|
2002 Annual Report Grassland Section |
Dickinson
Research Extension Center
1089 State Avenue Dickinson, ND 58601 |
Environmental
Factors that Affect Range Plant Growth, 1892-2001
Llewellyn
L. Manske PhD
Range Scientist
North
Dakota State University
Dickinson Research
Extension Center
Environmental factors affect
range plant growth. The three most ecologically important environmental factors
affecting rangeland plant growth are light, temperature, and water (precipitation).
Plant growth and development are controlled by internal regulators that are
modified according to environmental conditions. A research project was conducted
to describe the three most important environmental factors in western North
Dakota and to identify some of the conditions and variables that limit range
plant growth. Rangeland managers should consider these factors during the development
of long-term management strategies (Manske 2002).
Light is the most important
ecological factor affecting plant growth. Light is necessary for photosynthesis,
and changes in day length (photoperiod) regulate the phenological development
of rangeland plants. Changes in the day length function as the timer or trigger
that activates or stops physiological processes initiating growth and flowering
and that starts the process of hardening for resistance to low temperatures
in fall and winter. The tilt of the earth's axis in conjunction with the earth's
annual revolution around the sun produces the seasons and changes the length
of daylight in temperate zones. Dickinson (Fig. 1) has nearly
uniform day and night lengths (12 hours) during only a few days, near the vernal
and autumnal equinoxes, 20 March and 22 September, respectively, when the sun's
apparent path crosses the equator as the sun travels north or south, respectively.
The shortest day length (8 hours, 23 minutes) occurs at winter solstice, 21
December, when the sun's apparent path is farthest south of the equator. The
longest day length (15 hours, 52 minutes) occurs at summer solstice, 21 June,
when the sun's apparent path is farthest north of the equator. The length of
daylight changes during the growing season, increasing from about 13 hours in
mid April to nearly 16 hours in mid June, then decreasing to around 11 hours
in mid October (Fig. 1).
Temperature, an approximate
measurement of the heat energy available from solar radiation, is a significant
factor because both low and high temperatures limit plant growth. Most plant
biological activity and growth occur within only a narrow range of temperatures,
between 32oF (0oC) and 122oF (50oC).
The long-term (110-year) mean annual temperature in the Dickinson, North Dakota,
area is 40.8oF (4.9oC) (Table 1). January
is the coldest month, with a mean temperature of 11.0oF (-11.7oC).
July and August are the warmest months, with mean temperatures of 68.6oF
(20.3oC) and 67.0oF (19.4oC), 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). Perennial grassland plants are capable of growing for 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. Winter dormancy
in perennial plants is not total inactivity but reduced activity.
Water (precipitation) is
essential for all plants and is an integral part of living systems. Water is
ecologically important because it is a major force in shaping climatic patterns
and biochemically important because it is a necessary component in physiological
processes. Plant water stress limits growth. Water stress can vary in degree
from a small decrease in water potential to the lethal limit of desiccation.
The long-term (110-year) annual precipitation for the area of Dickinson, North
Dakota, is 16.05 inches (407.58 mm). The growing season precipitation (April
to October) is 13.58 inches (344.93 mm), 84.61% of the annual precipitation.
June has the greatest monthly precipitation, at 3.58 inches (90.95 mm). The
seasonal distribution of precipitation (Table 2) shows the
greatest amount of precipitation occurring in the spring (7.33 inches, 45.67%)
and the smallest amount occurring in winter (1.53 inches, 9.50%). Total precipitation
received in November through March averages less than 2.5 inches (15.33%). The
precipitation received in May, June, and July accounts for 50.72% of the annual
precipitation (8.14 inches).
Of the past 110 years (1892
to 2001), 14 (12.73%) were drought years, receiving 75% or less of the long-term
mean precipitation level. Fifteen (13.64%) were wet years, receiving 125% or
more of the long-term mean precipitation level. Eighty-one years (73.64%) received
normal annual precipitation amounts, between 75% and 125% of the long-term mean.
Of the past 110 growing seasons, 17 (15.45%) were drought growing seasons, 20
(18.18%) were wet growing seasons, and 73 (66.36%) received precipitation at
normal levels.
Temperature and precipitation
act together to affect the physiological and ecological status of range plants.
The balance between rainfall and potential evapotranspiration determines a plant's
biological situation. When rainfall is lower than evapotranspiration demand,
a water deficiency exists. The ombrothermic graph technique (Emberger et al.
1963), which plots mean monthly temperature and monthly precipitation on the
same axis, was used to identify months with water deficiency conditions during
1892-2001 (Manske 2002). The long-term ombrothermic graph for the Dickinson
area (Fig. 2) shows near water deficiency conditions for August,
September, and October, a finding indicating that range plants generally may
have difficulty growing and accumulating biomass during these 3 months. Favorable
water relations occur during May, June, and July, a period during which range
plants should be capable of growing and accumulating herbage biomass.
Drought years occurred
during 12.7% of the past 110 years, and 15.5% of the growing seasons were drought
growing seasons. The 110-year period (1892 to 2001) contained a total of 660
growing- season months. Water deficiency conditions occurred
during 215 of these, a finding indicating that during 32.58% of the growing
season months, or for an average of 2.0 months during every 6.0-month growing
season, range plants were under water stress and therefore limited in growth
and herbage biomass accumulation. Water deficiency occurred in May and June
14.6% and 9.1 % of the time, respectively. Water deficiency conditions occurred
in July less than 40% of the time. Water deficiency conditions occurred in August,
September, and October more than 50% of the time: 50.9% of the time in August,
50.9 % of the time in September, and 49.1% of the time in October. Water deficiency
conditions lasting a month or more cause plants to experience water stress severe
enough to reduce herbage production. These levels of water stress are a major
factor limiting the quantity and quality of plant growth in western North Dakota
and can limit livestock production if not considered during the development
and implementation of long-term grazing management strategies.
Acknowledgment
I am grateful to Sheri Schneider for assistance in processing the weather data, compilation of the tables and figures, and production of this manuscript. I am grateful to Amy M. Kraus and Naomi J. Thorson for assistance in preparation of this manuscript.
Literature Cited
Emberger, C., H.
Gaussen, M. Kassas, and A. dePhilippis. 1963. Bioclimatic map of the
Mediterranean Zone,
explanatory notes. UNESCO-FAO. Paris. 58p.
Manske, L.L. 2002. Environmental factors to consider during planning of management for range plants in the Dickinson, North Dakota, region, 1892-2001. NDSU Dickinson Research Extension Center. Range Research Report DREC 02-1018e. Dickinson, ND. 37p.
Tables and Figures
Figure 1. Annual pattern of daylight duration at Dickinson, North Dakota.
Figure 2. Ombrothermic diagram of long-term (1892-2001) mean monthly temperature and monthly precipitation at Dickinson, North Dakota.
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