Prolonged Summer Drought Research Articles

Patterns, timing, and environmental drivers of secondary growth in two physiologically distinct Mediterranean alpine shrub species

Alpine plants are particularly sensitive to climate change, and in the Mediterranean, less frequent winter cold and prolonged summer drought are expected to shift the growth patterns of species, altering their range and strategies to cope with these dual climatic stressors. However, adaptive strategies for drought and frost and their impact on performance of species are poorly explored, with critical timescales relevant for growth insufficiently reflected and a focus on a limited set of environmental drivers. Here, we explored the growth processes of two physiologically distinct Mediterranean alpine shrub species: Cytisus galianoi (green-stemmed species) and Astragalus granatensis (dimorphic species). By measuring the daily stem diameter changes of 26 specimens over six consecutive years (2015–2020) using dendrometers, as well as the corresponding soil temperature and soil moisture conditions, we identified bimodal annual growth patterns (i.e. two phases of growth), water-related timing of growth, and drought- and frost-related environmental constraints. By implementing correlation analyses, linear mixed effects models, and partial least-squares regression, we found pregrowth temperature and moisture drivers to be highly relevant for growth in both species, suggesting a temporal decoupling of growth and resource acquisition. However, the underlying mechanisms were contrasting. While the spring growth of C. galianoi was promoted by pregrowth winter conditions, the autumn growth of A. granatensis was promoted by pregrowth summer conditions. Thus, resource acquisition is likely to be optimized when the traits of species allow physiological activity at high gain and low costs, i.e. when adaptive mechanisms reduce resource consumption to cope with frost and drought. This is during winter for frost-tolerant green-stemmed species and during summer for drought-avoidant dimorphic species, leading to species-specific time windows of growth. Understanding these species-specific growth mechanisms contributes to answering the overarching question of when and how woody plants grow and helps in understanding their adaptability to future climate variability, particularly in sensitive alpine environments, where plant species are evolutionally adapted to physical peculiarities and reach their low-temperature limit.

Field-scale assessment of direct and indirect effects of soil texture on organic matter mineralization during a dry summer

Soil texture plays a crucial role in organic matter (OM) mineralization through both direct interactions with minerals and indirect effects on soil moisture. Separating these effects could enhance the modelling of soil organic carbon (SOC) dynamics under climate change scenarios. However, the attempts have been limited small-scale experiments. Here, we studied the effects of soil texture on added OM mineralization in loamy sand, loam and silt loam soils in nine agricultural fields in Flanders, Belgium. Soil moisture, temperature, groundwater table depth and the mineralization of 13C-labeled ryegrass were monitored in buried mesocosms for approximately three months during a dry summer. Ryegrass-C mineralization was lowest in the loamy sand (39 ± 7 %) followed by silt loam (48 ± 7 %) and loam (63 ± 5 %) soils, challenging the current clay%-based moderation of C-mineralization rates in soil models. Soil temperature was not influenced by soil texture, whereas soil moisture was indeed dependent on soil texture. It appears that capillarity sustained upward water supply from groundwater to the topsoil in loam and silt loam soils but not in loamy sand soil, although this difference in capillary rise could not fully explain the higher moisture content in loam than that in silt loam soils. Additionally, soil texture only impacted remnant added ryegrass pieces (>500 μm) but not the finer ryegrass-derived SOC (<500 μm), which might point at the important indirect control of texture on OM mineralization during prolonged summer drought. However, these effects are only manifested during drought when no other factors (e.g., groundwater depth or subsurface water flows) exert an overriding impact on the soil water balance. Overall, our findings highlight the need to properly incorporate the indirect effects of soil texture on OM mineralization into soil carbon models to accurately predict soil C stocks under future climate change scenarios.

The Novel Role of Tyrosinase Enzymes in the Storage of Globally Significant Amounts of Carbon in Wetland Ecosystems.

