Dry Spring Research Articles

Asymptotic trends in time-varying oscillatory period for a dual-staged torsional system

The primary goal of this article is to propose a new analysis tool that estimates the asymptotic trends in the time-varying oscillatory period of a non-linear mechanical system. The scope is limited to the step-response of a torsional oscillator containing a dry friction element and dual-staged spring. Prior work on the stochastic linearization techniques is extended and modified for application in time domain. Subsequently, an instantaneous expected value operator and the concept of instantaneous effective stiffness are proposed. The non-linear system is approximated at some instant during the step-response by a linear time-invariant mechanical system that utilizes the instantaneous effective stiffness concept. The oscillatory period of the non-linear step-response at that instant is then approximated by the natural period of the corresponding linear system. The proposed method is rigorously illustrated via two computational example cases (a near backlash and near pre-load non-linearities), and the necessary digital signal processing parameters for time domain analysis are investigated. Finally, the feasibility and applicability of the proposed method is demonstrated by estimating the softening and hardening trends in the time-varying oscillatory period of the measured response for two laboratory experiments that contain clearance elements and multi-staged torsional springs.

The North Atlantic Oscillation synchronises fruit production in western European forests

Weather and its lagged effects have been associated with interannual variability and synchrony of fruit production for several tree species. Such relationships are used often in hypotheses relating interannual variability in fruit production with tree resources or favourable pollinating conditions and with synchrony in fruit production among sites through the Moran effect (the synchronisation of biological processes among populations driven by meteorological variability) or the local availability of pollen. Climatic teleconnections, such as the North Atlantic Oscillation (NAO), representing weather packages, however, have rarely been correlated with fruit production, despite often being better predictors of ecological processes than is local weather. The aim of this study was to test the utility of seasonal NAO indices for predicting interannual variability and synchrony in fruit production using data from 76 forests of Abies alba, Fagus sylvatica, Picea abies, Pseudotsuga menziesii, Quercus petraea, and Q. robur distributed across central Europe. Interannual variability in fruit production for all species was significantly correlated with seasonal NAO indices, which were more prominently important predictors than local meteorological variables. The relationships identified by these analyses indicated that proximal causes were mostly responsible for the interannual variability in fruit production, supporting the premise that local tree resources and favourable pollinating conditions are needed to produce large fruit crops. Synchrony in fruit production between forests was mainly associated with weather and geographical distance among sites. Also, fruit production for a given year was less variable among sites during warm and dry springs (negative spring NAO phases). Our results identify the Moran effect as the most likely mechanism for synchronisation of fruit production at large geographical scales and the possibility that pollen availability plays a role in synchronising fruit production at local scales. Our results highlight the influence of the NAO on the patterns of fruit production across western Europe.

Durum Wheat Productivity in Response to Soil Water and Soil Residual Nitrogen Associated with Previous Crop Management

Green manure may have potential uses in dryland agroecosystems. This study determined the effect of green manure on residual soil water and soil N and the subsequent durum wheat (Triticum turgidum L.) performance in comparison with the effect of preceding dry pea (Pisum sativum L.), silage pea (Pisum sativum L.), and spring wheat (T. aestivum L.). Three green manures [black lentil (Lens culinaris Medik.), chickling vetch (Lathyrus sativus L.), and forage pea (Pisum sativum L.)] were grown in 2006, 2007, and 2008, along with pea, wheat, and a summerfallow (check) in Saskatchewan, Canada. Durum wheat was grown the year after these treatments. At durum wheat planting, the green manure treatments had the same amount of water in the 0‐ to 1.2‐m soil profile as the summerfallow in 2007 and 2009 but less water compared with the summerfallow in 2008. Summerfallow provided highest soil N among all treatments in 2007 and 2008 but lower soil N than green manure treatments in 2009. Green manure treatments increased subsequent durum wheat grain yield by 19% compared with preceding silage pea or dry pea and by 54% compared with preceding spring wheat but decreased yield by 12% compared with summerfallow treatments. The green manure with late planting and termination enhanced soil‐water conservation and offered input‐saving advantages compared with the early‐ and mid‐planting and termination treatments. Overall, green manure treatments enhanced soil water storage, provided N benefits, and increased subsequent durum wheat yield compared with crops harvested for grain.Core Ideas Green manures conserve a similar amount of water in the 0‐ to 1.2‐m soil profile as summerfallow. Green manures provides less N benefits as summerfallow to the crops the following year. Durum wheat after green manures yielded 19 to 54% more than the other preceding crops. Delay planting of green manures offers water‐saving advantages in dry environments.

