Turf Ecology and competition

The interaction between turfgrasses, their management, and the environment are complex and dynamic, and any ecological discussion concerning turfgrass communities is strongly influenced by these interactions.

Environments are dictated largely by climate, soil and geographic factors.

Unlike naturally occurring climaxes vegetation, turfgrasses persist only in settings managed by humans.

Human endeavor is an intrinsic part of the turfgrass ecosystem.

Turfgrass communities reflect intensive human management practices that control the natural tendencies of grasses.

Ecological aspects of establishing turfgrasses

The principal environmental factors that influence the establishment of turfgrasses are temperature, moisture, light, wind, edaphology (soil), and geography.

Soil and geographic factors are the most stable.

Climatic and environmental conditions are usually predictable for different seasons and locations.

Types of turfgrass communities desired

In choosing turfgrass species, consideration must be given to the species tolerance of climate, environmental stresses, potential uses, and anticipated cultural levels.

Based on climatic adaptation , turfgrass species have been placed into four categories; grasses adapted for cool humid region, warm humid region, cool arid region, and warm arid region (Ward, 1969).

Within a climatic region, species should be selected that are tolerant to the varying environmental stresses.

For example, terraces and slopes facing south are warmer and drier than those facing north. Species with heat and drought tolerance should be selected for South-facing slopes (McKee et al., 1965; Ward, 1969).

Tolerances to environmental stresses also very between cultivars of the same species.

Kentucky bluegrasses that have been selected from warmer rather than cooler regions are generally more tolerant of high temperatures because of lower nitrate nitrogen absorption and higher carbohydrate levels (Watschke et al., 1970).

Turfgrasses are, in essence, communities of plants that may be monostands, which consist of a single cultivar, or a polystands, which is composed of two or more cultivars of the same species (blends) or of different species (mixtures) (Beard, 1973).

Although polystand communities may not provide the uniform turf textural quality and growth responses of monostands, they do offered advantages.

Properly formulated polystand communities provide a wider range of environmental stress and pest tolerance than monostand communities.

Funk and Dickson (1981) indicated that performance of turfgrass polystands is determined by their relative cultivar composition, which may shift with time.

Grasses best adapted to their chosen environment eventually dominate the polystand community.

These authors reported that the cultivars Brunswick and Touchdown comprised 68% and 23% of plant population 7 yrs after being established in a blend of 38 Kentucky bluegrasses in New Jersey.

In a more recent study, Lickfeldt et al., (2002) planted a blend of 25% Blacksburg, 50% Midnight, and 25% Unique Kentucky bluegrass on a Illinois fairway and rough.

After a 2 years in the fairway and 1 year or in the rough the blend was measured to be approximately 14% Blacksburg, 46% Midnight, and 40% Unique Kentucky bluegrass in either site.

In highway slopes stabilization, it is necessary to alter species components to obtain rapid vegetative cover at various seasons to minimize erosion (Blazer, 1963; Blazer and Woodruff, 1968).

Botanical composition of polystands of perennial species is also influenced by the environment at planting time.

Hall (1980) found August through September and March through April seeding dates of tall fescue-Kentucky bluegrass mixtures enhance the tall fescue component when compared to October and November seeding dates.

Propagation sources

Propagation of turfgrass can be accomplished with seed or vegetative plant parts.

Seed is a most convenient propagating material in that it is easily stored and transported.

Vegetative planting materials, such as stolons, sprigs, plugs, and sod, must be used for propagating turfgrasses that do not produce viable seed or that cannot be a reproduce genotypically from seed.

The environment to which propagation material is exposed prior to establishment has an influence with its viability.

Seed is a living organism and continues to respire using its stored organic food.

Seed viability is affected principally by temperature and moisture conditions during storage.

Use the rule of 100 for best seed storage.

Temperature (F°) + relative humidity (%) = 100

Cool temperatures and low humidity conserve organic food enabling seed to remain viable for longer periods.

Vegetative propagules are more perishable than seed.

Propagules must be handle with more care than seed from harvest time to planting.

If these harvested plant parts are packed together, they may overheat and reduce storage life or ability to propagate (Darrah and Powell, 1977; King, 1970; King and Beard, 1972).

Temperature may be the most critical factor in storage of vegetative material because it not only increases respiration rate but also enhances microbial activity (Bailey, 1940).