Over the last millennia, wetlands have been sequestering carbon from the atmosphere via photosynthesis at a higher rate than releasing it and, therefore, have globally accumulated 550 × 1015 g of carbon, which is equivalent to 73% of the atmospheric carbon pool. The accumulation of organic carbon in wetlands is effectuated by phenolic compounds, which suppress the degradation of soil organic matter by inhibiting the activity of organic-matter-degrading enzymes. The enzymatic removal of phenolic compounds by bacterial tyrosinases has historically been blocked by anoxic conditions in wetland soils, resulting from waterlogging. Bacterial tyrosinases are a subgroup of oxidoreductases that oxidatively remove phenolic compounds, coupled to the reduction of molecular oxygen to water. The biochemical properties of bacterial tyrosinases have been investigated thoroughly in vitro within recent decades, while investigations focused on carbon fluxes in wetlands on a macroscopic level have remained a thriving yet separated research area so far. In the wake of climate change, however, anoxic conditions in wetland soils are threatened by reduced rainfall and prolonged summer drought. This potentially allows tyrosinase enzymes to reduce the concentration of phenolic compounds, which in turn will increase the release of stored carbon back into the atmosphere. To offer compelling evidence for the novel concept that bacterial tyrosinases are among the key enzymes influencing carbon cycling in wetland ecosystems first, bacterial organisms indigenous to wetland ecosystems that harbor a TYR gene within their respective genome (tyr+) have been identified, which revealed a phylogenetically diverse community of tyr+ bacteria indigenous to wetlands based on genomic sequencing data. Bacterial TYR host organisms covering seven phyla (Acidobacteria, Actinobacteria, Bacteroidetes, Firmicutes, Nitrospirae, Planctomycetes, and Proteobacteria) have been identified within various wetland ecosystems (peatlands, marshes, mangrove forests, bogs, and alkaline soda lakes) which cover a climatic continuum ranging from high arctic to tropic ecosystems. Second, it is demonstrated that (in vitro) bacterial TYR activity is commonly observed at pH values characteristic for wetland ecosystems (ranging from pH 3.5 in peatlands and freshwater swamps to pH 9.0 in soda lakes and freshwater marshes) and toward phenolic compounds naturally present within wetland environments (p-coumaric acid, gallic acid, protocatechuic acid, p-hydroxybenzoic acid, caffeic acid, catechin, and epicatechin). Third, analyzing the available data confirmed that bacterial host organisms tend to exhibit in vitro growth optima at pH values similar to their respective wetland habitats. Based on these findings, it is concluded that, following increased aeration of previously anoxic wetland soils due to climate change, TYRs are among the enzymes capable of reducing the concentration of phenolic compounds present within wetland ecosystems, which will potentially destabilize vast amounts of carbon stored in these ecosystems. Finally, promising approaches to mitigate the detrimental effects of increased TYR activity in wetland ecosystems and the requirement of future investigations of the abundance and activity of TYRs in an environmental setting are presented.

Short-term changes of the δ15N natural abundance of aboveground biomass and topsoil in temperate grassland – Effects of prolonged summer drought

Analysis of isotopic signatures of nitrogen (15N) is a widely used method for identifying and tracking pathways of N losses in agriculture. It is not yet clear what drives the development of the isotopic signal over time after change of management in grasslands. In a three-year field experiment in cut grassland on a sandy soil in northwest Germany, we investigated the response of isotopic signals in aboveground biomass and topsoil to different fertilizer management regimes: cattle slurry (org.) and calcium ammonium nitrate (CAN, syn.) were applied at different rates (0 kg N ha−1; 120 kg N ha−1 org.; 240 kg N ha−1 org.; 240 kg N ha−1 org./syn.; 360 kg N ha−1 org./syn.). Samples of aboveground biomass and topsoil were taken at the end of vegetation period every year. For aboveground biomass, the effects of fertilizer treatment on the isotopic signal were already evident in the second year and δ15N values ranged from 0.933‰ to 8.049‰. In the topsoil, the δ15N values ranged from 3.670‰ to 5.322‰ and a differentiation in δ15N values among fertilizer treatments could be proven for the third year of the study. We conclude that isotopic signals in aboveground biomass and topsoil can already be detected after a period of 2–3 years. Drought conditions can boost the detection of isotopic signals as they favor NH3 losses and lead to a reduced sink function of biomass and, consequently, large N surpluses.

