Early Spring Rainfall Research Articles

Plant Growth and Physiological Responses to Improved Irrigation and Fertilization Management for Young Peach Trees in the Southeastern United States

New peach orchards in the southeastern United States are often not irrigated until 3 or 4 years after planting. During those years, the only water comes from rainfall. Droughts in the region are becoming more common, making irrigation more important. At the same time, fertilization practices follow recommendations developed decades ago and may not be optimal for current production practices. This research aimed to investigate the effect of different irrigation and fertilization practices on young ‘Julyprince’ trees grafted onto ‘Guardian™’ rootstock. The treatments consisted of irrigated vs. nonirrigated trees, drip- vs. microsprinkler-irrigated trees, and four different fertilizer levels (25%, 50%, 100%, and 200%; with 100% = current fertilizer recommendations). Responses to the treatments varied by year. In 2016, below-average rainfall (severe drought as classified by the U.S. Drought Monitor) was recorded throughout the year. This severe drought reduced the growth of nonirrigated trees compared with irrigated trees (average reductions of 56% in canopy volume, 39% in trunk cross-sectional area, 39% in leaf and stem water potential, and 40% in leaf photosynthesis). The adverse effects on tree growth and physiological responses of the 2016 season carried over to 2017, which was characterized by a short period of below-average rainfall in early spring. Nonirrigated trees displayed advanced budbreak progression; reduced commercial yield (10.9 vs. 13.4 kg/tree for nonirrigated vs. irrigated trees); and smaller trunk cross-sectional area (54.0 vs. 70.1 cm2) and canopy volume (8.9 vs. 10.9 m3) compared with irrigated trees. In 2018, rainfall was like the historical average throughout the year. Major differences continued to be trunk cross-sectional area (103.4 vs. 126.7 cm2) and canopy volume (15.8 vs. 17.8 m3), with nonirrigated trees being smaller than irrigated trees. No major or consistent differences were found between drip vs. microsprinkler irrigation or among fertilizer levels during the 3 years of the experiment. During the first years of orchard establishments, irrigation resulted in increased plant growth, commercial yield, and superior water status (higher values of water potential) compared with no irrigation, especially when rainfall was below the historical average. Although no major differences were found between the irrigation systems, drip irrigation used 35% less water than microsprinkler irrigation. While different fertilizer levels did not induce major differences in young trees’ growth and yield, potential economic savings and long-term effects of reduced fertilizer applications are being monitored as trees mature.

Interdecadal Variation of Early Spring Rainfall Over the Southeastern Edge of the Tibetan Plateau

AbstractThis study investigates the interdecadal variation in early spring rainfall over the southeastern edge of the Tibetan Plateau (SEETP) during 1979–2015 and its possible causes using mainly Asian Precipitation–Highly‐Resolved Observational Data Integration Toward Evaluation daily precipitation data set, ERA5 reanalysis data and the atmospheric general circulation model ECHAM6. A significant interdecadal change in early spring rainfall over the SEETP occurred with its node around 1998/1999. The early spring rainfall over the SEETP was heavier in 1979–1998 than in 1999–2015, reflecting a reduction in moderate and heavy rain during the early spring. The covariability of the Pacific Decadal Oscillation (PDO), Atlantic Multidecadal Oscillation (AMO), and Indian Ocean Basin Mode (IOBM) is an important driver of the interdecadal decrease in early spring rainfall over the SEETP. When the PDO is in a cold phase, and the AMO and IOBM are in positive phases, an anomalous cyclone is excited over the Bay of Bengal‐western Pacific in the lower to middle troposphere. Anomalous easterly winds on the north flank of the anomalous cyclone lead to the interdecadal decrease in early spring rainfall over the SEETP by reducing water vapor transport from the Indian Ocean and producing divergence over the SEETP. This diagnosis of the key physical process linking the associated sea surface temperature anomalies with the interdecadal variation in early spring rainfall over the SEETP is supported by numerical experiments.

