WSC 101 LE 2

• Plant ecologists


  Study the reciprocal arrangements between plants and their environment
• Goal of plant ecologists


  Understand how climate, soil or edaphic, and biotic factors affect plant growth, development, and distribution
• Ecology


  The study of the interactions between individuals and their environment
• Weed ecology


  Differs from ecology only in that the organisms being studied are weeds
• Weed scientists


  Concerned with how weed management affects weed and crop growth and development
• Weed ecology


  • Gives special emphasis to the adaptive mechanisms that enable weeds to survive and prosper under conditions of maximum disturbance
  • Studies the growth and adaptations that enable weeds to exploit niches in environments disturbed by people who must practice agriculture
• The most successful weed management programs will be developed on a foundation of adequate ecological understanding
• Three important weed-environment interactions

  • Climate
  • Soil
  • Biota or living organisms
• Association of weeds and crops

  Determined largely by the degree of competition offered by a particular crop and weed
• Degree of competition


  Affected by biotic factors such as plant composition, diseases occurrence, toxins released, and animals to which includes insects, the soil fauna, and of course man
• Important factors that determine a weed's ecological interactions

  • Light
  • Temperature
  • Water
  • Wind
  • Humidity
  • Seasonal aspects - the climate
• Purple nutsedge


  Thrives in humid tropics and subtropics with some into sub-humid temperate regions
• Yellow nutsedge


  Does well in sub-humid tropics and warm, temperate regions
• Light intensity, quality, and duration

  • Affect weed presence and survival
  • Photoperiodic responses govern flowering and determine the time of seed maturation
  • If light is too intense or days too long or short, particular weeds won't flower and a species may not endure
• Temperature, particularly soil temperature

  • A primary determinant of seed germination and survival
  • Air and soil temperature are important determinants of species distribution and ecological interactions
• Weeds are found in the environment they prefer, and weed control or weed management often may be aided with changing the environment
• Irrigation and tillage are major environmental changes that lead to shifts in species composition in the affected areas
• Edaphic


  Comes from the Greek edaphos, meaning "soil" or "ground"
• Soil factors


  • Water
  • Aeration
  • Temperature
  • pH
  • Fertility
  • Fertility sources
  • Cropping system and associated practices
• Many weeds do well in soils too low in fertility for crop production, but others grow only in well-fertilized soil
• Most weeds can be found in soils differing widely in physical characteristics, moisture content, and pH
• Soil pH is an important determinant of what plants grow in an area, but no generalizations can be made about the influence of pH on weeds
• The effect of soil structure, water-holding capacity, texture, and nutrient level are also important factors in determining weed growth
• Field topography such as altitude, slope that determines sun exposure also determine of what weeds grow
• Competition


  A negative form of interaction where individuals make simultaneous demands that exceed limited resources and , which both suffer, one individual suffers less
• Competition between crops and weeds

  • Why weeds are controlled
  • If weeds were just there and benign, we wouldn't care as much about them
  • They cause harm to crops by competing with them
• Weeds have the capacity to obtain more water in the soil because they have more extensive root system that can penetrate deeper layer of the soil to extract water and nutrients
• Leaf area index (LAI)


  • The total leaf area per unit ground area
  • Quantifies the amount of leaf material in a canopy
• Importance of LAI


  • For canopy light harvest
  • Phenology
  • Canopy structure
  • Transpiration
  • Scaling processes
• Relationship of light transmission and LAI

  The higher the LAI (meaning more leaf density) the lesser the light transmission
• Factors that affect the magnitude of crop-weed competition

  • For weeds: species, density, distribution, and the length of time that the weed is present in an area
  • For crops: density, its distribution (including spacing between rows and spacing in the row), and duration (when thinning/weeding will be implemented)
  • All these factors (both crops and weeds) are modified by soil (edaphic) and climatic conditions
• Differences in competitive ability of weed species and biotypes

  • Brassica sp. (wild mustard) can reduce the yield of sugarbeet more than Setaria glauca (yellow foxtail)
  • Convolvulus arvensis (bindweed), a perennial weed, was found to be more competitive than annual weed species because of their deep roots and early, heavy shoot growth
• Characteristics that make weeds more competitive

  • Germination patterns differ markedly and sometimes erratically, causing differences in their potential for competition from year to year
  • Emergence and growth also vary from slow and even, to rapid and almost unpredictable
  • Different species and biotypes appear to respond differentially to various environmental conditions
• Phenotypic plasticity


  The ability of an organism to change its phenotype in response to environmental conditions
• • Weeds that emerge with the crop must be removed before the end of Period 1 (maximum weed-infested period) to prevent them from reducing crop yield
  • The crop must be kept clean throughout Period 2 (the critical period for weed–crop competition)
  • Later emerging weeds (Period 3) have little effect on crop yield
• Early literatures assumed that crop yield and weed density was linear, which is not correct
• Zimdahl (1980) proposed that the relationship was curvilinear, which is also wrong as it fails to predict that at high density crop yield will be reduced to zero
• Sigmoidal relationship between crop yield and weed density

  • At very low weed densities, there is no effect on crop yield, and as weed density increases, while there may be effect it is not very obvious
  • As weed density continues to increase, crop yield drops quickly but never goes completely to zero
  • Even at very high densities, weeds do not eliminate all crop plants
• It is very difficult to measure the effect of weeds especially in a large area
• Allelopathy


  • First coined by Molisch in 1937, who is considered as the father of allelopathy
  • The term was refined by Rice (1984) and in 1996, the International Allelopathy Society broaden the definition of allelopathy