Tree hazards compounded by successive climate extremes after masting in a small endemic tree, Distylium lepidotum, on subtropical islands in Japan.

Ongoing global warming increases the frequency and severity of tropical typhoons and prolonged drought, leading to forest degradation. Simultaneous and/or successive masting events and climatic extremes may thus occur frequently in the near future. If these climatic extremes occur immediately after mass seed reproduction, their effects on individual trees are expected to be very severe because mass reproduction decreases carbohydrate reserves. While the effects of either a single climate extreme or masting alone on tree resilience/growth have received past research attention, understanding the cumulative effects of such multiple events remains challenging and is crucial for predicting future forest changes. Here, we report tree hazards compound by two successive climate extremes, a tropical typhoon and prolonged drought, after mass reproduction in an endemic tree species (Distylium lepidotum Nakai) on oceanic islands. Across individual trees, the starch stored within the sapwood of branchlets significantly decreased with reproductive efforts (fruit mass/shoot mass ratio). Typhoon damage significantly decreased not only the total leaf area of apical shoots but also the maximum photosynthetic rates. During the 5‐month period after the typhoon, the mortality of large branchlets (8–10‐mm diameter) increased with decreasing stored starch when the typhoon hit. During the prolonged summer drought in the next year, the recovery of total leaf area, stored starch, and hydraulic conductivity was negatively correlated with the stored starch at the typhoon. These data indicate that the level of stored starch within branchlets is the driving factor determining tree regrowth or dieback, and the restoration of carbohydrates after mass reproduction is synergistically delayed by such climate extremes. Stored carbohydrates are the major cumulative factor affecting individual tree resilience, resulting in their historical effects. Because of highly variable carbohydrate levels among individual trees, the resultant impacts of such successive events on forest dieback will be fundamentally different among trees.

Expression, Purification, and Characterization of a Well-Adapted Tyrosinase from Peatlands Identified by Partial Community Analysis.

In peatlands, bacterial tyrosinases (TYRs) are proposed to act as key regulators of carbon storage by removing phenolic compounds, which inhibit the degradation of organic carbon. Historically, TYR activity has been blocked by anoxia resulting from persistent waterlogging; however, recent events of prolonged summer drought have boosted TYR activity and, consequently, the release of carbon stored in the form of organic compounds from peatlands. Since 30% of the global soil carbon stock is stored in peatlands, a profound understanding of the production and activity of TYRs is essential to assess the impact of carbon dioxide emitted from peatlands on climate change. TYR partial sequences identified by degenerated primers suggest a versatile TYR enzyme community naturally present in peatlands, which is produced by a phylogenetically diverse spectrum of bacteria, including Proteobacteria and Actinobacteria. One full-length sequence of an extracellular TYR (SzTYR) identified from a soda-rich inland salt marsh has been heterologously expressed and purified. SzTYR exhibits a molecular mass of 30 891.8 Da and shows a pH optimum of 9.0. Spectroscopic studies and kinetic investigations characterized SzTYR as a tyrosinase and proved its activity toward monophenols (coumaric acid), diphenols (caffeic acid, protocatechuic acid), and triphenols (gallic acid) naturally present in peatlands.

