Contrasting Precipitation Patterns Research Articles

Precipitation before Flowering Determined Effectiveness of Leaf Removal Timing and Irrigation on Wine Composition of Merlot Grapevine.

Grapevine productivity, and berry and wine flavonoid concentration, depend on the interactions of cultivar, environment, and applied cultural practices. We characterized the effects that mechanical leaf removal and irrigation treatments had on the flavonoid concentration of ‘Merlot’ (Vitis vinifera, L.) grape berries and wines in a hot climate over two growing seasons with contrasting precipitation patterns. Leaves were removed by machine, either at prebloom (PBLR), or at post-fruit-set (PFLR), or not removed (control) and irrigation was either applied as sustained deficit irrigation (SDI) at 0.8 of crop evapotranspiration (ETc) from budbreak to fruit set, or regulated deficit irrigation (RDI) at 0.8 ETc from bud break to fruit set, 0.5 ETc from fruit set to veraison, and 0.8 ETc from veraison to harvest, of ETc In 2014, PFLR reduced the leaf area index (LAI) compared to control. The RDI decreased season-long leaf water potential (ΨInt) compared to SDI. However, in 2015, none of the treatments affected LAI or ΨInt. In 2014, berry flavonoid concentrations were reduced by PBLR as well as SDI. SDI increased the flavonoid concentrations in wine, and PFLR increased some wine flavonols in one season. No factor affected the concentrations of wine proanthocyanidins or mean degree of polymerization. Thus, mechanical PFLR and RDI may increase berry flavonoid accumulation without yield reduction, in red wine grapes cultivars grown in hot climates when precipitation after bud break is lacking. However, spring precipitation may influence the effectiveness of these practices as evidenced by this work in a changing climate.

The interactive effects of nitrogen addition and increased precipitation on gross ecosystem productivity in an alpine meadow

AbstractThe ecological consequences of precipitation change and increased atmospheric nitrogen (N) deposition have profound impacts on ecosystem CO2 exchange in grassland ecosystems. Water and N can largely influence grassland productivity, community composition and ecosystem functions. However, the influences of water and N addition on the ecosystem CO2 exchange of alpine grassland ecosystems remain unclear. A field manipulative experiment with water and N additions was conducted in an alpine meadow on the Tibetan Plateau over 4 years with contrasting precipitation patterns. There were four treatments: control (Ctrl), N addition (N), water addition (W) and N and water addition (NW), each replicated three times. N addition, but not water addition, increased gross ecosystem productivity (GEP), plant biomass, community cover and community-weighted mean height. The responses of ecosystem CO2 exchange to water and N addition varied between the wet and dry years. Water addition had a positive effect on net ecosystem carbon exchange (NEE) due to a larger increase in GEP than in ecosystem respiration (ER) only in the dry year. On the contrary, N addition significantly enhanced ecosystem CO2 exchange only in the wet year. The increased GEP in N addition was attributed to the larger increase in NEE than ER. Moreover, N addition stimulated NEE mainly through increasing the cover of dominant species. Our observations highlight the important roles of precipitation and dominant species in regulating ecosystem CO2 exchange response to global environmental change in alpine grasslands.

Application of Fractions of Crop Evapotranspiration Affects Carbon Partitioning of Grapevine Differentially in a Hot Climate

Majority of viticulture regions are located in mid-latitudes characterized by weather variability and stressful environments relying on irrigation for mitigating environmental stress during the growing season and to ensure a profitable yield. The aim of this study was to characterize the response of grapevine (Vitis vinifera L. cv. Cabernet Sauvignon) to different applied water amounts based on the replacement of fractions of crop evapotranspiration (ETc) during two growing seasons with contrasting precipitation patterns. The experiment consisted of three irrigation treatments based on the weekly replacement of 25, 50, and 100% of ETc. Grapevine stem water potential decreased during the growing season reaching its lowest value (-1.5 and -1.2 MPa, respectively) at harvest in the more stressed vines (25 and 50% ETc). Leaf gas exchange variables were measured during the two seasons and 100% ETc had the highest rates of photosynthesis and stomatal conductance and better instantaneous water use efficiency, also resulting in higher leaf chlorophyll and carotenoid content. Mineral nutrient content for nitrogen and potassium increased linearly with the increase in applied water. At harvest, no differences were observed in the number of clusters per vine; however, the 25% ETc had the lowest berry size and yield per vine with no difference in sugar content of berry. Conversely, sugar allocation to reserve organs was highly affected by applied water leading to different shoot to root biomass partitioning, where shoot:root ratio, leaf non-structural carbohydrates, and photosynthetic pigments increased with greater applied water. Likewise sucrose:N ratio and root non-structural carbohydrates decreased with the lower applied water. Altogether, carbon allocation between the source and sink organs likely controlled the response of grapevines to water deficits in a hot climate, and replacing 50% ETc was sufficient to sustain the grapevine performance given the enhancement of sugar transport, which could slow down the detrimental effect of water deficits on yield.

