Showing papers by "Swiss Federal Institute for Forest, Snow and Landscape Research published in 2013"

Three decades of global methane sources and sinks

Abstract: Methane is an important greenhouse gas, responsible for about 20% of the warming induced by long-lived greenhouse gases since pre-industrial times. By reacting with hydroxyl radicals, methane reduces the oxidizing capacity of the atmosphere and generates ozone in the troposphere. Although most sources and sinks of methane have been identified, their relative contributions to atmospheric methane levels are highly uncertain. As such, the factors responsible for the observed stabilization of atmospheric methane levels in the early 2000s, and the renewed rise after 2006, remain unclear. Here, we construct decadal budgets for methane sources and sinks between 1980 and 2010, using a combination of atmospheric measurements and results from chemical transport models, ecosystem models, climate chemistry models and inventories of anthropogenic emissions. The resultant budgets suggest that data-driven approaches and ecosystem models overestimate total natural emissions. We build three contrasting emission scenarios-which differ in fossil fuel and microbial emissions-to explain the decadal variability in atmospheric methane levels detected, here and in previous studies, since 1985. Although uncertainties in emission trends do not allow definitive conclusions to be drawn, we show that the observed stabilization of methane levels between 1999 and 2006 can potentially be explained by decreasing-to-stable fossil fuel emissions, combined with stable-to-increasing microbial emissions. We show that a rise in natural wetland emissions and fossil fuel emissions probably accounts for the renewed increase in global methane levels after 2006, although the relative contribution of these two sources remains uncertain. © 2013 Macmillan Publishers Limited.

Continental-scale temperature variability during the past two millennia

Abstract: Past global climate changes had strong regional expression To elucidate their spatio-temporal pattern, we reconstructed past temperatures for seven continental-scale regions during the past one to two millennia The most coherent feature in nearly all of the regional temperature reconstructions is a long-term cooling trend, which ended late in the nineteenth century At multi-decadal to centennial scales, temperature variability shows distinctly different regional patterns, with more similarity within each hemisphere than between them There were no globally synchronous multi-decadal warm or cold intervals that define a worldwide Medieval Warm Period or Little Ice Age, but all reconstructions show generally cold conditions between ad 1580 and 1880, punctuated in some regions by warm decades during the eighteenth century The transition to these colder conditions occurred earlier in the Arctic, Europe and Asia than in North America or the Southern Hemisphere regions Recent warming reversed the long-term cooling; during the period ad 1971–2000, the area-weighted average reconstructed temperature was higher than any other time in nearly 1,400 years

Eemian interglacial reconstructed from a Greenland folded ice core

Abstract: Efforts to extract a Greenland ice core with a complete record of the Eemian interglacial (130,000 to 115,000 years ago) have until now been unsuccessful. The response of the Greenland ice sheet to the warmer-than-present climate of the Eemian has thus remained unclear. Here we present the new North Greenland Eemian Ice Drilling ('NEEM') ice core and show only a modest ice-sheet response to the strong warming in the early Eemian. We reconstructed the Eemian record from folded ice using globally homogeneous parameters known from dated Greenland and Antarctic ice-core records. On the basis of water stable isotopes, NEEM surface temperatures after the onset of the Eemian (126,000 years ago) peaked at 8 +/- 4 degrees Celsius above the mean of the past millennium, followed by a gradual cooling that was probably driven by the decreasing summer insolation. Between 128,000 and 122,000 years ago, the thickness of the northwest Greenland ice sheet decreased by 400 +/- 250 metres, reaching surface elevations 122,000 years ago of 130 +/- 300 metres lower than the present. Extensive surface melt occurred at the NEEM site during the Eemian, a phenomenon witnessed when melt layers formed again at NEEM during the exceptional heat of July 2012. With additional warming, surface melt might become more common in the future.

