Extreme Drought Years Research Articles

Comparison of water-use patterns for non-native and native woody species in a semiarid sandy region of Northeast China based on stable isotopes

In the semiarid sandy region of Northeast China, non-native woody species such as Pinus sylvestris var. mongolica L. and Populus x xiaozhuanica W.Y. Hsu & Liang frequently suffer dieback during years of extreme drought events due to water deficiency. In contrast, dieback does not usually occur in native woody species, e.g., Pinus tabuliformis Carr., Ulmus pumila L., and shrub Caragana microphylla Lam. However, the mechanism of water-use patterns between the non-native and native woody species remain unclear, which limits our understanding of the mechanisms underlying dieback of non-native woody species. During two consecutive growing seasons in 2014 and 2015, sources of water uptake by two non-native (P. mongolica and P. xiaozhuanica) and three native (P. tabuliformis, U. pumila and C. microphylla) woody species were determined using the stable isotopes of δ2H and δ18O in the twig xylem water, soil water at various depths (0–200 cm), groundwater (water from capillary fringe or beneath the water table) and precipitation, coupled with soil moisture and groundwater level measurements. Results showed that both P. mongolica and P. tabuliformis trees used the 0–100 cm soil water in the spring and the 0–40 cm and 0–100 cm soil water in summer of 2014 and autumn of 2015, respectively, when soil moisture was high. In contrast, in summer of 2015 and autumn of 2014, when soil moisture was low, P. mongolica trees used both the 0–100 cm soil water and groundwater, whereas P. tabuliformis trees used the 0–200 cm soil water and both the 40–200 cm soil water and relatively limited groundwater, respectively. The P. xiaozhuanica trees used the 40–200 cm or the 100–200 cm soil water and groundwater, whereas U. pumila trees mainly used the 0–100 cm or 0–200 cm soil water, except in autumn when soil moisture was high. P. xiaozhuanica trees used the 0–200 cm soil water, while U. pumila trees used the 0–40 cm soil water. The shrub C. microphylla mainly used the 0–200 cm soil water during the measurement period irrespective of soil moisture conditions. Compared with the native woody species, which mainly used soil water, the non-native tree species relied on groundwater besides soil water, especially under low soil moisture conditions. Therefore, a sharp decline in groundwater level during extreme drought years might have a strong negative impact on the growth and survival of non-native woody species.

100% Renewable energy planning with complementarity and flexibility based on a multi-objective assessment

The decarbonization of the power sector has led many academic, governmental and non-governmental institutions to design scenarios with 100% renewable energy and several options to compensate the mismatch between demand and generation have been presented. This paper addresses the benefits to renewable energy integration ensured by the complementarity, and flexibility options, such as demand-side management and batteries. A non-linear multi-objective problem is proposed that maximizes the complementarity and minimizes the total expansion cost. The model also optimizes the battery integration, complementarity between sources and regions, costs, demand-side management, hydropower storage requirements and spatial technology distribution with hourly resolution. The methodology is implemented for the Brazilian case and a scenario able to guarantee three consecutive years of extreme drought in 2050 is presented, without the need of new large reservoirs. Such scenario is ensured with 10% of solar energy, 43% of wind energy, 4.3% of biomass, 41% of hydropower and 1.7% of demand-side management.

Rogation ceremonies: a key to understanding past drought variability in northeastern Spain since 1650