Arctic soil water chemistry in dry and wet tundra subject to snow addition, summer warming and herbivory simulation

Multiple and rapid environmental changes in the Arctic have major consequences for the entire ecosystem. Soil water chemistry is one component with important implications for understanding climate feedbacks, plant growth, microbial turnover and net greenhouse gas emissions. Here we assess the contrasting growing season soil water chemistry in a Low arctic Greenlandic mesic tundra heath and a fen, which have been subjected to factorial treatments of summer warming using open top chambers (OTCs), snow addition using snow fences, which increase soil temperature in late winter, and shrub removal mimicking herbivory attack. Dissolved Organic Carbon (DOC) and plant nutrients, including NO3−, NH4+, PO42+ and total dissolved N were measured during multiple growing seasons (2013–2016) to quantify the treatment effects on nutrient availability in two dominating, but contrasting, vegetation types.Ambient nutrient concentrations in the mesic tundra heath decreased throughout the growing season and increased during senescence, while concentrations were highest during peak growing season in the fen. The content of NH4+ and DOC were highest in the fen, whereas NO3− was highest in the mesic tundra heath. The fen had no seasonal pattern.Summer warming in the mesic tundra heath did not change the availability of nutrients, but in combination with shrub removal, both NO3− and DOC concentrations increased, likely due to reduced plant uptake. Shrub removal alone increased NO3− in one growing season, and, combined with snow addition, increased DOC. Significant effects of shrub removal were mostly found in 2016. Snow addition combined with summer warming increased DOC and total N concentrations and highlights the potential loss of dissolved C from the ecosystem.In the fen, shrub removal alone and combined with summer warming decreased DOC. Snow addition alone and in combination with summer warming similarly decreased DOC.In the mesic tundra heath, shrub removal caused higher soil water contents in all years. In the dry and warm 2016, it meant <10% soil water content in controls and 15–20% in shrub removal plots during the peak growing season, which may have relieved soil moisture limitation on mineralization rates in the latter.We conclude that soil water chemistry is vegetation-specific, and that treatment effects are surprisingly limited when comparing multiple years with contrasting precipitation patterns. Herbivory may have larger impact in very dry, warm summers and, together with extreme weather events, exert similar or larger effects than four years of temperature manipulations. The effects of summer warming or increased winter snow depend on ecosystem type and moisture status of the soil. The combination of multi-year and multi-site studies therefore seem important for understanding future biogeochemical dynamics in Arctic landscapes.

Effects of contrasting precipitation patterns on the trajectory of actively growing and inactive microbial communities after rewetting

Predicted shifts in precipitation patterns could impact soil microbial activity, and thereby terrestrial ecosystem functioning. In a Mediterranean grassland soil which had been subjected to contrasting precipitation patterns, we investigated the response of active and inactive bacterial and fungal communities to rewetting over time, using 18O stable isotope probing. Altered precipitation patterns prior to rewetting had little impact on the trajectories over time of the active and inactive bacterial communities after rewetting, as bacteria died or were recruited from the inactive to the active community. The duration of the dry summer conditions affected the diversity and phylogenetic clustering of the inactive microbial community and its functional potential, likely indicating long-lasting effects on ecosystem stability.