Pteridium aquilinum performance is driven by climate, soil and land-use in Southwest Asia

Growing anthropogenic impacts on natural and semi-natural ecosystems have created a network of degraded sites throughout the world. These disturbed ecosystems are often colonized by invasive plants such as Pteridium aquilinum, which is one of the most widespread plants worldwide. In northern Iran, P. aquilinum is often found invading newly-created habitats formed after anthropogenic disturbance. This study aimed to assess the relationship between P. aquilinum performance and a range of environmental factors (soils, climate, topography and land-use) in northern Iran. In fifteen sites dominated by P. aquilinum, that spanned the regional distribution of P. aquilinum, we measured its cover, density and biomass. Pteridium aquilinum was found to occur in a variety of land-uses including abandoned agricultural lands, degraded forests and upland rangelands. The performance of P. aquilinum varied significantly between the sites and it performed better in a moderate-Mediterranean climate. Variation partitioning confirmed the importance of climate followed by soil and land-use in explaining the performance of P. aquilinum. Topographic variables did not show any significant effect on P. aquilinum. Both temperature and rainfall affected P. aquilinum performance. Density was correlated positively with early-spring rainfall whereas biomass and cover were found to have positive correlation with temperature. There was also a gradient of soil texture/pH/N/P influencing P. aquilinum. Frond density and biomass were correlated positively with sand content, N and P, but negatively with pH, lime and bulk density. Thus, soil conditions alongside temperature in spring and early summer could explain P. aquilinum cover, but for density, rainfall in early spring was the most important factor, suggesting that in northern Iran P. aquilinum performance appears to be intermediate compared to responses reported for temperate (temperature-controlled) and tropical climates (rainfall-controlled).

Macro-HyProS: A new macro-scale hydrologic processes simulator for depression-dominated cold climate regions

This study introduces a new Macro-scale Hydrologic Processes Simulator (Macro-HyProS), tailored for cold climate regions and depression-dominated terrains. Macro-HyProS is a grid-based hydrologic model of a unique structure to deal with hydrologic complexities in depression-dominated cold climate regions. The model runs on a daily time step and incorporates a LEGO-fashion horizontal layout to account for sub-grid land use heterogeneity. On the vertical layout, each grid consists of different bands, each of which is responsible for simulating specific hydrologic processes. Macro-HyProS employs improved methodologies to account for snow accumulation and ablation, depressions, and frozen ground condition. The Red River of the North Basin (RRB) was selected to highlight the capabilities of Macro-HyProS in describing the spatiotemporal complexities associated with depression-dominated cold climate regions. Results indicated that the model provided a satisfactory performance, having the Nash–Sutcliffe efficiency coefficients of 0.56 and 0.72 in the calibration and validation periods, respectively. The modeling results accentuated the impacts of frozen ground, snowmelt, and depressions in the generation of surface runoff over the basin. On average, the RRB experienced 145 days of frozen or partially frozen ground conditions per year, which coincided with early spring rainfall and snowmelt events. Due to the widespread frozen ground conditions over the basin, up to 60% of the generated snowmelt on specific days contributed to surface runoff, whereas infiltration had a smaller portion. Also, the results revealed that the depression-dominated areas acted as “regulators” of surface runoff, particularly in early springs. Macro-HyProS provides spatiotemporal simulations of various hydrologic processes to facilitate a better understanding of macro-scale hydrologic trends.