Several factors have been associated with sod heating:

soil temperature at harvest
amount of leaf area present
N nutritional levels
soil moisture content (Beard, 1969, 1973)

Darrah and Powell (1977) concluded that the detrimental effects of temperature on stored sod can be reduced by harvesting early in the morning, removing excess clippings, and low N fertility.

Soil preparation

In preparing for planting turfgrasses, it is imperative that adequate surface and subsurface drainage are provided to ensure optimum soil conditions for establishment, and to allow for future maintenance of the site.

For rapid establishment of turfgrasses , proper soil pH and an adequate supply of soil nutrients are essential.

Many researchers have concluded that P application is especially important for rapid seedling development in new turfgrass stands.

To create a good nutrient environment for turfgrass species and to encourage deep rooting, it is recommended that lime and P be incorporated into the top 10 to 15 cm of soil ( Musser and Perkins, 1969; Beard , 1973 ; Turgeon, 1980).

King and Skogley, (1969) reported better establishment for a Kentucky bluegrass-creeping red fescue mixture was surface applied P rather than incorporated into the upper 10 cm of soil.

It has also been documented that N fertility improves turfgrass establishment (King and Skogley, 1969).

When steep slopes, such as highway corridor cut and fills, are to be establishment to turf species, the grading and construction procedures can alter the microenvironment.

A rough, loose and undulating soil surface will permit precipitation to infiltrate rapidly, thus conserving soil moisture and reducing water runoff and erosion (Wright et al. , 1978).

The roughness of the soil surface permits sloughing of loose soil that covers seed creates a more favorable moisture situation, which stimulates germination, seedling growth, and rate of vegetative cover (Perry et al., 1975).

Stair-step grading of steep slopes further encourages precipitation infiltration and impedes water surface movement to minimize erosion and sedimentation.

This grading technique helps create a favorable environment for establishing vegetation on harsh xeric conditions.

Soil contact with seed or vegetative propagules

Covering seed with soil moderates the temperature surrounding them, enhances moisture conditions for germination , and decreases injury due to drying (Musser and Perkins , 1969).

Ensuring good seed-soil contact through light rolling after seeding ensures more successful establishment by shortening germination time, which may allow seedling turf to become more competitive with germinating weed species.

If seeds are planted too deeply, the developing seedlings may exhaust carbohydrates within the seed endosperm and die before leaves emerge and manufacture photosynthate.

Also, seeds that require light for germination such as Kentucky bluegrass and zoysiagrass should not be planted too deeply.

Small seeded species should not be planted deeper than 6 to 12 mm, while species with large seeds may be covered to approximately 20 mm.

It is also imperative that vegetative propagules have good soil contact to ensure rapid rooting during establishment.

When sprigs are planted in rows, they should be lightly topdressed with soil, leaving about 50% of the leaf tissue above the surface (Beard , 1973).

Sprig-soil contact may be accomplished by topdressing or pressing the sprigs into the soil with a steel mat or a straight disk-like roller.

Mulching materials

One of the most effective cultural practices used to enhance turfgrass establishment is mulching.

Mulches reduce surface soil compaction resulting from the impact of raindrops or irrigation, reduces runoff and increases the infiltration rate.

Mulches conserve soil moisture by moderating soil temperature and reducing evaporation.

Barkley, et al. (1965) reported that straw mulch applied at 2800 kg ha^-1 lowered temperatures by 7°C and preserved an average of 35% more moisture in the upper inch of soil than when no mulch was used.

In regions of limited rainfall where supplemental irrigation is not available, cereal grains are often planted the season prior to establishing the desirable grasses.

After the cereal grain is harvested in the fall, a 15 to 20 cm stumble is left to enhance moisture retention during winter, and desirable permanent grasses are seeded into the stubble following spring.

Most mulching materials are applied to the soil surface and may be broadly classified as:

textured
chemical
covers (mesh, blankets, and tarps).

The best mulches for turf establishment are textured material such as straw, hay , pine needles, wood cellulose fiber, recycled paper, and shredded woodbark.

Chemical mulches such as polyvinyl, polyvinyl acetate, pectin and elastomeric polymer emulsions have been used in specialized situations (Hotenstein, 1969).

Barkley et al.(1965) reported that a dark-colored emulsion mulch used in mid-summer conserved moisture, but cause soil temperatures to rise unfavorably reducing seedling stands when compared to non-mulch treatment.

Perry et al. (1975 and Wright et al. (1975) reported that chemical binders and soil stabilizers did not control erosion, moderate temperatures, or conserve soil moisture as well as straw or wood cellulose fiber mulches.

