Years Of Low Precipitation Research Articles

Plant functional traits are dynamic predictors of ecosystem functioning in variable environments

Abstract A central goal at the interface of ecology and conservation is understanding how the relationship between biodiversity and ecosystem function (B–EF) will shift with changing climate. Despite recent theoretical advances, studies which examine temporal variation in the functional traits and mechanisms (mass ratio effects and niche complementarity effects) that underpin the B–EF relationship are lacking. Here, we use 13 years of data on plant species composition, plant traits, local‐scale abiotic variables, above‐ground net primary productivity (ANPP), and climate from the alpine tundra of Colorado (USA) to investigate temporal dynamics in the B–EF relationship. To assess how changing climatic conditions may alter the B–EF relationship, we built structural equation models (SEMs) for 11 traits across 13 years and evaluated the power of different trait SEMs to predict ANPP, as well as the relative contributions of mass ratio effects (community‐weighted mean trait values; CWM), niche complementarity effects (functional dispersion; FDis) and local abiotic variables. Additionally, we coupled linear mixed effects models with Multimodel inference methods to assess how inclusion of trait–climate interactions might improve our ability to predict ANPP through time. In every year, at least one SEM exhibited good fit, explaining between 19.6% and 57.2% of the variation in ANPP. However, the identity of the trait which best explained ANPP changed depending on winter precipitation, with leaf area, plant height and foliar nitrogen isotope content (δ15N) SEMs performing best in high, middle and low precipitation years, respectively. Regardless of trait identity, CWMs exerted a stronger influence on ANPP than FDis and total biotic effects were always greater than total abiotic effects. Multimodel inference reinforced the results of SEM analysis, with the inclusion of climate–trait interactions marginally improving our ability to predict ANPP through time. Synthesis. Our results suggest that temporal variation in climatic conditions influences which traits, mechanisms and abiotic variables were most responsible for driving the B–EF relationship. Importantly, our findings suggest that future research should consider temporal variability in the B–EF relationship, particularly how the predictive power of individual functional traits and abiotic variables may fluctuate as conditions shift due to climate change.

Stabilization of the pine increment during recent years of low precipitation and high temperatures by removal of the spruce lower storey

In stands dominated by Scots pine in Central European nutrient-poor sandy forest sites, deterioration of pine health due to recent development in climate change has been observed, whereas the vitality of the Norway spruce growing in the lower storey of such stands has remained stable. The study aims to analyse the impact of the spruce lower storey on pine increments and performance in relation to drought stress. Wood core samples were collected using an increment borer from mature pines growing in deep, sandy soils with ground water at a depth that was unreachable by the roots in three plot pairs (blocks) consisting of a pine-spruce plot and a plot with the lower-storey spruce removed. The increments of earlywood and latewood were measured separately. Health status and soil moisture were also evaluated. Spruce removal increased the radial growth (tree ring width – TRW) of the pines, and this manifested earlier in the latewood. Spruce removal significantly changed relationship of TRW and umbrothermic index of released pines. The treatment also increased pine foliation, while increases of topsoil moisture in the transects were insignificant. The study indirectly proves the existence of inter-species competition in the rooting zone for water in soils where the ground water is unreachable by the roots. The flat root system of spruce creates a barrier that reduces the amount of water available for pines with deeper growing roots. To reduce the risk of the negative impact of unfavourable climate change on pine as the target species in commercial mature mixed pine forests with similar site and stand conditions, lower-storey spruce reduction can be recommended, even though this might locally and temporarily lessen the biodiversity of the forest stand.

