The Web of Life (81) Plant Competition


The survival and growth of an individual plant may be strongly influenced by competition with its neighbors. ~ American ecologist David Tilman

Plants are well aware of their situation. They constantly assess their prospects and grow in a decided direction.

If plants were unable to discriminate between unoccupied soil and those containing competitors, they would soon be eliminated by fitter individuals. ~ Anthony Trewavas

Because they sense their neighbors, trees do not crowd each other. Growing trees avoid already-shaded areas and shape their growth so that their leaves have their time in the Sun.

Branch shoots sprout a few leaves, getting feedback about local conditions. With a poor response the plant withers the branch.

Similarly, as trees grow up, in a race with neighbors to sunny heights, they wither the lower branches that no longer fetch enough light energy.

Flowers, shrubs, and trees all change their phenotype intelligently: foraging for resources, competitively excluding potential rivals, and constructing as fulsomely as possible.

Competition for resources is a way of life for wild plants as much as it is for all animals.

Plants are limited in their ability to choose their neighbours, but they are able to orchestrate a wide spectrum of rational competitive behaviours that increase their prospects to prevail under various ecological settings. Through the perception of neighbours, plants are able to anticipate probable competitive interactions and modify their competitive behaviours to maximize their long-term gains.

Specifically, plants can minimize competitive encounters by avoiding their neighbours; maximize their competitive effects by aggressively confronting their neighbours; or tolerate the competitive effects of their neighbours. The adaptive values of these options are expected to depend strongly on the plants’ evolutionary background, and to change dynamically according to their past development, and relative sizes and vigour. ~ Israeli evolutionary botanist Ariel Novoplansky

Soybeans were tested in soil boxes with removable partitions. With the partition in place, each plant had its own soil and experienced no competition. These were soil owners.

Remove the partition and each plant has double the space available but faced competition in having to share soil with another plant. These were soil sharers.

In the absence of competition, the soybeans constrained their roots. But when sharing soil, competition drove the plants to increase root production. They even confrontationally turned their roots in the direction of the rival.

Competition has its costs. Owners had a much higher shoot-root ratio, and nearly double the seed yield of sharers. But plants cannot bear the prospect of handing over soil resources that may be forgone at a later date.

The canopy of every forest signifies a fight for light. That competition begins underground.

Spotted knapweed, native to Eurasia, caught a ride to North America. An inveterate land grabber, knapweed blankets entire slopes, pushing out native vegetation. Its weapon: catechin, a phenol which can retard plant growth. Having grown used to knapweed’s ways, European plants neighboring knapweed are not bothered by its catechin seeps, but some North American plant species have not adapted, and so are overwhelmed. But not all. Lupin and blanketflower fight back by exuding extra oxalate: 4 times the normal level for blanketflower, and up to 40 times normal for lupine. Oxalate neutralizes catechin and extends a blanket of protection not only to the defender, but also neighbors.

Many plants employ secondary metabolites to establish territories for themselves and their offspring. Various phytochemicals, including oils, alkaloids, steroids, terpenes, and coumarin derivatives, are poisonous to botanical rivals.

Black walnut and eucalyptus trees are exemplary of fierce allelopathic warriors via root secretions. Allelopathy is the production of biochemicals intended to affect the health of other organisms.

Crabgrass does not just crowd out other plants. It kills them with herbicides released from its roots.

California purple sage pops terpene into the air, where it disperses and is then absorbed in the soil. This promotes clearing zones around sage shrubs.

Trees, particularly conifers, actively release terpenes in warm weather. The terpenes act as a natural cloud seeding, encouraging cloud formation. Trees regulate forest temperature by creating clouds that block sunlight.

Heath allelopathically inhibits pine trees by chemically inhibiting the soil fungi that share a symbiotic relationship with pine roots. The Scottish harvested the pine forests in Scotland at the advent of the Industrial Revolution as fuel for industry. The land is now colonized by heath, which thwart pine reforestation efforts by foiling the fungi needed for pines to make a comeback.