Abstract. In the northeast of the Iberian Peninsula, few studies have reconstructed drought occurrence and variability for the pre-instrumental period using documentary evidence and natural proxies. In this study, we compiled a unique dataset of rogation ceremonies – religious acts asking God for rain – from 13 cities in the northeast of Spain and investigated the annual drought variability from 1650 to 1899 CE. Three regionally different coherent areas (Mediterranean, Ebro Valley, and Mountain) were detected. Both the Barcelona and the regional Mediterranean drought indices were compared with the instrumental series of Barcelona for the overlapping period (1787–1899), where we discovered a highly significant and stable correlation with the Standardized Precipitation Index of May with a 4-month lag (r=-0.46 and r=-0.53; p<0.001, respectively). We found common periods with prolonged droughts (during the mid and late 18th century) and extreme drought years (1775, 1798, 1753, 1691, and 1817) associated with more atmospheric blocking situations. A superposed epoch analysis (SEA) was performed showing a significant decrease in drought events 1 year after the volcanic events, which might be explained by the decrease in evapotranspiration due to reduction in surface temperatures and, consequently, the higher availability of water that increases soil moisture. In addition, we discovered a common and significant drought response in the three regional drought indices 2 years after the Tambora volcanic eruption. Our study suggests that documented information on rogation ceremonies contains important independent evidence to reconstruct extreme drought events in areas and periods for which instrumental information and other proxies are scarce. However, drought index for the mountainous areas (denoted Mountain later in the text) presents various limitations and its interpretation must be treated with caution.

Six co-occurring conifer species in northern Idaho exhibit a continuum of hydraulic strategies during an extreme drought year

As growing seasons in the northwestern USA lengthen, on track with climate predictions, the mixed conifer forests that dominate this region will experience extended seasonal drought conditions. The year of 2015, which had the most extreme drought for the area on record, offered a potential analogue of future conditions. During this period, we measured the daily courses of water potential and gas exchange as well as the hydraulic conductivity and vulnerability to embolism of six dominant native conifer species, Abies grandis, Larix occidentalis, Pinus ponderosa, Pinus monticola, Pseudotsuga menziesii and Thuja occidentalis, to determine their responses to 5 months of record-low precipitation. The deep ash-capped soils of the region allowed gas exchange to continue without significant evidence of water stress for almost 2 months after the last rainfall event. Midday water potentials never fell below −2.2 MPa in the evergreen species and −2.7 MPa in the one deciduous species. Branch xylem was resistant to embolism, with P50 values ranging from −3.3 to −7.0 MPa. Root xylem, however, was more vulnerable, with P50 values from −1.3 to −4.6 MPa. With predawn water potentials as low as −1.3 MPa, the two Pinus species likely experienced declines in root hydraulic conductivity. Stomatal conductance of all six species was significantly responsive to vapour pressure only in the dry months (August–October), with no response evident in the wet months (June–July). While there were similarities among species, they exhibited a continuum of isohydry and safety margins. Despite the severity of this drought, all species were able to continue photosynthesis until mid-October, likely due to the mediating effects of the meter-deep, ash-capped silty-loam soils with large water storage capacity. Areas with these soil types, which are characteristic of much of the northwestern USA, could serve as refugia under drier and warmer future conditions.

Photosynthetic capacity, nutrient and water status following precommercial thinning in Anatolian black pine

Changes in gas exchange, photosynthetic pigments, midday xylem water potential (Ψmd) and needle nutrient status of the trees and environmental conditions following precommercial thinning (PCT) in the naturally regenerated Pinus nigra Arn. subsp. pallasiana (Lamb.) Holmboe stand was studied in forest districts of the western Mediterranean region of Turkey. An experiment involving two PCT treatments (spacing varied between 3.0 to 3.5 m and between 2.0 to 2.5 m) and one control (unthinned) was established in late April 2015. Different physiological responses to environmental changes following thinning were observed during the next three growing seasons. The soil water content (SWC) and temperature, Ψmd, chlorophyll pigments, needle N, P, and K concentrations, and gas exchange parameters were affected by PCT and the season. Compared to the unthinned plots, PCT increased the SWC and the soil temperature. The SWC was higher in the 3–3.5 m spaced plots. Increases in soil temperatures were generally similar between PCT treatments. The Ψmd, net photosynthetic rate (A), transpiration rate (E) and stomatal conductance (gs) of the trees in the thinned plots increased significantly compared to unthinned trees. This increase was attributed to increases in the SWC and decreases in soil temperature. The PCT with 3–3.5 m spacing had the highest Ψmd and A or similar to the 2–2.5 m spacing. However, the A, E, and gs was higher at 2–2.5 m spacing in the dry period (July) of the second and third years after PCT. Needle N (May 2015), P (September 2015), and K (July 2016) concentrations increased after the PCT with 3–3.5 m spacing compared with the unthinned and the 2–2.5 m spacing. Needle K concentration was negatively correlated with Ψmd and positively with WUE. A, gs and E were positively correlated with needle P concentration but not correlated with needle N concentration. The chlorophyll content of the unthinned trees remained high in the spring, similar to the 3–3.5 m spacing in the summer. The Ψmd, A, gs, and E varied seasonally in both unthinned and thinned plots, with the higher values in the spring and lower values in summer. According to the short-term results, the PCT with the 2–2.5 m spacing appeared more appropriate when the 3–3.5 m spacing might negatively impact soil, water, and photosynthetic parameters in extreme drought years and management objectives such as improvement of stem quality are considered.