Decomposition in Mixed Beech Forests in the South-Western Alps Under Severe Summer Drought

Climate and plant litter diversity are major determinants of carbon (C) and nitrogen (N) cycling rates during decomposition. Yet, how these processes will be modified with combined changes in climate and biodiversity is poorly understood. With a multisite field experiment, we studied the interactive effects of reinforced and prolonged summer drought (using rainout shelters) and tree species mixing on leaf litter decomposition in beech forests in the French Alps. Forests included monospecific stands of Fagus sylvatica, Abies alba and Quercus pubescens and two-species mixtures composed of beech and one of the other species. We hypothesized (1) slower C and N release during decomposition in response to experimentally prolonged summer drought, but (2) less so in mixed compared to monospecific tree stands, due to indirect canopy effects and direct litter mixing effects. Litter lost 35% of initial C and 15% of N on average across all sites and litter types over 30 months of decomposition. The experimentally prolonged summer drought led to weakly slower C loss but had no effect on N loss. Tree species mixing did not alter drought effects on decomposition but had non-additive effects on C and N loss, which were dominated by direct litter mixing rather than indirect plot-specific tree species composition effects. Our data suggest relatively small effects of reinforced and prolonged summer drought on decomposition, possibly because process rates are generally slow during summer and because microsite variability is larger than the effects of rainfall exclusion. The dominant contribution of litter mixing to the overall effect of plot-specific tree species mixtures on decomposition supports the importance of microsite conditions for C and N dynamics during decomposition, which should be accounted for more explicitly in climate and biodiversity change predictions.

Admixing Fir to European Beech Forests Improves the Soil Greenhouse Gas Balance

Research highlights: The admixture of fir to pure European beech hardly affected soil-atmosphere CH4 and N2O fluxes but increased soil organic carbon (SOC) stocks at a site in the Black Forest, Southern Germany. Background and objectives: Admixing deep-rooting silver fir has been proposed as a measure to increase the resilience of beech forests towards intensified drying-wetting cycles. Hence, the goal of this study was to quantify the effect of fir admixture to beech forests on the soil-atmosphere-exchange of greenhouse gases (GHGs: CO2, CH4 and N2O) and the SOC stocks by comparing pure beech (BB) and mixed beech-fir (BF) stands in the Black Forest, Germany. Materials and methods: To account for the impact of drying-wetting events, we simulated prolonged summer drought periods by rainout shelters, followed by irrigation. Results: The admixture of fir to pure beech stands reduced soil respiration, especially during autumn and winter. This resulted in increased SOC stocks down to a 0.9 m depth by 9 t C ha−1 at BF. The mixed stand showed an insignificantly decreased sink strength for CH4 (−4.0 under BB and −3.6 kg C ha−1 year−1 under BF). With maximal emissions of 25 µg N m−2 h−1, N2O fluxes were very low and remained unchanged by the fir admixture. The total soil GHG balance of forest conversion from BB to BF was strongly dominated by changes in SOC stocks. Extended summer droughts significantly decreased the soil respiration in both BB and BF stands and increased the net CH4 uptake. Conclusions: Overall, this study highlights the positive effects of fir admixture to beech stands on SOC stocks and the total soil GHG balance. In view of the positive impact of increased SOC stocks on key soil functions such as water and nutrient retention, admixing fir to beech stands appears to be a suitable measure to mitigate climate change stresses on European beech stands.

No carbon “bet hedging” in pine seedlings under prolonged summer drought and elevated CO2