Impacts of the Tibetan Plateau on Asian Climate

Abstract Professor Yanai is remembered in our hearts as an esteemed friend. Based on his accomplishments in tropical meteorology and with his flashes of insight he led his group at the University of California, Los Angeles, in the 1980s and 1990s to explore the thermal features of the Tibetan Plateau (TP) and its relation to the Asian monsoon, and he brought forward the TP meteorology established by Ye Duzheng et al. in 1957 to a new stage. In cherishing the memory of Professor Yanai and his great contribution to the TP meteorology, the authors review their recent study on the impacts of the TP and contribute this chapter as an extension of their chapter titled “Effects of the Tibetan Plateau” published by Yanai and Wu in 2006 in the book The Asian Monsoon. The influence of a large-scale orography on climate depends not only on the mechanical and thermal forcing it exerts on the atmosphere, but also on the background atmospheric circulation. In winter the TP possesses two leading heating modes resulting from the relevant dominant atmospheric circulations, in particular the North Atlantic Oscillation and the North Pacific Oscillation. The prevailing effect of the mechanical forcing of the TP in wintertime generates a dipole type of circulation, in which the anticyclonic gyre in the middle and high latitudes contributes to the warm inland area to the west, and the cold seashore area to the east, of northeast Asia, whereas the cyclonic gyre in low latitudes contributes to the formation of a prolonged dry season over central and southern Asia and moist climate over southeastern Asia. Such a dipole circulation also generates a unique persistent rainfall in early spring (PRES) over southern China. In 1980s, Yanai and his colleagues analyzed the in situ observation and found that the constant potential temperature boundary layer over the TP can reach about 300 hPa before the summer monsoon onset. This study supports these findings, and demonstrates that such a boundary layer structure is a consequence of the atmospheric thermal adaptation to the surface sensible heating, which vanishes quickly with increasing height. The overshooting of rising air, which is induced by surface sensible heating, then can form a layer of constant potential temperature with a thickness of several kilometers. The thermal forcing of the TP on the lower tropospheric circulation looks like a sensible heat–driven air pump (SHAP). It is the surface sensible heating on the sloping sides of the plateau that the SHAP can effectively influence the Asian monsoon circulation. In spring the SHAP contributes to the seasonal abrupt change of the Asian circulation and anchors the earliest Asian summer monsoon onset over the eastern Bay of Bengal. In summer, this pumping, together with the thermal forcing over the Iranian Plateau, produces bimodality in the South Asian high activity in the upper troposphere, which is closely related to the climate anomaly patterns over South and East Asia. Because the isentropic surfaces in the middle and lower troposphere intersect with the TP, in summertime the plateau becomes a strong negative vorticity source of the atmosphere and affects the surrounding climate and even the Northern Hemispheric circulation via Rossby wave energy dispersion. Future prospects in related TP studies are also addressed.

Large-Scale Processes Associated with Inter-Decadal and Inter-Annual Early Spring Rainfall Variability in Taiwan

Early spring (March - April) rainfall in Taiwan exhibits evident and distinct inter-annual and inter-decadal variability. The inter-annual varibility has a positive correlation with the El Nino/Southern Oscillation while the inter-decadal variability features a phase change beginning in the late 1970s, coherent with the major phase change in the Pacific decadal oscillation. Rainfall variability in both timescales is regulated by large-scale processes showing consistent dynamic features. Rainfall increases are associated with positive sea surface temperature (SST) anomalies in the tropical eastern Pacific and negative SST anomalies in the tropical central Pacific. An anomalous lower-level divergent center appears in the tropical central Pacific. Via a Rossby-wave-like response, an anomalous lower-level anticyclone appears to the southeast of Taiwan over the Philippine Sea-tropical western Pacific region, which is accompanied by an anomalous cyclone to the north-northeast of Taiwan. Both circulation anomalies induce anomalous southwesterly flows to enhance moisture flux from the South China Sea onto Taiwan, resulting in significant moisture convergence nearby Taiwan. With enhanced moisture supplied by anomalous southwesterly flows, significant rainfall increases occur in both inter-annual and inter-decadal timescales in early spring rainfall on Taiwan.

Geomorphological-related heterogeneity as reflected in tree growth and its relationships with climate of Monte Desert Prosopis flexuosa DC woodlands