Climate change impacts on Three Gorges Reservoir impoundment and hydropower generation

Climate change is expected to alter regional hydrological regimes, affecting the operation and performance of reservoirs and hydropower facilities. This study examines the impacts of climate change on the performance of the Three Gorges Reservoir (TGR) by means of a detailed daily reservoir regulation and hydropower model, linked to a physically-based hydrological model, and driven by an ensemble of five General Circulation Models (GCMs) under three Representative Concentration Pathways (RCP2.6, RCP4.5 and RCP8.5). As precipitation in the basin is expected to increase, the projected mean annual inflow and hydropower generation of the TGR will increase by 3.3–5.6% and 0.9–2.3% in 2040–2065, 7.9%-15.2% and 5.2–8.1% in 2080–2099 respectively. These increases are only statistically significant for 2080–2099 and are seasonally concentrated in the spring before the flood season and the early autumn during the end of the flood season. However, the inter-annual variation of power generation will increase specially in the dry season. The reservoir performance is highly sensitive to the changes in the seasonal distribution and extremes of streamflow. Increases in streamflow that occur in the flood season cause significant increases in the amount of spilled water and advance the time when the reservoir reaches the normal storage level. Additionally, increases in both the inter-annual variation of inflow and the intensity of inflow shortages during extreme drought years in the impounding stage drive decreases in the fully filled rate and average water storage level in the dry season. The utilization rate of water resources under projected extreme streamflow is expected to decrease, reshaping the response of power generation to climate change into a non-linear pattern where increases in streamflow do not proportionally translate to increases in power generation. These findings highlight the complexity of hydropower management and production under future climate change scenarios, motivating the need for introducing detailed regulating models for impact assessment studies and adaptive adjustment of the reservoir management to combat climate change.

Extreme drought causes fish recruitment failure in a fragmented Great Plains riverscape

AbstractDryland stream fishes are adapted to highly connected habitats with unpredictable hydrologic conditions, including frequent low flows and sometimes extreme drought. The low flow recruitment hypothesis predicts that some fishes spawn in main channel habitats during low flows when water temperatures and prey densities are high. However, extreme low flows during drought periods might be disruptive even among fishes whose life histories otherwise benefit from lower flows. We studied recruitment dynamics of six fishes (family Cyprinidae) at 15 sites in a fragmented Great Plains riverscape in Kansas, USA, during 2 years of extreme drought. We tracked the chronology of gonadal development and age 0 recruitment to test the hypothesis that recruitment by fishes that broadcast spawn planktonic ova would be less successful compared with fishes that spawn demersal or adhesive ova. We found all six taxa were reproductively active but recruitment was evident for only four. The two species for which recruitment was not evident, peppered chub (Macrhybopsis tetranema) and silver chub (Macrhybopsis storeriana), are confirmed or suspected pelagic‐broadcast spawning fishes that have declined in other fragmented and dewatered Great Plains riverscapes. Our data highlight the potential for extreme low flows within isolated stream fragments to cause complete or near‐complete recruitment failure for pelagic‐broadcast spawning fishes, especially those with small population sizes. Failed recruitment during extreme drought may be related to spawning mode, ova characteristics, or both. Our work informs management of fish diversity in drought‐prone riverscapes by establishing mechanistic linkages among extreme drought, fish recruitment, and assemblage structure.