YIELD FORMATION CAPACITY, SOIL WATER CONSUMPTION PROPERTY, AND PLANT WATER USE EFFICIENCY OF WHEAT UNDER WATER-SAVING CONDITIONS IN NORTH CHINA PLAIN

The drastically reduced underground water and shortage of the water resource have caused limitation for the winter wheat (T. aestivum) production in North China Plain. In this study, using a drought-tolerant wheat cultivar Shimai 18 as material, we investigated the yield formation capacity, soil water consumption property, and the plant water use efficiency (WUE) of wheat under various irrigation treatments. Our results indicated that the yield capacities of wheat varied largely across irrigation treatments in three growth seasons with contrasting precipitation patterns. The wheat yields showed constant in growth season with more rainfall and lowered to some extent in seasons with regular or less rainfall under I1 (water-saving condition with one time of irrigation) compared with I3 (regular irrigation with three times of irrigation). Along with increase of the irrigation times, plant WUE lowered drastically in all of three growth seasons examined, together with significantly increased evapotranspiration (ET) amount and relative constant of soil water deposits. These results suggested that the water-saving management is feasible in the winter wheat production in North China Plain. Additionally, the plant leaf area index (LAI), dry matter accumulation, and biomass partitioning rate of vegetative tissues during late growth stage (from flowering to maturity) were consistent with the yield formation capacities of the wheat cultivar. Our investigation indicates that the water-saving management using drought tolerant cultivars is effective in production of winter wheat in North China Plain, which can drastically improve the plant WUE and maintain the sustainable wheat productivity under reduced irrigation conditions.

Ammonia-Oxidizing Archaea Are More Resistant Than Denitrifiers to Seasonal Precipitation Changes in an Acidic Subtropical Forest Soil.

Seasonal precipitation changes are increasingly severe in subtropical areas. However, the responses of soil nitrogen (N) cycle and its associated functional microorganisms to such precipitation changes remain unclear. In this study, two projected precipitation patterns were manipulated: intensifying the dry-season drought (DD) and extending the dry-season duration (ED) but increasing the wet-season storms following the DD and ED treatment period. The effects of these two contrasting precipitation patterns on soil net N transformation rates and functional gene abundances were quantitatively assessed through a resistance index. Results showed that the resistance index of functional microbial abundance (-0.03 ± 0.08) was much lower than that of the net N transformation rate (0.55 ± 0.02) throughout the experiment, indicating that microbial abundance was more responsive to precipitation changes compared with the N transformation rate. Spring drought under the ED treatment significantly increased the abundances of both nitrifying (amoA) and denitrifying genes (nirK, nirS, and nosZ), while changes in these gene abundances overlapped largely with control treatment during droughts in the dry season. Interestingly, the resistance index of the ammonia-oxidizing archaea (AOA) amoA abundance was significantly higher than that of the denitrifying gene abundances, suggesting that AOA were more resistant to the precipitation changes. This was attributed to the stronger environmental adaptability and higher resource utilization efficiency of the AOA community, as indicated by the lack of correlations between AOA gene abundance and environmental factors [i.e., soil water content, ammonium (NH4+) and dissolved organic carbon concentrations] during the experiment.

Quantifying the rise of the Himalaya orogen and implications for the South Asian monsoon

Abstract We reconstruct the rise of a segment of the southern flank of the Himalaya-Tibet orogen, to the south of the Lhasa terrane, using a paleoaltimeter based on paleoenthalpy encoded in fossil leaves from two new assemblages in southern Tibet (Liuqu and Qiabulin) and four previously known floras from the Himalaya foreland basin. U-Pb dating of zircons constrains the Liuqu flora to the latest Paleocene (ca. 56 Ma) and the Qiabulin flora to the earliest Miocene (21–19 Ma). The proto-Himalaya grew slowly against a high (∼4 km) proto–Tibetan Plateau from ∼1 km in the late Paleocene to ∼2.3 km at the beginning of the Miocene, and achieved at least ∼5.5 km by ca. 15 Ma. Contrasting precipitation patterns between the Himalaya-Tibet edifice and the Himalaya foreland basin for the past ∼56 m.y. show progressive drying across southern Tibet, seemingly linked to the uplift of the Himalaya orogen.

