3.3.97
Weedy Life Cycles
limit.html
Competition:
Limiting Resources of the Environment
Introductory Concepts
Early plant growth:
-continued growth depends on its ability to extract energy, water, nutrients
and gases from the environment
-resources are limited
Limiting resources are intimately related to each other and act in concert
-water and nutrients in the soil solution
-water, CO2 and light through the leaves
Light as a Resource Factor
Supply of light most reliable of the environmental resources: regularity
of diurnal and annual cycles
Light can not accumulate
Light varies in intensity, duration, quality, direction and angle of incidence
both in daily and annual cycles
Light interception by the plant canopy:
-intensity of light decreases, and quality altered, as it penetrates down
through the canopy of leaves
-light passing down throught the canopy is not a continuous gradient but
a moving, dappled pattern of direct light added to a backround of diffuse
light
-leaves vary between species and on the same plant in the angle at which
they are borne and consequently in the time of day at which they cast the
greatest shadow
Photosynthesis in the canopy of a population
-in a canopy upper leaves become saturated as light intensity increases
-lower leaves in the shade may still respond to increased light that penetrates
through the light-saturated upper canopy
Canopies as a population of leaves, not individuals
-population acquires a holistic physiology within which the individual plant
is subordinated to the physiology of the whole
-plant populations adjust their structure and growth rate to the resources
available; perfect adjustment is impossible because the environment changes;
canopies are compromises, balancing respiration and photosynthesis
-neighbor effects in light competition occur between individual leaves,
not individual plants, permitting individual leaves to act as discrete units
often interfering directly with other parts of the same plant
Water as a Consumable Resource
Plants draw water primarily from stored moisture, a buffer against the uncertainty
of its availability
Plants act as wicks; with time and plant growth (leaf area, root system
size), the size of this wick increases, increasing water loss rates
Light and water are intimately related
-heat from solar radiation drives transpiration while quanta drive photosynthesis
-leaf stomatal (leaf pores) function links both together: it is impossible
to separate the roles of light and water in limitations to plant growth
-intimate role of CO2 uptake with these phenomena
Root systems and water use
-dense populations suffer water shortages earlier in life than sparse populations
-sparse populations cover the surface later with leaves, water conserved
longer
-sparse populations suffer less neighbor stress by shading and nutrient
depletion, the resulting more vigorous plants develop more extensive root
systems than densely packed populations and therefore tap a larger water
reservoir
-variance in shoot development above ground is reflected in below ground
root system growth: water stress is sensed differentially by different individuals
-greater interference occurs between root systems of different individuals
in a population than between parts of one root system, in contrast to the
potentially greater interference between leaves on the same plant above
ground
-Species differences can result in root system differences
-earlier germinating species can use up water resources sooner than later
species
-different species may exploit different zones of the soil profile and avoid
interference for water
Mineral Nutrients as Resource Factors
Plants obtain mineral resources from the soil, and many of the conditions
that govern their availability are similar to those affecting water
Soil minerals are held in the soil by physical and chemical linkages with
insoluable soil components and are in a rapid, dynamic, equilibrium with
ions in the soil solution
-when nutrients are removed by a root, there is a local lowering of its
concentration, a diffusion gradient is created, nutrients diffuse along
this gradient
The transpiration stream in plants from leaves, through the vascular system
to the roots, and then the soil creates a mass flow of soil solution towards
the roots
-mass flow and nutrient diffusion in the soil maximize nutrient flow towards
plants with the greatest growth
-"luxury" consumption of nutrients:
Mycorrhizal associations on roots can increase both water and nutrient uptake
Very difficult to identify the effects of nutrients as a limiting resource
in plant populations
-intimate relationship of water and nutrient availability
-supplying nutrients to plants may just speed up the time that light becomes
limiting to growth
-enhanced fertility may result in increased root system size speeding up
the time water availability becomes limiting to growth
Gases as Limiting Resource Factors
Carbon dioxide as a limiting resource factor
Amount of carbon dioxide supplied to a leaf can control the rate of photosynthesis
in that leaf, therefore it can be a limiting resource to plant growth
Differences in carbon metabolism: C3 and C4 plant species
-C3/C4 plants utilize different biochemical pathways to assimilate CO2
-greater efficiency of C4 plants reflected not in producing much larger
plants but in making more effective water-conserving plant with a greater
reproductive efficiency
Oxygen as a limiting resource factor
Oxygen may become limiting to plant growth below ground, in the soil
-local zones of O2 depletion may arise in water-saturated soils
-of all the resources needed by plants, oxygen is the least likely to limit
growth, least likely to be limited by neighbors
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