Plant Interactions

Water Freedom System

Survive Global Water Shortages

Get Instant Access

Interactions between plants are one of the driving forces controlling plant community structure. The nature, importance, and demonstration of these interactions generate considerable discussion amongst ecologists.

Interaction between plants resulting in increased plant growth is called facilitation, which is most frequently viewed as one species making the environment less harsh to which the second species responds favorably. An example of facilitation is hydraulic lift. In arid environments surface soils become too dry to support shallow-rooted species, but deep-rooted species persist. In the cold deserts of North America, big sage (Artemesia tridentata) brings water to the surface from deeper layers. Some of this water then diffuses into the soil and supports herbs beneath the shrubs.

Negative interactions between plants occur when one or both plants grow poorly in the presence of the other. The term 'competition' is frequently used to describe any negative interaction; however, this use does not describe the mechanism of the interaction. If the interaction is a result of two plants utilizing the same limiting resource such as water, light, or nutrients, then interaction is called 'resource competition' or 'scramble competition'. Typically the species differ in their ability to utilize the resource or to tolerate the result of reduction of the resource. Competition for water was demonstrated in a study of the interaction between the perennial bluebunch wheatgrass (Agropyron spicatum) and the introduced annual cheatgrass (Bromus tectorum) in the Palouse prairie grasslands of Northwestern USA, where most of the limited precipitation falls as winter snow. Cheatgrass grows in the winter, exhausts the water supply, and survives the summer drought as seed, while the bluebunch wheatgrass grows slowly in winter, experiences low water availability for its summer growth, and dies. This exemplifies the preemptive use of a resource and the tolerance of the result by cheatgrass.

Negative interactions may result if a substance is added to the environment by one plant with a detrimental effect on a second plant. This may be referred to as 'interference competition' or 'contest competition'. Plants add a wide range of chemicals to the environment that have negative effects and include salts, phenolics, terpenes, alkaloids, mustard oils, and cyanides. If the added material is an organic substance and has a negative effect, then the term 'allelopathy' is used to describe the interaction. A classic study of allelopathy by C. H. Muller described interaction of an aromatic shrub (Salvia leuco-phylla) with the surrounding annual grassland in which a zone immediately surrounding the shrubs was devoid of grasses, a second zone stretching out an additional 2 m had stunted grasses of a limited number of species, and a third zone of uninhibited grasses including wild oat (Avena fatua) and ripgut brome (B. diandrus) (Figure 1). Muller demonstrated that the shrubs produced a variety of volatile terpenes (e.g., cineole and camphor) with the potential to inhibit growth of seedlings. More commonly, allelopathic interactions result from water-soluble materials leaching from the canopy, roots, or litter. In desert communities the regular patterning of creosote bush (Larrea tridentata) may result from the toxic interactions of the roots. The demonstration of allelopathy is difficult, which makes the phenomenon controversial. To show that a negative interaction is allelopathic, an investigator must demonstrate no other biotic interaction (resource competition or herbivory) is causative, a chemical is

Creosote Allelopathy

Figure 1 Allelopathy demonstrated in shrubland-grassland interface in Santa Ynez Valley, CA, USA. At left is a stand of purple sage (Salvia leucophylla) surrounded by a bare zone with few or no grass species. An inhibition zone of 1- 2 m with foxtail fescue (Festuca megalura (=Vulpia myuros)) and soft chess (Bromus mollis) grading into uninhibited grassland of wild oat (Avena fatua) and ripgut brome (Bromus diandrus). The aromatic shrubs produce volatile compounds including cineole and camphor, which are toxic to the grasses.

Figure 1 Allelopathy demonstrated in shrubland-grassland interface in Santa Ynez Valley, CA, USA. At left is a stand of purple sage (Salvia leucophylla) surrounded by a bare zone with few or no grass species. An inhibition zone of 1- 2 m with foxtail fescue (Festuca megalura (=Vulpia myuros)) and soft chess (Bromus mollis) grading into uninhibited grassland of wild oat (Avena fatua) and ripgut brome (Bromus diandrus). The aromatic shrubs produce volatile compounds including cineole and camphor, which are toxic to the grasses.

produced and released in sufficient quantity and at time to be effective, and the target plant is susceptible to the toxin.

Was this article helpful?

0 0
Project Earth Conservation

Project Earth Conservation

Get All The Support And Guidance You Need To Be A Success At Helping Save The Earth. This Book Is One Of The Most Valuable Resources In The World When It Comes To How To Recycle to Create a Better Future for Our Children.

Get My Free Ebook


Post a comment