Severe Summer Drought Research Articles

Isotope‐aided modelling of ecohydrologic fluxes and water ages under mixed land use in Central Europe: The 2018 drought and its recovery

AbstractUnderstanding the interactions of vegetation and soil water under varying hydrological conditions is crucial to aid quantitative assessment of land‐use sustainability for maintaining water supply for humans and plants. Isolating and estimating the volume and ages of water stored within different compartments of the critical zone, and the associated fluxes of evaporation, transpiration, and groundwater recharge, facilitates quantification of these soil–plant‐water interactions and the response of ecohydrological fluxes to wet and dry periods. We used the tracer‐aided ecohydrological model EcH2O‐iso to examine the response of water ages of soil water storage, groundwater recharge, evaporation, and root‐uptake at a mixed land use site, in northeastern Germany during the drought of 2018 and in the following winter months. The approach applied uses a dynamic vegetation routine which constrains water use by ecological mechanisms. Two sites with regionally typical land‐use types were investigated: a forested site with sandy soils and a deep rooting zone and a grassland site, with loamier soils and shallower rooting zone. This results in much younger water ages (<1 year) through the soil profile in the forest compared to the grass, coupled with younger groundwater recharge. The higher water use in the forest resulted in a more pronounced annual cycle of water ages compared to the more consistent water age in the loamier soil of the grasslands. The deeper rooting zone of the forested site also resulted in older root‐uptake water usage relative to soil evaporation, while the grassland site root‐uptake was similar to that of soil evaporation. Besides more dynamic water ages in the forest, replenishment of younger soil waters to soil storage was within 6 months following the drought (cf. >8 months in the grassland). The temporal evaluation of the responsiveness of soil and vegetation interactions in hydrologic extremes such as 2018 is essential to understand changes in hydrological processes and the resilience of the landscape to the longer and more severe summer droughts predicted under future climate change.

Chitosan Oligosaccharide Addition to Buddhist Pine (Podocarpus macrophyllus (Thunb) Sweet) under Drought: Reponses in Ecophysiology and δ13C Abundance

Climate warming induces the necessity to increase the drought resistance of shade-obligate juvenile trees in sub-tropical forests. Chitosan oligosaccharide (COS) is a biopolymer derived from the marine resource that has attracted accumulative attention to induce and promote a plant’s resistance to abiotic stress. Buddhist pine (Podocarpus mascrophyllus (Thunb)Sweet) seedlings were cultured as the model material whose natural distribution in sub-tropical areas of China has suffered severe summer drought events in the last 113 years. A split-block design was conducted with a simulated drought event (drought vs. irrigated control), the COS addition, and two samplings at the ends of drought and re-watered treatments. The COS addition increased the resistance to drought by inducing a starch allocation towards roots where δ13C abundance and antioxidant enzyme activities were upregulated. The COS addition can promote biomass allocation to roots and increase the number of new roots. The COS addition to drought-treated Buddhist pine seedlings resulted in robust diameter growth. Therefore, COS is an available polymer to promote the resistance of Buddhist pine to drought. More work is suggested to clarify the dose of COS addition that can induce a prominent response of biomass accumulation and carbohydrate metabolism.

Is Pasture Cropping a Valid Weed Management Tool.

The aim of the present work was to study the feasibility of pasture cropping under the Mediterranean conditions prevailing in central Spain and its potential as a weed management tool. Three cropping systems were assessed: conventionally grown winter barley and winter barley in pasture cropping with two perennial summer species, Cynodon dactylon and Eragrostis curvula. The results showed that the growth of these two species in a pasture cropping system was limited by the severe drought conditions and high temperatures present during the summer in some of the study years. Although there were no differences in the establishment of winter barley in any of the treatments assessed, pasture cropping reduced winter barley yields up to 50%–60% in years with low rainfall in spring. Regarding weed control, pasture cropping showed a significant suppression of the total weed density and number of weed species. As a conclusion, pasture cropping can be considered as a valid weed management tool. However, the economic feasibility of this system under the climatic conditions of central Spain (characterized by a high risk of severe summer droughts) is still not clear. The availability of supplemental irrigation may reduce competition between pastures and winter crops and ensure a profitable production of summer pastures.