Across the Central Monte Desert district in Argentina, landform and soil variability drive radial growth of Prosopis flexuosa and its relation with precipitation. Desert forests grow under diverse ecological conditions, mainly resulting from the spatial heterogeneity of drylands with consequences on tree growth and its interactions with climate. In the Monte Desert, geomorphological processes generate landform and soil variability, determining the distribution and growth of plant species. Prosopis flexuosa DC., a dominant tree species in the Central Monte Desert, grows in territories characterized by a high variability of landform and soil. We applied classical dendrochronological and statistical analysis to disentangle the effect of spatial heterogeneity upon the species radial growth and its further relation with precipitation fluctuations. Trees from 11 plots distributed in seven P. flexuosa forests encompassing the most important geomorphological/landform units in the Central Monte Desert were analyzed. Tree-ring development at both high and low frequencies reflects spatial landform variability. Soil heterogeneity drives ring growth within landform. Regionally, precipitation influences radial growth at the beginning and the end of the growing season, while locally dependent mechanisms related to landform/soil variability emerged. In this sense, the negative influence of late-summer precipitation found for a riparian chronology is a function of soil permeability. Ring growth at the paleo-river environment depends on late spring and early mid-summer precipitation, with within-landform differences probably related to soil heterogeneity. In the case of inter-dune and lowland units, radial growth depends on early spring rainfall. Our findings highlight the influence of the heterogeneity of desert environments on tree growth. The information is relevant to management and conservation policies, particularly for the forests of P. flexuosa in Argentine Monte.

Species‐specific water use by woody plants on the Edwards Plateau, Texas

ABSTRACTWoody encroachment is of great socio‐economic and ecological importance in semiarid savannas because it is presumed to alter the water balance in such ecosystems. We compared two encroachers, Ashe juniper (Juniperus ashei) and honey mesquite (Prosopis glandulosa), with the less invasive live oak (Quercus fusiformis) to ascertain whether soil depth or species‐specific water use strategy is most important in determining transpirational losses on shallow soils underlain by fractured limestone bedrock on the Edwards Plateau. Sap fluxes, leaf water potentials, soil moisture and meteorological information were monitored during spring 2009 at three sites with differing soil depths and were used in a non‐spatial soil–vegetation–atmosphere water transfer model to predict different rooting depths. Sap flux responses and soil water stress were compared using multivariate statistical analysis. At two relatively deep‐soil savanna sites, we found that dominant tree species had similar water use behaviour. However, at the shallow‐soil forest containing Ashe juniper and live oak, juniper transpiration was much higher early in the season than oak transpiration, indicating that juniper benefited more from early spring rainfall than oaks. The model predicts that maximum rooting depths for the woody vegetation were mostly constrained by site soil depths. This study provides new evidence that these species perform similarly on shallow soils and that their water use is restricted by soil depth. The only species‐specific strategy found important in determining transpirational losses was juniper's ability to withstand low soil water potentials in very shallow soils. Copyright © 2012 John Wiley & Sons, Ltd.

Effects of rainfall on soil moisture and water movement in a subalpine dark coniferous forest in southwestern China

AbstractWater content and movement in soil profile and hydrogen isotope composition (δD) of soil water, rainwater, and groundwater were examined in a subalpine dark coniferous forest in the Wolong National Nature Reserve in Sichuan, China, following rainfall events in 2003–2004. Light rainfall increased water content in the litter and at soil depth of 0–80 cm, but the increased soil water was lost in several days. Heavy rainfall increased soil water content up to 85% at depths of 0–40 cm. Following the light rainfall in early spring, the δD of water from the litter, humus, illuvial, and material layers decreased first and then gradually reached the pre‐rainfall level. In summer, light rainfall reached the litter humus, and illuvial layer, but did not hit the material layer. Heavy rainfall affected δD of water in all layers. The δD of soil interflow slightly fluctuated with rainfall events. The δD of shallow groundwater did not differ significantly among all rainfall events. Light rainfall altered the shape of δD profile curve of water in the upper layer of soil, whereas heavy rainfall greatly affected the shape of δD profile curve of water in all soil layers. Following the heavy rainfall, preferential flow initially occurred through macropores, decayed plant roots, and rocks at different depths of soil profile. With continuing rainfall, the litter and surface soil were nearly saturated or fully saturated, and infiltration became homogeneous and plug‐like. Forest soil water, particularly in deeper soil profile, was slightly affected by rainfall and, thus, can be a source of water supply for regional needs, particularly during dry seasons. Copyright © 2011 John Wiley & Sons, Ltd.