Abstract More frequent drought episodes are expected to cause higher mortality in isohydric tree species such as pines, because individuals close their stomata early during drought in order to maintain constant needle water potentials. It has been suggested that trees delay the ensuing carbon starvation by actively storing carbon at the expense of growth (“bet hedging”). Because such a strategy is only adaptive in drought‐prone regions, we hypothesise that the degree of carbon “bet hedging” should differ between ecotypes. We repeatedly measured the allocation of biomass, starch and soluble sugars to needles, stem and roots in seedlings of nine populations of Pinus sylvestris and Pinus nigra along a gradient from Central Europe to the Mediterranean. During two consecutive growing seasons, seedlings grown from seed were exposed to factorial combinations of 4 months of drought (D1, D2) and ambient/elevated CO2 (aCO2/eCO2). Drought‐stressed pine seedlings did neither increase starch concentrations, nor change biomass production or experience lower mortality under eCO2 compared to aCO2. By the end of D2, seedlings from drier origin had accumulated more starch but at the same time also more biomass than seedlings from wetter origin. Surprisingly, seedlings acclimatised to dry conditions after D1 so that mortality dropped to zero and drought effects on needle starch (P. sylvestris) and overall starch (P. nigra), respectively, disappeared after D2. Synthesis. The absence of a trade‐off between carbon storage (starch) and growth (biomass), and the patterns of mortality observed in seedlings growing under combined drought and eCO2 do not support the theory of carbon “bet hedging” in isohydric Pinus sylvestris and Pinus nigra. Results suggest that reduced growth and acclimatisation minimised seedling mortality in the second year. Acclimatisation might thus enable pine seedlings to resist a moderate increase in summer drought frequency expected in the future.

Revamping Pennsylvania's Water Law System

As a relatively “wet” state, most people think of Pennsylvania as having a bountiful water supply with over 86,000 miles of streams and rivers, an abundance of groundwater, and various economic sectors generally avoiding the water shortage difficulties of the western U.S. Think again. Based on predictions from various reports, notably the Pennsylvania Climate Impacts Assessment, Pennsylvania can expect warmer temperatures year-round, increased frequency of flood events, more rain than snow in winter, prolonged summer drought, and impacts to coldwater fisheries. Establishing minimum instream flows for Commonwealth waters offers a stepping stone for adapting to predicted changes.

Zoophagy in the Larvae of Ibero-Maghrebian Spade-foot Toads

Abstract: The ecological role of anuran larvae is usually defined as a primary consumer. Recent studies have shown, however, that some grazing–rasping species consume animal matter on a regular basis. We investigated zoophagy in two species of Spade-foot Toads (Pelobates cultripes and Pelobates varaldii). The larvae of both species showed no specific morphological adaptations for macrophagy but are very large and inhabit invertebrate-rich ponds under prolonged summer drought conditions. We hypothesized that both species would consume animals having high nutritional value, and that there would be no difference between the two species in terms of the animals consumed, because tadpoles are broad dietary generalists. We also hypothesized that the consumption of animals would vary during development based on the size limitations of the oral cavity. Examination of the intestinal contents of P. varaldii and P. cultripes indicated that they had consumed a wide range of invertebrates, as predicted. Differences in ...

Impact of summer drought on isoprenoid emissions and carbon sink of three Scots pine provenances.

Scots pine (Pinus sylvestris L.) provenances cover broad ecological amplitudes. In a greenhouse study, we investigated the impact of drought stress and rewetting on gas exchange for three provenances (Italy: Emilia Romagna; Spain: Alto Ebro; Germany: East-German lowlands) of 2-year old Scots pine seedlings. CO2, water vapour and isoprenoid exchange of stressed and control trees were quantified with a four-chamber dynamic-enclosure system in the controlled environment of a climate chamber. The three provenances showed distinct isoprenoid emission patterns and were classified into a non-Δ3-carene, with either high α-/β-pinene or β-myrcene fraction, and a Δ3-carene dominated type. Isoprenoid emission rates, net-photosynthesis and transpiration were reduced during summer drought stress and significantly recovered after rewetting. A seasonal increase of isoprenoid emission rates towards autumn was observed for all control groups. Compared with the German provenance, the Spanish and Italian provenances revealed higher isoprenoid emission rates and more plastic responses to drought stress and seasonal development, which points to a local adaptation to climate. As a result of drought, net carbon uptake and transpiration of trees was reduced, but recovered after rewetting. We conclude from our study that Scots pine isoprenoid emission is more variable than expected and sensitive to drought periods, likely impacting regional air chemistry. Thus, a provenance-specific emission assessment accounting for reduced emission during prolonged (summer) drought is recommend for setting up biogenic volatile organic compound emission inventories used in air quality models.

