Interactions, Plant-Plant

In plant communities each plant might interact in a positive, negative, or neutral manner. Plants often directly or indirectly alter the availability of resources and the physical habitat around them. Trees cast shade, moderate temperature and humidity, alter penetration of rain, aerate soil, and modify soil texture. Plant neighbors may buffer one another from stressful conditions, such as strong wind. Some plants make contributions to others even after they die. Trees in old-growth forests that fall and decompose ("nurse" logs) make ideal habitat for seeds to sprout, and such a log may be covered with thousands of seedlings. While effects on the physical habitat are consistent aspects of communities, plant-to-plant competition to preempt resources also takes place, and in some instances chemical interactions occur between species.

Commensalism occurs as one species lives in a direct association with another (the host), gaining shelter or some other environment requisite for survival and not causing harm or benefit to the host. Orchids and bromeliads (Neoregelia spp.) live on the trunk or branches of their host, gaining water and nutrients from the air or bark surface without penetrating host tissue. Stocky roots and xeromorphic leaves that help gain and retain water are characteristic of vascular epiphytes (epiphyte means to live upon another). Bryophyte, lichen, and fern epiphytes are so abundant in the tropical rain forest that they often embody more plant material than their host trees. Another facilitation is illustrated by seedling growth of the Saguaro cactus (Cereus giganteus ), which typically occurs in the shade of paloverde trees or other plants, which create a better water-relationship environment for the cactus and protect it from the negative effects of the intense sun. Farming practices often use "nurse" plants to create a temporary improvement in the environment for the main crop. For example, oat and alfalfa may be seeded together so that oat shades and maintains better soil surface moisture for the emerging alfalfa seedlings.

Direct plant-plant contacts that benefit both organisms are termed mutualism. Taking the broader view of plants to include microorganisms, a good example of this arrangement is the association of legumes and nitrogen-fixing bacteria that live within legume root nodules. The legume benefits by obtaining nitrogen from the bacteria, while the bacteria gain necessary carbohydrate energy from legume photosynthesis. The free-living bacteria actually change and become bacteroids, no longer able to live outside the roots. The vast majority of higher plants have fungal-root associations called mycorrhizae. The vascular plants benefit because the fungus is much better at absorbing and concentrating phosphorus (and perhaps other mineral nutrients) than the root tissue, while the fungus gains a source of carbon compounds from the plant.

Parasitic plant-plant interactions are harmful to the host. A number of plants (e.g., dodders, broomrakes, and pinedrops) do not contain chlorophyll and cannot photosynthesize. They parasitize green plants by penetrating the outer tissue of the host plant with haustoria (rootlike projections), which eventually tap the water and food-conducting tissue. Mistletoe also form haustoria but the primary function of these structures is obtaining water, as this partially parasitic plant is capable of manufacturing its own food by photosynthesis. Witchweed (Striga spp.) has green leaves but is an obligate parasitic weed that causes tremendous crop losses to tropical-origin cereal grain crops and legumes. Witchweed has evolved so that chemicals from the host plant have become signals for witchweed seed to germinate and attach to the host. Subsequently, witchweed penetrates the host roots and steals water, minerals, and hormones. Strangler fig is a tree that germinates high in the host tree and sends roots to the ground, eventually killing the host when the fig roots and vines surround and strangle the flow of sugars in the host.

It is rare that plants are unaffected by neighboring plants. Negative effects on one of the neighbors are referred to as interference, and they include competition and allelopathy . Competition, the situation in which one plant depletes the resources of the environment required for growth and reproduction of the other plant, is the most common plant-plant phenomenon in nature. Members of plant associations that are more successful at gaining major resources—water, nutrients, light, and space—have the advantage and typically dominate the community. Competitive advantage may result from a plant's season of growth, growth habit, or morphological features such as depth of rooting, and special physiological capabilities like differences in rate of photosynthesis. In contrast to competition, allelopathic interference is the result of a plant adding toxic chemicals to the environment that inhibit the growth and reproduction of associated species or those that may later grow in the area. Many negative effects on target species probably occur from a combination of competition and allelopathy. Chemicals released from one plant may also be a communication to other plants, causing germination (e.g., Striga ) or signaling defense responses to insect attack.

see also Allelopathy; Defenses, Chemical; Ecosystem; Interactions, Plant-Fungal; Interactions, Plant-Insect; Interactions, Plant-Vertebrate; Invasive Species; Mycorrhizae; Nitrogen Fixation; Parasitic Plants; Plant Community Processes; Symbiosis.

Frank A. Einhellig

James A. Rasmussen

Bibliography

Kareiva, Peter M., and Mark D. Bertness, eds. "Special Feature: Re-Examining the Role of Positive Interactions in Communities." Ecology 78, vol. 7 (1997).

Raven, Peter H., Ray F. Evert, and Susan E. Eichhorn. Biology of Plants, 6th ed. New York: W. H. Freeman and Company, 1999.