Driving factors of a vegetation shift from Scots pine to pubescent oak in dry Alpine forests

Abstract: An increasing number of studies have reported on forest declines and vegetation shifts triggered by drought. In the Swiss Rhone valley (Valais), one of the driest inner-Alpine regions, the species composition in low elevation forests is changing: The sub-boreal Scots pine (Pinus sylvestris L.) dominating the dry forests is showing high mortality rates. Concurrently the sub-Mediterranean pubescent oak (Quercus pubescens Willd.) has locally increased in abundance. However, it remains unclear whether this local change in species composition is part of a larger-scale vegetation shift. To study variability in mortality and regeneration in these dry forests we analysed data from the Swiss national forest inventory (NFI) on a regular grid between 1983 and 2003, and combined it with annual mortality data from a monitoring site. Pine mortality was found to be highest at low elevation (below 1000 m a.s.l.). Annual variation in pine mortality was correlated with a drought index computed for the summer months prior to observed tree death. A generalized linear mixed-effects model indicated for the NFI data increased pine mortality on dryer sites with high stand competition, particularly for small-diameter trees. Pine regeneration was low in comparison to its occurrence in the overstorey, whereas oak regeneration was comparably abundant. Although both species regenerated well at dry sites, pine regeneration was favoured at cooler sites at higher altitude and oak regeneration was more frequent at warmer sites, indicating a higher adaptation potential of oaks under future warming. Our results thus suggest that an extended shift in species composition is actually occurring in the pine forests in the Valais. The main driving factors are found to be climatic variability, particularly drought, and variability in stand structure and topography. Thus, pine forests at low elevations are developing into oak forests with unknown consequences for these ecosystems and their goods and services.

Quantifying uncertainty sources in an ensemble of hydrological climate-impact projections

Abstract: [1] The quantification of uncertainties in projections of climate impacts on river streamflow is highly important for climate adaptation purposes. In this study, we present a methodology to separate uncertainties arising from the climate model (CM), the statistical postprocessing (PP) scheme, and the hydrological model (HM). We analyzed ensemble projections of hydrological changes in the Alpine Rhine (Eastern Switzerland) for the near-term and far-term scenario periods 2024–2050 and 2073–2099 with respect to 1964–1990. For the latter scenario period, the model ensemble projects a decrease of daily mean runoff in summer (−32.2%, range [−45.5% to −8.1%]) and an increase in winter (+41.8%, range [+4.8% to +81.7%]). We applied an analysis of variance model combined with a subsampling procedure to assess the importance of different uncertainty sources. The CMs generally are the dominant source in summer and autumn, whereas, in winter and spring, the uncertainties due to the HMs and the statistical PP gain importance and even partly dominate. In addition, results show that the individual uncertainties from the three components are not additive. Rather, the associated interactions among the CM, the statistical PP scheme, and the HM account for about 5%–40% of the total ensemble uncertainty. The results indicate, in distinction to some previous studies, that none of the investigated uncertainty sources are negligible, and some of the uncertainty is not attributable to individual modeling chain components but rather depends upon interactions.

Drought response of five conifer species under contrasting water availability suggests high vulnerability of Norway spruce and European larch

Abstract: The ability of tree species to cope with anticipated decrease in water availability is still poorly understood. We evaluated the potential of Norway spruce, Scots pine, European larch, black pine, and Douglas-fir to withstand drought in a drier future climate by analyzing their past growth and physiological responses at a xeric and a mesic site in Central Europe using dendroecological methods. Earlywood, latewood, and total ring width, as well as the d13C and d18O in early- and latewood were measured and statistically related to a multiscalar soil water deficit index from 1961 to 2009. At the xeric site, d13C values of all species were strongly linked to water deficits that lasted longer than 11 months, indicating a long-term cumulative effect on the carbon pool. Trees at the xeric site were particularly sensitive to soil water recharge in the preceding autumn and early spring. The native species European larch and Norway spruce, growing close to their dry distribution limit at the xeric site, were found to be the most vulnerable species to soil water deficits. At the mesic site, summer water availability was critical for all species, whereas water availability prior to the growing season was less important. Trees at the mesic were more vulnerable to water deficits of shorter duration than the xeric site. We conclude that if summers become drier, trees growing on mesic sites will undergo significant growth reductions, whereas at their dry distribution limit in the Alps, tree growth of the highly sensitive spruce and larch may collapse, likely inducing dieback and compromising the provision of ecosystem services. However, the magnitude of these changes will be mediated strongly by soil water recharge in winter and thus water availability at the beginning of the growing season.