Scion–rootstock interactions influence the growth and behaviour of the grapevine root system in a heavy clay soil

Background and Aims Generally, grapevine roots have been less studied than the above-ground parts of the plant. Here we analyse scion–rootstock interactions in mature vines growing in a heavy clay soil in a climate characterised by severe summer drought to investigate the effect of the scion–rootstock interaction in a suboptimal soil. Methods and Results The rootstocks, 34 Ecole de Montpellier, 140 Ruggeri and 1103 Paulsen, were grafted onto Nerello Mascalese and Nero d'Avola scions and assessed along with self-rooted vines. Root distribution and root architecture were analysed using the profile wall method at 0, 60 and 120 cm from the row midline. Root density was greatest at a depth between 21 and 60 cm. The cumulative root fraction for root density registered a β value, a numerical quantity that summarises depth distribution, ranging between 0.932 and 0.962. Root number and density were significantly lower for the self-rooted vines compared to that of the grafted vines. Conclusions The scion genotypes affected most developmental parameters, including the diameter of the root system, the root density at 21–80 cm depth and the ratio of fine roots to coarse roots. Significance of the Study The scion plays an important role in grapevine root growth, development and distribution in a heavy clay soil, although the mechanism remains unclear.

Germination timing and chilling exposure create contingency in life history and influence fitness in the native wildflower Streptanthus tortuosus

Abstract The timing of life history events, such as germination and reproduction, influences ecological and selective environments throughout the life cycle. Many organisms evolve responses to seasonal environmental cues to synchronize these key events with favourable conditions. Often the fitness consequences of each life history transition depend on previous and subsequent events in the life cycle. If so, shifts in environmental cues can create cascading effects throughout the life cycle, which can influence fitness, selection on life history traits, and population viability. We examined variation in cue responses for contingent life history expression and fitness in a California native wildflower, Streptanthus tortuosus, by manipulating seasonal germination timing in a common garden experiment. We also manipulated chilling exposure to test the role of vernalization cues for seasonal life history contingency. Plants germinating early in the growing season in autumn were more likely to flower in the first year and less likely to perennate than later germinants in spring. First‐year reproduction and overall fitness was the highest for autumn cohorts. Sensitivity analyses showed that optimal germination date depended on survival beyond the first year and fruit production in later years. Experimental chilling exposure induced first‐year flowering in spring germinants, demonstrating that seasonal life history contingency is mediated by a vernalization requirement. This requirement reduced fitness of spring germinants without increasing survival or later fecundity and may be maladaptive. Such mismatches between cues and fitness may become more pervasive as predicted climate change reduces exposure to chilling, shortens growing seasons, and increases severity of summer drought. Synthesis. Shifts in germination timing in seasonal environments can cause cascading effects on trait expression and fitness that extend beyond the first year of the life cycle. Climate change is likely to shift seasonal conditions, influencing such life history contingency, with significant impacts on trait expression, fitness, and population persistence. These shifts may cause strong natural selection on cue sensitivity and life history expression, but it is an open question whether populations have the potential for rapid adaptation in response to this selection.

Snowpack influences spatial and temporal soil nitrogen dynamics in a western U.S. montane forested watershed

AbstractDeclines in winter snowpack have increased the severity of summer droughts in western U.S. forests, with the potential to also impact soil available nitrogen (N). To understand how snowpack controls spatiotemporal N availability, we examined seasonal N dynamics across elevation, aspect, and topographic position (hollow vs. slope) in a forested watershed in the northern Rocky Mountains. As expected, peak snow‐water equivalent (SWE) was generally greater at higher elevations and on north‐facing aspects. However, the effects of topographic position and snowdrift led to variability in snow accumulation at smaller spatial scales. Spatial patterns of the snowpack, in turn, influenced soil moisture and temperature, with greater SWE leading to generally higher soil moisture levels during the summer and smaller temperature fluctuations throughout the year. Wetter conditions in spring or fall generally supported greater inorganic N pools, but at the driest locations (low‐elevation slope), pulses of N mineralization in summer may have played important roles in overall N dynamics. More importantly, soil moisture during the summer appeared to be more influenced by antecedent snowpack from the previous year than by current‐year summer rain. Subsequently, N mineralization under snowpack may be strongly influenced by soil moisture and temperature conditions from the previous fall, before snowpack accumulation. Together, our results indicate that snowpack strongly influences N dynamics beyond the current growing season in western coniferous forests through mediation of soil moisture and temperature, and suggest that further decline in winter snowpack may affect these forests through constraints in both water and N availability.

