3.3.97
Weedy Life Cycles
density.html
Competition: The Effect of Density on
Yield, Mortality, Plant Form and Reproduction
Influences of Density on Growth
and Yield
Density-Yield Response: Relationship between yield of dry matter per unit
area and density of plants per unit area
-Low density: as plant numbers increase, yield per area increases
-With time, and/or greater numbers of plants per unit area, yield per unit
area becomes saturated, reaches holding capacity of that space
-variations in sowing density are largely compensated for by amount of growth
made by individual plants
-population behaves more as integrated system, reacting independently of
individuals, with individual behavior subordinate to that of the population
Plant to plant variation
-plants growing under density stress at harvest is a mixture of individuals
(few large dominants & a large number of suppressed, small, plants)
-danger in assuming average plant performance represents the commonest type,
or most typical, plant performance
The Importance of Early Emergence
The size of an individual in a plant population is largely determined in
the very early stages of plant establishment & competition
The weight of an individual is a function of:
a. starting capital: embryo plus some part of food reserve weight
b. relative growth rate of the genotype in the environment provided
c. length of time for which this growth rate is continued
d. restrictions on the rate or time of growth imposed by the presence, character
or arrangement of neighbors in the population
The amount of growth made by an individual is mostly determined by its emergence
relative to that of its neighbors (crop or weed), rather than by the actual
time at which it emerges, or the relative spatial arrangement of neighbors
The advantage which an early emerging seedling gains is far greater than
can be accounted for merely by the greater time that it has been allowed
to grow:
-early emergers capture a disproportionate share of environmental resources
-corresponding deprivation by late emergers
Once a difference between neighbors has been triggered, it becomes progressively
exaggerated with time
Influences of Density on Mortality
Two categories of mortality: density-independent and density-dependent
Density-dependent mortality: increasing risk of death associated with increasing
population density
Density-independent mortality example: mortality risk to a seedling from
being hit by a raindrop or hailstone
Mortality serves as a buffer against unrestrained population increase with
increasing density
Self-thinning: mortality due to density stress of neighbors of the same
species
-mortality greater in high fertility regimes
-survival greater in high light regimes versus low light regimes
-mechanism of self-thinning not understood; individuals most likely to die
are smallest and weakest
Populations derived from large seed (ex. velvetleaf) suffer more rapid mortality
than those derived from small seed: faster-growing, larger, more vigorous
seedlings produced a more intense density stress among themselves than in
populations of the same density but from smaller, slower growing, seedlings
Influence of Plant Density
on Form and Reproduction
Stresses of density:
-Plasticity (plastic growth of plant parts)
-Mortality
-Most density responses are negative: reduced plant size or increased death
risk
Plasticity. The individual plant responds to density stress in the same
way as a population of plants:
-varied birth and death rate of parts (leaves, branches, flowers, fruits)
-these plastic responses are unlike animals (changes in numbers)
Plasticity of numbers of units, not form of units:
-The form of repeating units of plant construction (leaves and flowers)
is tightly controlled and changes only slightly over a wide range of environments
-The number of these units, and thus the size of the whole plant, varies
greatly with both age and conditions
Dry weight of plant population compensates more or less perfectly with variations
in density, but the parts of individuals are not altered to the same extent
-growth of individuals under density stress results in differential allocation
of assimilates between different structures, and resulting differences in
the size of those parts
Optimal density for a particular product (seed, storage root, latex, etc.)
may be different than that density for dry matter production; example, corn:
optimal seed yield populations less than that for silage
Density stress: generally seen in reduction in number of plant parts produced
(branches, flowering nodes) and in part by organ abortion (death of old
leaves, abscission of flowers and pods)
Density stress example, wheat:
-vegetative parts that are stable: height, leaf width, stem diameter, no.
of spikelets/spike
-vegetative parts that are plastic: branching (tiller formation)
-reproductive parts that are stable: grains per ear, mean weight per grain
-reproductive parts that are plastic: fertile tillers per plant (ears)
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