Elevated CO2 increases tree‐level intrinsic water use efficiency: insights from carbon and oxygen isotope analyses in tree rings across three forest FACE sites

Abstract: Elevated CO₂ increases intrinsic water use efficiency (WUE(i) ) of forests, but the magnitude of this effect and its interaction with climate is still poorly understood. We combined tree ring analysis with isotope measurements at three Free Air CO₂ Enrichment (FACE, POP-EUROFACE, in Italy; Duke FACE in North Carolina and ORNL in Tennessee, USA) sites, to cover the entire life of the trees. We used δ¹³C to assess carbon isotope discrimination and changes in water-use efficiency, while direct CO₂ effects on stomatal conductance were explored using δ¹⁸O as a proxy. Across all the sites, elevated CO₂ increased ¹³C-derived water-use efficiency on average by 73% for Liquidambar styraciflua, 77% for Pinus taeda and 75% for Populus sp., but through different ecophysiological mechanisms. Our findings provide a robust means of predicting water-use efficiency responses from a variety of tree species exposed to variable environmental conditions over time, and species-specific relationships that can help modelling elevated CO₂ and climate impacts on forest productivity, carbon and water balances.

Above‐ and belowground linkages in Sphagnum peatland: climate warming affects plant‐microbial interactions

Abstract: Peatlands contain approximately one third of all soil organic carbon (SOC). Warming can alter above- and belowground linkages that regulate soil organic carbon dynamics and C-balance in peatlands. Here we examine the multiyear impact of in situ experimental warming on the microbial food web, vegetation, and their feedbacks with soil chemistry. We provide evidence of both positive and negative impacts of warming on specific microbial functional groups, leading to destabilization of the microbial food web. We observed a strong reduction (70%) in the biomass of top-predators (testate amoebae) in warmed plots. Such a loss caused a shortening of microbial food chains, which in turn stimulated microbial activity, leading to slight increases in levels of nutrients and labile C in water. We further show that warming altered the regulatory role of Sphagnum-polyphenols on microbial community structure with a potential inhibition of top predators. In addition, warming caused a decrease in Sphagnum cover and an increase in vascular plant cover. Using structural equation modelling, we show that changes in the microbial food web affected the relationships between plants, soil water chemistry, and microbial communities. These results suggest that warming will destabilize C and nutrient recycling of peatlands via changes in above- and belowground linkages, and therefore, the microbial food web associated with mosses will feedback positively to global warming by destabilizing the carbon cycle. This study confirms that microbial food webs thus constitute a key element in the functioning of peatland ecosystems. Their study can help understand how mosses, as ecosystem engineers, tightly regulate biogeochemical cycling and climate feedback in peatlands

Biogeochemical plant-soil microbe feedback in response to climate warming in peatlands

Abstract: Peatlands are important sinks for carbon dioxide, but how their biogeochemistry will be affected by climate warming is poorly understood. This study compares sites along an altitudinal gradient, simulating a natural gradient in soil temperature to elucidate plant–soil microbe feedback in response to a climate-induced change in vegetation.

Current near-to-nature forest management effects on functional trait composition of saproxylic beetles in beech forests.

Abstract: With the aim of wood production with negligible negative effects on biodiversity and ecosystem processes, a silvicultural practice of selective logging with natural regeneration has been implemented in European beech forests (Fagus sylvatica) during the last decades. Despite this near-to-nature strategy, species richness of various taxa is lower in these forests than in unmanaged forests. To develop guidelines to minimize the fundamental weaknesses in the current practice, we linked functional traits of saproxylic beetle species to ecosystem characteristics. We used continental-scale data from 8 European countries and regional-scale data from a large forest in southern Germany and forest-stand variables that represented a gradient of intensity of forest use to evaluate the effect of current near-to-nature management strategies on the functional diversity of saproxylic beetles. Forest-stand variables did not have a statistically significant effect on overall functional diversity, but they did significantly affect community mean and diversity of single functional traits. As the amount of dead wood increased the composition of assemblages shifted toward dominance of larger species and species preferring dead wood of large diameter and in advanced stages of decay. The mean amount of dead wood across plots in which most species occurred was from 20 to 60 m(3) /ha. Species occurring in plots with mean dead wood >60 m(3) /ha were consistently those inhabiting dead wood of large diameter and in advanced stages of decay. On the basis of our results, to make current wood-production practices in beech forests throughout Europe more conservation oriented (i.e., promoting biodiversity and ecosystem functioning), we recommend increasing the amount of dead wood to >20 m(3) /ha; not removing dead wood of large diameter (50 cm) and allowing more dead wood in advanced stages of decomposition to develop; and designating strict forest reserves, with their exceptionally high amounts of dead wood, that would serve as refuges for and sources of saproxylic habitat specialists.