Impacts of the 2017 flash drought in the US Northern plains informed by satellite-based evapotranspiration and solar-induced fluorescence

Drought is increasing in frequency and severity, exacerbating food and water security risks in an era of continued global warming and human population growth. Here, we analyzed a severe summer drought affecting the US Northern Plains region in 2017. We examined the spatial pattern and seasonal progression of vegetation productivity and water use in the region using satellite-based estimates of field-scale (30 m) cropland evapotranspiration (ET), county level annual crop production statistics, and GOME-2 satellite observations of solar-induced chlorophyll fluorescence (SIF). The cropland ET record shows strong potential to track seasonal cropland water demands spatially, with strong correspondence to regional climate variables in the Northern Plains. The GOME-2 SIF record shows significant but limited correlations with finer scale climate variability due to the coarse sensor footprint, but captured an anomalous regional productivity decline coincident with drought related decreases in crop production and ET. The drought contributed to an overall 25% reduction in cropland ET, 6% decrease in crop production, and 11% reduction in SIF productivity over the region from April to September in 2017 relative to the longer (2008–2017) satellite record. More severely impacted agricultural areas indicated by the US Drought Monitor exceptional drought (D4) category represented 11% of the region and showed much larger anomalous ET (20%–81%) and productivity (11%–73%) declines. The regional pattern of drought impacts indicated more severe productivity and ET reductions in the north central and southern counties with extensive agriculture, and less impact in the western counties of the Northern Plains. This study provides a multiscale assessment of drought related impacts on regional productivity and ET over a crop intensive region, emphasizing the use of global satellite observations capable of informing regional to global scale water and food security assessments.

Different Effects of Spring and Summer Droughts on Ecosystem Carbon and Water Exchanges in a Semiarid Shrubland Ecosystem in Northwest China

Extensive revegetation and conservation programs have been initiated in semiarid desert areas in northern China for over 10 years. However, our knowledge on how drought affects the desert ecosystem carbon (C) and water exchanges is limited. Based on eddy covariance measurements, we evaluated the effect of spring and summer droughts on ecosystem C and water exchanges over a semiarid Artemisia ordosica shrubland in Northwest China for 2014–2016. Over the 3 years, the ecosystem was a weak source of C (40 ± 32 g C m−2 y−1) (mean ± standard deviation) with evapotranspiration (ET) of 310 ± 65 mm y−1. Annual net ecosystem production was − 76, − 25 and − 18 g C m−2 y−1 and water use efficiency (WUE) [gross ecosystem production (GEP)/ET] was 0.92, 1.14 and 1.09 g C kg−1 H2O in 2014, 2015 and 2016, respectively, indicating greater C loss and suppressed WUE in 2014. We found that GEP, Re and ET were significantly suppressed in year 2014 when a severe spring drought occurred, as compared to the year 2016 with summer droughts. The differences in C and water fluxes between years 2014 and 2016 were greater than those between years 2015 and 2016, although the ecosystem suffered a severe summer drought in 2015. Interannual differences in GEP were likely due to differences in leaf area index and canopy conductance, both of which were decreased by drought. Our results indicated that spring drought appears to be more critical to the C balance than summer drought in this semiarid shrubland. This suggested that under future climate change with more variable precipitation and frequent droughts expected in northern China, the timing of drought may be critical to the C and water balances.