Enhanced priming of old, not new soil carbon at elevated atmospheric CO2

Rising atmospheric CO2 concentrations accompanied by global warming and altered precipitation patterns calls for assessment of long-term effects of these global changes on carbon (C) dynamics in terrestrial ecosystems, as changes in net C exchange between soil and atmosphere will impact the atmospheric CO2 concentration profoundly. In many ecosystems, including the heath/grassland system studied here, increased plant production at elevated CO2 increase fresh C input from litter and root exudates to the soil and concurrently decrease soil N availability. Supply of labile C to the soil may accelerate the decomposition of soil organic C (SOC), a phenomenon termed ‘the priming effect’, and the priming effect is most pronounced at low soil N availability. Hence, we hypothesized that priming of SOC decomposition in response to labile C addition would increase in soil exposed to long-term elevated CO2 exposure. Further, we hypothesized that long-term warming would enhance SOC priming rates, whereas drought would decrease the priming response.We incubated soil from a long-term, full-factorial climate change field experiment, with the factors elevated atmospheric CO2 concentration, warming and prolonged summer drought with either labile C (sucrose) or water to assess the impact of labile C on SOC dynamics. We used sucrose with a 13C/12C signature that is distinct from that of the native SOC, which allowed us to assess the contribution of these two C sources to the CO2 evolved. Sucrose induced priming of SOC, and the priming response was higher in soil exposed to long-term elevated CO2 treatment. Drought tended to decrease the priming response, whereas long-term warming did not affect the level of priming significantly.We were also able to assess whether SOC-derived primed C in elevated CO2 soil was assimilated before or after the initiation of the CO2 treatment 8 years prior to sampling, because CO2 concentrations were raised by fumigating the experimental plots with pure CO2 that was 13C-depleted compared to ambient CO2. Surprisingly, we conclude that sucrose addition primed decomposition of relatively old SOC fractions, i.e. SOC assimilated more than 8 years before sampling.

Drought effects on C, N, and P nutrition and the antioxidative system of beech seedlings depend on geographic origin

AbstractIn future, prolonged summer drought and heat will constitute a major risk for the cultivation of shallow‐rooting beech in Central Europe and will negatively affect the productivity of beech forests. In a pot experiment under controlled conditions, the influence of long‐term (28 d) water deprivation on nitrogen (N), carbon (C), phosphate (Pi), and ascorbate (ASC) concentrations was examined in leaves and fine roots of beech seedlings (Fagus sylvaticaL.) from six provenances originating from Central Europe (Germany: Neidenstein and Illertissen, intermediate habitats), the Balkan peninsula (Croatia: Zagreb and Gospic, wet habitats), and Southeast Europe (Bulgaria: Kotel, Greece: Paikos; dry habitats). The goal of the study was to identify beech provenances well adapted to water limitation during summer drought events. Our results suggest that N might be involved in the alleviation of water scarcity, whereas Pimight become a limiting factor for forest growth during drought periods. Drought stress resulted in significant changes of ASC pools in leaves and fine roots and the ASC redox state. Under well‐watered and under drought conditions, ASC in leaves was the most important factor causing differences between the provenances examined. Finally, a link between P nutrition and the capacity of antioxidative stress defense by ascorbate could be highlighted. Based on observations from this study, beech seedlings from three origins (Paikos, Zagreb, and Neidenstein) might constitute beech provenances well adapted to water shortage in summer. This conclusion is drawn from the high potential of these provenances to alleviate oxidative stress during water shortage.