Microclimate in forests with varying leaf area index and soil moisture: potential implications for seedling establishment in a changing climate

Abstract: Summary 1. Forest microclimate is crucial for the growth and survival of tree seedlings and understorey vegetation. This high ecological relevance contrasts with the poor functional and quantitative understanding of how the properties of forest ecosystems influence forest microclimate. 2. In a long-term (1998–2011) trial, we investigated how temporal patterns of microclimate below sparse and dense forest canopy related to those of nearby open areas and how this relationship was influenced by soil moisture and seasonality. Air temperature (T), vapour pressure deficit (VPD), soil matrix potential and leaf area index (LAI) were measured in a unique set-up of below-canopy and open-area meteorological stations at eleven distinct forest ecosystems, characteristic of subalpine and temperate climate zones. Data from these plots were analysed for the moderating capacity of the canopy, that is, the differences between below-canopy and open-area microclimate, with respect to (i) long-term means, (ii) dynamics within homogeneous moist- vs. dry-soil periods and (iii) diurnal patterns. 3. The long-term mean moderating capacity of the canopy was up to 3.3 °C for daily Tmax and 0.52 kPa for daily VPDmax, of which soil moisture status alone accounted for up to 1.2 ° C( Tmax) and 0.21 kPa (VPDmax). Below dense canopy (LAI > 4), the moderating capacity was generally higher when soils were dry and increased during dry-soil periods, particularly in spring and somewhat less in summer. The opposite pattern was found below sparse canopy (LAI < 4). At the diurnal level, moderating capacity below dense canopy was strongest in mid-afternoon and during dry-soil conditions, whereas peak moderation below sparse canopy occurred in mid-morning and during moist-soil conditions. 4. Synthesis. Our results suggest a threshold canopy density, which is probably linked to sitespecific water availability, below which the moderating capacity of forest ecosystems switches from supportive to unsupportive for seedling establishment. Under supportive moderating capacity, we understand a stronger mitigation during physiologically most demanding conditions for plant growth. Such a threshold canopy density sheds new light on forest resilience to climate change. Climate change may alter forest canopy density in a way that precludes successful establishment of tree species and ultimately changes forest ecosystem structure and functioning.

Biotic plant–soil feedbacks across temporal scales

Abstract: 1. Plant effects on soil biota can result in feedbacks affecting plant performance, with consequences for plant community and ecosystem dynamics on short and long time-scales. In addition, the strength and direction of plant-soil feedbacks depend on temporal shifts in abiotic environmental conditions. 2. We synthesize current knowledge on temporal aspects of plant-soil feedbacks and present new ideas to better understand and predict the effects of plant-soil feedbacks on community and ecosystem properties across temporal scales. 3. Explaining short-term temporal feedback dynamics requires us to better understand mechanistic linkages between plants, soil organisms and locally available resources. On the other hand, we need to refine our understanding of the context-dependency of plant-soil feedbacks, as the strength and direction of feedback interactions are influenced by 'external' temporal ecosystem dynamics, such as variation in soil resource availability after disturbance or during succession. 4. Synthesis. Based on our synthesis of temporal aspects of plant-soil feedbacks, we suggest three main avenues for future research: (i) how plant-soil feedbacks changes with ontogeny, (ii) how plant and soil organism traits drive temporal variation in plant-soil feedbacks and (iii) how environmental changes across temporal scales alter the strength and direction of plant-soil feedbacks.

Landscape connectivity, habitat structure and activity of bat guilds in farmland-dominated matrices

Abstract: Summary 1. Agricultural intensification has caused a decline in structural elements in European farmland, where natural habitats are increasingly fragmented. The loss of habitat structures has a detrimental effect on biodiversity and affects bat species that depend on vegetation structures for foraging and commuting. 2. We investigated the impact of connectivity and configuration of structural landscape elements on flight activity, species richness and diversity of insectivorous bats and distinguished three bat guilds according to species-specific bioacoustic characteristics. We tested whether bats with shorter-range echolocation were more sensitive to habitat fragmentation than bats with longer-range echolocation. We expected to find different connectivity thresholds for the three guilds and hypothesized that bats prefer linear over patchy landscape elements. 3. Bat activity was quantified using repeated acoustic monitoring in 225 locations at 15 study plots distributed across the Swiss Central Plateau, where connectivity and the shape of landscape elements were determined by spatial analysis (GIS). Spectrograms of bat calls were assigned to species with the software BATIT by means of image recognition and statistical classification algorithms. 4. Bat activity was significantly higher around landscape elements compared to open control areas. Short- and long-range echolocating bats were more active in well-connected landscapes, but optimal connectivity levels differed between the guilds. Species richness increased significantly with connectivity, while species diversity did not (Shannon’s diversity index). Total bat activity was unaffected by the shape of landscape elements. 5. Synthesis and applications. This study highlights the importance of connectivity in farmland landscapes for bats, with shorter-range echolocating bats being particularly sensitive to habitat fragmentation. More structurally diverse landscape elements are likely to reduce population declines of bats and could improve conditions for other declining species, including birds. Activity was highest around optimal values of connectivity, which must be evaluated for the different guilds and spatially targeted for a region’s habitat configuration. In a multispecies approach, we recommend the reintroduction of structural elements to increase habitat heterogeneity should become part of agri-environment schemes.