Importance of Early Season Conditions and Grazing on Carbon Dioxide Fluxes in Colorado Shortgrass Steppe

Understanding the influence of grazing management and environmental drivers on net ecosystem exchange of CO2 (NEE) is essential for optimizing carbon (C) uptake in rangelands. Herein, using 15 treatment-years (two 3-yr experiments, one with three grazing treatments, the other two) and Bowen ratio flux towers, we evaluated the influence of grazing intensity, soil water content (SWC), and plant cover (Normalized Difference Vegetation Index, or NDVI) on NEE in Colorado shortgrass steppe. Among several soil water and plant cover traits evaluated over 6-yr, early season (April, DOY 91–120) SWC and early season (DOY 130) NDVI were most highly correlated with NEE (−0.96 and −0.98, respectively) during the second quarter (April to June) of the year and also over the entire growing season (April to September; −0.97 and −0.96). Due to the strong effect of early-season SWC, an average of 166 g m−2 CO2 were lost in 2 yr with dry spring weather, compared with an average annual uptake of 218 g m−2 CO2 in 4 yr with more abundant early-season precipitation and plant cover. Grazing effects on NEE were also apparent. In one experiment, moderate grazing resulted in annual CO2 uptake of 267 g m−2 CO2 over 3 yr compared with essentially zero NEE in heavily grazed pasture. However, that treatment difference in annual NEE was only half that experienced between dry and wet years. Similar trends were observed in a second experiment, although results were insignificant. Results suggest that the recommended moderate grazing intensity for the Colorado shortgrass steppe is near optimal for CO2 uptake under season-long continuous grazing, with annual climatic variability sometimes being more influential. To enhance C sequestration in the western Great Plains of North America, grazing management strategies should emphasize flexible and adaptive practices that consider early-season SWC and promote vegetation cover during the key early spring growth period.

Salt tolerance, date of flowering and rain affect the productivity of wheat and barley on rainfed saline land

Over two growing seasons, we examined the effects of natural field salinity on grain production by a range of wheat and barley genotypes on rainfed (i.e. non-irrigated) land with a Mediterranean climate. Blocks of wheat and barley were grown at adjacent locations, on saline and non-saline sites. The two growing seasons differed in the amount of rain that fell in late Spring during grain filling, which strongly affected soil moisture, and in the salinity of the soil solution during grain filling. On the non-saline site in 2009 (less rain in Spring) there were similar ranges of grain yield between the 27 barley and 90 wheat genotypes; the 5th–95th percentile ranges of grain yield for the two crops were 0.9–2.1tha−1 and 0.9–1.9tha−1 respectively. However on the saline site in this year, the average salinity of the soil solution during grain filling was ∼0.4M, and the earlier flowering barley genotypes had higher grain yields (5th–95th percentile 1.5–3.1tha−1) than the later flowering wheat genotypes (0.8–2.3tha−1). By contrast, in 2011 (dry early Spring, rain in late Spring), on the saline site the average salinity of the soil solution was less than 0.2M during grain filling, and the 320 wheat and 14 barley genotypes had similar 5th to 95th percentile ranges in grain yield to each other under saline conditions (barley 1.1–3.1tha−1; wheat 1.3–2.9tha−1). Comparisons of genotypes common to both sites showed that there were wheat (e.g. Mace, Tammarin Rock, Binnu) and barley cultivars (e.g. Mundah, Parent 19) with consistently higher yields under saline conditions. We conclude that grain yield by cereals on saltland is associated with the severity of salinity, the adaptation of genotypes to local conditions, their salt tolerance, and (in seasons with a dry spring) early flowering.

