Automated Rainout Shelter Research Articles

Drought and grazing III: Root dynamics and germinable seed bank

Drought and herbivory frequently influence North American rangelands. While these influences may temporarily reduce vegetative cover, their mutual influence on the available seedbanks which might occupy new safe sites is unclear. We examine effects of drought and grazing upon pre- and post-drought plant root distribution and germinable seed bank to determine 1) if the response of root distributions to drought depends upon grazing use and 2) if the presence of germinable seeds is altered significantly by drought and grazing. Using twelve, 5 X 10 m non-weighing lysimeters with an automated rainout shelter, we documented root intercepts in situ using a minirhizotron from 1993-1996. Seed bank samples were incubated in a greenhouse to determine seedling emergence. Roots were fewer in shallow soil layers in grazed plots than ungrazed plots by the end of the study, irrespective of drought. Roots in deeper (Bw horizon) soil layers were fewer during drought, but were not influenced by grazing. Seed bank composition results suggest that perennial grasses were a small portion of the seed bank. Cool-season annual grass seeds accumulated after drought. Without drought, forb seed banks increased with grazing. Thus while shallow roots may decrease during drought, in the year following drought grazing may decrease aboveground net primary production, and allow large accumulations of cool-season annual grass seed in a northern mixed grass prairie.

Drought and Grazing: I. Effects on Quantity of Forage Produced

This research addresses the hypothesis that grazing intensity during and following drought can dramatically alter community level, post-drought recovery patterns. Research was conducted during the 1993 through 1996 growing seasons at the Fort Keogh Livestock and Range Research Laboratory located near Miles City, Mont. Study plots were twelve, 5 × 10-m non-weighing lysimeters constructed in 1992 on a gently sloping (4%) clayey range site. An automated rainout shelter was constructed to control the amount of precipitation received on 6 lysimeters during the 1992 growing season. We conclude from study results that the independent and combined effects of the imposed late spring to early fall drought and associated grazing treatments were minimal relative to soil water dynamics and aboveground net primary production although both grazing treatments reduced herbage standing crops. We attribute the absence of a strong response to the drought to its timing (i.e., late growing season) in that most herbage production in these cool-season dominated grasslands is completed by early summer. Thus, annual production processes in these grasslands avoided the major impacts of the drought. The results do not provide convincing evidence, however, that would lead us to completely reject our original hypothesis. Rather, they simply provide evidence that these grasslands are well adapted to surviving late growing season drought with or without intensive grazing by ungulates.

Black Bean Sensitivity to Water Stress at Various Growth Stages

Producers relying on the traditional central Great Plains cropping system of winter wheat (Triticum aestivum L.)-fallow could increase crop yield and resource sustainability by changing to more diverse crop rotations. Crops suited to the variable timing and amount of precipitation characteristic of this region need to be identified so that rotations can be diversified. This study was conducted to determine the effects of water deficit at various growth stages on leaf area index, plant height, yield components, soil water extraction pattern, and seasonal water use of black bean (Phaseolus vulgaris L.). The study was conducted during the 1995 and 1996 growing seasons near Akron, CO, on a Rago silt loam (fine, montrnorillonitic, mesic Pachic Argiustoll). An automated rainout shelter was used to eliminate precipitation. Irrigation treatments consisted of withholding water at vegetative, reproductive, or grain-filling growth stages. All treatments received 18.3 cm of water during the growing season. Water stress during the vegetative growth stage produced the shortest plants with the least leaf area. Soil water extraction and total water use were also least when water was withheld during the vegetative stage. Seed yield was reduced because of reductions in pods per plant and/or seeds per pod when water stress occurred during the reproductive stage. Black bean appears to have potential as a dryland rotation crop in the central Great Plains when soil water profiles are near field capacity at planting and normal to above normal precipitation is expected.