Impact of Drought and Wildfires in Recent Trends of Diarrhetic Shellfish Toxins in Cockles from Northwest Portugal and Its Similarities with Sardine Stock Trends in the Period 2001–2022

In Portugal, high levels of diarrhetic shellfish toxins (DSTs) originating from Dinophysis spp. are accumulated in bivalves, mainly on the northwest coast (NWC). The occurrence of DSTs in spring and early summer is positively related to precipitation. A decrease in average precipitation after 2003 led to a reduction in DSTs after 2008. However, the accumulation of DSTs in the NWC increased in the period 2020–2022. The hydrological year of 2022 was marked by extreme drought. In cockles from the endemic area of Ria de Aveiro, 37.3% of the weeks tested presented samples above the regulatory limit (RL). The previous record was 31.9% in 2005, also an extremely dry year. The average percentage of weeks above the RL in severe drought years surpassed both the low and high precipitation years. In severe dry years, toxicity was dominated mainly by D. acuta toxins. While D. acuminata grows after the abundant river discharges in spring, D. acuta is capable of growing in summer during upwelling favourable conditions. In the last two decades, extensive forest wildfires were recurrent, but the area burnt reduced sharply after 2017 following tighter fire control measures. Low levels of DSTs or low percentage of weeks with cockles above the RL were related to high burnt areas in the previous year. The recent increase in 2020–2022 of DSTs, a planktonic biomarker in bivalves, was also coincident with the similar temporal increase in either the recruitment or the biomass of Sardina pilchardus, a planktivorous fish in decline after 2005/2006. Both low sardine recruitment and biomass were coincident with low precipitation or high burnt areas. Wildfires degrade coastal water quality, as seems reflected both in the cockle’s DST and the sardine stock time series.

Transformation of tall-tussock grasslands and soil water dynamics in the Flooding Pampa

Cow-calf production is the main practice within marginal agricultural lands of the world, like the Flooding Pampa of Argentina, where it promotes the transformation of native tall-tussock grasslands Paspalum quadrifarium into native short-grass grasslands or sown pastures. The effect of these land use changes on water dynamics are not well understood, especially in regions subjected to marked interannual drought and flooding cycles. Here we measured soil properties (infiltration rate, bulk density and soil organic matter), rainfall interception by the canopy, and soil moisture during two years with different annual rainfall. Then, we parameterized a hydrological model (HYDRUS) for inferring consequences of soil water fluxes on water regulation. Infiltration rate was significantly higher in native tall-tussock grasslands than native short-grass grasslands and sown pastures, bulk density was significantly lower in native tall-tussock grasslands than native short-grass grasslands and sown pastures, and soil organic matter was significantly higher in native tall-tussock grasslands than sown pastures. Simulated water dynamics during years of low annual precipitation (summer rainfall deficit), show that transpiration and evaporation from native short-grass grasslands represented 59 % and 23 % of total water balance, whereas transpiration and evaporation from native tall-tussock grasslands represented 70 % and 12 %, respectively. This result reflects the high productive capacity of native tall-tussock grasslands under dry conditions. In contrast, under high annual precipitation (excess during fall and winter), transpiration and evaporation from native short-grass grasslands represented 48 % and 26 % of total water balance, whereas in native tall-tussock grasslands represented 35 % and 9 %, respectively. These results suggest a low capacity of native tall-tussock grasslands to evacuate water excess, especially during fall and winter. The observed differences in water fluxes between native tall-tussock grasslands and native short-grass grasslands are important to understand water dynamics under different climatic conditions and could be useful for adaptation to climate change through ecosystem-based management.

Can Litter (Dead Herbage) Management Affect the Production and Composition of a Desert Steppe Community?

To examine the effect of litter on ANPP and species composition (using ground cover) in a Desert Steppe community by removing or adding litter, during plant dormancy, in a single event in either fall or spring. Litter was removed or added in three intensity treatments (heavy, moderate, control—undisturbed) as the main plot and season (fall or spring) as the secondary treatment in a split-plot design with five replications. The experiments were repeated in each of 5 years while three of those were resampled twice. The year effect was analyzed by classifying them into high or low precipitation categories and including those in the statistical model. We found few treatment effects one year after treatment and no persistent effect. Therefore, we focus our examination on the first year only. The total ANPP of individual plant types, or their proportions, were not affected by litter treatment or its interaction with season of treatment or precipitation category. Only the ground cover of selected species was influenced by the treatment. The ground cover of Stipa breviflora was greater with heavy litter removal in fall but unaffected by litter removal in spring while Neopallasia pectinata had a greater cover with moderate or heavy removal in years when precipitation was low. Litter addition resulted in a greater ground cover of Neopallasia pectinata and reduced the cover of Convolvulus ammannii in years of low precipitation. The marginal effectiveness of litter treatments on the plan community in the Desert Steppe suggests that it need not be a factor for consideration in grazing management.