Clear polyethylene covers with small holes on 10-cm centers to permit infiltration of water have been used to enhance microenvironment for seedling establishment of fine turf areas under suboptimal temperatures.

These covers limit evaporation and permit solar radiation to warm soil (Beard , 1973).

Because of heat buildup, plastic covers should not be used when temperatures are optimum or higher.

Irrigation

Once newly sown seeds have imbibed water they are prone injury from moisture stress.

Imbibition of water by seeds generally decreases as soil moisture tensions increase causing germination of most plant species to decrease.

Depending on weather conditions, supplemental irrigation will generally enhance turfgrass seedling development, even when mulched adequately.

Newly seeded or vegetatively propagated turf may benefit from a light irrigation several times a day to maintain adequate moisture near the soil surface until the turfgrass is well rooted into the soil (Marsh, 1969).

Pest control

Often areas scheduled for turfgrass establishment contained pests that potentially compete with the desired turfgrass species.

Pests such as weed seeds, undesirable vegetation, soil-borne disease organisms, insects, and nematodes may be need to be eliminated or controlled prior to the establishment of the turfgrasses.

Fumigants such as methyl bromide or chloropicrin have been used to control troublesome organisms.

Weeds are generally the pest that gives the most competition to turf seedlings.

Undesirable plant seeds and vegetative propagules remaining in the soil prior to planting may become dominant species.

In addition to fumigants, nonselective herbicides and selected premergence herbicides may be used to control infestations.

Seeding and sprigging rates of desirable turfgrass species influence the competition of weeds in newly established turf areas.

Brede and Duich (1981) reported less annual bluegrass encroachment when several Kentucky bluegrass were sown at 388 or 1033 rather than at 64 or 94 pure live seeds dm^-2.

Ecological aspects of mature turfgrass stands (from Turfgrass Management & Ecology, Danneberger, 1993)

Grime (1980) concluded that species, cultivars and populations can be classified according to their ecological strategies.

These ecological indicators include the distribution, life-history, phenology, reproductive biology, and physiology of turfgrass species.

The ecology of mature turfgrass communities, however, is strongly influenced by management. So much so, that turfgrass stands are adapted to environmental conditions through the cultural manipulation of species competition.

Population dynamics

Before any discussion on species competition, it is important to understand the population dynamics of a turfgrass community.

A turfgrass population is a group of plants of the same species occupying a given space or habitat.

Habitat is where the organism lives and is characterized by the environmental conditions present.

A turfgrass community is the population of all of the species, including plants and other organisms living in the same habitat.

Finally the system itself, called a turfgrass ecosystem, is all species living in a habitat and its subsequent interaction with the biotic and abiotic components of the environment.

A niche is the range of abiotic and biotic conditions in which a species can live and reproduce.

A fundamental niche is the entire range of resources and conditions under which an organism can survive.

In contrast a realized niche is considerably narrower than the fundamental and is the range of resource and conditions under which an organism can survive with competition.

If two competing organisms overlap in their realized niches and enough differences are present, the to species will coexist.

If the two competing organisms overlap in their realized niches, but differentiation is not present, one of the organisms will be driven from the habitat.

An example of one organism driven from the habitat is a turfgrass community composed of Kentucky bluegrass and annual bluegrass. If mowig height is lowered to a point were the Kentucky bluegrass was not competitively adapted (outside its realized niche) it would not survive in the presence of annual bluegrass.

Stability and complexity

Stability refers to the communities ability to resist change in the presence of a disturbance.

Stability is measured by the speed at which a community returns to its former state.

Critical to the ability to resist change is the duration of the disturbance.

For example, on a creeping bentgrass putting green the stability of the community is demonstrated when a stress, such as lowering the mowing height below optimum, is imposed.

If the stress is later alleviated by rising the mowing height and the community was not altered from its prestress state, the community is stable.

If the stress resulted in an invasion of competing species such as weeds, the communities considered unstable.

Similar effects occur in high-traffic areas.

Short durations of traffic on athletic field may cause damage, but the turf generally returns to its original state.

If repeated traffic occurs, however, the turfgrass community will not return to its original composition, but instead develop a considerably different form.

Ecosystem complexity is indicated by the number of turf species.

High numbers of species and their associated interactions result in greater stability. The interaction between the turf pathogen "take-all patch" and the soil is an example of the complex leading to stability. (Smith et al., 1989)

In a recently fumigated soil, this pathogen is often found to be pervasive causing disruption to the turfgrass community.

