ALBERT

Description: Plant invasion is an emerging driver of global change worldwide. We aimed to disentangle its impacts on plant-soil nutrient concentrations. We conducted a meta-analysis of 215 peer-reviewed articles and 1233 observations. Invasive plant species had globally higher N and P concentrations in photosynthetic tissues but not in foliar litter, in comparison to their native competitors. Invasive plants were also associated with higher soil C and N stocks and N, P and K availabilities. The differences in N and P concentrations in photosynthetic tissues and in soil total C and N, soil N, P and K availabilities between invasive and native species decreased when the environment was richer in nutrient resources. The results thus suggested higher nutrient resorption efficiencies in invasive than in native species in nutrient-poor environments. There were differences in soil total N concentrations but not in total P concentrations, indicating that the differences associated to invasive plants were related with biological processes, not with geochemical processes. The results suggest that invasiveness is not only a driver of changes in ecosystem species composition but that it is also associated with significant changes in plant-soil elemental composition and stoichiometry. This article is protected by copyright. All rights reserved.

Description: During the eruption of the ice-covered Eyjafjallajökull volcano, a series of images from an airborne Synthetic Aperture Radar (SAR) were obtained by the Icelandic Coast Guard. Cloud obscured the summit from view during the first three days of the eruption, making the weather-independent SAR a valuable monitoring resource. Radar images revealed the development of ice cauldrons in a 200 m thick ice cover within the summit caldera, as well as the formation of cauldrons to the immediate south of the caldera. Additionally, radar images were used to document the subglacial and supraglacial passage of floodwater to the north and south of the eruption site. The eruption breached the ice surface about four hours after its onset at about 01:30 UTC on 14 April 2010. The first SAR images, obtained between 08:55 and 10:42 UTC, show signs of limited supraglacial drainage from the eruption site. Floodwater began to drain from the ice cap almost 5.5 h after the beginning of the eruption, implying storage of meltwater at the eruption site due to initially constricted subglacial drainage from the caldera. Heat transfer rates from magma to ice during early stages of cauldron formation were about 1 MW m−2 in the radial direction and about 4 MW m−2 vertically. Meltwater release was characterized by accumulation and drainage with most of the volcanic material in the ice cauldrons being drained in hyperconcentrated floods. After the third day of the eruption, meltwater generation at the eruption site diminished due to an insulating lag of tephra.

Description: Northern Europe supports large soil organic carbon (SOC) pools and has been subjected to high frequency of land-use changes during the past decades. However, this region has not been well represented in previous large-scale syntheses of land-use change effects on SOC, especially regarding effects of afforestation. Therefore, we conducted a meta-analysis of SOC stock change following afforestation in Northern Europe. Response ratios were calculated for forest floors and mineral soils (0-10 cm and 0-20/30 cm layers) based on paired control (former land use) and afforested plots. We analyzed the influence of forest age, former land use, forest type and soil textural class. Three major improvements were incorporated in the meta-analysis: analysis of major interaction groups, evaluation of the influence of non-independence between samples according to study design and mass correction. Former land use was a major factor contributing to changes in SOC after afforestation. In former croplands, SOC change differed between soil layers and was significantly positive (20%) in the 0-10 cm layer. Afforestation of former grasslands had a small negative (non-significant) effect indicating limited SOC change following this land-use change within the region. Forest floors enhanced the positive effects of afforestation on SOC, especially with conifers. Meta-estimates calculated for the periods 〈 30 years and 〉 30 years since afforestation revealed a shift from initial loss to later gain of SOC. The interaction group analysis indicated that meta-estimates in former land use, forest type and soil textural class alone were either offset or enhanced when confounding effects among variable classes were considered. Furthermore, effect sizes were slightly overestimated if sample dependence was not accounted for and if no mass correction was performed. We conclude that significant SOC sequestration in Northern Europe occurs after afforestation of croplands and not grasslands, and changes are small within a 30 years perspective. This article is protected by copyright. All rights reserved.