Influence of precipitation change and topography characteristics on the development of farmland gully in the black soil region of northeast China

Gully erosion in cultivated land has seriously damaged land resources and reduced crop production. The long-term changes of gully erosion have not been well understood because of limited continuous observed data on the gully development process, which further causes difficulty in establishing universal models of gully erosion rates. Hillslope gullies and valley floor gullies were continuously monitored by RTK GPS and Unmanned Aerial Vehicle (UAV) from 2002 to 2021 in the Heshan farm of Songnen black soil region of northern China. UAV-generated DEMs, historical remote sensing images, and daily precipitation data were combined to analyze the influence of precipitation changes and topographic differences on the development process of gully erosion. The results showed that gully erosion had strong variability between years due to the difference in annual precipitation. The volumetric increment in high precipitation years was 6.5 times and 16.5 times that in normal precipitation years and low precipitation years, respectively, and the variability of valley floor gullies was greater than that of hillslope gullies. The annual average headcut retreat of valley floor gullies and hillslope gullies was 13.1 m y-1 and 4.2 m y-1, respectively, but the volumetric soil loss was mainly contributed by gully bottom deepening and gully sidewall collapsing. The ratio of volumetric soil loss from the gully head to total volumetric soil loss was the smallest in high precipitation years, followed by normal precipitation years and low precipitation years because more runoff in high precipitation years enhanced erosion kinetic energy, which accelerated gully bottom deepening and gully sidewall collapsing. Topography and rainfall were the important influencing factors for the continuous gully development. The maximum value of 3-day accumulative rainfall best explained valley floor gully erosion, and the accumulative value of erosive rainfall best accounted for hillslope gully erosion. A statistical model for estimating volumetric soil loss rate (△V) was developed that included as independent variables drainage area (A) and rainfall characteristic index (RI): △V = 1.571*10-7*（A*RI）1.983 (R2 = 0.98).

Cooling by Cyprus Lows of Surface and Epilimnion Water in Subtropical Lake Kinneret in Rainy Seasons

Comparison between high-precipitation (HP) years and low-precipitation (LP) years led to our main findings which are as follows: Cyprus lows are instrumental in the cooling of surface and epilimnion water in subtropical Lake Kinneret and in the cooling of eastern Mediterranean surface water. Cyprus lows are responsible for cold weather, rainfall, and for an increase in cloudiness causing a decrease in solar radiation over the eastern Mediterranean and north Israel (including Lake Kinneret). In the daytime, comparison between HP and LP years of Kinneret surface water temperature (SWT) and epilimnion water temperature (WT) showed water cooling of up to 2 °C in HP years. This study was carried out using the 21-year period of satellite and in-situ data: (1) MODIS 1 km × 1 km resolution records of SWT, in (2) shipboard measurements of WT vertical profiles down to a depth of ~40 m (2000–2020). We found that a decrease in solar radiation caused by Cyprus lows (due to an increase in cloudiness) was the main factor contributing to Kinneret water cooling. In winter (December–January) when solar radiation (SR) was minimal, no water cooling was observed: the WT difference between HP and LP years was insignificant. However, in spring (March–April) when SR increased and became the main factor contributing to water heating, water cooling was observed: SWT and epilimnion WT, averaged over the HP years, was lower by ~2 °C and ~1.4 °C, respectively, than SWT and epilimnion WT, averaged over the LP years. Not only was water cooling observed in Lake Kinneret, but also in eastern Mediterranean surface water. Comparison of SWT over the eastern Mediterranean between the same HP and LP years in spring showed SWT cooling by ~1.2 °C. This is evidence of the regional character of the daytime water-cooling phenomenon caused by Cyprus lows.