The dual effect of sand-covered biocrusts on annual plants: Increasing cover but reducing individual plant biomass and fecundity

It is generally accepted that biological soil crusts promote the growth of vascular plants. Nevertheless, contradictory results were recently found for annual plants in the extreme desert dunefield of the Negev (average precipitation of 95 mm) during extreme drought years, explained by the lower available water content (AWC) under the low-albedo biocrusted surfaces at the interdune (ID). Trying to assess whether the biomass and fecundity of the annual plants is higher at the non-crusted dune crest (CR) in comparison to ID also during years with near-average precipitation, surface stability, rain, AWC, fine (silt and clay) content (FC), and soil organic carbon (SOC) were measured in a pair of plots established in CR and ID in two dunes and interdunes. The research was conducted for three years (2009/2010, 2013/14 and 2014/15), having a mean precipitation of 109.4 mm. At the end of each winter, sampling of the annual plants (cover, density, species composition, biomass) took place at CR and ID at 30 cm-diameter miniplots (miniplot-scale), while the biomass and fecundity of 50 randomly-chosen individuals that abundantly inhabited both habitats during the same year were measured at parallel transects (individual plant-scale). Significantly higher cover and generally higher diversity characterized the plants at ID at the miniplot-scale, explained by the higher surface stability. Although characterized by lower FC and SOC, significantly higher biomass and fecundity characterized the individual plants at CR, explained by the higher AWC. Biocrusts were found to have a dual effect on the annual plants: positive due to increased surface stability (cover and for most years also diversity) at the miniplot-scale, and negative due to reduced AWC (low biomass and fecundity) at the individual plant-scale. The higher biomass and fecundity of the individual plants at CR point to the cardinal role played by water (rather than SOC) in this low-nutrient extreme desert. As far the annual plants are concerned, these findings also point at the fact that moderate crust destruction (such as by light trampling) may not necessarily be hazardous to the ecosystem, as it may increase fecundity and subsequently germination success.

Experimental warming amplified opposite impacts of drought vs. wet extremes on ecosystem carbon cycle in a tallgrass prairie

Climate warming is leading to greater precipitation variability, resulting in increased frequency and intensity of both drought and wet extremes. However, how these extreme events interact with climate warming and hay-harvest in grasslands to impact ecosystem functions has not yet been well explored. In this study, we took advantage of a long-term experiment to examine how climate warming and clipping (i.e., mimicking hay harvest) regulated impacts of naturally occurring drought and wet extremes on ecosystem CO2 fluxes of a tallgrass prairie in the Great Plains, USA. Warming resulted in net ecosystem carbon release (i.e., positive net ecosystem CO2 exchange, NEE) in the extreme drought year of 2011, but significantly enhanced net carbon uptake in the extremely wet year of 2015 in comparison with NEE in normal years. Warming-induced carbon release in the drought year was due to significantly enhanced ecosystem respiration (ER) from mid-summer to early-autumn, whereas warming-enhanced NEE in the wet year was due to an increase in aboveground net primary production (ANPP) compared to those in normal years. Drought diminished warming-induced increases in ANPP to about one sixth of that in the wet year in the unclipped plots. Interestingly, clipping offset the drought-mediated ecosystem carbon loss by increasing GPP and weakened the wet-enhanced ANPP. Overall, our results suggest that a future, warmer climate may exacerbate carbon losses in terrestrial ecosystems during drought extremes but stimulate the ecosystem carbon sink under wet extremes.

