Springer Science and Business Media LLC

Northern lakes are ice-covered for considerable portions of the year, where carbon dioxide (CO2) can accumulate below ice, subsequently leading to high CO2 emissions at ice-melt. Current knowledge on the regional control and variability of below ice partial pressure of carbon dioxide (pCO2) is lacking, creating a gap in our understanding of how ice cover dynamics affect the CO2 accumulation below ice and therefore CO2 emissions from inland waters during the ice-melt period. To narrow this gap, we identified the drivers of below ice pCO2 variation across 506 Swedish and Finnish lakes using water chemistry, lake morphometry, catchment characteristics, lake position, and climate variables. We found that lake depth and trophic status were the most important variables explaining variations in below ice pCO2 across the 506 lakes. Together, lake morphometry and water chemistry explained 53% of the site-to-site variation in below ice pCO2. Regional climate (including ice cover duration) and latitude only explained 7% of the variation in below ice pCO2. Thus, our results suggest that on a regional scale a shortening of the ice cover period on lakes may not directly affect the accumulation of CO2 below ice but rather indirectly through increased mobility of nutrients and carbon loading to lakes. Thus, given that climate-induced changes are most evident in northern ecosystems, adequately predicting the consequences of a changing climate on future CO2 emission estimates from northern lakes involves monitoring changes not only to ice cover but also to changes in the trophic status of lakes.

This study evaluated the hydrologic sensitivity of vernal pool ecosystems in the Central Valley of California to climatic changes projected for 2100. A vernal pool water-balance model was used to evaluate rain-fed vernal pools at four locations under future conditions projected by two contrasting global climate models. The potential for change in the duration of continuous inundation, frequency of reproductively suitable inundation events, and the seasonal distribution of inundation was quantified. The potential impact of hydrologic changes varied by species and by location. Three scales of response were identified: (a) At the regional scale, pools in the middle of the Central Valley near Merced were the most responsive to climatic changes. (b) At the local scale, smaller, shallower pools had the greatest potential to change the distribution of reproductively suitable habitat available to branchiopods. (c) At the individual pool scale, changes in precipitation will dominate changes in temperature, resulting in relatively linear responses in the duration of inundation. The ecological impact of these changes will be determined by a balance between the increasing suitability of vernal pools for branchiopod predators and the hydrologic improvement of currently marginal habitats.

Increased aridity may have severe effects on productivity of dry forests. However, it remains unclear to what degree the positive effects of elevated CO2 (both increased carboxylation rates and enhanced water-use efficiency) may offset the negative effects of drought and climate warming. In forest ecosystems, it is particularly challenging to evaluate CO2 effects on productivity because the impacts of climate variability, competition, and management, combine to have longlasting effects on stand-level productivity. Here we address this problem using a unique long-term database containing repeated inventories of wood biomass for every decade from 1912 to 2002 in a pine forest (Pinus pinaster Ait.) in central Spain (≈7,500 ha.). The approach is based upon a combination of statistical analyses of long-term historical management data and mechanistic modeling which allows us to evaluate the effects of potential CO2 fertilization, climate, and stand structure on woody net primary production (W-NPP). We found a significant negative effect of drought on W-NPP during the first half of the twentieth century that diminishes at the turn of the century. Simulations with a process-based ecosystem model, ORCHIDEE, suggest that wood production under conditions that included CO2 fertilization produced a more highly correlated long-term W-NPP than simulations keeping CO2 values in preindustrial levels. Interestingly, however, the CO2 effect was only apparent when accounting for confounding factors such as competition and management legacies. Identifying CO2 fertilization on forest growth is a critical issue, and requires partitioning CO2 effects from confounding factors that have jointly shaped stand dynamics and carbon balance during the twentieth century.

Restoration of channelized streams by returning coarse sediment from stream edges to the wetted channel has become a common practice in Sweden. Yet, restoration activities do not always result in the return of desired biota. This study evaluated a restoration project in the Vindel River in northern Sweden in which practitioners further increased channel complexity of previously restored stream reaches by placing very large boulders (>1 m), trees (>8 m), and salmonid spawning gravel from adjacent upland areas into the channels. One reach restored with basic methods and another with enhanced methods were selected in each of ten different tributaries to the main channel. Geomorphic and hydraulic complexity was enhanced but the chemical composition of riparian soils and the communities of riparian plants and fish did not exhibit any clear responses to the enhanced restoration measures during the first 5 years compared to reaches restored with basic restoration methods. The variation in the collected data was among streams instead of between types of restored reaches. We conclude that restoration is a disturbance in itself, that immigration potential varies across landscapes, and that biotic recovery processes in boreal river systems are slow. We suggest that enhanced restoration has to apply a catchment-scale approach accounting for connectivity and availability of source populations, and that low-intensity monitoring has to be performed over several decades to evaluate restoration outcomes.