Interactive effects of precipitation and above canopy nitrogen deposition on understorey vascular plants in a jack pine (Pinus banksiana) forest in northern Alberta, Canada

Elevated nitrogen (N) deposition in the bituminous sands region of northern Alberta, Canada is localized but expected to increase over time. Here we seek to determine the effects of above canopy N deposition on understorey vascular plants in a jack pine (Pinus banksiana) stand in a five-year experimental study. Aqueous N (ammonium nitrate) was applied four times annually (May through October) via helicopter above the canopy between 2011 and 2015 across a narrow but environmentally relevant N deposition gradient (0, 5, 10, 15, 20 and 25 kg N ha−1 yr−1). Changes in vascular plant species richness, diversity and total vascular cover were best explained by throughfall water flux, but the positive responses to precipitation decreased with increasing N application. Arctostaphylos uva-ursi and Maianthemum canadense showed positive cover increases in wet years; however, the positive cover expansion at ≥5 kg N ha−1 yr−1 treatments was suppressed relative to controls. Total cover expansion was muted in low precipitation years in treatments ≥10 kg N ha−1 yr−1. In contrast, Vaccinium vitis-idaea cover changes ≥10 kg N ha−1 yr−1 were consistently negative. There were no differences in soil net N mineralization rates, plant foliar N or NO3− leaching among treatments. We conjecture the extensive moss/lichen layer of the forest floor that accumulates most of incoming N in throughfall allows them to outcompete vascular plants for water during higher precipitation years, effectively reducing vascular cover expansion relative to controls. This work suggests the response of vascular plants in xeric jack pine ecosystems may interact with climate and these interactions should be considered in risk assessment studies.

The role of internal nitrogen loading in supportingnon‐N‐fixing harmful cyanobacterial blooms in the water column of a large eutrophic lake

AbstractWestern Lake Erie cyanobacterial harmful algal blooms (cyanoHABs) occur every summer as a result of anthropogenic nutrient loading. Although the physiological importance of nitrogen (N) in supporting bloom biomass and toxin production is established, the role of internal N recycling in the water column to support bloom maintenance is not as well understood. Over three field seasons (2015–2017), we collected water from western Lake Erie and employed bottle incubations with15N‐ammonium () enrichments to determine regeneration and potential uptake rates in the water column. Potential uptake rates followed spatial and seasonal patterns, with greatest rates measured nearest the Maumee River inflow and during peak bloom months (August and September). Regeneration followed a similar spatial pattern but was greatest in early summer (June and July) and supported ~ 20–60% of potential demand during the height of the bloom. Basin‐wide internal regeneration during the April–October period could supply at 60–200% of annual external N loading to the western basin. These results help explain how non‐N‐fixing cyanoHABs in Lake Erie and other large, eutrophic lakes continue producing biomass and N‐rich toxins long after spring nutrient loads are exhausted or transported to other areas. Internal N loads are ultimately driven by external N loads; in low precipitation years, external nutrient loads result in smaller blooms, producing less substrate for subsequent internal N loads. Overall, these findings, along with others, confirm that both internal and external N loading must be considered when evaluating cyanoHAB management strategies.

Shifts in dominance and complementarity between sessile oak and beech along ecological gradients

Abstract Whether tree species benefit from growing in a mixed forest depends on the relative importance of positive versus negative interactions, which varies according to abiotic conditions. In mixture with sessile oak Quercus petraea [Matt.] Liebl., European beech Fagus sylvatica L. is often competitively dominant. Sessile oak, however, is more resistant to water deficit stress. Under water‐limiting conditions, facilitation or even competition exerted by oak may gain in importance. We investigated the relative importance of dominance and complementarity in seven triplets of pure and mixed forests in Belgium's temperate oceanic climate. We analysed ring width data of 145 oaks and 138 beech trees along three distinct gradients: a spatial (i) and a temporal (ii) water availability gradient and a temporal gradient in growing conditions (iii). Gradient (ii) was represented by a precipitation variable, obtained by defining a large set of candidate climate variables over variable time frames, narrowed down using a species and site‐specific lasso model. Growing conditions on a temporal gradient (iii) were represented by standardized tree growth. Two sets of linear mixed‐effects models were used. Growth models assess mixing effects on ring width along gradients (i) and (ii). Mixing index models test for all three gradients how they affect the relative difference in average ring width of trees in mixed and pure stands. Beech trees grew faster than oak trees, and mixing further increased beech growth while decreasing oak growth, except on drier sites where the negative effect on oak growth disappeared. Low precipitation years, in contrast, reduced the beneficial effect of mixing on beech. In years of low growth, the positive mixing effect on beech growth was reinforced, and the negative mixing effect on oak growth decreased. Synthesis. Mixing buffered against growth limitations in general, though this depends on the nature of the limitation and the gradient over which it is measured. On dry sites, competitive dominance of beech was replaced by higher complementarity between species. During dry years, however, oak did not profit from a decreased mixing benefit of beech.