Early Spring Dry Spell in the Southeastern Margin of the Tibetan Plateau

The climatic features of the distinctive early-spring dry spell in the southeastern margin of the Tibetan Plateau and the relationship between this dry spell and topographic forcing are investigated. The results show that the early-spring dry spell is characterized by the lowest relative humidity and highest evaporation in the year and by an extremely low amount of rainfall in March and April. The dry spell is a regional phenomenon, as centers of high relative humidity and large rainfall amounts are located to both the east and west of the dry region. In the major dry region, more than 80% of the days in March and April are “dry” days, during which the daily precipitation amounts to no more than 0.1 mm. The mean duration of a dry period between two consecutive rainfall events is found to be 7 days. Further analyses indicate that the relationship between the prevailing westerly flow and the topographic conditions plays an important role in the formation of the dry spell. Strong orographic uplift occurs as the westerly wind flows over the north-south-oriented mountain ranges to the west of the dry region; this in turn causes heavy rainfall over the windward side. After flowing over the mountains, the air subsides over the dry region on the leeward side, suppressing precipitation and increasing the surface air temperature. On the basis of sensitivity experiments carried out using a regional climate model, the role of the topographic conditions in modulating the local climate is confirmed. Reduction in the height of the western mountains as well as lowering of the elevation of the eastern highlands can lead to significant increases in the early-spring rainfall over the dry region.

Advance of apple and pear tree full bloom dates in response to climate change in the southwestern Cape, South Africa: 1973–2009

Studies from throughout the world have demonstrated that tree phenophases are becoming earlier in spring and are closely associated with rising temperatures. Despite many such studies from the Northern Hemisphere, similar comparative work has not been forthcoming from the Southern Hemisphere or from Africa. In addition, few studies have demonstrated the possible role of changes in precipitation and associated soil moisture to driving fruit tree phenophases. Here we provide changes of mean full bloom dates for three apple Malus domestica (Golden Delicious, Sayaka, Granny Smith) and one pear Pyrus communis (Bon Chrétien) cultivars in the southwestern Cape of South Africa. These phenological changes are correlated with temperature and precipitation changes in the same region between 1973 and 2009. Significant early spring (August/September) temperature increases of +0.45 °C/decade are associated with a mean full bloom advance of 1.6 d/decade over the last 37 years. Golden Delicious apple trees have the strongest sensitivity (+4.2 d/°C) to climate change in the region, whilst Granny Smith apple trees have the lowest (+2.4 d/°C). Although winter and early spring rainfall has also decreased over this time, such decreases are not significant. However, significant correlations are found for both temperature and rainfall when comparing with the mean full bloom dates, and it is proposed that both variables operate synergistically to influence mean full bloom dates in the southwestern Cape.

Climate variations and phenological stages modulate ozone damages in field-grown wheat. A three-year study with eight modern cultivars in Po Valley (Northern Italy)

Abstract A three-year field study of leaf symptoms on eight wheat cultivars was carried out in Po Valley (Northern Italy). The survey aimed at showing a possible relationship between climate conditions (temperature and rainfall) and surface ozone levels with the evolution and severity of symptoms due to air pollution (very likely due to ozone) as assessed with open-top chambers (OTC) facilities and histo-cytochemical investigations. Symptoms consisted of small regular chlorotic spots, usually appearing in the last decade of April and evolving, in some cultivars, in necrotic elliptical lesions. The surveys (March–June, each year) indicated that symptom severity, besides being related to the sensitivity of each cultivar, also depended on the phenological stage in which critical ozone AOT40 (Accumulated dose Over a Threshold of 40 ppb) levels were reached. In 2007, the exceptionally mild winter and the heavy early spring rainfall caused a worsening of symptom severity, probably due to the higher rate of gas exchanges and, hence, to an enhanced pollutant uptake. Finally, the analysis of yield data indicated that less symptomatic cultivars had the lowest yield, particularly when the ozone levels were the highest, suggesting a strong reduction of the stomatal conductance that excluded both the pollutant entry and the rate of carbon assimilation. Our findings underline once again the importance of considering ozone flux rather than AOT40 levels to assess the pollutant effects on crops, particularly in the Mediterranean basin.

