Ombrothermic Interpretation of Range Plant Water Deficiency from Temperature and
Precipitation Data at the Ranch Headquarters of the
Dickinson Research Extension Center in Western North Dakota, 1982-1999

Llewellyn L. Manske PhD Range Scientist
Tables and Figures Compiled by Sheri Schneider
North Dakota State University
Dickinson Research Extension Center

Introduction

Plant growth and development are controlled by internal regulators that are modified according to environmental conditions. Length of daylight, temperature, precipitation, seasonal precipitation pattern, soil moisture, and evaporation are the environmental factors affecting plant growth in a region. Ecologically, the three most important environmental factors that affect rangeland plant growth are light, temperature, and water (precipitation). Native vegetation and naturalized plants function as meteorologic instruments capable of measuring all the integrated climatic factors. The type of vegetation in a region is a result of the total effect of the long-term climatic factors for that region. Plant communities experience annual dynamic changes in response to annual climatic variability. Successful management of the grassland ecosystems of a region requires knowledge of the climatic factorsí effect on plant growth and of the relationships between plant growth and the climate of a region. This paper will attempt to describe the most important climatic factors at the Ranch Headquarters of the Dickinson Research Extension Center (DREC) in western North Dakota by using historical weather data and to point out some of the conditions and variables that limit plant growth. These factors should be considered during the development of long-term grassland management strategies.

Study Area

The study area is the region around the Ranch Headquarters of the Dickinson Research Extension Center (DREC), Dunn County, western North Dakota, USA.

Native vegetation in western North Dakota is the Wheatgrass-Needlegrass Type (Barker and Whitman 1988, Shiflet 1994) of the mixed grass prairie.

Methods

Temperature and precipitation data were taken from historical climatological data collected from June 1981 through December 1999 at the Ranch Headquarters of the Dickinson Research Extension Center, latitude 47E 14' N, longitude 102E 50' W, Dunn County, near Manning, North Dakota.

A technique reported by Emberger et al. (1963) was used to develop water deficiency months data from historical temperature and precipitation data. The water deficiency months data were used to identify months with conditions unfavorable for plant growth. This method plots mean monthly temperature (oC) and monthly precipitation (mm) on the same axis, with the scale of the precipitation data at twice that of the temperature data. The temperature and precipitation data are plotted against an axis of time. The resulting ombrothermic diagram shows general monthly trends and identifies months with conditions unfavorable for plant growth. Water deficiency conditions exist during months when the precipitation data bar drops below the temperature data curve and plants are under water stress. Plants are under temperature stress when the temperature curve drops below the freezing mark (0oC).

Definition of Terms

Drought conditions exist when precipitation amounts for a month, growing season, or annual period are 75% or less of the long-term mean. Wet conditions exist when precipitation amounts for a month, growing season, or annual period are 125% or greater of the long-term mean. Normal conditions exist when precipitation amounts for a month, growing season, or annual period are greater than 75% and less than 125% of the long-term mean. Water stress occurs in plants when the rate at which water is lost through transpiration exceeds the rate at which water is replaced by absorption. A water deficiency period exists when the amount of rainfall is lower than potential evapotranspiration demand. The freeze-free period is the number of days between the average date of the last occurrence of 32oF or lower in the spring and the average date of the first occurrence of 32oF or lower in the fall and is approximately the length of growing season for annually seeded plants. The growing season for perennial plants roughly coincides with the period between the first 5 consecutive days in spring and the last 5 consecutive days in fall with mean daily temperature at or above 32oF (0oC).

Results and Discussion

Temperature

Temperature is an approximate measurement of the heat energy available from solar radiation. At both low and high levels temperature limits plant growth. Most plant biological activity and growth occur within only a narrow range of temperatures, between 32EF (0EC) and 122EF (50EC). Low temperatures limit biological reactions because water becomes unavailable when it is frozen and because levels of available energy are inadequate. However, respiration and photosynthesis can continue slowly at temperatures well below 32EF if plants are "hardened". High temperatures limit biological reactions because the complex structures of proteins are disrupted or denatured.