The impact of geomorphology on groundwater recharge in a semi-arid mountainous area

The impact of geomorphological structures on rainwater infiltration and groundwater recharge in the mountainous terrain of a semi-arid region was studied in the southern part of the Judean Western Mountain Aquifer (WMA) in Israel and the Palestinian Authority. The study was conducted in two main typical geomorphological landscape units which include exposed rock outcrops on the hillslope, and human-made agricultural soil terraces in a first-order stream channel. We measured rainwater infiltration through the unsaturated zone in the two geomorphological setups using vadose zone monitoring systems (VMS), that enabled continuous tracking of rainwater percolation and tracer transport through the unsaturated zone with respect to the rain patterns.The results from three years of continuous monitoring of water percolation through the unsaturated zone indicated a quick and intense percolation pattern in the rocky terrain compared to that measured in the deep soil of the terrace. The precipitation patterns, as reflected from the rain distribution and intensities, appeared to have a significant impact on deep water percolation and ultimately on groundwater recharge. In the rocky terrain, rain events of high intensity, even during drought years of low cumulative precipitation, resulted in deep water infiltration, which ultimately turned into groundwater recharge. On the other hand, infiltration through deep soils, as prevailed in terraces, valleys, and high plains, was found to be very limited, subjected to intense evaporation and ultimately result by reduced groundwater recharge. It appears that the value of annual cumulative precipitation is of a lower importance to groundwater recharge while high rain intensity is the most dominant factor responsible for deep water infiltration.Tracer migration patterns in the unsaturated zone revealed that precipitation on the rock outcrops promotes the generation of local runoff that rapidly drains into the subsurface through infiltration hot spots. It has been observed that these hot spots are often marked by prickly burnet shrubs (Sarcopoterium spinosum), which thrive year-round on the water abundancy in the soil pockets beneath the impervious rock on land surface. An important outcome of this study is the suggested strong coupling between geomorphological features and groundwater recharge. Application of variable infiltration functions to the different geomorphological characteristics of each terrain may improve groundwater recharge models. Climate changes in recent years, expected to lead to decreased annual precipitations and increased evaporation in parallel with an increase in rainfall intensities in the semi-arid mountainous areas of the eastern Mediterranean. Yet according to our results, the expected increase in rain intensity may actually result in an increase in groundwater recharge.

Intensified burn severity in California’s northern coastal mountains by drier climatic condition

The severity of wildfire burns in interior lands of western US ecosystems has been increasing. However, less is known about its coastal mountain ecosystems, especially under extreme weather conditions, raising concerns about the vulnerability of these populated areas to catastrophic fires. Here we examine the fine-scale association between burn severity and a suite of environmental drivers including explicit fuel information, weather, climate, and topography, for diverse ecosystems in California’s northern coastal mountains. Burn severity was quantified using Relative difference Normalized Burn Ratio from Landsat multispectral imagery during 1984–2017. We found a significant increasing trend in burned areas and severity. During low-precipitation years, areas that burned had much lower fuel moisture and higher climatic water deficit than in wetter years, and the percentage of high-severity areas doubled, especially during the most recent 2012–2016 drought. The random forest (RF) machine learning model achieved overall accuracy of 79% in classifying categories of burn severity. Aspect, slope, fuel type and availability, and temperature were the most important drivers, based on both classification and regression RF models. We further examined the importance of drivers under four climatic conditions: dry vs. wet years, and during two extended drought periods (the 2012–2016 warmer drought vs. the 1987–1992 drought). During warm and dry years, the spatial variability of burn severity was a mixed effect of slope, long-term minimum temperature, fuel amount, and fuel moisture. In contrast, climatic water deficit and short-term weather became dominant factors for fires during wetter years. These results suggest that relative importance of drivers for burn severity in the broader domain of California’s northern coastal mountains varied with weather scenarios, especially when exacerbated by warm and extended drought. Our findings highlight the importance of targeting areas with high burn severity risk for fire adaptation and mitigation strategies in a changing climate and intensifying extremes.