Changes in precipitation over the La Plata Basin, projected by CLARIS-LPB regional models

CR Climate Research Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsSpecials CR 68:169-182 (2016) - DOI: https://doi.org/10.3354/cr01388 Contribution to the CR Special: 'CLARIS-La Plata Basin: climate variability and uncertainties' Changes in precipitation over the La Plata Basin, projected by CLARIS-LPB regional models Iracema Fonseca de Albuquerque Cavalcanti*, Virginia Piccinini Silveira Center of Weather Forecasting and Climate Studies/National Institute for Space Research (CPTEC/INPE), Rodovia Presidente Dutra, Km 40, CEP: 12630-000, Cachoeira Paulista, SP, Brazil *Corresponding author: iracema.cavalcanti@cptec.inpe.br ABSTRACT: The La Plata Basin (LPB) is an important region for social and economic sectors of South America, mainly as a source of water and for agriculture. Regional models are used to analyze changes in precipitation over South America in order to provide more details of the projected changes in specific regions. Results of regional models from the CLARIS-LPB project are analyzed to assess changes and uncertainties in future projections for the LPB compared to the base period. Results from several models are taken into account in order to assess uncertainties. The models agree in projecting more precipitation in the whole basin and in an increased frequency of extreme wet austral summers. The majority of models agree in projections of drier conditions in the upper LPB and an increase in the frequency of extreme dry austral winters and springs. However, in the lower LPB the projections indicate an increase in the frequency of extreme wet winters and springs. In austral autumn the uncertainties are high in the upper LPB. The uncertainties are low regarding increases in the frequency of rainy days in the middle and lower LPB and in the maximum number of consecutive dry days in the upper LPB. The projected patterns of austral summer anomaly precipitation variability obtained from empirical orthogonal function analysis display the same dipole between southern LPB and the South Atlantic Convergence Zone as in the base period, but increased anomalies in the northern center. In the austral winter the only anomaly signal simulated in the base period remains in the future projections, but the maximum variability is displaced southwards. KEY WORDS: Precipitation extremes · Regional models · La Plata Basin · Climate change · Frequency of extremes · Precipitation variability · Precipitation indices Full text in pdf format PreviousNextCite this article as: Cavalcanti IFA, Silveira VP (2016) Changes in precipitation over the La Plata Basin, projected by CLARIS-LPB regional models. Clim Res 68:169-182. https://doi.org/10.3354/cr01388 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in CR Vol. 68, No. 2-3. Online publication date: May 04, 2016 Print ISSN: 0936-577X; Online ISSN: 1616-1572 Copyright © 2016 Inter-Research.

The spatiotemporal patterns of surface soil moisture in Northeast China based on remote sensing products

Northeast China (NEC) has become one of China's most obvious examples of climate change because of its rising warming rate of 0.35 °C/10 years. As the indicator of climate change, the dynamic of surface soil moisture (SSM) has not been assessed yet. We investigated the spatiotemporal dynamics of SSM in NEC using a 32-year SSM product and found the following. (1) SSM displayed the characteristics of being dry in the west and wet in the east and decreased with time. (2) The seasonal difference was found for the temporal dynamics of SSM: it increased in summer and decreased in spring and autumn. (3) For all four regions studied, the temporal dynamics of SSM were similar to those of the whole of NEC, but with different rates of SSM change. Moreover, SSM in regions B and D had a lower spatial variance than the other two regions because of the stable spatial pattern of cropland. (4) The change rates for SSM were consistent with that observed for the warming rates, which indicated that SSM levels derived from remote sensing data will correlate with climate change. In summary, a wetter summer and a drier spring and autumn were observed in NEC over the past 30 years.

Characterizing volatile organic compounds in leachate from Gohagoda municipal solid waste dumpsite, Sri Lanka

Fate and transport of volatile organic compounds (VOCs) in landfill leachate causes environmental pollution and human health concern. These VOCs can directly be emitted to the atmosphere and also can end up in leachate plume from open solid waste dumpsites. However, no data exists on the VOCs in landfill leachate in Sri Lanka. Hence, the purpose of this study was to assess the variation of VOCs in landfill leachate at the Gohagoda Municipal Solid Waste (MSW) dumpsite, Kandy, Sri Lanka in a dry (Spring) and a wet (Autumn) month. A list of the key offensive VOCs covering 13 chemicals in leachate was quantified by using a head space–gas-chromatography–mass-spectrometry (HS-GC–MS). Results indicated that benzene and toluene were most commonly observed in both sampling periods, ranging from 1.78–21.7 and 1.73–20.2µgL−1, respectively. 4-Isopropyltoluene had the highest concentrations of the VOCs investigated in both sampling periods (38.0 and 129µgL−1 in spring and autumn, respectively). Totally, 9 different VOCs out of 13 listed were detected in the landfill leachate. In comparison, the number of VOCs and total VOC concentrations were higher at all sampling locations in spring except GSV1 sampling point. Thereby, it appears that a time series analysis of VOCs during the dry and wet months may provide a better understanding of the degree and risk of pollution of the Mahaweli River water.