Assessing the effects of drought and “Grain for Green” Program on vegetation dynamics in China's Loess Plateau from 2000 to 2014

China's Loess Plateau (CLP) has experienced severe water loss and soil erosion over the past few decades. To prevent water loss and soil erosion and improve ecological environment, the Grain for Green Project (GGP) was launched in 1999. Meanwhile, the frequency and severity of drought in CLP has shown an increasing trend in the recent years. However, few studies have addressed the coupled effects of drought and the GGP on vegetation dynamics in this area. Based on the Standardized Precipitation Evapotranspiration Index (SPEI) and Normalized Difference Vegetation Index (NDVI), this study assesses the change trends of drought and vegetation dynamics and the role of droughts and GGP on trends in vegetation dynamics. Results show that: (1) At the regional scale, annual and seasonal NDVI show a significant increase from 2000 to 2014; however, drought change trends were insignificant. (2) At the pixel scale, annual, spring, summer, and autumn NDVI increased in the CLP from 2000 to 2014, with 51.92%, 50.04%, 74%, and 62.69% of the study area showing a significant increase, respectively, with p < 0.05. Summer drought had strongest effect on vegetation dynamics. (3) Drought was one of the primary reasons for the decreasing trend of NDVI in the northern of the CLP. The severe and extreme drought (SPEI<−1.5) in summer of 2001 and 2005 reduced the NDVI by 22.44% and 9.98%, respectively. (4) The residual analysis results indicate that 21.54% of the CLP was affected by drought. 43.64% and 34.82% of the CLP experienced human induced improvement and degradation, respectively. (5) The GGP had an important influence on vegetation dynamics, strong correlations between the cumulative afforestation area and annual NDVI in Yan'an and Yulin from 2000 to 2013 were found, with r = 0.96 and 0.65. The spatial pattern of the cumulative afforestation area ratio matched quite well with the NDVI change trend.

Cutting the Gordian Knot of abiotic stress in grapevine: From the test tube to climate change adaptation.

In Mediterranean climate areas, the available scenarios for climate change suggest an increase in the frequency of heat waves and severe drought in summer. Grapevine (Vitis vinifera L.) is a traditional Mediterranean species and is the most valuable fruit crop in the world. Currently, viticulture must adjust to impending climate changes that are already pushing vine‐growers toward the use of irrigation, with the concomitant losses in wine quality, and researchers to study tolerance to stress in existing genotypes. The viticulture and winemaking worlds are in demand to understand the physiological potential of the available genotypes to respond to climate changes. In this review, we will focus on the cross‐talk between common abiotic stresses that currently affect grapevine productivity and that are prone to affect it deeper in the future. We will discuss results obtained under three experimental stress conditions and that call for specific responses: (1) acclimatization of in vitro plantlets, (2) stress combinations in controlled conditions for research purposes, (3) extreme events in the field that, driven by climate changes, are pushing Mediterranean species to the limit. The different levels of tolerance to stress put in evidence by the plasticity of phenotypic and genotypic response mechanisms, will be addressed. This information is relevant to understand varietal adaptation to impending climate changes and to assist vine growers in choosing genotypes and viticulture practices.

Effects of A Severe Drought On Summer Abundance, Growth, and Movement of Cutthroat Trout In A Western Oregon Headwater Stream

Winter snowpack depth, snowmelt timing, and snowmelt duration are projected to change in the future, leading to increased frequency and severity of drought in the Pacific Northwest. In summer 2015, stream flows throughout the Pacific Northwest were at record low levels because of low winter snowpack conditions consistent with these climate projections. We explored effects of the 2015 low-snowpack-associated drought on Coastal Cutthroat Trout (Oncorhynchus clarkii clarkii) abundance, growth, and movement patterns in two 100m reaches ( 350 y old forests) of an unnamed perennial western Oregon headwater stream before (2 y) and during a severe drought. We found that the abundance of Cutthroat Trout declined substantially during the drought year, regardless of habitat availability, riparian forest age, or stream wood structure. Fish growth during summer was consistently negative during all 3 y of the study in both reaches. During the drought year, estimated abundance and total biomass of Cutthroat Trout declined in both reaches compared to the 2y prior. In all 3 y, the majority (76%) of fish in the reach with a young riparian forest stand moved >2 m from their release point. In contrast, across the 3 study years, only 26% of fish on average moved >2 m from their release point in the old-growth reach, which had more large wood and pool area. Across both reaches, in the non-drought years, most fish moved into pools (32.4%), but some moved to riffles (23.3%). During the drought year, of the fish that were recaptured, only upstream movement to pools were observed. There were no observed movements of recaptured fish to riffles. Overall, study results suggest that increasing severity of summer drought in the Pacific Northwest is likely to reduce the abundance of fish in small headwater streams, and the remaining fish preferentially use pool habitats such as those found in structurally complex streams.