Fine-root turnover rates of European forests revisited: an analysis of data from sequential coring and ingrowth cores

Abstract: Forest trees directly contribute to carbon cycling in forest soils through the turnover of their fine roots. In this study we aimed to calculate root turnover rates of common European forest tree species and to compare them with most frequently published values. We compiled available European data and applied various turnover rate calculation methods to the resulting database. We used Decision Matrix and Maximum-Minimum formula as suggested in the literature. Mean turnover rates obtained by the combination of sequential coring and Decision Matrix were 0.86 yr−1 for Fagus sylvatica and 0.88 yr−1 for Picea abies when maximum biomass data were used for the calculation, and 1.11 yr−1 for both species when mean biomass data were used. Using mean biomass rather than maximum resulted in about 30 % higher values of root turnover. Using the Decision Matrix to calculate turnover rate doubled the rates when compared to the Maximum-Minimum formula. The Decision Matrix, however, makes use of more input information than the Maximum-Minimum formula. We propose that calculations using the Decision Matrix with mean biomass give the most reliable estimates of root turnover rates in European forests and should preferentially be used in models and C reporting.

The past ecology of Abies alba provides new perspectives on future responses of silver fir forests to global warming

Abstract: Paleoecology can provide valuable insights into the ecology of species that complement observation and experiment-based assessments of climate impact dynamics. New paleoecological records (e.g., pollen, macrofossils) from the Italian Peninsula suggest a much wider climatic niche of the important European tree species Abies alba (silver fir) than observed in its present spatial range. To explore this discrepancy between current and past distribution of the species, we analyzed climatic data (temperature, precipitation, frost, humidity, sunshine) and vegetation-independent paleoclimatic reconstructions (e.g., lake levels, chironomids) and use global coupled carbon-cycle climate (NCAR CSM1.4) and dynamic vegetation (LandClim) modeling. The combined evidence suggests that during the mid-Holocene (;6000 years ago), prior to humanization of vegetation, A. alba formed forests under conditions that exceeded the modern (1961–1990) upper temperature limit of the species by ;5–78C (July means). Annual precipitation during this natural period was comparable to today (.700–800 mm), with drier summers and wetter winters. In the meso-Mediterranean to sub-Mediterranean forests A. alba co-occurred with thermophilous taxa such as Quercus ilex, Q. pubescens, Olea europaea, Phillyrea, Arbutus, Cistus, Tilia, Ulmus, Acer, Hedera helix, Ilex aquifolium, Taxus, and Vitis. Results from the last interglacial (ca. 130 000–115 000 BP), when human impact was negligible, corroborate the Holocene evidence. Thermophilous Mediterranean A. alba stands became extinct during the last 5000 years when land-use pressure and specifically excessive anthropogenic fire and browsing disturbance increased. Our results imply that the ecology of this key European tree species is not yet well understood. On the basis of the reconstructed realized climatic niche of the species, we anticipate that the future geographic range of A. alba may not contract regardless of migration success, even if climate should become significantly warmer than today with summer temperatures increasing by up to 5–78C, as long as precipitation does not fall below 700–800 mm/yr, and anthropogenic disturbance (e.g., fire, browsing) does not become excessive. Our finding contradicts recent studies that projected range contractions under global-warming scenarios, but did not factor how millennia of human impacts reduced the realized climatic niche of A. alba.

Aluminum exclusion and aluminum tolerance in woody plants.