Spatial and temporal patterns of drought in Zambia

Drought acutely affects economic sectors, natural habitats and communities. Understanding the past spatial and temporal patterns of drought is crucial because it facilitates the forecasting of future drought occurrences and informs decision-making processes for possible adaptive measures. This is especially important in view of a changing climate. This study employed the World Meteorological Organization (WMO)-recommended standardized precipitation index (SPI) to investigate the spatial and temporal patterns of drought in Zambia from 1960 to 2016. The relationship between the occurrence of consecutive dry days (CDD; consecutive days with less than 1 mm of precipitation) and SPI was also investigated. Horizontal wind vectors at 850 hPa during the core of the rainy season (December–February) were examined to ascertain the patterns of flow during years of extreme and severe drought; and these were contrasted with the patterns of flow in 2007, which was a generally wet year. Pressure vertical velocity was also investigated. Based on the gamma distribution, SPI successfully categorized extremely dry (with a SPI value less than or equal to –2.0) years over Zambia as 1992 and 2015, a severely dry (–1.9 to–1.5) year as 1995, moderately dry (–1.4 to –1.0) years as 1972, 1980, 1987, 1999 and 2005, and 26 near normal years (–0.9 to 0.9). The occurrence of CDD was found to be strongly negatively correlated with SPI with a coefficient of –0.6. Further results suggest that, during wet years, Zambia is influenced by a clockwise circulating low-pressure zone over the south-eastern Angola, a second such zone over the northern and eastern parts, and a third over the Indian Ocean. In stark contrast, years of drought were characterized by an anti-clockwise circulating high-pressure zone over the south-western parts of Zambia, constraining precipitation activities over the country. Further, wet years were characterized by negative pressure vertical velocity anomalies, signifying ascending motion; while drought years were dominated by positive anomalies, signifying descending motion, which suppresses precipitation. These patterns can be used to forecast drought over Zambia and aid in strategic planning to limit the potential damage of drought.

Influence of Climate Variability and Reservoir Operation on Streamflow in the Yangtze River

Understanding the effects of climate variability and reservoir operation on runoff is important for shipping, irrigation and water supply services, especially during extreme drought years. After the operation of the Three Gorges Dam (TGD) began, the discharge processes in the mid-lower reaches of the Yangtze River were completely different from those during the pre-dam period. The measured hydrological data and the Mike 11-HD model were used to estimate the contributions of the TGD operation and climate variability to the variation in discharge during extreme drought years. The results are as follows: under the effects of the TGD operation and extreme drought, the special phenomenon of a “positive discharge anomaly in drought season and negative discharge anomaly in flood season” occurred compared with the conditions in the pre-dam period. During the flood season, the contributions of climate variation (TGD operation) to the changes in streamflow from Yichang station to Datong station were 86.6% (13.4%) and 80.7% (19.7%) in 2006 and 64.8% (35.2%) and 71.3% (28.7%) in 2011. During the dry season, the values were 81.2% (18.8%) and 93.9% (6.1%) in 2006 and 59.9% (40.1%) and 72.9% (27.1%) in 2011. Clearly, climate variation was the main reason for the variation in seasonal runoff. Furthermore, even in the 156 m and 175 m impoundments, climate variation was the dominant factor.

Drought ecohydrology alters the structure and function of benthic invertebrate communities in mountain streams

AbstractWe studied benthic macroinvertebrate communities in 12 mid‐elevation mountain streams in the Sierra Nevada, California. Data were collected at nine times (seven springs and two autumns) over a 14‐year period of variable hydrological conditions, including years of flood and extreme unprecedented drought.The taxonomic and trait structures of communities were similar during wet and average springs but became increasingly different as drought continued and smaller streams became intermittent.The density of total invertebrates, primarily chironomids, increased during the drought years, but the richness and abundance of mayfly, stonefly, and caddisfly (EPT) taxa declined during the late drought. During the late drought, the proportions of rheophilic, semivoltine taxa with clinger or swimmer behavioural habits decreased, whereas small, stress‐tolerant taxa with burrower and climber habits increased. Collector–gatherers dominated in all periods, but during the severe drought the relative abundance of micropredators increased and filterers, shredders, and grazers decreased.We also found greater changes in community structure between spring runoff and autumn base flow conditions in an average than a wet year. However, spring to autumn population growth rates were much greater during the wet than average year.Invertebrate richness and EPT abundance decreased and community structure showed large changes when stream discharge fell below 1–10 L/s, but remained relatively constant across a range of higher flows. During the severe drought, there were significant declines in the densities of 40% of the common EPT taxa compared to average and wet years, but chironomid, mite, and other invertebrate taxa showed variable responses.Invertebrate diversity, community structure, and ecosystem functions in small, headwater streams are especially vulnerable to drought conditions, which are expected to increase in frequency and intensity with climate change in the Sierra Nevada and other mountainous regions.