Periods with temperatures within the range for optimum plant growth are very limited in western North Dakota. The frost-free period is the number of days between the last day with minimum temperatures below 32oF (0oC) in the spring and the first day with minimum temperatures below 32oF (0oC) in the fall and is approximately the length of the growing season for annually seeded plants. The frost-free period for western North Dakota generally lasts for 120 to 130 days, from mid to late May to mid to late September (Ramirez 1972). Perennial grassland plants are capable of growing for periods longer than the frost-free period, but to continue active growth they require temperatures above the level that freezes water in plant tissue and soil. Many perennial plants begin active growth more than 30 days before the last frost in spring and continue growth after the first frost in fall. The growing season for perennial plants is considered to be between the first 5 consecutive days in spring and the last 5 consecutive days in fall with mean daily temperature at or above 32EF(0EC). In western North Dakota the growing season for perennial plants is considered to be generally from mid April through mid October. Low air temperature during the early and late portions of the growing season greatly limits plant growth rate. High temperatures, high evaporation rates, drying winds, and low precipitation levels after mid summer also limit plant growth.

The Ranch Headquarters of DREC in western North Dakota experience severe, windy, dry winters with little snow accumulation. The springs are relatively moist in most years, and the summers are often droughty but are interrupted periodically by thunderstorms. The long-term (18-year) mean annual temperature is 43.9EF (5.7oC) (Table 1). January is the coldest month, with a mean temperature of 13.4oF (-10.3oC). July and August are the warmest months, with mean temperatures of 73.0oF (20.4oC) and 72.6oF (20.3oC), respectively. Months with mean monthly temperatures below 32.0oF (0.0oC) are too cold for active plant growth. Low temperatures define the growing season for perennial plants, which is generally from mid April to mid October (6.0 months, 183 days). During the other 6 months each year, plants in western North Dakota cannot conduct active plant growth because of low temperatures. Soils are frozen to a depth of 3 to 5 feet for a period of 4 months (121 days) (Larson et al. 1968). The early and late portions of the 6- month growing season have very limited plant activity and growth. The period of active plant growth is generally 5.5 months (168 days).

Precipitation

Water (precipitation) is essential for all plants and is an integral part of living systems. Plant water stress limits growth. Plant water stress develops in plant tissue when the rate of water loss through transpiration exceeds the rate of water absorption by the roots. Water stress can vary in degree from a small decrease in water potential, as in midday wilting on warm, clear days, to the lethal limit of desiccation (Brown 1995).

Early stages of water stress slow shoot and leaf growth. Leaves show signs of wilting, folding, and discoloration. Tillering and new shoot development decrease. Root production may increase. Senescence of older leaves accelerates. Rates of cell wall formation, cell division, and protein synthesis decrease. As water stress increases, enzyme activity declines and the formation of necessary compounds slows or ceases. The

stomata begin to close; this reaction results in decreased rates of transpiration and photosynthesis. Rates of respiration and translocation decrease substantially with increases in water stress. When water stress becomes severe, most functions nearly or completely cease and serious damage occurs. Leaf and root mortality induced by water stress progresses from the tips to the crown. The rate of leaf and root mortality increases with increasing stress. Water stress can increase to a point that is lethal, resulting in damage from which the plant cannot recover. Plant death occurs when meristems become so dehydrated that cells cannot maintain cell turgidity and biochemical activity (Brown 1995).

The long-term (18-year) annual precipitation for the Ranch Headquarters of the Dickinson Research Extension Center in western North Dakota is 15.99 inches (406.19 mm). The long-term mean monthly precipitation is shown on Table 1. The growing-season precipitation (April to October) is 13.59 inches (345.19 mm) and is 85.00% of annual precipitation. June has the greatest monthly precipitation, at 3.06 inches (77.61 mm).