The divergent environmental characteristics of permanently-open, seasonally-open and normally-closed estuaries of south-western Australia

This study has compared the environmental characteristics of the basins and saline lower reaches of the tributaries of eight estuaries on the south coast of Western Australia, which differ in their degree of connectivity with the ocean. Although four estuaries between 115.1° and 121.8° E along that coast remain permanently open to the ocean, the others only become open when the volume of river discharge is sufficient to breach the prominent sand bars at their mouths, which occurs annually following heavy winter and early spring rainfall in some estuaries (seasonally open) and infrequently in others (normally closed). Estuaries to the west of 118.5° E are predominantly permanently open, e.g. Oyster Harbour, or seasonally open, e.g. Broke, Irwin and Wilson inlets, whereas those further east, e.g. Wellstead Estuary and Hamersley, Culham and Stokes inlets, where annual rainfall and thus discharge are much lower, only become open after exceptionally heavy discharge. In permanently and seasonally-open estuaries, pronounced haloclines and oxyclines are present in the narrow rivers but not the wide basins where the waters are subjected to wind-driven mixing. The extent of cyclical seasonal fluctuations in environmental conditions differed markedly among the three seasonally-open estuaries and between years in one of those systems. These differences reflected variations in the relationship between the volume of fluvial discharge, which is determined by a combination of the amount of local rainfall, catchment size and extent of clearing of native vegetation, and the amount of intrusion by marine waters, which is largely controlled by the size and duration of the opening of the estuary mouth. The mean seasonal salinities in the basins of the three normally-closed estuaries increased over three years of very low rainfall to 64 in the deepest of these estuaries (Stokes Inlet) to 145 in Hamersley Inlet and to 296 in the shallowest estuary (Culham Inlet). These results demonstrate that the environmental characteristics of estuaries on the south coast of Western Australia differ markedly, even among those of the same type, e.g. seasonally-open estuaries or normally-closed estuaries.

Flushing of nitrate from a forested watershed: An insight into hydrological nitrate mobilization mechanisms through seasonal high-frequency stream nitrate dynamics

The variability of hydrologically induced mobilization of nitrate during sequences of hydrological events in different seasons was studied through high-frequency measurements of streamwater nitrate concentrations accompanied by a detailed hydrometeorological monitoring system. The study was carried out in 2006, within the 42 km2 forested Padež watershed in the southwestern part of Slovenia, which is characterized by distinctive flushing, an almost torrential hydrological regime influenced by impermeable flysch geological settings. More than 15 recorded hydrographs which, in the hydrological and biogeochemical sense, differed substantially, disclosed a highly variable but at the same time a strong linkage between hydrological and biogeochemical controls of nitrate exports from the spatial perspective of a watershed. During most of the hydrographs, with the exception of early spring rainfall events, a positive relationship between the nitrate concentration and discharge was observed with peak nitrate concentrations having a time delay in the order of a few hours after the hydrograph peaks. Peak nitrate concentrations in periods of rainfall events span from 3.5 mg/l-N in late spring to 14 mg/l-N in the case of the autumn hydrograph. However, the dilution effect as a consequence of high event water contributions was observed in certain events. The role of specific hydrological events on the nitrate mobilization proved to be important as the size of the accumulated nitrate pool available for mobilization was large throughout most of the hydrographs. The biogeochemical environment of the forest soils presumably drastically alters the size of the available nitrate pool at the studied watershed and, together with the specific hydrological conditions, shifts the watershed from the flush-limited state in the late spring, summer and autumn to the source-limited state in winter and early spring.