Plant Selection by Bumble Bees (Hymenoptera: Apidae) in Montane Riparian Habitat of California.

Many bumble bee species (Bombus Latreille) have declined dramatically across North America and the globe, highlighting the need for a greater understanding of the habitat required to sustain or recover populations. Determining bumble bee species' plant selection is important for retaining and promoting high-quality plant resources that will help populations persist. We used nonlethal methods to sample 413 plots within riparian corridors and meadows in the Sierra Nevada of California for bumble bees during two summers following extremely low and normal precipitation years, respectively. We assessed the five most abundant bumble bee species' plant selection by comparing their floral use to availability. Additionally, we described the shift in plant selection between years for the most abundant species, Bombus vosnesenskii Radoszkowski. Bumble bee species richness was constant between years (13 species) but abundance nearly tripled from 2015 to 2016 (from 1243 to 3612 captures), driven largely by a dramatic increase in B. vosnesenskii. We captured bumble bees on 104 plant species or complexes, but only 14 were significantly selected by at least one bumble bee species. Each of the five most frequently captured bumble bee species selected at least one unique plant species. Plant blooming phenology, relative availability of flowers of individual plant species, and plant selection by B. vosnesenkii remained fairly constant between the two study years, suggesting that maintaining, seeding, or planting with these 'bumble bee plants' may benefit these five bumble bee species.

Precipitation amount and event size interact to reduce ecosystem functioning during dry years in a mesic grassland.

Ongoing intensification of the hydrological cycle is altering rainfall regimes by increasing the frequency of extreme wet and dry years and the size of individual rainfall events. Despite long-standing recognition of the importance of precipitation amount and variability for most terrestrial ecosystem processes, we lack understanding of their interactive effects on ecosystem functioning. We quantified this interaction in native grassland by experimentally eliminating temporal variability in growing season rainfall over a wide range of precipitation amounts, from extreme wet to dry conditions. We contrasted the rain use efficiency (RUE) of above-ground net primary productivity (ANPP) under conditions of experimentally reduced versus naturally high rainfall variability using a 32-year precipitation-ANPP dataset from the same site as our experiment. We found that increased growing season rainfall variability can reduce RUE and thus ecosystem functioning by as much as 42% during dry years, but that such impacts weaken as years become wetter. During low precipitation years, RUE is lowest when rainfall event sizes are relatively large, and when a larger proportion of total rainfall is derived from large events. Thus, a shift towards precipitation regimes dominated by fewer but larger rainfall events, already documented over much of the globe, can be expected to reduce the functioning of mesic ecosystems primarily during drought, when ecosystem processes are already compromised by low water availability.

Performance of AquaCrop model in simulating maize growth, yield, and evapotranspiration under rainfed, limited and full irrigation

