Ecophysiology of Grasses in Semi Arid Areas

Michael A. Smith, Professor and Extension Specialist, Renewable Resources

Grasses exhibit two photosynthetic pathways, C-3 and C-4, respectively referred to as cool season and warm season that respond differently to temperatures and moisture conditions. The majority of the following discussion focuses on cool season species constituting the majority of grass species occurring in Wyoming and much of the northern Great Plains and central/northern Rocky Mountain region. Common C-3, cool season grasses include wheatgrasses, needlegrasses, bluegrasses, and fescues. Common C-4, warm season grasses are blue grama, little and big bluestem, and prairie sandreed.

Grasses begin growth in spring when soil and daytime temperatures reach thresholds that trigger initiation of growth if soil moisture is available. the threshold temperature for growth initiation is somewhat species specific, but generally lower for cool season compared to warm season species. Growth involves the capture of solar energy by leaves and the production of carbohydrates, including structural materials, and proteins. A basic principle of growth is that sufficient leaf material must be present to capture sunlight. Initiation of growth is from stored energy but most growth is based on photosynthates produced in leaves nearest the growth meristems including the stem nodes and base of leaf sheaths and blades. Optimization of plant growth occurs when maximum leaf area is exposed to sunlight and shading of leaves is minimized. In semi arid environments shading can occur from shrubs or standing litter from previous season’s growth. Vegetative leaves and litter are important for protecting the soil from erosive raindrop impact and runoff and reducing surface wind velocity. Managing the residual vegetation amount (stubble height) can be a balance between soil protection needs and minimizing shading, but on non irrigated rangelands shading is a minimal issue except in ungrazed bunchgrasses such as bluebunch wheatgrass. Optimal leaf area maximizes growth and available animal forage while providing cover for protecting the soil surface.

The available moisture rather than soil fertility generally limits the amount of plant growth potential. Residual moisture from fall and winter usually results in a modest fairly consistent amount of plant growth. Subsequent moisture inputs generally determine total growth amount and the length of the green period. Data from near Saratoga, WY, 10-14" ppt. zone and 7200' elevation, indicates maximum growth is achieved when higher precipitation levels occur in April although May precipitation will add to growth potential. Plant maturation, full phenological development expressed by seed stalk production, occurs from near the end of May through late June in most plains environments of Wyoming. Subsequent growth after maturation of these C-3 grasses is negligible although adequate soil moisture maintains green leaves and forage quality later in the season. Mid summer precipitation may stimulate a regreening of plant material not already scenesed, but is unlikely to stimulate regrowth if the leaves have cured. Cooler temperatures and precipitation may cause limited leaf growth in fall. C-3 grasses have adapted a growth pattern to match the typical peak precipitation and cool temperatures of spring and have little response to the typically low and erratic precipitation of summer.

C-4 grasses are adapted to warmer climates with summer precipitation. C-4 plant growth may be initiated anytime adequate soil moisture or precipitation occurs during the warm season, even after extended dry periods. Research has shown that once C-4 plants have scenesed, at least 0.75 inches of precipitation in a short period of a 1-2 days is necessary to reinitiate growth. storms of a half inch during the summer help keep grass green and nutritious but account for little new production.

Drought and grazing can impact plant growth potential although severity of these factors is the primary determinant of severity of impact. In contrast to southern and southwestern regions of the US, northern regions seldom have a year or multi year droughts of such severity that some growth is not evident in spring due to water stored in soil from fall or winter precipitation. Moderate drought, meaning at least some winter or spring precipitation, has not been shown to affect the growth potential in subsequent years of predominantly C-3 plant communities with or without moderate grazing. Grazing has been shown to have a distinctly seasonal effect. Spring grazing before seed stalk elongation or grazing after seed production have little effect on subsequent year’s plant health or growth potential. Grazing during seed stalk growth will reduce subsequent year’s plant productivity when viewed at the single plant level. Moderate use at this time period in a plant community would affect various plant species differently depending on their phenology and would not affect every plant as many are not grazed. Grazing use before or after seed stalk elongation only reduces the standing crop of that season by the amount used. The degree of grazing use may impact soil surface protection and has been shown to reduce animal performance at heavier levels.

A prudent grazing manager might adopt a grazing strategy that varies the season of grazing to avoid using a location at the same season every year, particularly during seed stalk elongation. Heavy and repeated grazing has been shown to reduce the competitiveness of desirable forage grasses in favor of less palatable or more grazing resistant species, but moderate levels of use especially when combined with a periodic rotation of the season of use has been successful in a wide range of environments in maintaining or improving rangeland conditions.