The Influence of Mechanical and Thermal Forcing by the Tibetan Plateau on Asian Climate

Abstract This paper attempts to provide some new understanding of the mechanical as well as thermal effects of the Tibetan Plateau (TP) on the circulation and climate in Asia through diagnosis and numerical experiments. The air column over the TP descends in winter and ascends in summer and regulates the surface Asian monsoon flow. Sensible heating on the sloping lateral surfaces appears from the authors’ experiments to be the major driving source. The retarding and deflecting effects of the TP in winter generate an asymmetric dipole zonal-deviation circulation, with a large anticyclone gyre to the north and a cyclonic gyre to the south. Such a dipole deviation circulation enhances the cold outbreaks from the north over East Asia, results in a dry climate in south Asia and a moist climate over the Indochina peninsula and south China, and forms the persistent rainfall in early spring (PRES) in south China. In summer the TP heating generates a cyclonic spiral zonal-deviation circulation in the lower troposphere, which converges toward and rises over the TP. It is shown that because the TP is located east of the Eurasian continent, in summertime the meridional winds and vertical motions forced by the Eurasian continental-scale heating and the TP local heating are in phase over the eastern and central parts of the continent. The monsoon in East Asia and the dry climate in middle Asia are therefore intensified.

Major droughts in England and Wales, 1800–2006

The UK’s vulnerability to drought was high-lighted in 2006 as large parts of southernBritain coped with the impacts of twosuccessive dry winter/spring periods,followed by a hot and relatively dry summer.Particularly arid conditions during thesummers of 2006, 2003, 1995 and 1990 havereinforced the public perception thatdroughts are generally associated with hot,dry summers. Rising temperatures havesimilarly fuelled concern that droughts arebecoming more frequent. Support for sucha tendency may be found in the UK ClimateImpacts Programme (UKCIP) scenarios(UKWIR, 2003), which indicate an increasedlikelihood of wetter winters and more aridsummers in a warmer world. The UK climate,however, is naturally very variable and indi-vidual drought clusters can provide only avery limited insight into possible futuretrends. Importantly, in relation to waterresources, most of our reservoirs are refilledand groundwater storage replenished overthe November–April period when evapora-tion losses are modest. Deficiencies in win-ter and early spring rainfall can therefore becritical in creating subsequent water supplyand environmental stress. This is particularlytrue in the English Lowlands, where ground-water is the principal supply source andspring outflows sustain many rivers andwetlands throughout the summer andautumn.In an attempt to place recent droughtconditions into a wider historical perspec-tive, this study uses hydrological and docu-mentary evidence to extend the timeframewithin which drought episodes can be iden-tified and characterized. The magnitude andimpact of major drought episodes inEngland and Wales since 1800 are assessedand consideration is given to how droughtvulnerability has changed over the last 200years. Particular emphasis is placed onlonger term multi-season droughts – thosecapable of causing significant waterresources and environmental stress.

Summertime European heat and drought waves induced by wintertime Mediterranean rainfall deficit

The risk of extreme heat waves in Europe like the unprecedented one of summer 2003 is likely to increase in the future, calling for increased understanding of these phenomena. From an analysis of meteorological records over 58 years, we show that hot summers are preceded by winter rainfall deficits over Southern Europe. Subsequent drought and heat spreads northward throughout Europe in early summer, due to atmospheric transport of anomalously warm and dry air from Southern Europe in southerly wind episodes. This is shown by the observations and supported by mesoscale meteorological sensitivity simulations for Summer 1994. Moreover previous winter and early spring rainfall frequency in the Mediterranean regions is correlated with summer temperature in central continental Europe. These results emphasize the critical role of the water reservoir in the soil of continental Mediterranean areas for the maintenance of European climate.

Root deployment and shoot growth for two desert species in response to soil rockiness

Soil texture, as well as the presence of rocks, can determine the water status, growth, and distribution of plants in arid environments. The effects of soil rockiness and soil particle size distribution on shoot and root growth, root system size, rooting depth, and water relations were therefore investigated for the Crassulacean acid metabolism leaf succulent Agave deserti and the C(4) bunchgrass Pleuraphis rigida after precipitation events during the summer and winter/spring rainfall periods in the northwestern Sonoran Desert. The soils at the field site varied from sandy (<3% rocks by volume) to rocky (up to 35% rocks), with greater water availability at higher water potentials for sandy than for rocky soils. Although A. deserti was absent from the sandiest sites, its shoot and root growth during both rainfall periods were greatest in comparatively sandier sites and decreased as the soil rock content increased. Furthermore, A. deserti had twofold greater root surface area, root : leaf area ratio, and mean rooting depth at sandier than at rocky sites. As for A. deserti, shoot growth was greater for P. rigida at the sandier sites than at the rockier sites, even though its root surface area and mean rooting depth did not vary significantly. After early spring rainfall events, the leaf water potential for A. deserti did not differ between rocky and sandy sites, but transpiration rates were almost twofold greater at rocky than at sandy sites. During the same period, P. rigida had lower leaf water potentials and 25% lower transpiration rates at rocky than at sandy sites. The greater variability in the deployment of the root systems of A. deserti in response to soil rockiness may reflect its evergreen habit and slower growth, which allow it to endure periods of lower water availability than does P. rigida, whose leaves die during drought.