The performance assessments of AquaCrop model using long-term field-measured data are rare. In this study, the model was evaluated relative to maize growth, yield, and water use parameters/variables under different water stress conditions over six years (2005–2010) in Nebraska, USA. The model was calibrated and validated for full irrigation treatment (FIT), limited irrigation (50, 60, and 75% of FIT), and rainfed. Model default parameters provided very poor estimates for mostly all variables. After calibration, the model adequately simulated daily canopy cover (CC) for both 2009 and 2010 (NRMSEs ≤ 15.6%), except for slight discrepancies in 2010. However, the model overestimated the final biomass in 2009 due to overestimation of biomass development in the late growth stages. The simulation of final biomass was better in 2010 (NRMSE = 5.3%) than in 2009 (NRMSE = 31%). The model simulated grain yield quite well (NRMSE of 7.7 and 12.1%, EF of 0.8 and 0.7) during both calibration and validation, respectively, except for rainfed treatments in most years. The model was not able to simulate total soil-water accurately in most cases, except for 2009 growing season. The crop evapotranspiration (ETc) was simulated with good accuracy during 2007, 2009, and 2010 and the higher prediction error (up to 16.5%) was observed for dry years (2005 and 2006) and wet year (2008). The model performance declined substantially in conditions of water stress, excess water, and high evaporative demand. In conclusion, the AquaCrop simulated yield and ETc (with slight underestimation) quite well in some cases, but encountered substantial difficulties in simulating biomass and soil-water, especially during years of low precipitation and high evaporative demand as well as in wet year. Further evaluation of the model is needed using field measured evaporation and transpiration data to determine the accuracy of ETc partition by the model to better simulate soil-water and ETc, which are critical for the estimation of in-season irrigation requirements, biomass production, and yield.

Cover Crops Reduce Nitrate Leaching in Agroecosystems:A Global Meta-Analysis.

Cover crops are well recognized as a tool to reduce NO leaching from agroecosystems. However, their effectiveness varies from site to site and year to year depending on soil, cash and cover crop management, and climate. We conducted a meta-analysis using 238 observations from 28 studies (i) to assess the overall effect of cover crops on NO leaching and subsequent crop yields, and (ii) to examine how soil, cash and cover crop management, and climate impact the effect of non-leguminous cover crops on NO leaching. There is a clear indication that nonleguminous cover crops can substantially reduce NO leaching into freshwater systems, on average by 56%. Nonlegume-legume cover crop mixtures reduced NO leaching as effectively as nonlegumes, but significantly more than legumes. The lack of variance information in most published literature prevents greater insight into the degree to which cover crops can improve water quality. Among the factors investigated, we identified cover crop planting dates, shoot biomass, and precipitation relative to long-term mean precipitation as potential drivers for the observed variability in nonleguminous cover crop effectiveness in reducing NO leaching. We found evidence indicating greater reduction in NO leaching with nonleguminous cover crops on coarse-textured soils and during years of low precipitation (<90% of the long-term normal). Earlier fall planting and greater nonleguminous shoot biomass further reduced NO leaching. Overall, this meta-analysis confirms many prior studies showing that nonleguminous cover crops are an effective way to reduce NO leaching and should be integrated into cropping systems to improve water quality.

Leveraging Spatial and Temporal Variability to Probabilistically Characterize Nutrient Sources and Export Rates in a Developing Watershed

AbstractHybrid watershed models based on nonlinear regression are useful tools for estimating the magnitude of loading rates (i.e., export coefficients) for various pollutant sources within large‐scale river basins. Few such models, however, have incorporated temporal variability in either source distributions or climate, despite evidence that precipitation is the primary driver in interannual variability in loading rates. The model developed here includes changes in precipitation, land use, point source discharge, and livestock operations to capture temporal variability in nitrogen loads. Precipitation is incorporated directly in the formulation of export rates using coefficients that vary by source type. Instream and reservoir retention of nitrogen is included to account for nitrogen sinks within the watershed. A Bayesian hierarchical approach is employed to integrate uncertainty in loading estimates, include prior knowledge of parameters, address intrawatershed correlation, and estimate export coefficients probabilistically. We apply this method to three North Carolina river basins that have experienced substantial growth in urban development and livestock operations in the past few decades, and where eutrophication‐related water quality problems are common. Accounting for temporal variability constrains uncertainties in nonpoint source export coefficients by nearly 50%, relative to a spatial‐only model. Results indicate that livestock operations are a significant contributor of nitrogen throughout much of the study area. Precipitation is shown to have a larger influence on export rates for agricultural than for developed lands, creating a system dominated by agricultural total nitrogen during high precipitation years and by developed (urban) regions during low precipitation years.