Description: Understanding and predicting how global warming affects the structure and functioning of natural ecosystems is a key challenge of the 21 st century. Isolated laboratory and field experiments testing global change hypotheses have been criticised for being too small-scale and overly simplistic, whereas surveys are inferential and often confound temperature with other drivers. Research that utilises natural thermal gradients offers a more promising approach and geothermal ecosystems in particular, which span a range of temperatures within a single biogeographic area, allow us to take the laboratory into nature rather than vice versa . By isolating temperature from other drivers, its ecological effects can be quantified without any loss of realism, and transient and equilibrial responses can be measured in the same system across scales that are not feasible using other empirical methods. Embedding manipulative experiments within geothermal gradients is an especially powerful approach, informing us to what extent small-scale experiments can predict the future behaviour of real ecosystems. Geothermal areas also act as sentinel systems by tracking responses of ecological networks to warming and helping to maintain ecosystem functioning in a changing landscape by providing sources of organisms that are pre-adapted to different climatic conditions. Here, we highlight the emerging use of geothermal systems in climate change research, identify novel research avenues, and assess their roles for catalysing our understanding of ecological and evolutionary responses to global warming. This article is protected by copyright. All rights reserved.

Description: Terrestrial carbon cycle feedbacks to global warming are major uncertainties in climate models. For in-depth understanding of changes in soil organic carbon (SOC) after soil warming, long-term responses of SOC stabilisation mechanisms such as aggregation, organo-mineral interactions and chemical recalcitrance need to be addressed. This study investigated the effect of six years of geothermal soil warming on different SOC fractions in an unmanaged grassland in Iceland. Along an extreme warming gradient of +0 to ~+40°C, we isolated five fractions of SOC that varied conceptually in turnover rate from active to passive in the following order: particulate organic matter (POM), dissolved organic carbon (DOC), SOC in sand and stable aggregates (SA), SOC in silt and clay (SC-rSOC) and resistant SOC (rSOC). Soil warming of 0.6°C increased bulk SOC by 22±43% (0-10 cm soil layer) and 27±54% (20-30 cm), while further warming led to exponential SOC depletion of up to 79±14% (0-10 cm) and 74±8% (20-30) in the most warmed plots (~+40°C). Only the SA fraction was more sensitive than the bulk soil, with 93±6% (0-10 cm) and 86±13% (20-30 cm) SOC losses and the highest relative enrichment in 13 C as an indicator for the degree of decomposition (+1.6±1.5 ‰ in 0-10 cm and +1.3±0.8 ‰ in 20-30 cm). The SA fraction mass also declined along the warming gradient, while the SC fraction mass increased. This was explained by deactivation of aggregate-binding mechanisms. There was no difference between the responses of SC-rSOC (slow-cycling) and rSOC (passive) to warming, and 13 C enrichment in rSOC was equal to that in bulk soil. We concluded that the sensitivity of SOC to warming was not a function of age or chemical recalcitrance, but triggered by changes in bio-physical stabilisation mechanisms, such as aggregation. This article is protected by copyright. All rights reserved.

Description: The phenology of vegetation, particularly the length of the growing season (LOS; i.e. the period from greenup to senescence), is highly sensitive to climate change, which could imply potent feedbacks to the climate system, e.g. by altering the ecosystem carbon (C) balance. In recent decades, the largest extensions of LOS have been reported at high northern latitudes, but further warming-induced LOS extensions may be constrained by too short photoperiod or unfulfilled chilling requirements. Here, we studied subarctic grasslands, which cover a vast area and contain large C stocks, but for which LOS changes under further warming are highly uncertain. We measured LOS extensions of Icelandic subarctic grasslands along natural geothermal soil warming gradients of different age (short-term, where the measurements started after 5 years of warming and long-term, i.e. warmed since ≥50 years) using ground-level measurements of normalized difference vegetation index (NDVI). We found that LOS linearly extended with on average 2.1 days per °C soil warming up to the highest soil warming levels (ca. +10°C) and that LOS had the potential to extend at least one month. This indicates that the warming impact on LOS in these subarctic grasslands will likely not saturate in the near future. A similar response to short- and long-term warming indicated a strong physiological control of the phenological response of the subarctic grasslands to warming, and suggested that genetic adaptations and community changes were likely of minor importance. We conclude that the warming-driven extension of the LOSs of these subarctic grasslands did not saturate up to +10°C warming, and hence that growing seasons of high latitude grasslands are likely to continue lengthening with future warming (unless genetic adaptations or species shifts do occur). This persistence of the warming-induced extension of LOS has important implications for the C-sink potential of subarctic grasslands under climate change. This article is protected by copyright. All rights reserved.