Exploring tree crown spacing and slope interaction effects on fire behavior with a physics-based fire model

ABSTRACTA substantial proportion of forests in the Republic of Korea are characterized by dense, young-growth pine stands which are prone to burn, particularly in the dry spring and fall seasons. Forest fuel reduction by thinning is needed to reduce the risk of catastrophic forest fire in Korea, but thinning guidelines to effectively reduce crown fire potential are not well established, particularly in areas with steep slopes. We describe an exploration of the interaction of different tree spacings, forest age classes, and slopes using the Wildland–urban interface Fire Dynamics Simulator (WFDS), a physics-based fire behavior model. A total of 36 simulations were performed using two terrain slopes, three stand ages and six tree spacings. Our results suggest that tree spacing significantly affects how fires propagate through forest canopies. We observed large reductions in fire size and intensity when tree spacing approached crown width. Also, most crown fires in our simulation stopped spreading through the canopy in less than 100 m when tree spacing was at least 6 m. We found that a different range of tree spacings was required to limit fire size, intensity, and fire spread distance according to its age class and the terrain slope. Although this exploratory study is only a first step in assessing the potential effectiveness of thinning treatments on fire behavior in steep terrain, this study demonstrates a potential use of WFDS for testing fuel reduction thinning guidelines. Future studies should assess sensitivity to different wind speeds and a broader range of fuel characteristics.

Seasonal patterns of nitrogen cycling in subtropical short-hydroperiod wetlands: Effects of precipitation and restoration

In the event of increased frequency of extreme wet or dry events resulting from climate change, it becomes more important to understand the temporal dynamics of soil nitrogen (N) processes in ecosystems. Here, seasonal patterns of N cycling were characterized in subtropical wetlands in Everglades National Park, Florida, USA. Two restored sites and one reference site with different nutrient status, soil depth, and vegetation communities, were selected. Soil available N, microbial biomass, potential N mineralization and denitrification rates, enzyme activities of leucine aminopeptidase (LAP) and N-acetyl-β-d-glucosaminidase (NAG) were measured across the wet and dry seasons from 2010 to 2011. In general, most N processes were significantly correlated with soil water contents (P<0.05) which reflected the precipitation regime. The lower elevation and shallower soil (2–3cm depth) at the restored site may contribute to their higher soil water contents compared to the reference site with ~10cm soil depth, which further led to the earlier peaks of microbial biomass at the two restored sites. Potential N mineralization was positively correlated with LAP at the restored sites whereas with NAG at the reference site (P<0.05), implying that different vegetation composition may provide varying substrates for soil microbes. The build-up of nitrate in the dry spring of 2011 induced a pulse of denitrification after rewetting by a sudden rainfall, implying the presence of a hot moment of denitrification during the dry-rewetting transition period. The decrease of MBC:MBN ratio from dry to wet season indicates a possible microbial composition shift from fungi to bacteria, shedding lights on the potential contribution of fugal groups to denitrification in the dry season. Our study highlight that even under the same climate regime, the small-scale variations could affect the seasonal patterns of N cycling.

Effect of harvest on the different fertilization methods of asparagus (Asparagus officinalis L.)

The experiments were performed in the University of Debrecen Centre for Agricultural Research Institute of Nyíregyháza of 1500 m2 asparagus plantation in 2011. We were determind the number of shoots, plant height, fold thickness and yield of asparagus hybrid. I studied to the effects of various cropyear is addition Grolim hybrid asparagus growth and development. Compared to previous years, 2014 was unusually wet and cold weather conditions which causes the plantation has developed outstanding. The soil temperature early and dynamic increase due to, the harvest had started a one month earlier 2014 than the 2013. The experiment asparagus harvested from lower yields as a result of the extremely dry spring in 2015.