ANALYSIS OF HISTORICAL SERIES OF MAMIRAUÁ LAKE LEVEL (PRELIMINARY STUDY)

Amazonian floodplains can be defined as areas bathed by white or muddy rivers; Its main feature is the amount of suspended materials that are transported during the flood and drought dynamics of the rivers and lakes in the region. The objective of this work was to analyze the historical water level series of Mamirauá Lake. We used 25 years of data from the lake performed by the Mamirauá Institute Fluviometric Monitoring since 1990. The descriptive statistics technique was used for the maximum, minimum and average quotas of the monitoring years, as it sought to verify the level relationship. of the lake with the occurrence of extreme events that have happened over the years. The results obtained were: the lowest level of the data series, with a value of 21.7 m.a.n.m was recorded in 1995 and the highest in 1999 was 38.6 m.a.n.m., that year the Amazon basin suffered one of the largest floods related to the La Niña phenomenon. Given the results analyzed, the research concluded that the water level of Mamirauá Lake did not change in years of occurrence of extreme events, as was verified in the years of extreme drought (2005 and 2010).

Compound fire‐drought regimes promote ecosystem transitions in Mediterranean ecosystems

Abstract Understanding ecosystem responses to compound disturbance regimes and the influence of specific sequences of events in determining ecosystem shifts remains a challenge. We use a modelling framework for Mediterranean‐type ecosystems to assess the effects of fire–drought interactions on long‐term vegetation dynamics and to identify disturbance‐driven changes in trait composition (tree seeder vs. tree resprouter prevalence) and ecosystem state (forest vs. non‐forest). Changes in tree seeder and the tree resprouter dominance show nonlinear, threshold‐type trends over gradients of increasing compound disturbance frequency. Vegetation composition thresholds mostly occur in a narrow range of the compound fire–drought disturbance space. Additionally, trait compositional switches and the likelihood of sudden changes in ecosystem state are promoted by fire‐drought interactions. Distinct sequences of disturbance events cause vegetation transitions, disrupting ecosystem resilience, even under moderate recurrence of individual disturbances. An extreme drought year followed by one or two large fire events promotes shifts from resprouter‐ to seeder dominance. Contrastingly, a large crown fire followed by an extreme drought promotes changes from seeder to resprouter dominance. This disturbance sequence is also a mechanism strong enough to trigger sudden shifts in ecosystem state (from forest to non‐forest). Synthesis. Thresholds of change in vegetation composition occur over a narrow range of the modelled gradients of compound fire‐drought recurrence, and the loss of ecosystem resilience is contingent on particular sequences of disturbance events. Overall, our findings highlight that disturbance interactions define the relative location of tipping points in ecosystem state, and that effects and feedbacks of compound disturbance regimes increase the long‐term likelihood of sudden ecosystem shifts and, therefore, uncertainty in predicting vegetation state.

Water level variation characteristics under the impacts of extreme drought and the operation of the Three Gorges Dam

Under the influence of a climate of extreme drought and the Three Gorges Dam (TGD) operation, the water levels in the middle and lower reaches of the Yangtze River in 2006 and 2011 changed significantly compared with those in the extreme drought years of 1978 and 1986. To quantitatively analyze the characteristics of water level variations in 2006 and 2011, a new calculation method was proposed, and the daily water level and discharge from 1955–2016 were collected in this study. The findings are as follows: in 2006 and 2011, the water level in the dry season significantly increased, but that in the flood season obviously decreased compared with the levels in 1978 and 1986. Here, we described this phenomenon as “no low-water-level in dry season, no high-water-level in flood season”. Based on the calculation method, the contributions of climate variability and the Three Gorges Dam operation to water level variations in the middle and lower reaches of the Yangtze River were calculated, and the contributions indicated that climate variability was the main reason for the phenomenon of “no low-water-level in dry season, no high-water-level in flood season” instead of flood peak reduction in the flood season and drought runoff implementation in the dry season, which are both induced by TGD.