Exploring the Origins of Snow Drought in the Northern Sierra Nevada, California

Abstract The concept of snow drought is gaining widespread interest as the climate of snow-dominated mountain watersheds continues to change. Warm snow drought is defined as above- or near-average accumulated precipitation coinciding with below-average snow water equivalent at a point in time. Dry snow drought is defined as below-average accumulated precipitation and snow water equivalent at a point in time. This study contends that such point-in-time definitions might miss important components of how snow droughts originate, persist, and terminate. Using these simple definitions and a variety of observations at monthly, daily, and hourly time scales, the authors explore the hydrometeorological origins of potential snow droughts in the northern Sierra Nevada from water years 1951 to 2017. This study finds that snow droughts can result from extreme early season precipitation, frequent rain-on-snow events, and low precipitation years. Late-season snow droughts can follow persistent warm and dry periods with effects that depend upon elevation. Many snow droughts were characterized by lower snow fractions and midwinter peak runoff events. These findings can guide improved evaluations of historical and potential future snow droughts, particularly with regards to how impacts on water resources and mountain ecosystems may vary depending on how snow droughts originate and evolve in time.

Patch burning on tall-grass native prairie does not negatively affect stocker performance or pasture composition

The purpose of this study was to determine stocker BW gain on patch-burned native tall-grass prairie while also determining plant species influenced by fire. The study was conducted in a split-block experimental design where treatments consisted of a yearly spring burn on the pastures (CON) or patch burning of one-third pasture per year (PB). Stocker steers grazed the pastures using a three-quarter-season (~114 d) grazing period from about mid-April to mid-August from 2006 to 2012. Steer ADG, final weight, and total BW gain were not different by treatment (P > 0.35). However, when comparing treatment effects with precipitation classification (high, average, low), cattle on PB had a greater ADG (P = 0.02; 0.10 kg/d), final weight (P = 0.07; 12 kg), and total BW gain (P = 0.02; 11.8 kg) in low precipitation years (2011 and 2012). Overall, patch burning provides similar BW gains as yearly burning on native tall-grass prairie, while providing a BW gain advantage in low precipitation years. The switchgrass (Panicum virgatum) population declined (P < 0.05) on CON treatment, whereas the population of other perennial grasses increased. The amount of annual grasses, including hairy crabgrass (Digitaria sanguinalis) and yellow foxtail (Setaria pumila), increased (P < 0.05) under PB. Botanical composition shifts were similar on patch-burn pastures and full-burn pastures, with the exception of increasing annual grasses with patch burning.

Growing-season precipitation since 1872 in the coastal area of subtropical southeast China reconstructed from tree rings and its relationship with the East Asian summer monsoon system

Tree rings from temperate zones of the world have provided abundant palaeo- ecological and paleo-hydroclimatic information. However, tree rings from subtropical to tropical regions remain relatively scarce, which greatly limit our fully understanding about the climate change issues. In the present work, tree-ring-width (TRW) measurements of Masson pine from Fujian province, the coastal area of subtropical southeast China were successfully crossdated and a TRW STD chronology was developed from 1854 to 2012. Significantly positive correlation was identified between the tree rings and April–November total precipitation (r=0.71, p<0.01). The reconstructed April–November precipitation exhibited two comparatively wet (1876–1886 and 1957–1962) and one comparatively dry (1986–2004) periods. An evident drying trend since 1959 was seen and it was mitigated after 1993. Most of the extreme low-precipitation years in the reconstruction were supported by the historical records. As revealed by the spatial correlation patterns, our precipitation reconstruction was also consistent with other hydroclimatic records along the coastal areas of southeast China, proving its ability to capture the large-scale hydrological signal in southeast China (mainly refers to the south of the middle-lower reaches of Yangtze River). The reconstructed precipitation showed significant correlation with the East Asian summer Monsoon (EASM) index. Moreover, it also indicated simultaneous variation with the monsoon precipitation in North China on a decadal scale, implying that growing season precipitation variations in both regions were influenced by the EASM strength. This work highlights the potential of using tree-ring width to reconstruct precipitation in subtropical southeast China, while the relevant issues about precipitation variation in this region is far from resolved.