Invasive Asclepias syriaca can have facilitative effects on native grass establishment in a water‐stressed ecosystem

AbstractQuestionWhat is the effect of invasive common milkweed (Asclepias syriaca L.) on the germination and early establishment of native grass species during open sand grassland vegetation recovery in old‐fields?LocationFülöpháza Sand Dune Area, Hungary.MethodsA small‐scale experiment was carried out in a sandy old‐field infested with Asclepias. We designated 36 2 m × 2 m plots in patches of Asclepias. We seeded two native grass species Festuca vaginata and Stipa borysthenica in 12 plots each (a third of the plots was left unseeded). We applied repeated mechanical removal of Asclepias shoots on half of the plots for two growing seasons. The number and above‐ground cover of the two grass seedlings were evaluated for two growing seasons.ResultsThe number and above‐ground cover of Festuca and Stipa seedlings did not increase after applying Asclepias shoot removal during the 2 years of the study. We found lower seedling number and cover of Festuca in plots with Asclepias shoot removal in the second year, when a severe summer drought occurred at the study site. The number and cover of the Stipa seedlings did not differ between plots with Asclepias shoot removal and control plots throughout the experiment.ConclusionsWe did not find any negative effects of the presence of the invasive Asclepias during open sand grassland regeneration in terms of germination and early establishment of the dominant grass species. We even detected a nurse effect of Asclepias on Festuca, where the shade of Asclepias may have mitigated the unfavourable abiotic conditions for Festuca caused by summer drought. This mitigation was not observed in the case of Stipa, which can better tolerate summer droughts. Our results suggest that Asclepias control is not required for a successful open sand grassland restoration in the early phase of vegetation recovery, and restoration efforts should focus on the mitigation of propagule limitation of native grasses. However, further information is needed about the effects of Asclepias on other elements of the biota and in later phases of secondary succession.

Determination of the Growth Performance and Soil Protection Ability of the Caper (Capparis sp) in Eroded Site in Semi-Arid Region of Artvin, Turkey

As a result of the statistical analysis, there were significant differences in terms of runoff, sediment transport amount and sediment concentration according to season and years, but no significant differences were observed between bare area and caper experimental sites. It has been determined that the adaptation of the caper in the eroded soil and its development is not at the desired level. It is thought that severe summer drought during the growing season negatively affects the survival and development of the caper saplings in the first years of planting. The survival rate of the caper and its low development could not prevent the erosive effect of the rain. It can be said that severe summer drought and soil characteristics (e.g: soil texture, soil water level) have significant effects on growing and survival rates of caper saplings. It can be more useful to use combination with additional erosion control techniques (e.g., soil protection techniques) to make erosion control in this kind of sites where there is irregular and inadequate rainfall. In this study, the growth performance and soil protection ability of the Caper (Capparis sp) in eroded site in semi-arid region of Artvin, Turkey were investigated. For this purpose, the experimental design at the site included a randomized complete block with four replicates (a total of 8 experimental plots: 2 treatments × 4 replicates). Each plot was 20-m long×5-mwide, which is wide enough to minimize edge effects and large enough for downslope rills to develop. Between 2006-2008, water and sediment accumulated in sediment deposited tanks were measured. Then, with the help of these values, the runoff, the amount of transported sediment and the concentration of transported sediment were calculated. In addition, the development of the caper seedlings has been observed and recorded. As a result of the research, the average survival percent of caper sapling in 2006 was found to be 21%. While, the mean height of caper seedlings varied between 4.6 cm and 6.1 cm in 2006, it has been varied between 5.2 cm and 8.18 cm in 2007.