The seasonal distribution of precipitation (Table 2) shows the greatest amount of precipitation occurring in the spring (6.54 inches, 40.90%) and the least amount occurring in winter (1.54 inches, 9.63%). Total precipitation for the 5-month period of November through March averages less than 2.5 inches (15.00%). The precipitation received in the 3- month period of May, June, and July accounts for 48.41% of the annual precipitation (7.74 inches).

The annual and growing-season precipitation levels and percent of the long-term mean for 18 years (1982 to 1999) are shown in Table 3. Two drought years (11.11%) occurred between 1982-1999, with precipitation amounts of 75% or less of the long-term mean (Table 4). Three wet years (16.67%) had precipitation amounts of 125% or more of the long-term mean (Table 5). Annual precipitation amounts at normal levels, between 75% and 125% of the long-term mean, occurred during 13 years (72.22%) (Table 3). Two drought growing seasons (11.11%) occurred between 1982-1999 (Table 6). The area experienced 4 wet growing seasons (22.22%) (Table 7). Growing season precipitation amounts at normal levels, between 75% and 125% of the long-term mean, occurred during 12 years (66.67%) (Table 3). The 6-year period (1987-1992) was a long period with near-drought conditions. The average annual precipitation for these 6 years was 12.16 inches (308.86 mm), only 76.05% the long-term mean. The average growing- season precipitation for the 6-year period was 10.03 inches (254.76 mm), only 73.80% the long-term mean (Table 3).

Temperature and Precipitation

Temperature and precipitation act together to affect the physiological and ecological status of range plants. The biological situation of a plant at any time is determined by the balance between rainfall and potential evapotranspiration. The higher the temperature, the greater the rate of evapotranspiration and the greater the need for rainfall to maintain homeostasis. When the amount of rainfall received is less than potential evapotranspiration demand, a water deficiency exists. Under water deficiency conditions, plants are unable to absorb adequate water to match the transpiration rate, and plant water stress develops. Range plants have mechanisms that help reduce the damage from water stress, but some degree of reduction in herbage production occurs.

The ombrothermic graph technique reported by Emberger et al. (1963) was intended to identify monthly periods in which water deficiency conditions exist. This technique assumes that most plants experience some level of water stress during water deficiency periods. This technique is not sensitive enough to identify the degree of water stress experienced by plants or the level of long-term damage. This technique also cannot identify water deficiency periods shorter than 1 month because most temperature and precipitation data are collected and summarized by meteorologists on a monthly basis. This characteristic in the data set forces a default assumption that water deficiency conditions shorter than a month do not cause long-lasting negative effects and that short-term water stress causes minimal damage from which plants recover. It also assumes that stored soil water is adequate to compensate for plant transpiration losses during periods of water deficiency shorter than a month.

Monthly periods with water deficiency conditions are identified on the ombrothermic graphs when the precipitation data bar drops below the temperature data curve. On the ombrothermic graphs, periods during which plants are under low-temperature stress are indicated when the temperature curve drops below the freezing mark of 0.0o C (32.0o F). The long-term ombrothermic graph for the Ranch Headquarters (Fig. 1) shows near water deficiency conditions exist for the months of August, September, and October. This finding indicates that range plants generally may have a difficult time growing and accumulating herbage biomass during these 3 months. Favorable water relations occur during May, June, and July, a condition indicating that range plants should be able to grow and accumulate herbage biomass during these 3 months.

The ombrothermic relationships for the Ranch Headquarters of the Dickinson Research Extension Center in western North Dakota are shown for each month from June 1981 through December 1999 in Figure 2. The 18-year period (1982 to 1999) had a total of 108 months during the growing season. Of these growing-season months, 38.0 months had water deficiency conditions, which indicates that range plants were under water stress during 35.19% of the growing-season months (Fig. 2, Tables 8 and 9): this amounts to an average of 2.11 months during every 6.0-month growing season range plants have been limited in growth and herbage biomass accumulation because of water stress. The converse indicates that only 3.89 months of an average year have conditions in which plants can grow without water stress.