As time passes and a greater diversity of soil organisms become present , antagonism occurs results in a reduction in disease severity.

This increase in soil organism diversity results in more stability, and is reflected in fewer disease symptoms.

Minimal maintenance turfs are characterized by high species diversity and numerous interactions and are complex and stable.

Devastating pest epidemics in low maintenance situations are unlikely due to the complexity.

In intensely maintain turfgrass systems such as a Kentucky bluegrass lawn, plant species diversity and certain specific organisms (pests) are discouraged leading to less complexity.

Ironically, as management levels increases complexity decreases, more management inputs are required to maintain a stable turf system.

Small, incremental increases in complexity do not always lead to an associated increase in stability.

For example, seeding mixtures containing Kentucky bluegrass and perennial ryegrass are promoted by retailers as a mean to enhance stability over a single species.

Yet, results from a number of studies comparing mixtures of a monoculture have shown little difference. (Donald, 1963)

The natural tendency of a ecosystem is towards diversity.

Therefore, turf managers use cultural practices to resist this trend.

Environmental vs. density-dependent dynamics

Two approaches to studying population dynamics are common in turfgrass systems.

The first approach examines population growth and development as controlled by the environment. This approach is most evident in the development of models for predicting turfgrass disease and insect outbreaks.

Models for predicting anthracnose (Danneberger et al., 1984) are built on statistical methods that measure the relationship between certain environmental factors and disease incidence.

However, little biological information on the population level or developmental stage of the organism is considered.

The second approach to population dynamics is a study of density-dependent effects.

A population growth rate is governed either directly or indirectly by the resource level and by the presence of predators.

Environmental conditions are of less importance in density-dependent interactions.

Two major outcomes can occur in density-dependent interactions.

In the first, the community and predator population fluctuate but neither results in species extinction.

In the second, predator population's fluctuate wildly sometimes resulting in species extinction as resources are exhausted.

Population growth

Changes in population size are the results of births and immigration, minus deaths and emigration.

If the number of births for a population are greater than deaths and the trend remains constant, the population will grow at an exponential rate.

To sustain exponential growth, unlimited resources and a favorable environment must be present.

Fortunately, this is not the case or the Earth would be covered with dandelions, Japanese beetles and humans.

For example, if a Kentucky bluegrass turf is susceptible to the pathogen causing melting out and conditions are favorable for disease development, the pathogen will initially began by infecting a few leaves.

This cycle of infection and reproduction continues unabated at an exponential rate.

Eventually the pathogen numbers will begin to level off.

Because of the finite amount of healthy plant tissue available, the pathogen population eventually decreases as the amount of viable tissue available to support the pathogen decreases.

Logistic equation

The most common equation used in ecology to describe population growth is the logistic equation developed by Lotka in the early part of the 20th century.

The logistic equation is expressed as:

dN/dt = the growth rate of the population r = the rate of population increase N = the population size K = the carrying capacity of the system

The carrying capacity is the term used to describe the maximum number of individuals or plants a given habitat can support.

N: Effects on pests

The larger the initial population, the sooner the population reaches its carrying capacity.

One strategy with pest management is to reduce the initial population of pathogens or destructive insects to delay the growth of the pests population.

Reduction in pathogen inoculum or insect populations is often accomplished by genetic resistance or destroying plant debris that harbors pest and pesticide applications.

A few examples of inoculum reduction are:

Reducing rust through frequent mowing of leaves thus eliminating spores before they mature.
Reducing the spread of St. Augustine declined virus by cleaning mowing equipment.
Using powery mildew resisting cultivars of Kentucky bluegrass in shaded conditions.
Diminishing the presence of adult Kentucky bluegrass billbug or the black turfgrass atenius in a spring with an insecticide application.

Assessing the population level of pests is often difficult , especially for diseases.

For management strategies to be effective, consideration of the population's growth rate is important.

If the relative growth rate of a pathogen is high, reduction in an initial inoculum has minimal effects.

For example, pythium propagules in soil and thatch are the source for blight epidemics.

Although reduction of thatch may be a logical mean of reducing the pythium blight incidents, this is probably not effective given pythium can develop explosively from a very tiny population.

r: Rate of growth

Rate of reproduction greatly influences population levels.

If reproduction or "births" is equal to mortality, no population growth occurs.

If reproduction is greater than mortality than the population will increase.

Rate with regard to pest problems is governed by their reproductive potential (generation time), the availability and susceptibility of the host and environmental conditions of the interaction.