Spatial and seasonal dynamics of soil loss ratio in mountain rangelands of south-western Kyrgyzstan

Vegetation cover is the main factor of soil loss prevention. The C-factor of the RUSLE (Revised Universal Soil Loss Equation) was predicted with NDVI, ground data and exponential regression equation for mountain rangelands of Kyrgyzstan. Time series of C-factor, precipitation and temperature were decomposed into seasonal and trend components with STL (seasonal decomposition by loess) to assess their interrelations. C-factor, precipitation and temperature trend components indicated significant lagged correlation, whereas seasonal components indicated more complex relations with climate factors which can be promoting as well as limiting factors for vegetation development, depending on the season. Rainy springs and hot summers may increase soil loss dramatically, whereas warm and dry springs with rainy summers can decrease it. Steep slopes indicated higher soil loss ratio, whereas flat areas were better protected by vegetation.

Tick exposure and extreme climate events impact survival and threaten the persistence of a long-lived lizard.

Assessing the impacts of multiple, often synergistic, stressors on the population dynamics of long-lived species is becoming increasingly important due to recent and future global change. Tiliqua rugosa (sleepy lizard) is a long-lived skink (>30 years) that is adapted to survive in semi-arid environments with varying levels of parasite exposure and highly seasonal food availability. We used an exhaustive database of 30 years of capture-mark-recapture records to quantify the impacts of both parasite exposure and environmental conditions on the lizard's survival rates and long-term population dynamics. Lizard abundance was relatively stable throughout the study period; however, there were changing patterns in adult and juvenile apparent survival rates, driven by spatial and temporal variation in levels of tick exposure and temporal variation in environmental conditions. Extreme weather events during the winter and spring seasons were identified as important environmental drivers of survival. Climate models predict a dramatic increase in the frequency of extreme hot and dry winter and spring seasons in our South Australian study region; from a contemporary probability of 0.17 up to 0.47-0.83 in 2080 depending on the emissions scenario. Our stochastic population model projections showed that these future climatic conditions will induce a decline in the abundance of this long-lived reptile of up to 67% within 30 years from 2080, under worst case scenario modelling. The results have broad implications for future work investigating the drivers of population dynamics and persistence. We highlight the importance of long-term data sets and accounting for synergistic impacts between multiple stressors. We show that predicted increases in the frequency of extreme climate events have the potential to considerably and negatively influence a long-lived species, which might previously have been assumed to be resilient to environmental perturbations.

Seed predation and climate impacts on reproductive variation in temperate forests of the southeastern USA.

Climatic effects on tree recruitment will be determined by the interactive effects of fecundity and seed predation. Evaluating how insect and vertebrate seed predators mediate tree reproductive responses to climate depends on long-term studies of seed production, development, and predation. In this study, our objectives were to (1) assess the effects of interannual climate variation on seed abortion rates, (2) assess the impact of seed density on predation rates, and (3) examine the degree to which density-dependent seed predation would amplify or dampen interannual variation in fecundity associated with seed abortion. We used a 19-year study of seed abortion and pre-dispersal predation rates by insects and vertebrates (birds and rodents) for five temperate tree species across forest plots from the North Carolina Piedmont to the Southern Appalachian Mountains in the southeastern USA. We found that rates of seed abortion and predation increased reproductive variation for oaks (Quercus species). Probability of seed abortion was greatest during years with cool, dry springs. Responses of seed predation on Quercus species to current year's seed density varied by species, but exhibited positive density-dependence to previous year's seed density consistent with numerical responses of seed predators. Seed abortion and predation rates for two drupe species responded little to variation in climate or seed density, respectively. Given that predation increased interannual variation in seed availability and the negative density-dependence to previous year's seed density, our results indicate that consistent numerical responses of oak seed predators may amplify interannual variation due to climate-mediated processes like seed abortion.

Re-evaluating sowing time of spring canola (Brassica napus L.) in south-eastern Australia—how early is too early?