Abstract: The aluminium (Al) cation Al3+ is highly rhizotoxic and is a major stress factor to plants on acid soils, which cover large areas of tropical and boreal regions. Many woody plant species are native to acid soils and are well adapted to high Al3+ conditions. In tropical regions, both woody Al accumulator and non-Al accumulator plants occur, whereas in boreal regions woody plants are non-Al accumulators. The mechanisms of these adaptations can be divided into those that facilitate the exclusion of Al3+ from root cells (exclusion mechanisms) and those that enable plants to tolerate Al3+ once it has entered the root and shoot symplast (internal tolerance mechanisms). The biochemical and molecular basis of these mechanisms have been intensively studied in several crop plants and the model plant Arabidopsis. In this review, we examine the current understanding of Al3+ exclusion and tolerance mechanisms from woody plants. In addition, we discuss the ecology of woody non-Al accumulator and Al accumulator plants, and present examples of Al3+ adaptations in woody plant populations. This paper complements previous reviews focusing on crop plants and provides insights into evolutionary processes operating in plant communities that are widespread on acid soils.

Life‐history constraints in grassland plant species: a growth‐defence trade‐off is the norm

Abstract: Plant growth can be limited by resource acquisition and defence against consumers, leading to contrasting trade-off possibilities. The competition-defence hypothesis posits a trade-off between competitive ability and defence against enemies (e.g. herbivores and pathogens). The growth-defence hypothesis suggests that strong competitors for nutrients are also defended against enemies, at a cost to growth rate. We tested these hypotheses using observations of 706 plant populations of over 500 species before and following identical fertilisation and fencing treatments at 39 grassland sites worldwide. Strong positive covariance in species responses to both treatments provided support for a growth-defence trade-off: populations that increased with the removal of nutrient limitation (poor competitors) also increased following removal of consumers. This result held globally across 4 years within plant life-history groups and within the majority of individual sites. Thus, a growth-defence trade-off appears to be the norm, and mechanisms maintaining grassland biodiversity may operate within this constraint.

Selection and treatment of data for radiocarbon calibration: an update to the international calibration (intcal) criteria

Abstract: High-quality data from appropriate archives are needed for the continuing improvement of radiocarbon cali- bration curves. We discuss here the basic assumptions behind 14C dating that necessitate calibration and the relative strengths and weaknesses of archives from which calibration data are obtained. We also highlight the procedures, problems, and uncer- tainties involved in determining atmospheric and surface ocean 14C/12C in these archives, including a discussion of the vari- ous methods used to derive an independent absolute timescale and uncertainty. The types of data required for the current IntCal database and calibration curve model are tabulated with examples.

Central European hardwood trees in a high‐CO2 future: synthesis of an 8‐year forest canopy CO2 enrichment project

Abstract: Rapidly increasing atmospheric CO2 is not only changing the climate system but may also affect the biosphere directly through stimulation of plant growth and ecosystem carbon and nutrient cycling. Although forest ecosystems play a critical role in the global carbon cycle, experimental information on forest responses to rising CO2 is scarce, due to the sheer size of trees. Here, we present a synthesis of the only study world-wide where a diverse set of mature broadleaved trees growing in a natural forest has been exposed to future atmospheric CO2 levels (c. 550ppm) by free-air CO2 enrichment (FACE). We show that litter production, leaf traits and radial growth across the studied hardwood species remained unaffected by elevated CO2 over 8years. CO2 enrichment reduced tree water consumption resulting in detectable soil moisture savings. Soil air CO2 and dissolved inorganic carbon both increased suggesting enhanced below-ground activity. Carbon release to the rhizosphere and/or higher soil moisture primed nitrification and nitrate leaching under elevated CO2; however, the export of dissolved organic carbon remained unaltered.Synthesis. Our findings provide no evidence for carbon-limitation in five central European hardwood trees at current ambient CO2 concentrations. The results of this long-term study challenge the idea of a universal CO2 fertilization effect on forests, as commonly assumed in climate-carbon cycle models.

Subordinate plant species enhance community resistance against drought in semi-natural grasslands

Abstract: According to the insurance hypothesis, more diverse plant communities are more likely to be resistant to drought. Whilst many experiments have been carried out to determine the effects of plant diversity on plant community insurance, the results are still contradictory. Here, we conducted a drought experiment where we tested whether the presence of subordinate species increases plant community insurance. In Swiss Jura grassland, we combined a removal experiment of subordinate species with a summer drought event using rainout shelters. Plant community composition was determined after the drought and based on biomass measurements; we estimated resistance, recovery and resilience of the plant community for each combination of treatments. Moreover, to assess drought impacts on water-use efficiency (WUE), we analysed carbon isotope ratios (13C values) in plant leaves of two dominants and two subordinates collected at the end of the drought period. We showed that subordinate species are more resistant to drought and increased community resistance by enhancing their above-ground biomass production during the imposed drought. These patterns were associated with decreased competitiveness of dominant species whose biomass decreased during drought. Significant increase in 13C values in plant tissue under drought indicated a better WUE for the measured species. Interestingly, the WUE was significantly higher in plots where subordinates were removed. Recovery and resilience were not affected by the summer drought, but the absence of subordinates reduced overall above-ground biomass in both watered and drought plots. Synthesis. We demonstrated that, independent of plant diversity, the presence of drought-resistant subordinate species increases plant community insurance against drought and, hence, is important for the functioning of grassland ecosystems.