Genetic-Based Susceptibility of a Foundation Tree to Herbivory Interacts With Climate to Influence Arthropod Community Composition, Diversity, and Resilience.

Understanding how genetic-based traits of plants interact with climate to affect associated communities will help improve predictions of climate change impacts on biodiversity. However, few community-level studies have addressed such interactions. Pinyon pine (Pinus edulis) in the southwestern U.S. shows genetic-based resistance and susceptibility to pinyon needle scale (Matsucoccus acalyptus). We sought to determine if susceptibility to scale herbivory influenced the diversity and composition of the extended community of 250+ arthropod species, and if this influence would be consistent across consecutive years, an extreme drought year followed by a moderate drought year. Because scale insects alter the architecture of susceptible trees, it is difficult to separate the direct influences of susceptibility on arthropod communities from the indirect influences of scale-altered tree architecture. To separate these influences, scales were experimentally excluded from susceptible trees for 15 years creating susceptible trees with the architecture of resistant trees, hereafter referred to as scale-excluded trees. Five patterns emerged. (1) In both years, arthropod abundance was 3-4X lower on susceptible trees compared to resistant and scale-excluded trees. (2) Species accumulation curves show that alpha and gamma diversity were 2-3X lower on susceptible trees compared to resistant and scale-excluded trees. (3) Reaction norms of arthropod richness and abundance on individual tree genotypes across years showed genotypic variation in the community response to changes in climate. (4) The genetic-based influence of susceptibility on arthropod community composition is climate dependent. During extreme drought, community composition on scale-excluded trees resembled susceptible trees indicating composition was strongly influenced by tree genetics independent of tree architecture. However, under moderate drought, community composition on scale-excluded trees resembled resistant trees indicating traits associated with tree architecture became more important. (5) One year after extreme drought, the arthropod community rebounded sharply. However, there was a much greater rebound in richness and abundance on resistant compared to susceptible trees suggesting that reduced resiliency in the arthropod community is associated with susceptibility. These results argue that individual genetic-based plant-herbivore interactions can directly and indirectly impact community-level diversity, which is modulated by climate. Understanding such interactions is important for assessing the impacts of climate change on biodiversity.

Estimates of benthic invertebrate community variability and its environmental determinants differ between snapshot and trajectory designs

Long-term data sets are essential for biodiversity research and monitoring. Researchers use 2 major approaches in the study of temporal variability of biological communities: 1) the trajectory approach (monitoring sites across several consecutive years) and 2) the snapshot approach (comparing sites among few sampling events several years apart). We used data on benthic macroinvertebrate communities in 23 near-pristine forested streams to compare these 2 approaches for different study periods ranging from 3 to 14 y. We asked whether the level of temporal turnover and the identity of the best explanatory variables underlying it were comparable across studies based on differing approaches, study periods, or total duration. The 2 approaches yielded partly different stories about the level of community variability and its environmental correlates. With the snapshot approach, variation in community similarity and factors explaining it reflected short-term (e.g., year-specific) conditions, which could be misinterpreted as long-term trends, the difference being most evident for periods that began or ended in an extreme drought year. Our results imply that snapshot studies may lead to ambiguous conclusions, whereas the trajectory approach yielded more consistent results. Trajectory data of differing length showed minor differences, apart from studies with the shortest durations. Overall, our results suggest that time sequences of ∼6 y of trajectory data (i.e., 6 generations for most benthic invertebrates in boreal streams) may be needed for the among-year similarity of macroinvertebrate communities in near-pristine streams to stabilize. If temporal replication is limited (snapshots/very short time sequences) the outcome depends strongly on the particular years included in a comparison. Based on our results, we advise caution when basing conclusions on a comparison of a few (e.g., just 2) occasions several years apart or on very short time sequences.