Most growing seasons have months with water deficiency conditions. In only 1 of the 18 years (Fig. 2, Table 8) did water deficiency conditions not occur in any of the six growing-season months. In each growing-season month of 1982, the amounts and distribution of the precipitation were adequate to prevent water stress in plants. Eleven years (61.11%) had water deficiency for 0.5 to 2.5 months during the growing season. Five years (27.78%) had water deficiency conditions for 3.0 to 4.0 months during the growing season. One year (5.56%), 1988, had water deficiency conditions for 5.0 months during the growing season. None of the 18 years had water deficiency conditions for all 6.0 months of the growing season (Fig. 2, Table 8). The 6-year period (1987-1992) was a long period with low precipitation; during this period, water deficiency conditions existed for an average of 3.1 months during each growing season, which amounts to 51.39% of this periodís growing-season months (Fig 2, Table 8).

May, June, and July are the 3 most important precipitation months and therefore constitute the primary period of production for range plant communities. May and June are the 2 most important months for dependable precipitation. Only 2 (11.11%) of the 18 years had water deficiency conditions during May, and 3 years (16.67%) had water deficiency conditions during June. None of the years had water deficiency conditions in both May and June. Seven (38.89%) of the 18 years had water deficiency conditions in July (Table 9).

Most of the growth in range plants occurs in May, June, and July (Goetz 1963, Manske 1994). Peak aboveground herbage biomass production usually occurs during the last 10 days of July, a period that coincides with the time when plants have attained 100% of their growth in height (Manske 1994). Range grass growth coincides with the 3- month period of May, June, and July, when 48.4% of the annual precipitation occurs.

August, September, and October are not dependable for positive water relations. August and September had water deficiency conditions in 44.44% and 66.67% of the years, respectively, and October had water deficiency conditions in 38.89% of the years (Table 9). August 1996 had only 0.01 inches of precipitation greater than the amount that would have technically classified the month as water deficient. Visual observations of range grasses with wilted, senescent leaves in August indicate that most plants experience some level of water stress when conditions approach those of water deficiency. August, September, and/or October had water deficiency conditions during 89% of the growing seasons in the previous 18 years (Fig 2, Table 8). These 3 months make up 42% of the growing season, and they had water deficiency conditions on the average of 50.00% of the time (Table 9). The water relations in August, September, and October limit range plant growth and herbage biomass accumulation.

Water Stress

Water stress in plants occurs during water deficiency periods, which develop when the amount of rainfall is less than evapotranspiration demand. Water stress in range plants can be minor to severe and can last from a few hours to several years. Periods in which 75% or less of the long-term mean precipitation is received are called droughts. Drought conditions are traditionally considered to be long periods, i.e. 12 months for a full year or 6 months for a complete growing season, but water deficiency periods of 1 month are long enough to limit herbage production greatly and warrant consideration and recognition.

Over the last 18 years, drought years occurred 11.1% of the time. Drought growing seasons occurred 11.1% of the time. Water deficiency months occurred 35.19% of the time. Water deficiency occurred in the months of May and June 11% and 17% of the time, respectively. July had water deficiency conditions less than 39% of the time. August, September, and October had water deficiency conditions more than 50% of the time. Water deficiency periods lasting for a month place plants under water stress severe enough to reduce herbage biomass production. These levels of water stress are a major factor limiting the quantity and quality of plant growth in the Ranch Headquarters region and can limit livestock production if not considered during the development and implementation of long-term grazing management strategies.

Light

Light is necessary for plant growth because light is the source of energy for photosynthesis. Plant growth is affected by variations in quality, intensity, and duration of light. The quality of light (wavelength) varies from region to region, but the quality of sunlight does not vary enough in a given region to have an important differential effect on the rate of photosynthesis. However, the intensity (measurable energy) and duration (length of day) of sunlight change with the seasons and affect plant growth. Light intensity varies greatly with the season and with the time of day because of changes in the angle of incidence of the sunís rays and the distance light travels through the atmosphere. Light intensity also varies with the amount of humidity and cloud cover because atmospheric moisture absorbs and scatters light rays.