Tree migration-rates: narrowing the gap between inferred post-glacial rates and projected rates

Abstract: Faster-than-expected post-glacial migration rates of trees have puzzled ecologists for a long time. In Europe, post-glacial migration is assumed to have started from the three southern European peninsulas (southern refugia), where large areas remained free of permafrost and ice at the peak of the last glaciation. However, increasing palaeobotanical evidence for the presence of isolated tree populations in more northerly microrefugia has started to change this perception. Here we use the Northern Eurasian Plant Macrofossil Database and palaeoecological literature to show that post-glacial migration rates for trees may have been substantially lower (60–260 m yr–1) than those estimated by assuming migration from southern refugia only (115–550 m yr–1), and that early-successional trees migrated faster than mid- and late-successional trees. Post-glacial migration rates are in good agreement with those recently projected for the future with a population dynamical forest succession and dispersal model, mainly for early-successional trees and under optimal conditions. Although migration estimates presented here may be conservative because of our assumption of uniform dispersal, tree migration-rates clearly need reconsideration. We suggest that small outlier populations may be a key factor in understanding past migration rates and in predicting potential future range-shifts. The importance of outlier populations in the past may have an analogy in the future, as many tree species have been planted beyond their natural ranges, with a more beneficial microclimate than their regional surroundings. Therefore, climate-change-induced range-shifts in the future might well be influenced by such microrefugia.

Modeling root reinforcement using a root-failure Weibull survival function

Abstract: . Root networks contribute to slope stability through complex interactions with soil that include mechanical compression and tension. Due to the spatial heterogeneity of root distribution and the dynamics of root turnover, the quantification of root reinforcement on steep slopes is challenging and consequently the calculation of slope stability also. Although considerable progress has been made, some important aspects of root mechanics remain neglected. In this study we address specifically the role of root-strength variability on the mechanical behavior of a root bundle. Many factors contribute to the variability of root mechanical properties even within a single class of diameter. This work presents a new approach for quantifying root reinforcement that considers the variability of mechanical properties of each root diameter class. Using the data of laboratory tensile tests and field pullout tests, we calibrate the parameters of the Weibull survival function to implement the variability of root strength in a numerical model for the calculation of root reinforcement (RBMw). The results show that, for both laboratory and field data sets, the parameters of the Weibull distribution may be considered constant with the exponent equal to 2 and the normalized failure displacement equal to 1. Moreover, the results show that the variability of root strength in each root diameter class has a major influence on the behavior of a root bundle with important implications when considering different approaches in slope stability calculation. Sensitivity analysis shows that the calibration of the equations of the tensile force, the elasticity of the roots, and the root distribution are the most important steps. The new model allows the characterization of root reinforcement in terms of maximum pullout force, stiffness, and energy. Moreover, it simplifies the implementation of root reinforcement in slope stability models. The realistic quantification of root reinforcement for tensile, shear and compression behavior allows for the consideration of the stabilization effects of root networks on steep slopes and the influence that this has on the triggering of shallow landslides.

What Kind of Entrepreneurship Drives Regional Development in European Non-core Regions? A Literature Review on Empirical Entrepreneurship Research

Abstract: Regional policies across Europe aim at stimulating regional development in non-core regions through fostering entrepreneurship. However, the policies applied in non-core regions and the concepts of entrepreneurship these policies are based on differ. Therefore, the goal of this review is to identify different understandings of entrepreneurship and their role for regional development processes in European non-core regions. To this end, empirical studies investigating entrepreneurship in European non-core regions from 1999 to 2011 were analysed. The results of the analysis are presented along three drivers and outcomes of entrepreneurship identified inductively from the literature: innovation, social capital and institutional change. We made out seven different types of entrepreneurship in European non-core regions. These seven types of entrepreneurship comprise particular mechanisms through which they stimulate regional development. Further research should study the interplay between these different mechan...