Optimising the sowing date of canola (Brassica napus L.) in specific environments is an important determinant of yield worldwide. In eastern Australia, late April to early May has traditionally been considered the optimum sowing window for spring canola, with significant reduction in yield and oil in later sown crops. Recent and projected changes in climate, new vigorous hybrids, and improved fallow management and seeding equipment have stimulated a re-evaluation of early-April sowing to capture physiological advantages of greater biomass production and earlier flowering under contemporary conditions. Early–mid-April sowing generated the highest or equal highest yield and oil content in eight of nine field experiments conducted from 2002 to 2012 in south-eastern Australia. Declines in seed yield (–6.0% to –6.5%), oil content (–0.5% to –1.5%) and water-use efficiency (–3.8% to –5.5%) per week delay in sowing after early April reflected levels reported in previous studies with sowings from late April. Interactions with cultivar phenology were evident at some sites depending on seasonal conditions. There was no consistent difference in performance between hybrid and non-hybrid cultivars at the earliest sowing dates. Despite low temperatures thought to damage early pods at some sites (&lt;−2°C), frost damage did not significantly compromise the yield of the early-sown crops, presumably because of greater impact of heat and water-stress in the later sown crops. A validated APSIM-Canola simulation study using 50 years of weather data at selected sites predicted highest potential yields from early-April sowing. However, the application of a frost-heat sensitivity index to account for impacts of temperature stress during the reproductive phase predicted lower yields and higher yield variability from early-April sowing. The frost–heat-limited yields predicted optimum sowing times of mid-April at southern sites, and late April to early May at the northern sites with lower median yield and higher yield variability in crops sown in early April. The experimental and simulation data are potentially compatible given that the experiments occurred during the decade of the Millennium drought in south-eastern Australia (2002–10), with dry and hot spring conditions favouring earlier sowing. However, the study reveals the need for more accurate and validated prediction of the frost and heat impacts on field-grown canola if simulation models are to provide more accurate prediction of attainable yield as new combinations of cultivar and sowing dates are explored.

Determinants of nectar production in heather

With the aim of contributing to the development of decision support tools for migratory beekeepers in their planning for hive movements to heathlands in time and space, models were developed from analyses of heather flowering, nectar secretion, sugar concentration, and weight of bee hives in relation to meteorological parameters across two seasons in Denmark. The number of heather flowers was significantly lower in 2013 compared with 2012, reflecting the very long-lasting winter combined with the very dry spring period of 2013. The analyses showed that the initial number of flowers is determined by the amount of accumulated day-degree in early May, as well as the total rainfall prior to flowering and that subsequent changes in the number of flowers is governed by daily temperature and rainfall. The amount of sugar secreted per heather flower was constant throughout the observation period in 2012 and significantly higher than in 2013 when sugar secretion rate decreased in the course of the observation period. The analyses revealed that total rainfall prior to flowering, in addition to both the relative humidity and the difference between night and day temperatures in the period of secretion, is of importance for the sugar secretion rate. Fewer honey bees foraged in heather in 2013 compared with 2012, reflecting the poorer heather quality and translating directly into similar differences in weight gains of hives. Our results are useful for beekeepers in their evaluation of expected heather honey yield and thus the profitability in relocating bee colonies to heathland areas.

El Kowm Oasis, human settlement in the Syrian Desert during the Pleistocene

The area of the El Kowm oasis in the centre of today's Syria is a unique showcase of Middle Eastern prehistory not only for its geographic position within the Arabian Desert, but also for the depth of its history. In fact the core area, about a dozen kilometres across, with the numerous natural springs were an important attractor for game and humans, visiting regularly this region since more than 1.8 million years. All over the periods, from the Lower Palaeolithic to the arrival of farmers, definitely arid conditions prevailed, perfectly showing human adaption to this particular environment, as has been clearly demonstrated by several extensive excavations of exceptionally rich Palaeolithic sites. Exceptional preservation conditions due to a particular geological setting in the context of active or dry springs offer an extraordinary resolution of human behaviour within a limited landscape during the complete period of the Pleistocene, illustrating cognitive capacities of early man to cope readily with arid environments, challenge the migration routes proposed by Out of Africa diffusion models, typically considered to be corridors with a superior environmental potential.

Initial Stages of Tundra Shrub Litter Decomposition May Be Accelerated by Deeper Winter Snow But Slowed Down by Spring Warming