Carbon Isotope Dicrimination: Potential in Screening Cool‐Season Grasses for Water‐Limited Environments

Carbon isotope discrimination (∆) has been shown to be negatively related to water use efficiency (WUE) in several C3 crop species and has been proposed as a criterion to select for improved WUE in plant breeding programs. This study was conducted to determine if ∆ is related to WUE in crested wheatgrass [Agropyron desertorum (Fischer ex Link) Schultes] and AItai wildrye [Leymus angustus (Trin.) Pilger] and to evaluate the influence of drought on the magnitude of genetic variability and broad‐sense heritability of ∆ among clones of crested wheatgrass. In a greenhouse pot experiment, ∆ was negatively related to WUE in crested wheatgrass (r = −0.73**, significant at P < 0.01) and Altai wildrye (r −0.81**). In field studies, a line‐source sprinkler system operated in conjunction with an automated rainout shelter imposed a continuously variable water application on 29 vegetatively propagated clones of crested wheatgrass. Three water levels along the water application gradient were selected for sampling and forage dry weights and ∆ values were determined for two harvests in 1986 and one harvest in 1987. In the field experiment, ∆ declined as less water was applied and was positively associated with forage yield at each of the three harvests (r = 0.59**, 0.44**, and 0.80**, respectively). Significant variability for ∆ was detected among the clones for the two peak‐season harvests, hut not for the late‐season harvest in 1986. Broad‐sense heritability for ∆ was 0.49 in the analysis of data combined over harvests and water levels. Genetic variances and broad‐sense heritabilities (H) for ∆ were generally greater at the high (H = 0.52) and intermediate (H = 0.52) water applications than at the low water application (H = 0.37), and coefficients of variation for ∆ were typically less than 4.5%. Selection for ∆ appears to be a promising screening tool for the development of crested wheatgrass populations that are productive and make efficient use of limited water resources.

Infrared Thermometry to Measure single Leaf Temperatures for Quantification of Water Stress in Sunflower

AbstractQuantification of water stress by infrared thermometry is routinely done on complete plant canopies to minimize the influence of viewed soil surface. When plants are small or widely spaced, canopies may not be complete. This study was conducted to determine if Crop Water Stress Index (CWSI) values computed from infrared thermometer measurements of single‐leaf temperatures of sunflower [Helianthus annuus (L.) ‘Triumph 566DW’] were correlated with other measures of plant water stress. Plants were grown under an automated rainout shelter and subjected to four water treatments ranging from 0.0 to 100% replacement of evapotranspirational losses. The soil type is a Rago silt loam (fine, montmorillonitic, mesic Pachic Argiustoll). Statistically significant correlations were found between CWSI calculated from single‐leaf temperatures and stomatal conductance, CO2 exchange rate, leaf water potential, transpiration rate, and percent available water in the active root zone. Calculating CWSI from single‐leaf temperatures measured with an infrared thermometer can provide a rapid means of assessing plant water status in incomplete canopies.

WATER STRESS AND GENOTYPIC EFFECTS ON EPICUTICULAR WAX PRODUCTION OF ALFALFA AND CRESTED WHEATGRASS IN RELATION TO YIELD AND EXCISED LEAF WATER LOSS RATE

Epicuticular wax production has been found to be associated with higher seed yield of drought-stressed crops. The present study was conducted to determine the relationship between epicuticular wax and forage yield in two range plant species. A line-source sprinkler system under an automated rainout shelter imposed an irrigation water gradient on 30 clones of alfalfa (Medicago sativa L.) from two source populations and 30 clones of crested wheatgrass (Agropyron desertorum [Fisch. ex Link] Schult.). Leaf epicuticular wax was determined gravimetrically at three contrasting water levels. Epicuticular wax production increased for all clones as water application declined in all three harvests for alfalfa and in the first harvest for crested wheatgrass. At the low water level, clones of creeping-rooted Spredor 2 alfalfa produced more epicuticular wax per unit leaf area than noncreeping-rooted NC-83-1 clones. Within the three water levels, epicuticular wax production and yield were not correlated in alfalfa for any harvest. Yield and epicuticular wax were negatively correlated at the medium and low water levels for crested wheatgrass at Harvest 1 and positively correlated at the high water level for Harvest 2 (aftermath). No consistent relationship was found between epicuticular wax production and cuticular transpiration as estimated by water loss rate of excised leaves of alfalfa and crested wheatgrass.Key words: Medicago sativa, Agropyron desertorum, line-source sprinkler, drought, cuticular transpiration

Automated Rainout Shelter for Controlled Water Research

Automated Rainout Shelter for Controlled Water Research