Strong evidence for a pan-Arctic expansion of upright shrubs comes from analysis of satellite imagery, historical photographs, vegetation plots, and growth rings. However, there are still uncertainties related to local-scale patterns of shrub growth, resulting interactions among vegetation functional groups, and the relative roles of disturbance and climate as drivers of observed change. Here, we present evidence that widespread and rapid shrub expansion and lichen declines over a 15,000 km2 area of the western Canadian Arctic have been driven by regional increases in temperature. Using 30 m resolution Landsat satellite imagery and high resolution repeat color-infrared aerial photographs, we show that 85% of the land surface has a positive 1985–2011 trend (P < 0.05) in NDVI, making this one of the most intensely greening regions in the Arctic. Strong positive trends (>0.03 NDVI/decade) occurred consistently across all landscape positions and most vegetation types. Comparison of 208, 1:2,000 scale vertical air photo pairs from 1980 and 2013 clearly shows that this greening was driven by increased canopy cover of erect dwarf and tall shrubs, with declines in terricolous lichen cover. Disturbances caused by wildfires, exploratory gas wells, and drained lakes all produced strong, yet localized increases in NDVI due to shrub growth. Our analysis also shows that a 4°C winter temperature increase over the past 30 years, leading to warmer soils and enhanced nutrient mineralization provides the best explanation for observed vegetation change. These observations thus provide early corroboration for modeling studies predicting large-scale vegetation shifts in low-Arctic ecosystems from climate change.

Lakes and reservoirs globally are experiencing unprecedented changes in land use and climate, depleting dissolved oxygen (DO) in the bottom waters (hypolimnia) of these ecosystems. Because DO is the most energetically favorable terminal electron acceptor (TEA) for organic carbon mineralization, its availability controls the onset of alternate TEA pathways (for example, denitrification, manganese reduction, iron reduction, sulfate reduction, methanogenesis). Low DO concentrations can trigger organic carbon mineralization via alternate TEA pathways in the water column and sediments, which has important implications for greenhouse gas production [carbon dioxide (CO2) and methane (CH4)]. In this study, we experimentally injected supersaturated DO into the hypolimnion of a eutrophic reservoir and measured concentrations of TEAs and terminal electron products (TEPs) in the experimental reservoir and an upstream reference reservoir. We calculated the electron equivalents yielded from each TEA pathway and estimated the contributions of each TEA pathway to organic carbon processing in both reservoirs. DO additions to the hypolimnion of the experimental reservoir promoted aerobic respiration, suppressing most alternate TEA pathways and resulting in elevated CO2 accumulation. In comparison, organic carbon mineralization in the reference reservoir’s anoxic hypolimnion was dominated by alternate TEA pathways, resulting in both CH4 and CO2 accumulation. Our ecosystem-scale experiments demonstrate that the alternate TEA pathways that succeed aerobic respiration in lakes and reservoirs can be manipulated at the ecosystem scale. Moreover, changes in the DO dynamics of freshwater lakes and reservoirs may result in concomitant changes in the redox reactions in the water column that control organic carbon mineralization and greenhouse gas accumulation.

High rates of deforestation in the Brazilian Amazon have the potential to alter the storage and cycling of carbon (C) and nitrogen (N) across this region. To investigate the impacts of deforestation, we quantified total aboveground biomass (TAGB), aboveground and soil pools of C and N, and soil N availability along a land-use gradient in Rondônia, Brazil, that included standing primary forest, slashed primary and secondary forest, shifting cultivation, and pasture sites. TAGB decreased substantially with increasing land use, ranging from 311 and 399 Mg ha–1 (primary forests) to 63 Mg ha–1 (pasture). Aboveground C and N pools declined in patterns and magnitudes similar to those of TAGB. Unlike aboveground pools, soil C and N concentrations and pools did not show consistent declines in response to land use. Instead, C and N concentrations were strongly related to percent clay content of soils. Concentrations of NO3-N and NH4-N generally increased in soils following slash-and-burn events along the land-use gradient and decreased with increasing land use. Increasing land use resulted in marked declines in NO3-N pools relative to NH4-N pools. Rates of net nitrification and N-mineralization were also generally higher in postfire treatments relative to prefire treatments along the land-use gradient and declined with increasing land use. Results demonstrate the linked responses of aboveground C and N pools and soil N availability to land use in the Brazilian Amazon; steady reductions in aboveground pools along the land-use gradient were accompanied by declines in inorganic soil N pools and transformation rates.