No role for xylem embolism or carbohydrate shortage in temperate trees during the severe 2015 drought

AbstractTemperate forests are predicted to experience an increased frequency and intensity of climate change‐induced summer droughts and heat waves in the near future. Yet, while previous studies clearly showed a high drought sensitivity of different temperate tree species, the vulnerability of the physiological integrity of these trees remains unclear.Here, we assessed the sensitivity of six temperate tree species to severe water limitation during three consecutive growing seasons, including the exceptional 2015 central European summer drought and heat wave. Specifically, we assessed stem increment growth, sap flow, water potentials, hydraulic vulnerability, and nonstructural carbohydrate contents in leaves and branches to determine how mature temperate trees responded to this exceptional weather event and how the observed responses relate to variation in xylem embolism and carbohydrate economy.We found that the trees' predawn water potentials reached their minimum values during the 2015 summer drought and most species reduced their sap flow by up to 80%. Also, increment growth was strongly impaired with the onset of the drought in all species. Despite the strong responses in the trees' growth and water relations, all species exhibited minimum midday shoot water potentials well away from values associated with severe embolism (P50). In addition, we detected no distinct decrease in nonstructural carbohydrate contents in leaves, bark, and stems throughout the drought event.Synthesis. This study shows that mature individuals of six common central European forest tree species strongly reacted to a severe summer drought by reducing their water consumption and stopping growth. We found, however, no indications for xylem embolism or carbohydrate depletion in these trees. This suggests, that xylem embolism formation and carbohydrate reserve depletion are not routine in temperate trees during seasonal strong drought and reveals a low vulnerability of the physiological integrity of temperate trees during drought events as we describe here.

Climatic anomaly and its impact on vegetation phenology, carbon sequestration and water-use efficiency at a humid temperate forest

Changing climate, especially extreme weather event, is exerting considerable impacts on the hydrological and biogeochemical processes in forests worldwide. A deep understanding of climate change–terrestrial feedbacks is essential to predict future regional/global carbon and water budgets, which can be used to develop potential strategies for forest management. In this study, totally 11 years of eddy covariance tower measurements of CO2 and H2O fluxes, as well as the relevant environmental variables were analyzed to reveal the effects of climate anomalies on vegetation phenology, carbon sequestration and ecosystem water-use efficiency (WUE) dynamics in a humid temperate deciduous forest. Warmer spring temperatures altered the phenological phases with the green-up date advanced approximately 3.5 days per °C, and the extended growing season of about 3 days per °C, reaching the peak in 2012. Because of spring temperature anomaly, the shift from carbon source to sink occurred nearly 40 days earlier than usual. But the abnormal carbon dynamics happened during the peak growth period. Correlation analyses indicated that the amount of precipitation dominantly controlled the capacity of forest carbon sequestration (NEP) in this area. Therefore, the subsequent water scarcity owing to the extremely dry summer together with heatwave severely decreased the forest NEP by about 64.1%. Further analyses implied that the sharp reduction in gross primary production (GPP) rather than ecosystem respiration (Re) resulted to the decrease in NEP. Both GPP and evatranspiration (ET) were larger during the springtime in 2012 than those at adjacent years. But severe summer drought reduced the ecosystem WUE and yielded the lowest GPP and ET. Further work must focus on improving the recognition of forest feedback to climate systems.

Drought and surface‐level solar radiation predict the severity of outbreaks of a widespread defoliating insect