Weather extremes and plantation crops in the humid tropics

Monsoon rainfall across the State of Kerala was declining since last 60 years (cyclic trend of 40-60 years is also noticed with annual/monsoon rainfall) while rise in temperature is evident. Of course, rate of increase in temperature was alarming across the High ranges (where cardamom, coffee and tea are grown) due to deforestation. It is also true to some extent along the Coast (low land) due to increase in sea surface air temperature. The decade 1981-90 was the driest and warmest decade. The year 1987 was the warmest year across Kerala. The State as a whole was moving from wetness to dryness within the Humid Climate ((B4-B3 as per the Thornthwaite’s climate classification). Among weather extremes, summer drought, monsoon flood, strong wind (blows in Palghat Gap from November to February, other than cyclonic wind across the State), hailstorms, unusual rains, landslides and warming may adversely affect plantation crops’ production and its quality. Heat wave and cold waves are not relevant with reference to plantation crops under the Humid Tropics. The effect of summer drought on coconut yield was noticed in 1983, 2004 and 2013. In the case of coffee, it appears that crop matures early in recent years due to increase in temperature and climate change. The quality of coffee and black pepper, nut size in coconut and cashew are also influenced due to global warming and climate change. In the case of black pepper, the mortality rate is high in young pepper vines due to prolonged summer drought as noticed in summer 1983, 2004 and 2013. Mixed cropping/integrated farming is suggested to sustain crop income against adverse weather on long run under projected climate change scenario.

Genetic and physiological differences of European beech provenances (F. sylvatica L.) exposed to drought stress

Prolonged summer droughts constitute to a major risk for the cultivation of beech in Central Europe. Therefore the identification of beech ecotypes that cope with growth conditions expected in future is highly desirable for forest practitioners. In a pot experiment under controlled conditions, the influence of longterm water deprivation on growth, root/shoot ratio, nitrogen (N)- and carbon (C) assimilation of seedlings from 6 European beech (Fagus sylvatica) provenances, originating from Central Europe (Germany), the Balkan Peninsula (Croatia), and south-east Europe (Bulgaria, Greece) were examined. Genetic diversity and relationships between the provenances were analysed by molecular markers such as nuclear EST microsatellites (EST-SSRs), SNPs and chloroplast microsatellites. Genetic diversity within provenances was high and highlighted close relationships between plants from Greece, Croatia and Germany, whereas beech from Bavaria (Germany) seemed to be admixed with genotypes from Bulgaria. Significant changes in fine root δ13C and C/N ratio as well as the intrinsic Water Use Efficiency (iWUE) demonstrate a better adaptability to future environmental conditions of beech ecotypes genetically related to the Greek beech population.

Hyphal growth in ingrowth mesh bags in Fagus sylvatica, Quercus petraea and Pinus pinaster stands in France

Our objective was to quantify the production of ectomycorrhizal (ECM) hyphae throughout the growing season. For this purpose, we used ingrowth mesh bags (30 μm mesh filled with 40 g sand) in top soils of Fagus sylvatica, Quercus petraea and Pinus pinaster forests in France. Installations were done at three or four different growth phases at each site and mesh bags were retrieved after one to three months. Ingrowing ECM hyphae measured 250–2257 mm bag−1 on average after one to three months. Specific hyphal length was 14.6–42.9 km g−1 of hyphal mass and differed among sites. Hyphal colonization was generally greater after three months than after one month regardless of season or species. Ingrowth was lower in autumn relative to the growth in summer (Fagus) or in spring and summer (Quercus). In Pinus, growth in winter was lower than in other seasons. These differences in ingrowth may relate to differences in seasonal carbon allocation, but may also result from differences between soils and/or climate such as the prolonged summer drought, affecting in particular the Pinus site.

Effects of warming and drought on potential N2O emissions and denitrifying bacteria abundance in grasslands with different land-use.