The greatest variation in intensity of light received by range plants results from the various degrees of shading from other plants. Most range plants require full sunlight or very high levels of sunlight for best growth. Shading from other plants reduces the intensity of light that reaches the lower leaves of an individual plant. Grass leaves grown under shaded conditions become longer but narrower, thinner (Langer 1972, Weier et al. 1974), and lower in weight than leaves in sunlight (Langer 1972). Shaded leaves have a reduced rate of photosynthesis, which decreases the carbohydrate supply and causes a reduction in growth rate of leaves and roots (Langer 1972). Shading increases the rate of senescence in lower, older leaves. Accumulation of standing dead leaves ties up carbon and nitrogen. Decomposition of leaf material through microbial activity can take place only after the leaves have made contact with the soil. Standing dead material not in contact with the soil does not decompose but breaks down slowly as a result of leaching and weathering. Under ungrazed treatments the dead leaves remain standing for several years, slowing nutrient cycles, restricting nutrient supply, and reducing soil microorganism activity in the top 12 inches of soil. Standing dead leaves shade early leaf growth in spring and therefore slow the rate of

growth and reduce leaf area. Long-term effects of shading, such as that occurring in ungrazed grasslands and under shrubs or leafy spurge, reduce the native grass species composition and increase composition of shade-tolerant or shade-adapted replacement species like smooth bromegrass and Kentucky bluegrass.

Day-length period (photoperiod) is one of the most dependable cues by which plants time their activities in temperate zones. Day-length period for a given date and locality remains the same from year to year. Changes in the photoperiod function as the timer or trigger that activates or stops physiological processes bringing about growth and flowering of plants and that starts the process of hardening for resistance to low temperatures in fall and winter. Sensory receptors, specially pigmented areas in the buds or leaves of a plant, detect day length and night length and can activate one or more hormone and enzyme systems that bring about physiological responses (Odum 1971, Daubenmire 1974, Barbour et al. 1987).

The tilt of the earthís axis in conjunction with the earthís annual revolution around the sun produces the seasons and changes the length of daylight in temperate zones. The phenological development of rangeland plants is triggered by changes in the length of daylight. Vegetative growth is triggered by photoperiod and temperature (Langer 1972, Dahl 1995), and reproductive initiation is triggered primarily by photoperiod (Roberts 1939,

Langer 1972, Leopold and Kriedemann 1975, Dahl 1995) but can be slightly modified by temperature and precipitation (McMillan 1957, Leopold and Kriedemann 1975, Dahl and Hyder 1977, Dahl 1995). Some plants are long-day plants and others are short-day plants. Long-day plants reach the flower phenological stage after exposure to a critical photoperiod and during the period of increasing daylight between mid April and mid June. Generally, most cool-season plants with the C3 photosynthetic pathway are long-day plants and reach flower phenophase before 21 June. Short-day plants are induced into flowering by day lengths that are shorter than a critical length and that occur during the period of decreasing day length after mid June. Short-day plants are technically responding to the increase in the length of the night period rather than to the decrease in the day length (Weier et al. 1974, Leopold and Kriedemann 1975). Generally, most warm-season plants with the C4 photosynthetic pathway are short-day plants and reach flower phenophase after 21 June.

The annual pattern in the change in daylight duration follows the calendar and is the same every year for each region. Grassland management strategies based on phenological growth stages of the major grasses can be planned by calendar date after the relationships between phenological stage of growth of the major grasses and time of season have been determined for a region with consideration of a possible variation of about Ī 7 days to accommodate annual potential modification from temperature and precipitation (Manske 1980).

Conclusion

The vegetation in a region is a result of the total effect of the long-term climatic factors for that region. Ecologically, the three most important environmental factors that affect rangeland plant growth are light, temperature, and water (precipitation).