AbstractDefoliating insects are a major factor impacting tree growth in temperate and boreal forests, but the effects of climate change on the severity and frequency of outbreaks of these insects are not well understood. Dendrochronological reconstruction of forest tent caterpillar (Malacosoma disstria) outbreaks on trembling aspen (Populus tremuloides) in southwestern Manitoba, Canada, from 1851 to 2010 was used to examine the effects of changes in temperature, incident surface‐level solar radiation, and drought severity on the cyclicity, frequency, and severity of outbreaks. Outbreak severity was determined by comparing the tree‐ring widths of trembling aspen with those of white spruce (Picea glauca), which is not a forest tent caterpillar host. Wavelet analysis suggested the cyclicity of outbreaks increased after 1930. From 1930 to 2010, the dominant period length of outbreak cycles steadily increased over time from three to seven years, coinciding with long‐term declines in drought severity. The strength of forest tent caterpillar suppression of trembling aspen growth in a given year was inversely related to incident solar radiation and drought severity during late spring (the larval feeding period) of the same year. The effect of late spring drought on the impact of forest tent caterpillar on ring growth differed from that of summer drought. There was a nonlinear relationship between growth suppression in a given year and summer drought severity in the previous year; slightly above‐average drought severity was associated with reduced impacts of the defoliator, whereas severe drought was associated with intense growth suppression. Strong suppression of aspen growth observed during outbreaks from 1983 to 2010 may be partly explained by observed long‐term trends of decreasing atmospheric transmittance of solar radiation (a phenomenon termed “global dimming”) and increasing soil moisture availability during late spring. However, as the severity and frequency of summer droughts are expected to increase over the 21st century, the nonlinear relationship between summer drought severity and growth suppression the following year suggests a future in which summer droughts precede severe outbreaks that threaten the health of boreal forest hardwood trees.

Hydraulic and mechanical dysfunction of Norway spruce sapwood due to extreme summer drought in Scandinavia

Abstract Projected climate change scenarios such as frequently occurring dry summer spells are an enormous threat to the health of boreal conifer forests. We identified visible features indicating wood with tracheids predisposed for hydraulic and mechanical dysfunction in Norway spruce, suggest why this is formed during severe summer drought and hypothesised on mechanism that would cause tracheid collapse and stem cracks. Trees from southern Sweden that showed signs of severe reaction to drought, i.e. stem cracks along the trunk, were compared to healthy, undamaged trees. Rings investigated included those formed in 2006, a year with an extremely dry summer season in the study region. In southern Norway, we investigated trees with and without drought-induced top dieback symptoms. We analysed anatomical features such as tracheid lumen diameter, thickness of cell wall and its various layers (S1, S2 and S3), applied Raman imaging in order to get information on the lignin distribution in the cell wall and the compound middle lamellae and performed hydraulic flow and shrinkage experiments. Although tracheids in annual rings with signs of collapse had higher tangential lumen diameters than those in “normal” annual rings, we conclude that collapse of tracheid walls depends mainly on wall thickness, which is genetically determined to a large extent. Spruce trees that produce earlywood with extremely thin cell walls can develop wall collapse and internal cracks under the impact of dry spells. We also present a new diagnostic tool for detecting individuals that are prone to cell wall collapse and stem cracks: Lucid bands, i.e. bands in the fresh sapwood with very thin cell walls and inhomogeneous lignin distribution in the S-layers and the compound middle lamellae that lost their hydraulic function due to periods of severe summer drought. The detection of genotypes with lucid bands could be useful for an early selection against individuals that are prone to stem cracks under the impact of severe summer drought, and also for early downgrading of logs prone to cracking during industrial kiln drying.

Climate change enhances the severity and variability of drought in the Pearl River Basin in South China in the 21st century

Drought is expected to increase in frequency, severity, and duration in the future due to global warming and this will greatly threaten the future socio-economic development. This study evaluates the spatiotemporal variation of future drought (2016–2100) in the Pearl River Basin (PRB) using the Palmer drought severity index (PDSI), the Variable Infiltration Capacity (VIC) model, and future climate projections based on the general circulation models (GCMs) from phase 5 of the Coupled Model Intercomparison Project (CMIP5) under three representative concentration pathway (RCP) scenarios. The results show that future drought conditions are expected to be more serious than historical (1960–2015) drought conditions in the PRB, especially under RCP8.5. The severity and variability of future drought conditions are also higher than for the historical period. The west Guangxi and South Guizhou provinces exhibit the highest increment in drought severity under the three RCP scenarios. At the seasonal scale, drought severity shows the highest increment in the winter with little change in summer; the summer drought severity increases in most areas of the PRB but the increment of the drought severity is lower than the winter. The number of future drought events is expected to decrease but the duration and severity are expected to increase, especially in the mid-west PRB and under high greenhouse gas (GHG) emission scenarios. In summary, in most areas of the PRB, the severity and variability of drought are expected to increase in the 21st century, especially in the mid-west PRB and an increase in evapotranspiration is assumed to be the main underlying cause. Irrigation projects and agricultural production in the PRB might face serious threats of drought in the future.