Interannual Variability in Dry Mixed-Grass Prairie Yield: A Comparison of MODIS, SPOT, and Field Measurements

Remote sensing is often used to assess rangeland condition and biophysical parameters across large areas. In particular, the relationship between the Normalized Difference Vegetation Index (NDVI) and above-ground biomass can be used to assess rangeland primary productivity (seasonal carbon gain or above-ground biomass “yield”). We evaluated the NDVI–yield relationship for a southern Alberta prairie rangeland, using seasonal trends in NDVI and biomass during the 2009 and 2010 growing seasons, two years with contrasting rainfall regimes. The study compared harvested biomass and NDVI from field spectrometry to NDVI from three satellite platforms: the Aqua and Terra Moderate Resolution Imaging Spectroradiometer (MODIS) and Système Pour l’Observation de la Terre (SPOT 4 and 5). Correlations between ground spectrometry and harvested biomass were also examined for each growing season. The contrasting precipitation patterns were easily captured with satellite NDVI, field NDVI and green biomass measurements. NDVI provided a proxy measure for green plant biomass, and was linearly related to the log of standing green biomass. NDVI phenology clearly detected the green biomass increase at the beginning of each growing season and the subsequent decrease in green biomass at the end of each growing season due to senescence. NDVI–biomass regressions evolved over each growing season due to end-of-season senescence and carryover of dead biomass to the following year. Consequently, mid-summer measurements yielded the strongest correlation (R2 = 0.97) between NDVI and green biomass, particularly when the data were spatially aggregated to better match the satellite sampling scale. Of the three satellite platforms (MODIS Aqua, MODIS Terra, and SPOT), Terra yielded the best agreement with ground-measured NDVI, and SPOT yielded the weakest relationship. When used properly, NDVI from satellite remote sensing can accurately estimate peak-season productivity and detect interannual variation in standing green biomass, and field spectrometry can provide useful validation for satellite data in a biomass monitoring program in this prairie ecosystem. Together, these methods can be used to identify the effects of year-to-year precipitation variability on above-ground biomass in a dry mixed-grass prairie. These findings have clear applications in monitoring yield and productivity, and could be used to support a rangeland carbon monitoring program.

Ephemeral plants mediate responses of ecosystem carbon exchange to increased precipitation in a temperate desert

The ecological consequences of increased precipitation on ecosystem carbon (C) exchange are gaining increasing concern, especially in the context of ongoing climate change in temperate deserts. In this study, a field manipulative experiment was conducted to assess the effects of increased precipitation and nitrogen (N) addition on net ecosystem C exchange (NEE) in a temperate desert in northwestern China during two years with contrasting precipitation patterns (2011 and 2012). Increased precipitation decreased ecosystem C release by nearly 50% in the wet year of 2011, whereas ecosystem C release was increased in the dry year of 2012 because of the disproportional stimulation of gross ecosystem productivity (GEP) and ecosystem respiration (ER) by increased precipitation. N addition had no impact on NEE because of the slight responses of both GEP and ER to N addition. During the wet year, most of the precipitation occurred during the growing season of ephemeral plants, which profoundly stimulated plant growth and led to a higher response of GEP than ER to increased precipitation. As a result, positive effects of increased precipitation on NEE occurred in this year. During the dry year, the majority of the precipitation fell post the ephemeral growing season and only a small increase in herb biomass was observed. However, the response of ER to increased precipitation was larger than that of GEP, leading to a more negative NEE in 2012, as compared to 2011. C release was thus stimulated by increased precipitation in 2012. Irrespective of precipitation treatment, N addition weakly decreased C release because of the negligible stimulation of GEP and ER, as well as the slight response of ephemeral biomass. The responses of NEE to increased precipitation showed no difference between interplant spaces and beneath the dominant shrubs because of the similar responses of plant growth, GEP and ER between sites. Overall, this study shows that the responses of NEE to projected increasing precipitation depend on the coupling of precipitation timing and plant growing season.

Effects of spring drought on carbon sequestration, evapotranspiration and water use efficiency in the songnen meadow steppe in northeast China

AbstractGlobal climate change projections suggest an increasing frequency of droughts and extreme rain events in the steppes of the Eurasian region. Using the eddy covariance method, we measured carbon and water balances of a meadow steppe ecosystem in Northeast China during 2 years which had contrasting precipitation patterns in spring seasons in 2007 and 2008. The meadow steppe sequestrated only 64·2 gC m−2 year−1 in 2007 compared to 160·5 gC m−2 year−1 in 2008, due to a severe spring drought in 2007. The 2007 spring drought resulted in a dramatic reduction of leaf area index (LAI) and aboveground net primary productivity (ANPP). However, the meadow steppe still acted as a carbon sink in 2007. The strength of the sink was much greater than that in the typical steppes in Central Mongolia and Inner Mongolia. Spring drought also caused a reduction of plant transpiration (Tr) and total ecosystem evapotranspiration (ET). However, the suppression of ET in 2007 was relatively small in comparison to gross ecosystem productivity (GEP) reduction. Thus, ecosystem water use efficiency (WEU) (GEP/ET) in 2007 was reduced to 5·0 gCO2 kg−1 H2O or 75% of that of 2008. We concluded that spring drought detrimentally impacted meadow steppe ecosystem by reducing leaf areas, biomass, GEP, WUE and associated increases in soil evaporation (Es) that might aggravate soil salinization of the Songnen Plain. Copyright © 2011 John Wiley &amp; Sons, Ltd.

