What functional types of plants do agricultural flora consist of? Agricultural communities consist of intentionally introduced crop species, as well as plants not intended or desired by humans to be present. Baker (1965; p. 147) has defined weeds as:
"a plant is a weed if, in any specified geographical area, its populations grow entirely or predominantly in situations markedly disturbed my man (without, of course, being deliberately cultivated plants)."
Closely allied to this view of weeds are those defining:
colonizing species (a plant, typically r-selected, which invades and colonizes a new habitat or territory, Lincoln et al., 1998)
feral plants (a plant that has reverted to the wild from a state of cultivation or domestication; wild, not cultivated or domesticated; Lincoln et al., 1998)
agrestals (growing on arable land, Lincoln et al., 1998)
ruderals (a plant inhabiting a disturbed site. Lincoln et al., 1998).
The definition of:
invasive species: organism undergoing a mass movement or encroachment from one area to another; Lincoln et al., 1998
has evolved in response to environmental, human health and nativistic goals in (U.S.) governmental public policy:
invasive species: an alien species whose introduction does or is likely to cause economic or environmental harm or harm to human health, Anonymous, 1999; a species that is non-native (or alien) to the ecosystem under consideration and whose introduction causes or is likely to cause economic or environmental harm or harm human health, Anonymous, 2004) (Dekker, 2005).
Any basis for understanding agroecosystem biodiversity must encompass the biology of all these overlapping types of plants.
Other Weed Definitions
1: anthropomorphic (plants relationship to us as humans); tells us something about how we
view nature;
2: "a plant out of place" (WSSA, 1956);
3: "competitive and aggressive behavior" (Brenchley, 1920);
4: "persistance and resistance to control" (Gray, 1879);
5: "useless, unwanted, undesirable" (Bailey, 1941);
6: "a plant whose virtues have not yet been discovered" (Emerson, 1878);
7: "appearing without being sown or cultivated" (Brenchley, 1920);
8: "a plant that grows spontaneously in a habitat greatly modified by human
action" (Harper, 1944);
9: "unsightly" (Thomas, 1956)
Weeds Defined by Their Traits
Weeds have also been defined by the traits or characteristics they
possess. In this course we will develop the idea of important weedy traits
in detail.
The "Ideal Weed Concept" (Baker, 1974):
Ideal characteristics of the worst weeds (Baker, 1965, 1974):
Seed bank:
-Germination requirements fulfilled in many environments.
-Discontinuous germination (internally controlled) and great longevity of seed
Vegetative:
-Rapid growth through vegetative phase to flowering
-If a perennial, vigorous vegetative reproduction or regeneration from fragments
-If a perennial, brittlenenss so as not to be drawn from ground easily
-Ability to compete interspecifically by special means (rosette, choking growth,
allelochemicals)
Reproductive:
-Continuous seed production for as long as growing conditions permit.
-Self-compatibility but not complete autogamy or apomixy
-Cross-pollination, when it occurs, by unspecialized visitors or wind
-Very high seed output in favorable environmental circumstances
-Production of some seed in wide range of environmental conditions; tolerance and
plasticity
-Adaptations for short-distance and long-distance dispersal
Adaptive Characteristics of Agronomic Weeds (Patterson, 1985)
Related to physiology, growth, and competitiveness
-High relative growth rates in seedling stage
-High rates of photosynthesis
-Rapid development of exploitative root systems
-Rapid partitioning of photosynthate into new leaf area production
-Rapid vegetative growth to reproductive phase
-Special "weapons" for interference
-Freedom from environmental constraints ("general purpose genotype"); high
capacity for acclimation to changing environment
Related to reproductive phase
-Breeding systems that provide some outcrossing but also allow self-fertilization
-Copious seed production under favorable conditions with some seed production occurring
over a range of favorable and stressful conditions
-Pollination by wind or generalized insect visitors
Related to cultural practices
-Morphological and physiological similarity to crop
-Timing of seed maturity to coincide with crop harvest
-Resistance or tolerance to chemical herbicides
-Resistance to mechanical control; regeneration from rhizomes or other vegetative
propagules
-Seed dormancy, longevity in soil; discontinuous germination over long periods of time