Description: [1]  In total, Icelandic ice caps contain ~3,600 km 3 of ice, which if melted would raise sea level by ~1 cm. Here, we present an overview of mass changes of Icelandic ice masses since the end of the 19th century. They have both gained and lost mass during this period. Changes in ice volume have been estimated both through surface mass balance measurements (performed annually since ~1990) and differencing of digital elevation models derived from various satellite and airborne observations. While the glaciers showed little mass loss as the 20th century began, losses increased rapidly after 1925, peaked in the 1930s and 1940s and remained significant until the 1960s. After being near-zero or even positive during the 1980s andearly 1990s, glacier mass budgets declined considerably, and have since the mid-1990s shown an average annual loss of 9.5 ± 1.5 Gt a –1 , contributing ~0.03 mm a –1 to sea level rise. Since 1995 inter-annual variability in mass loss is high, ranging from 2.7 to 25.3 ± 1.5 Gt a –1 , corresponding to surface mass balances of -0.2 to -2.2 ± 0.15 m we a –1 . This variability is driven by climate fluctuations and also by transient reduction of albedo due to volcanic eruptions.

Description: Iceland's glaciers are particularly sensitive to climate change, and their margins respond to trends in air temperature. Most Icelandic glaciers have been in retreat since c . 1990, and almost all since 1995. Using ice-front measurements, photographic and geomorphological evidence, we examined the record of ice-front fluctuations of Virkisjökull–Falljökull, a steep high-mass-turnover outlet glacier in maritime SE Iceland, in order to place recent changes in a longer-term (80-year) context. Detailed geomorphological mapping identifies two suites of annual push moraines: one suite formed between c . 1935 and 1945, supported by lichenometric dating; the other between 1990 and 2004. Using moraine spacing as a proxy for ice-front retreat rates, we show that average retreat rates during the 1930s and 1940s (28 m a −1 ) were twice as high as during the period from 1990 to 2004 (14 m a −1 ). Furthermore, we show that both suites of annual moraines are associated with above-average summer temperatures. Since 2005, however, retreat rates have increased considerably – averaging 35 m a −1 – with the last 5 years representing the greatest amount of ice-front retreat (∼190 m) in any 5-year period since measurements began in 1932. We propose that this recent, rapid, ice-front retreat and thinning in a decade of unusually warm summers has resulted in a glaciological threshold being breached, with subsequent large-scale stagnation of the glacier terminus (i.e. no forward movement) and the cessation of annual push-moraine formation. Breaching this threshold has, we suggest, caused further very rapid non-uniform retreat and downwasting since 2005 via a system feedback between surface melting, glacier thinning, decreased driving stress and decreased forward motion.

Description: The Holuhraun eruption in 2014-15 was the largest in Iceland for more than 200 years. It resulted in emissions of large quantities of volcanic gases into the atmosphere (11 Mt SO 2 , 0.1 Mt HCl and 0.05 Mt HF). During the eruption the volcanic gases had major effects on F, SO 4 and to a lesser extent Cl concentrations in precipitation throughout Iceland, effects not observed in recent decades. The concentrations of F, Cl and SO 4 (n = 705) reached values of 444 μM, 12,270 μM and 17,324 μM during the eruption and were on average ~20 times higher for F and SO 4 and much lower for Cl compared to pre-eruption times. The concentrations of major cations (Si, Na, K, Ca, Mg, Al and Fe) (n = 151) in the precipitation, were taken as having originated from seawater spray and dissolution of rock dust and aerosol. Based on the mixing model developed here, it is demonstrated that the source of the enrichment of F and SO 4 was indeed the volcanic gas emissions with 〉60-100 mol% of SO 4 and F in the precipitation originated from volcanic gas, whereas the Cl originated mostly from seawater spray, making the volcanic gas input of Cl relatively less important than for F and SO 4 . The results showed that large volcanic eruptions can have major effects on atmospheric chemistry and impact the composition of precipitation.