Insights into the Drought and Heat Avoidance Mechanism in Summer-Dormant Mediterranean Tall Fescue.

Summer dormancy is an evolutionary response that some perennial cool-season grasses adopted as an avoidance strategy to escape summer drought and heat. It is correlated with superior survival after severe summer droughts in many perennial grass species originating from Mediterranean environments. Understanding the genetic mechanism and environmental determinants of summer dormancy is important for interpreting the evolutionary history of seasonal dormancy and for the development of genomic tools to improve the efficiency of genetic selection for this important trait. The objectives of this research are to assess morphological and biochemical attributes that seem to be specific for the characterization of summer dormancy in tall fescue, and to validate the hypothesis that genes underlying stem determinacy might be involved in the mechanism of summer dormancy. Our results suggest that vernalization is an important requirement in the onset of summer dormancy in tall fescue. Non-vernalized tall fescue plants do not exhibit summer dormancy as vernalized plants do and behave more like summer-active types. This is manifested by continuation of shoot growth and high root activity in water uptake during summer months. Therefore, summer dormancy in tall fescue should be tested only in plants that underwent vernalization and are not subjected to water deficit during summer months. Total phenolic concentration in tiller bases (antioxidants) does not seem to be related to vernalization. It is most likely an environmental response to protect meristems from oxidative stress. Sequence analysis of the TFL1 homolog CEN gene from tall fescue genotypes belonging to summer-dormant and summer-active tall fescue types showed a unique deletion of three nucleotides specific to the dormant genotypes. Higher tiller bud numbers in dormant plants that were not allowed to flower and complete the reproductive cycle, confirmed that stem determinacy is a major component in the mechanism of summer dormancy. The number of variables identified in these studies as potential players in summer dormancy in tall fescue including vernalization, TFL1/CEN, water status, and protection from oxidative stress are a further confirmation that summer dormancy is a quantitative trait controlled by several genes with varying effects and prone to genotype by environment interactions.

Quantitative losses vs. qualitative stability of ectomycorrhizal community responses to 3years of experimental summer drought in a beech-spruce forest.

Forest ecosystems in central Europe are predicted to face an increasing frequency and severity of summer droughts because of global climate change. European beech and Norway spruce often coexist in these forests with mostly positive effects on their growth. However, their different below-ground responses to drought may lead to differences in ectomycorrhizal (ECM) fungal community composition and functions which we examined at the individual root and ecosystem levels. We installed retractable roofs over plots in Kranzberg Forest (11°39'42″E, 48°25'12″N; 490m a.s.l.) to impose repeated summer drought conditions and assigned zones within each plot where trees neighboured the same or different species to study mixed species effects. We found that ECM fungal community composition changed and the numbers of vital mycorrhizae decreased for both tree species over 3 drought years (2014-2016), with the ECM fungal community diversity of beech exhibiting a faster and of spruce a stronger decline. Mixed stands had a positive effect on the ECM fungal community diversity of both tree species after the third drought year. Ectomycorrhizae with long rhizomorphs increased in both species under drought, indicating long-distance water transport. However, there was a progressive decline in the number of vital fine roots during the experiment, resulting in a strong reduction in enzyme activity per unit volume of soil. Hydrolytic enzyme activities of the surviving ectomycorrhizae were stable or stimulated upon drought, but there was a large decline in ECM fungal species with laccase activity, indicating a decreased potential to exploit nutrients bound to phenolic compounds. Thus, the ectomycorrhizae responded to repeated drought by maintaining or increasing their functionality at the individual root level, but were unable to compensate for quantitative losses at the ecosystem level. These findings demonstrate a strong below-ground impact of recurrent drought events in forests.