Spatial and temporal patterns of water availability in a grass–shrub ecotone and implications for grassland recovery in arid environments

AbstractEncroachment of woody shrubs into historic desert grasslands is a major problem throughout the world. Conversion of grasslands to shrub‐dominated systems may result in significant alteration of biogeochemical processes and reduced resource availability in shrub interspaces, making grassland recovery difficult. Soil petrocalcic horizons, which are common in arid and semi‐arid regions globally, have been shown to dramatically alter plant water availability. To assess how soil water dynamics are affected by woody encroachment in petrocalcic soils under contrasting precipitation patterns, we conducted a 3‐year replicated study in a mixed shrub–grass system measuring soil water in unvegetated interspaces and under mesquite canopies. Plots were instrumented with time‐domain reflectometry (TDR) moisture probes, both above and within the soil petrocalcic horizon. Soils under both cover types maintained large increases in available water content for several months during a wetter than normal winter and summer (increases of 0·08–0·16 m3 m−3). Interspace soils absorbed significantly greater quantities of water during the winter and retained more water into the spring than soils under shrubs. In contrast, soils under shrubs initially absorbed greater volumes of water during and following summer rains. Differing seasonal dynamics were attributed to interactions between the unique properties of soils with petrocalcic horizons and canopy‐induced variability in evapotranspiration. Observed patterns of plant‐available water do not support the hypothesis of greater resource availability under shrubs. Similar or greater water availability in shrub interspaces indicates that concentration of soil water under shrubs may not be a process limiting grass recovery on these soils. Published in 2010 by John Wiley &amp; Sons, Ltd.

Synoptic Circulation and Land Surface Influences on Convection in the Midwest U.S. “Corn Belt” during the Summers of 1999 and 2000. Part II: Role of Vegetation Boundaries

Abstract In Part I of this observational study inquiring into the relative influences of “top down” synoptic atmospheric conditions and “bottom up” land surface mesoscale conditions in deep convection for the humid lowlands of the Midwest U.S. Central Corn Belt (CCB), the composite atmospheric environments for afternoon and evening periods of convection (CV) versus no convection (NC) were determined for two recent summers (1999 and 2000) having contrasting precipitation patterns and amounts. A close spatial correspondence was noted between composite synoptic features representing baroclinity and upward vertical motion with the observed precipitation on CV days when the “background” (i.e., free atmosphere) wind speed exceeded approximately 10 m s−1 at 500 hPa (i.e., “stronger flow”). However, on CV days when wind speeds were &amp;lt;∼10 m s−1 (i.e., “weaker flow”), areas of increased precipitation can be associated with synoptic composites that are not so different from those for corresponding NC days. From these observations, the presence of a land surface mesoscale influence on deep convection and precipitation is inferred that is better expressed on weaker flow days. Climatically, a likely candidate for enhancing low-level moisture convergence to promote deep convection are the quasi-permanent vegetation boundaries (QPVBs) between the two major land use and land cover (LULC) types of crop and forest that characterize much of the CCB. Accordingly, in this paper the role of these boundaries on summer precipitation variations for the CCB is extracted in two complementary ways: 1) for contrasting flow day types in the summers 1999 and 2000, by determining the spatially and temporally aggregated land surface influence on deep convection from composites of thermodynamic variables [e.g., surface lifted index (SLI), level of free convection (LFC), and lifted condensation level (LCL)] that are obtained from mapped data of the 6-h NCEP–NCAR reanalyses (NNR), and 0000 UTC rawinsonde ascents; and 2) for summer seasons 1995–2001, from the statistical associations of satellite-retrieved LULC boundary attributes (i.e., length and width) and precipitation at high spatial resolutions. For the 1999 and 2000 summers (item 1 above), thermodynamic composites determined for V(500) categories having minimal differences in synoptic meteorological fields on CV minus NC (CV − NC) days (i.e., weaker flow), show statistically significant increases in atmospheric moisture (e.g., greater precipitable water; lower LCL and LFC) and static instability [e.g., positive convective available potential energy (CAPE)] compared to NC days. Moreover, CV days for both weaker and stronger background flow have associated subregional-scale thermodynamic patterns indicating free convection at the earth’s surface, supported by a synoptic pattern of at least weakly upward motion of air in the midtroposphere in contrast to NC days. The possibility that aerodynamic contrasts along QPVBs readily permit air to be lofted above the LFC when the lower atmosphere is moist, thereby assisting or enhancing deep convection on CV days, is supported by the multiyear analysis (item 2 above). In early summer when LULC boundaries are most evident, precipitation on weaker flow days is significantly greater within 20 km of boundaries than farther away, but there is no statistical difference on stronger flow days. Statistical relationships between boundary mean attributes and mean precipitation change sign between early summer (positive) and late summer (negative), in accord with shifts in the satellite-retrieved maximum radiances from forest to crop areas. These phenological changes appear related, primarily, to contrasting soil moisture and implied evapotranspiration differences. Incorporating LULC boundary locations and phenological status into reliable forecast fields of lower-to-midtropospheric humidity and wind speed should lead to improved short-term predictions of convective precipitation in the Corn Belt and also, potentially, better climate seasonal forecasts.

