conditions persisted in this area well beyond 11.5 ka, suggesting the effects of the Younger Dryas event may have been longer lived in the western Sierra Nevada than in Greenland. However, a shifting drip center and corresponding change in seepage water routing may have influenced the trace element records between 10.6 and 9.6 ka. Linkages between northern high-latitude climate and precipitation in the Sierra Nevada suggested here could indicate that, under conditions of continued global warming, this drought-prone region may experience a reduction in Pacific-sourced moisture.
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TUNDRA’S BURNING
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More than 20,000 lightning strikes were recorded on the North Slope of Alaska in 2007. Some struck the vast stretches of lakes; some hit the
treeless tundra. And one of them torched into life the largest and longest-lasting tundra fire recorded in the state’s history. The blaze, which started near the Anaktuvuk River on 16 July, burned 7,000 hectares a day at its peak, and eventually consumed 100,000 hectares, an area larger than that of New York City. It finally stopped burning in early October, smothered by thick snow. Arctic lightning fire
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