Some cultural practices are devised to minimize pest problems and may reduce the growth rate.

Any cultural practice that creates environmental conditions unfavorable for reproduction and growth will reduce r.

Samples of cultural practices that reduce r include:

Increasing air movement through removal of brush around a green, or improving drainage to reduce conditions favorable for pythium blight.
Irrigating a turf to promote natural fungal control of some insects.
Applying adequate amounts of nitrogen to reduce the growth rate of anthracnose.

K: Carrying capacity

Turfgrass managers have the capability of raising or lowering the carrying capacity of the system through managed inputs.

For example, well-fertilized and watered turf can support more plants and potentially more pests such as white grubs and bluegrass billbugs than a turf receiving minimal fertilization.

Under natural conditions limits to growth such as temperature, competition for limited resources and predators frequently restrict the population's ability to reach the carrying capacity.

In turf, however, management practices allow growth to be maintain close to the carrying capacity or increase it through nutritional programs and pest control.

Intraspecific competition

Intraspecific competition is the interaction between individuals of the same species brought about by shared requirements for limited resources.

Competition can occur among individuals of grasses, weeds, pathogens, insects and beneficial organisms.

Intraspecific competition in grasses may be detrimental to plants because of a decreased rate of resource uptake per plant, decreased growth and increased susceptibility to pests.

Competitive effects are measure predominantly through weight accumulation and tillering numbers.

Biomass, which is the weight of living material, is often expressed in ecological studies as the weight per individual.

Since the number of individual plants is so high for turf, a population biomass per given area is generally used.

Plants do not directly interact with each other, but compete indirectly through close association.

A plant can gain advantage over a neighboring plant by utilizing resources earlier or with greater efficiency.

This indirect interaction with plants creates a "resource depletion zone" and is termed exploitation (Bergon et al., 1990).

The intensity of this competition is dependent on the amount of resources available and the plant density.

Intraspecific competition in turf develops progressively from establishment to maturity.

In a newly seeded turf very little competition occurs among seedlings.

As the seedlings developed and grow, mortality increases as the individual tillers began to compete for resources.

Plants begin to grow, but at a slower rate than they would if they were widely spaced.

At this point growth is no longer independent of density.

Self-thinning

Competition can reach a point in a turfgrass stand were growth can only occur at the deaths of others.

This process is termed self thinning.

The self-thinning line can also be thought of as the carrying capacity.

Higher population densities will reach the self-thinning line faster than lower population densities.

Cultural practices and effects of the dynamics of self thinning

Mary Lush (1990) has looked at the impact of cultural practices on turfgrass populations in relation to the self thinning principal.

Mowing height influences community makeup and the amount of resources needed for maintenance.

At high cutting heights, the plant density decreases and biomass increases.

Moisture influences the biomass and density of a turf.

If moisture is limited and turfgrass is occurring below the self thinning line, the addition of water will increase both the biomass and density.

Nitrogen applied to the turf can raise the carrying capacity of the turf community.

For example at a low nitrogen level, an increase in nitrogen to a medium level will result in a positive increase in density and biomass.

Interaction of mowing and nitrogen on density and biomass

The association between nitrogen fertilization and mowing heights on putting greens is routinely faced by golf course superintendents.

Additions of nitrogen will raise the self-thinning line, so they population will not reach the carrying capacity.

If there is a desire to increase greens speed, mowing height is often lowered.

This will increase plant density, but the carrying capacity might still not be achieved.

Interspecific competition

Interspecific competition is the battle between individuals of different species for limited resources.

The principles underlying interspecific competition may be found by looking closely at mixed turfgrass stands.

In the cool-humid region Kentucky bluegrass-perennial ryegrass mixtures are common.

From a competitive standpoint, perennial ryegrass has a significant advantage over Kentucky bluegrass by monopolizing resources such as light in nutrients in the initial seedling development stage.

This results in zones of resource completion for the slower Kentucky bluegrass.

In one combination of 'Kenblue' Kentucky bluegrass the amount of seed in the mixture exceeded 90%, but the actual percentage of Kentucky bluegrass plants in the established stand was 30% or less compared to ryegrass.

Brede and Duich (1984) found that a Kentucky bluegrass/perennial ryegrass mix was less than 70% of Kentucky bluegrass resulted in inadequate establishment of the bluegrass.

They also indicated that mixes with greater than 95% Kentucky bluegrass resulted in poor dispersion of ryegrass and cause unsightly patches.