Tree growth response along an elevational gradient: climate or genetics?

Abstract: Environment and genetics combine to influence tree growth and should therefore be jointly considered when evaluating forest responses in a warming climate. Here, we combine dendroclimatology and population genetic approaches with the aim of attributing climatic influences on growth of European larch (Larix decidua) and Norway spruce (Picea abies). Increment cores and genomic DNA samples were collected from populations along a ~900-m elevational transect where the air temperature gradient encompasses a ~4 °C temperature difference. We found that low genetic differentiation among populations indicates gene flow is high, suggesting that migration rate is high enough to counteract the selective pressures of local environmental variation. We observed lower growth rates towards higher elevations and a transition from negative to positive correlations with growing season temperature upward along the elevational transect. With increasing elevation there was also a clear increase in the explained variance of growth due to summer temperatures. Comparisons between climate sensitivity patterns observed along this elevational transect with those from Larix and Picea sites distributed across the Alps reveal good agreement, and suggest that tree-ring width (TRW) variations are more climate-driven than genetics-driven at regional and larger scales. We conclude that elevational transects are an extremely valuable platform for understanding climatic-driven changes over time and can be especially powerful when working within an assessed genetic framework.

Informing landscape planning and design for sustaining ecosystem services from existing spatial patterns and knowledge

Abstract: Over the last decade we have seen an increased emphasis in environmental management and policies aimed at maintaining and restoring multiple ecosystem services at landscape scales. This emphasis has resulted from the recognition that management of specific environmental targets and ecosystem services requires an understanding of landscape processes and the spatial scales that maintain those targets and services. Moreover, we have become increasingly aware of the influence of broad-scale drivers such as climate change on landscape processes and the ecosystem services they support. Studies and assessments on the relative success of environmental policies and landscape designs in maintaining landscape processes and ecosystem services is mostly lacking. This likely reflects the relatively high cost of maintaining a commitment to implement and maintain monitoring programs that document responses of landscape processes and ecosystem services to different landscape policies and designs. However, we argue that there is considerable variation in natural and human-caused landscape pattern at local to continental scales and that this variation may facilitate analyses of how environmental targets and ecosystem services have responded to such patterns. Moreover, wall-to-wall spatial data on land cover and land use at national scales may permit characterization and mapping of different landscape pattern gradients. We discuss four broad and interrelated focus areas that should enhance our understanding of how landscape pattern influences ecosystem services: (1) characterizing and mapping landscape pattern gradients; (2) quantifying relationships between landscape patterns and environmental targets and ecosystem services, (3) evaluating landscape patterns with regards to multiple ecosystem services, and (4) applying adaptive management concepts to improve the effectiveness of specific landscape designs in sustaining ecosystem services. We discuss opportunities as well as challenges in each of these four areas. We believe that this agenda could lead to spatially explicit solutions in managing a range of environmental targets and ecosystem services. Spatially explicit options are critical in managing and protecting landscapes, especially given that communities and organizations are often limited in their capacity to make changes at landscape scales. The issues and potential solutions discussed in this paper expand upon the call by Nassauer and Opdam (Landscape Ecol 23:633–644, 2008) to include design as a fundamental element in landscape ecology research by evaluating natural and human-caused (planned or designed) landscape patterns and their influence on ecosystem services. It also expands upon the idea of “learning by doing” to include “learning from what has already been done.”

High-Resolution Vertical Profiles of X-Band Polarimetric Radar Observables during Snowfall in the Swiss Alps

Abstract: An X-band polarimetric radar was deployed in the eastern Swiss Alps at an altitude of 2133 m. Radar measurements were complemented with several weather stations deployed in an altitude range from 1500 to 3100 m as well as with a fixed GPS ground station that was used to infer integrated water vapor estimates. Around 8000 vertical profiles of polarimetric radar observables above the melting layer collected during two months are analyzed. First, the behavior of the mean profiles of reflectivity at horizontal polarization Zh, differential reflectivity Zdr, copolar cross correlation ρhv, and specific differential phase shift Kdp are interpreted from a microphysical point of view. It is shown that the whole evolution of snowflakes, from pristine crystals at temperatures around −30°C to dendritic crystals around −15°C, to large aggregates around 0°C, is well captured by the polarimetric radar variables. In a second step, the profiles are analyzed as functions of high and low water vapor and snow accumul...