Plant growth is limited by both low and high temperatures and occurs within only a narrow range of temperatures, between 32E and 122E F. Perennial plants have a 6-month growing season, between mid April and mid October. Diurnal temperature fluctuations of warm days and cool nights are beneficial for plant growth. Cool-season plants have lower optimum temperatures for photosynthesis than do warm-season plants, and cool-season plants do not use water as efficiently as do warm-season plants. Temperature affects evaporation rates, which has a dynamic effect on the annual ratios of cool-season to warm-season plants in the plant communities. A mixture of cool- and warm-season plants is highly desirable because the the grass species in a mixture of cool- and warm-season species have a wide range of different optimum temperatures and the herbage biomass production is more stable over wide variations in seasonal temperatures.

Water is essential for living systems. Average annual precipitation is 16.0 inches at the Ranch Headquarters, with 85.0% occurring during the growing season and 48.4% occurring in May, June, and July. Plant water stress occurs when the rate of water loss through transpiration exceeds the rate of replacement by absorption. Years with drought conditions have occurred 11.1% of the time during the past 18 years. Growing seasons with drought conditions have occurred 11.1% of the time.

Water deficiencies exist when the amount of rainfall received is less than evapotranspiration demand. Temperature and precipitation data can be used in ombrothermic graphs to identify monthly periods with water deficiencies. During the past 18 years, 35.2% of the growing-season months had water deficiency conditions that placed range plants under water stress: range plants were limited in growth and herbage biomass accumulation for an average of 2.10 months during every 6-month growing season. May, June, and July had water deficiency conditions 11.11%, 16.67%, and 38.89% of the time, respectively. August, September, and October had water deficiency conditions 44.44%, 66.67% and 38.89% of the time, respectively. One month with water deficiency conditions causes plants to experience water stress severe enough to reduce herbage biomass production.

Light is the most important ecological factor because it is necessary for photosynthesis. Changes in time of year and time of day coincide with changes in the angle of incidence of the sunís rays; these changes cause variations in light intensity. Daylight duration oscillation for each region is the same every year and changes with the calendar. Shading of sunlight by cloud cover and from other plants affects plant growth. Day-length period is important to plant growth because it functions as a trigger to physiological processes in plants. Most cool-season plants reach flower phenophase between mid May and mid June. Most warm-season plants flower between mid June and mid September.

Most of the growth in range grasses occurs in May, June, and July. In western North Dakota, 100% of range grass leaf growth in height and 91% to 100% of range flower stalk growth in height are completed by 30 July. Peak aboveground herbage biomass production usually occurs during the last 10 days of July, a period that coincides with the time during which plants are attaining 100% of their height. Most range grass growth occurs during the 3-month period of May, June, and July, when 48.4% of the annual precipitation occurs.

Grassland management should be based on phenological growth stages of the major grasses and can be planned by calendar date. Management strategies for a region should consider the environmental factors that affect and limit range plant growth.

Acknowledgment

I am grateful to Sheri Schneider for assistance in processing the weather data, compilation of the tables and figures, and word processing this manuscript. I am grateful to Amy M. Kraus and Naomi J. Thorson for assistance in preparation of this manuscript.

Report Tables and Figures
(click on Table or Figure name)

Table 1. Long-term (1892-1999) mean monthly temperature and monthly precipitation at Dickinson, ND.

Table 2. Seasonal percentage of mean annual precipitation distribution (1892-1999).

Table 3. Precipitation for total year and growing season and percent of long-term mean (LTM) for Ranch Headquarters DREC weather data, (1982-1999).

Table 4. Years with precipitation amounts for 12 months with 75% or less of the long-term mean (LTM).

Table 5. Years with precipitation amounts for 12 months with 125% or more of the long-term mean (LTM).

Table 6. Years with precipitation amounts for the growing season with 75% or less of the long-term mean (LTM).

Table 7. Years with precipitation amounts for the growing season with 125% or more of the long-term mean (LTM).

Table 8. Months when temperature and precipitation conditions caused water stress for perennial plants (1982-1989, 1990-1999).

Table 9. Months when temperature and precipitation conditions caused water stress for perennial plants for 108 years (1892-1999).