The Arctic climate is projected to change during the coming century, with expected higher air temperatures and increased winter snowfall. These climatic changes might alter litter decomposition rates, which in turn could affect carbon (C) and nitrogen (N) cycling rates in tundra ecosystems. However, little is known of seasonal climate change effects on plant litter decomposition rates and N dynamics, hampering predictions of future arctic vegetation composition and the tundra C balance. We tested the effects of snow addition (snow fences), warming (open top chambers), and shrub removal (clipping), using a full-factorial experiment, on mass loss and N dynamics of two shrub tissue types with contrasting quality: deciduous shrub leaf litter (Salix glauca) and evergreen shrub shoots (Cassiope tetragona). We performed a 10.5-month decomposition experiment in a low-arctic shrub tundra heath in West-Greenland. Field incubations started in late fall, with harvests made after 249, 273, and 319 days of field incubation during early spring, summer and fall of the next year, respectively. We observed a positive effect of deeper snow on winter mass loss which is considered a result of observed higher soil winter temperatures and corresponding increased winter microbial litter decomposition in deep-snow plots. In contrast, warming reduced litter mass loss during spring, possibly because the dry spring conditions might have dried out the litter layer and thereby limited microbial litter decomposition. Shrub removal had a small positive effect on litter mass loss for C. tetragona during summer, but not for S. glauca. Nitrogen dynamics in decomposing leaves and shoots were not affected by the treatments but did show differences in temporal patterns between tissue types: there was a net immobilization of N by C. tetragona shoots after the winter incubation, while S. glauca leaf N-pools were unaltered over time. Our results support the widely hypothesized positive linkage between winter snow depth and litter decomposition rates in tundra ecosystems, but our results do not reveal changes in N dynamics during initial decomposition stages. Our study also shows contrasting impacts of spring warming and snow addition on shrub decomposition rates that might have important consequences for plant community composition and vegetation-climate feedbacks in rapidly changing tundra ecosystems.

Reduced stem growth, but no reserve depletion or hydraulic impairment in beech suffering from long-term decline

Under non-extreme drought conditions, reduced stem growth is not associated with reserve or hydraulic impairment in beech suffering from long-term decline. Global change is expected to increase the frequency and the intensity of drought events in temperate ecosystems. In some regions, this might be associated with an increase in tree decline. Of the ecophysiological mechanisms that have been proposed to explain tree decline, an impairment of the vascular transport system and/or carbon function are two of the most discussed. In a context of long-term decline caused by droughts, we investigated the functional differences between the carbon, nitrogen, and hydraulic functions of healthy and declining mature beech (Fagus sylvatica L.) trees. The study was carried out over two consecutive years with contrasting water availabilities. The radial growth of declining trees was clearly less than that of healthy trees, due to a lower growth rate, associated during the wet year with a shorter growth period. Leaf functional characteristics and hydraulic parameters (native embolism and cavitation vulnerability curves) were similar in healthy and declining trees. However, at the end of a growing season characterized by a dry spring, carbon reserves concentrations in young branches of declining trees were lower than those in healthy trees, though they recovered during the following non-constraining growing season. Our results did not indicate carbon starvation, nitrogen deficiency, or hydraulic failure. However, there seems to be some compensation mechanism related to reserve dynamics in the remaining living tissue of the declining trees. This study shows that the climate conditions of successive years are probably crucial for these functional adjustments to be operational.

Seasonal microbial and nutrient responses during a 5-year reduction in the daily temperature range of soil in a Chihuahuan Desert ecosystem.

High daily temperature range of soil (DTRsoil) negatively affects soil microbial biomass and activity, but its interaction with seasonal soil moisture in regulating ecosystem function remains unclear. For our 5-year field study in the Chihuahuan Desert, we suspended shade cloth 15 cm above the soil surface to reduce daytime temperature and increase nighttime soil temperature compared to unshaded plots, thereby reducing DTRsoil (by 5 ºC at 0.2 cm depth) without altering mean temperatures. Microbial biomass production was primarily regulated by seasonal precipitation with the magnitude of the response dependent on DTRsoil. Reduced DTRsoil more consistently increased microbial biomass nitrogen (MBN; +38%) than microbial biomass carbon (MBC) with treatment responses being similar in spring and summer. Soil respiration depended primarily on soil moisture with responses to reduced DTRsoil evident only in wetter summer soils (+53%) and not in dry spring soils. Reduced DTRsoil had no effect on concentrations of dissolved organic C, soil organic matter (SOM), nor soil inorganic N (extractable NO3 (-)-N + NH4 (+)-N). Higher MBN without changes in soil inorganic N suggests faster N cycling rates or alternate sources of N. If N cycling rates increased without a change to external N inputs (atmospheric N deposition or N fixation), then productivity in this desert system, which is N-poor and low in SOM, could be negatively impacted with continued decreases in daily temperature range. Thus, the future N balance in arid ecosystems, under conditions of lower DTR, seems linked to future precipitation regimes through N deposition and regulation of soil heat load dynamics.