BIRD–LANDSCAPE RELATIONS IN THE CHIHUAHUAN DESERT: COPING WITH UNCERTAINTIES ABOUT PREDICTIVE MODELS

During the springs of 1995–1997, we studied birds and landscapes in the Chihuahuan Desert along part of the Texas–Mexico border. Our objectives were to assess bird–landscape relations and their interannual consistency and to identify ways to cope with associated uncertainties that undermine confidence in using such relations in conservation decision processes. Bird distributions were often significantly associated with landscape features, and many bird–landscape models were valid and useful for predictive purposes. Differences in early spring rainfall appeared to influence bird abundance, but there was no evidence that annual differences in bird abundance affected model consistency. Model consistency for richness (42%) was higher than mean model consistency for 26 focal species (mean 30%, range 0–67%), suggesting that relations involving individual species are, on average, more subject to factors that cause variation than are richness–landscape relations. Consistency of bird–landscape relations may be influenced by such factors as plant succession, exotic species invasion, bird species' tolerances for environmental variation, habitat occupancy patterns, and variation in food density or weather. The low model consistency that we observed for most species indicates the high variation in bird–landscape relations that managers and other decision makers may encounter. The uncertainty of interannual variation in bird–landscape relations can be reduced by using projections of bird distributions from different annual models to determine the likely range of temporal and spatial variation in a species' distribution. Stochastic simulation models can be used to incorporate the uncertainty of random environmental variation into predictions of bird distributions based on bird–landscape relations and to provide probabilistic projections with which managers can weigh the costs and benefits of various decisions. Uncertainty about the true structure of bird–landscape relations (structural uncertainty) can be reduced by ensuring that models meet important statistical assumptions, designing studies with sufficient statistical power, validating the predictive ability of models, and improving model accuracy through continued field sampling and model fitting. Uncertainty associated with sampling variation (partial observability) can be reduced by ensuring that sample sizes are large enough to provide precise estimates of both bird and landscape parameters. By decreasing the uncertainty due to partial observability, managers will improve their ability to reduce structural uncertainty.

Role of the Fall Armyworm (Lepidoptera: Noctuidae) Pheromone and Other Factors in the Capture of Bumblebees (Hymenoptera: Aphidae) by Universal Moth Traps

The International Pheromone Systems Universal moth trap (Unitrap), when baited with the pheromone of fall armyworm, Spodoptera frugiperda (J. E. Smith), has intermittently been observed to capture large numbers of bumblebees in addition to fall armyworm males. Studies were undertaken in Graham County, N.C., in 1988 and 1989 to determine the contributing factors. Investigation revealed that the yellow color of the traps was attractive to bumblebees, but the color alone did not influence their subsequent movement into the trap and resulting capture. Fall armyworm pheromone alone, particularly when used in combination with a No-Pest resin strip containing dichlorovos insecticide (to kill trapped insects), significantly increased the capture of bumblebees in yellow Unitraps. The green Unitrap alone was totally unattractive to bumblebees. Unitraps captured the greatest number of bumblebees when positioned adjacent to, and at the same height as, pollinating corn tassels. The number of captured bumblebees declined progressively as Unitrap height was lowered at 1-meter intervals from 4 to 1 m. Eleven species of bumblebees were identified from Unitrap captures. Abundant early spring rainfall in 1989 appeared to be primarily responsible for substantial reductions in the number of ground-nesting species of bumblebees during the summer.