Changing Characteristics of Droughts over Kerala, India: Inter-Annual Variability and Trend

Realized changes in the climate over Kerala received more attention during recent times under the back to back drought conditions prevailing over the state. The year 2016 registered as one of the worst and extreme drought year over Kerala in recent past. This extreme drought condition may be attributed to the combined effect of realized changes in the mean climate state as well as the climate variability.Both the recent and historic drought events over Kerala is examined and its possible impacts on different sectors like agriculture and water resource are studied using the standardized precipitation index (SPI). Historical occurrence of both the short term and long term water stress associated with deficit rainfall during monsoon and post-monsoon season is presented in the paper. It is found that south west monsoon rainfall over Kerala is decreasing over south Kerala during recent decades, whereas the post-monsoon rainfall is found to be increasing especially over south Kerala. There exists a weak relationship between Pacific SST and Kerala rainfall. Trend analysis of SPI on different time scale showed negative trend and that negative trend increases with increasing time scale.

Tree rings reveal long-term changes in growth resilience in Southern European riparian forests

The gradual increase in temperature over the last few decades is one of the major consequences of global change. It is also projected that drought frequency and intensity in the Mediterranean region will increase, promoting changes in plant responses to environmental conditions and ultimately species distribution. Studying past growth trends can help understand future impacts of climate change on species-function and predict alterations in how species are distributed. This study sought to evaluate growth trends in riparian tree species to assess both their resistance and resilience responses to, and their complementary strategies in the face of, climatic and hydrological changes. Their supporting ecosystem role in riverine ecosystems and their representativeness in the study region led to the selection of the species Alnus glutinosa (L.) Gaertn. and Fraxinus angustifolia Vahl for this purpose.Yearly growth curves were obtained for coexisting A. glutinosa and F. angustifolia, sampled in 2009 in a riparian forest in a Southern Portuguese river basin. Standardized Precipitation Evapotranspiration Index (SPEI) was calculated to select drought events, identified as years of extreme and severe drought combined with adverse river discharge conditions. Temporal trends in Basal Area Increments (BAI) were explored for both species across 1970–2009, particularly during drought years. Tree growth response was associated with long-term SPEI, minimum temperature, rainfall and discharge.Both species presented an increase in BAI until the 1980s, apparently triggered by the rise in minimum temperatures, which lengthened the growing season. However, after the 80′s this trend in tree growth has been reversed due to the reduced precipitation associated with increased drought intensity. Similar tree growth responses to drought were observed in both species, but responses to extreme drought events were species-specific. Distinct long-term resilience trends between species suggest that under the current climate change scenario, mesic species (e.g. A. glutinosa) will gradually be replaced by more drought-tolerant species, such as F. angustifolia.Finally, further research should be undertaken to provide information on physiological and anatomical changes of riparian tree species in the Mediterranean region.

Soil Moisture, Grass Production and Mesquite Resprout Architecture Following Mesquite Above-Ground Mortality

Honey mesquite (Prosopis glandulosa) is an invasive native woody plant in the southern Great Plains, USA. Treatments used to slow the invasion rate have either killed the plant (“root-kill”) or killed above-ground tissue (“top-kill”). Top-killing provides temporary suppression, but stimulates multi-stemmed regrowth. This study from north central Texas quantified soil moisture, grass production and mesquite resprout architecture following a mechanical clearing treatment that top-killed mesquite (cleared) compared to untreated mesquite woodland (woodland) over a 10-year period. During an extreme drought at 5 and 6 years post-clearing, soil moisture at 60-cm depth became lower in cleared than in woodland, suggesting that, as early as 5 years after top-kill, water use by regrowth mesquite could be greater than that by woodland mesquite. Perennial grass production was greater in cleared treatments than in woodland treatments in all years except the extreme drought years. Mesquite regrowth biomass increased numerically each year and was independent of annual precipitation with one exception. During the year 5 and 6 drought, mesquite stopped lateral expansion of larger stems and increased growth of smaller stems and twigs. In summary, top-killing mesquite generated short-term benefits of increased grass production, but regrowth created potentially negative consequences related to soil moisture.