Fig. 1. Ombrothermic diagram of long-term (1982-1999) mean monthly temperature and monthly precipitation at Dickinson, North Dakota.

Fig 2a. Ombrothermic diagram of 1981-1989 mean monthly temperature and monthly precipitation at Ranch Headquarters, Dickinson, North Dakota.

Fig 2b. Ombrothermic diagram of 1990-1999 mean monthly temperature and monthly precipitation at Ranch Headquarters, Dickinson, North Dakota.

 

Literature Cited

Barbour, M.G., J.H. Burk, and W.D. Pitts. 1987. Terrestrial plant ecology. The Benjamin/Cummings Publishing Co., CA. 634p.

Barker, W.T., and W.C. Whitman. 1988. Vegetation of the Northern Great Plains. Rangelands 10:266-272.

Brown, R.W. 1995. The water relations of range plants: adaptations to water deficits. Pages 291-413. in D.J. Bedunah and R.E. Sosebee (eds.). Wildland plants: physiological ecology and developmental morphology. Society for Range Management. Denver, CO.

Dahl, B.E., and D.N. Hyder. 1977. Developmental morphology and management implications. Pages 257-290. in R.E. Sosebee (ed.). Rangeland plant physiology. Range Science Series No. 4. Society for Range Management. Denver, CO.

Dahl, B.E. 1995. Developmental morphology of plants. Pages 22-58. in D.J. Bedunah and R.E. Sosebee (eds.). Wildland plants: physiological ecology and developmental morphology. Society for Range Management. Denver, CO.

Daubenmire, R.F. 1974. Plants and Environment. John Wiley and Sons, New York, NY. 422p.

Dickinson Research Center. 1981-1999. Temperature and precipitation weather data.

Emberger, C., H. Gaussen, M. Kassas, and A. dePhilippis. 1963. Bioclimatic map of the Mediterranean Zone, explanatory notes. UNESCO-FAO. Paris. 58p.

Goetz, H. 1963. Growth and development of native range plants in the mixed grass prairie of western North Dakota. M.S. Thesis, North Dakota State University, Fargo, ND. 165p.

Langer, R.H.M. 1972. How grasses grow. Edward Arnold, London, Great Britain. 60p.

Leopold, A.C., and P.E. Kriedemann. 1975. Plant growth and development. McGraw-Hill Book Co., New York, NY. 545p.

Larson, K.E., A.F. Bahr, W. Freymiller, R. Kukowski, D. Opdahl, H. Stoner, P.K. Weiser, D. Patterson, and O. Olson. 1968. Soil survey of Stark County, North Dakota. U.S. Government Printing Office, Washington, DC. 116p.+ plates.

Manske, L.L. 1980. Habitat, phenology, and growth of selected sandhills range plants. Ph.D. Thesis, North Dakota State University, Fargo, ND. 154p.

Manske, L.L. 1994. Problems to consider when implementing grazing management practices in the Northern Great Plains. NDSU Dickinson Research Extension Center. Range Management Report DREC 94-1005, Dickinson, ND. 11p.

McMillan, C. 1957. Nature of the plant community. III. Flowering behavior within two grassland communities under reciprocal transplanting. American Journal of Botany 44 (2): 144-153.

Odum, E.P. 1971. Fundamentals of ecology. W.B. Saunders Company, Philadelphia, PA. 574p.

Ramirez, J.M. 1972. The agroclimatology of North Dakota, Part 1. Air temperature and growing degree days. Extension Bulletin No. 15. North Dakota State University, Fargo, ND. 44p.

Roberts, R.M. 1939. Further studies of the effects of temperature and other environmental factors upon the photoperiodic response of plants. Journal of Agricultural Research 59(9): 699-709.

Shiflet, T.N. (ed.). 1994. Rangeland cover types. Society for Range Management, Denver, CO. 152p.

Weier, T.E., C.R. Stocking, and M.G. Barbour. 1974. Botany: an introduction to plant biology. John Wiley and Sons, New York, NY. 693p.