Tree phenology in montane forests of southern Ecuador can be explained by precipitation, radiation and photoperiodic control

Abstract:We investigated the effect of seasonality on tree phenology in the tropical montane rain forest of southern Ecuador and analysed possible triggering factors. Two hypotheses were tested: (1) Interspecific synchronization of flowering and fruiting phenology is higher at study sites with pronounced rainfall seasonality compared with sites within perhumid forests. (2) Proximate causes for flowering in closely situated seasonal and perhumid sites are either photoperiodicity or climatic factors. Two nearby study sites with contrasting precipitation patterns were selected at the same altitude east and west of the western Cordillera. Eighty trees from 13 species were observed over a 2.5-y period. Three species were common to both study sites. Phenological and climate data were collected and cross-correlated by conducting a time-series analysis. At the perhumid site, very clear annual patterns of phenological behaviour could be observed for most of the selected rain-forest tree species, but with a poor interspecific synchronization. On the nearby seasonal site in contrast, most species showed high synchrony in their phenological behaviour coinciding with the dry season. There is strong evidence that flowering is induced not by one factor alone: we identified photoperiodic control, radiation and precipitation as possible proximate causes for both sites. Our results confirm studies which state that these factors are closely linked to each other in the tropics.

Kabuli chickpea as a winter-sown crop in northern Syria: moisture relations and crop productivity

In the Mediterranean region, chickpeas are traditionally spring-sown since humid winter conditions encourage the development of Aschochyta blight which often causes complete crop failure. Lines resistant to this blight have been selected at the International Centre for Agricultural Research in Dry Areas (ICARDA) and thus winter planting has become feasible. The best selection, ILC 482, was sown in both winter and spring at three locations in northern Syria with contrasting precipitation patterns. Crop growth and soil moisture analyses were undertaken on all treatments, and relevant meteorological data were collected at each location. At all locations maximum greenarea and dry-matter production of the winter-sown crop was nearly double that of the spring sowing. Large differences were also observed between sites, with green-area and dry-matter production decreasing with precipitation. Depth of profile recharge, amount of extractable moisture and crop evapotranspiration also decreased with precipitation, but only small differences in these moisture variables were observed between winter and spring sowing. Duration of green-area production, as determined by the onset of rapid leaf senescence, was closely related to the fraction of extractable moisture in the soil profile, rapid senescence occurring in all treatments when extractable moisture fell below 40% of its maximum value. Differences in the maximum rates of green-area production are discussed both in relation to the depth of profile recharge, hence the depth of rooting and moisture extraction and to the ambient evaporative demand. The differences observed in green-area and dry-matter production was clearly reflected in final seed yield. The components of yield, number of pods, percentage of empty pods and seed size are related to the variations in moisture stress experienced by the crop.