Increased warming in spring and prolonged summer drought may alter soil microbial denitrification. We measured potential denitrification activity and denitrifier marker gene abundances (nirK, nirS, nosZ) in grasslands soils in three geographic regions characterized by site-specific land-use indices (LUI) after warming in spring, at an intermediate sampling and after summer drought. Potential denitrification was significantly increased by warming, but did not persist over the intermediate sampling. At the intermediate sampling, the relevance of grassland land-use intensity was reflected by increased potential N2O production at sites with higher LUI. Abundances of total bacteria did not respond to experimental warming or drought treatments, displaying resilience to minor and short-term effects of climate change. In contrast, nirS- and nirK-type denitrifiers were more influenced by drought in combination with LUI and pH, while the nosZ abundance responded to the summer drought manipulation. Land-use was a strong driver for potential denitrification as grasslands with higher LUI also had greater potentials for N2O emissions. We conclude that both warming and drought affected the denitrifying communities and the potential denitrification in grassland soils. However, these effects are overruled by regional and site-specific differences in soil chemical and physical properties which are also related to grassland land-use intensity.

Long-term multifactorial climate change impacts on mesofaunal biomass and nitrogen content

Rising atmospheric CO2 concentration accompanied by temperature increases and altered precipitation patterns calls for assessment of long-term effects of these climatic changes on soil organisms that are essential for ecosystem functioning.In a long-term, full-factorial climate change field experiment, with factors elevated atmospheric CO2 concentration, warming and prolonged summer drought, we assessed the responses of Collembola, oribatid and mesostigmatic mites and enchytraeid worms after 8 years of treatment. Both the biomasses and N content of oribatid and mesostigmatic mites increased at elevated CO2, or tended do so. In contrast, enchytraeid N content decreased at elevated CO2. Soil microbial biomass N pool and litter C:N ratio also increased with elevated CO2, which suggests that mite biomasses are more coupled to microbial biomass, whereas enchytraeid biomass to a larger extent is governed by litter nitrogen concentration, i.e. litter quality. Structural equation modelling confirmed the positive coupling between soil microbial N content and oribatid biomass and further between oribatid and mesostigmatic biomass. The SEM also revealed a negative relationship between microbial N content and enchytraeid biomass.The biomass of all mesofaunal groups was reduced by spring drought, especially when combined with warming. Enchytraeid and especially collembolan biomass suffered greater drought declines than mite biomasses.We conclude that under long-term elevated CO2 exposure, energy and elements will to a larger extent pass through decomposer organisms such as oribatid mites, which are based on food sources with relatively high nitrogen content.After eight years of repeated spring drought events, soil mesofauna did not show signs of adaptation to acute stress effects imposed by drought. However, Collembola and enchytraeids were more drought-sensitive than mites, and although the soil temperature increase in warmed treatments was very modest, warming exacerbated the drying of soil and thus also the negative drought impact on soil mesofauna.

What makes Haloxylon persicum grow on sand dunes while H. ammodendron grows on interdune lowlands: a proof from reciprocal transplant experiments

Determining the mechanisms underlying the spatial distribution of plant species is one of the central themes in biogeography and ecology. However, we are still far from gaining a full understanding of the autecological processes needed to unravel species distribution patterns. In the current study, by comparing seedling recruitment, seedling morphological performance and biomass allocation of two Haloxylon species, we try to identify the causes of the dune/interdune distribution pattern of these two species. Our results show the soil on the dune had less nutrients but was less saline than that of the interdune; with prolonged summer drought, soil water availability was lower on the dune than on the interdune. Both species had higher densities of seedlings at every stage of recruitment in their native habitat than the adjacent habitat. The contrasting different adaptation to nutrients, salinity and soil water conditions in the seedling recruitment stage strongly determined the distribution patterns of the two species on the dune/interdune. Haloxylon persicum on the dunes had lower total dry biomass, shoot and root dry biomass, but allocated a higher percentage of its biomass to roots and possessed a higher specific root length and specific root area by phenotypic traits specialization than that of Haloxylon ammodendron on the interdune. All of these allowed H. persicum to be more adapted to water stress and nutrient shortage. The differences in morphology and allocation facilitated the ability of these two species to persist in their own environments.