Plant Density, Plant Form and Community Diversity

"Neighboring plants interfere with each other's activities according to their age, size and distance apart. Such density stress affects the birth rates and death rates of plant parts. As plants in a population develop, the biomass produced becomes limited by the rate of availability of resources so that yield per unit area becomes independent of density-the carrying capacity of the environment. The stress of density increases the risk of mortality to whole plants as well as their parts and the rate of death becomes a function of the growth rate of the survivors. Self-thinning in populations of single species regularly follows a 3/2 power equation that relates the mean weight per plant to the density of survivors. Density-stressed populations tend to form a hierarchy of dominant and +/- stressed subordinate individuals. The death risk is concentrated within the classes of suppressed individuals."  Harper (1977): summary, p. xvi; Ch. 6, pp. 195-235.

"The effects of density do not fall equally on all parts of a plant. In general the size of parts (e.g., leaves or seeds) is much less plastic than the number of parts (e.g., branches). The stress created by the proximity of neighbors may be absorbed in an increased mortality risk for whole plants or their parts, reduced reproductive output, reduced growth rate, delayed maturity and reproduction. The term density is used in a special sense here, to signify the integrated stresses within a community rather than the number of individuals per unit area"  Harper (1977): summary, p. xvi; Ch. 7, pp. 195-235.

A.  Influences of plant density on growth and yield
B.  Influences of plant density on mortality
C.  Influence of plant density on form and reproduction

Influences of Plant Density on Growth and Yield

Density stress is the integrated stresses within a community produced by neighbors on each other, and includes plastic growth and well as the altered risk of death.

The population-like structure of an individual plant also responds to density stress: varied birth and death rate of parts, leaves, branches, flowers, fruits; unlike animals (numbers).

Density yield response: Relationship between yield of dry matter per unit area and density of plants per unit area (e.g., Bromus sp. at 3 levels of N fertilization; Donald 1951)
·        Early in growth, and at low numbers of plants per unit area, number of plants and yield are directly, linearly related.
·        With time, and/or greater numbers of plants per unit area, yield per unit area becomes independent of plant number: saturated yield for holding capacity of that space
     -"law of constant yield" (Kira et al., 1953)
     -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 have a skewed distribution of individual plant weights:
·        Skewing of the frequency distribution (numbers of plants versus weight per plant) increases with time and with increasing density (plants per unit area)
·        At harvest: heirarchy of individuals established with few large dominants and a large number of suppressed, small, plants
·        Danger in assuming average plant performance represents the commonest type, or most typical, plant performance

The place an individual occupies within the hierarchy of a plant population is largely determined in the very early stages of plant establishment and development (the critical role of relative time of emergence):
·        The weight of an individual is a function of:
    -starting capital: embryo plus some part of food reserve weight
    -relative growth rate of the genotype in the environment provided
    -length of time for which this growth rate is continued
    -restrictions on the rate or time of growth imposed by the presence, character or arrangement of neighbors in the population

·        "Percentage emergence ranking": index of the position in emergence ranking an individual occupies in a population "sown" at the same time

·        The amount of growth made by and individual is more directly determined by its order in the sequence of emergence than by the actual time at which it emerges, or the relative spatial arrangement of neighbors

 greater time that it has been allowed to grow; space capture:
    -capture of disproportionate share of environmental resources by early emergers
    -corresponding deprivation by late emergers

·        Once a difference between neighbors has been triggered, it becomes progressively exaggerated with time

·        Pre-emption of space (resources) by developing seedlings (see below) (Ross, 1968)

Influences of Plant Density on Mortality

Two categories of mortality: density-independent and density-dependent
·        Density-dependent mortality: increasing risk of death associated with increasing population density

·        Pre-emption of space (resources) by developing seedlings (Ross, 1968)
    -"self-thinning": density-dependent mortality in a plant population
    -"alien-thinning": density-dependent mortality in one species that can be ascribed to the stress of density of an associated species

·        Density-independent mortality example: mortality risk to a seedling from being hit by a raindrop or hailstone

Population density may enhance seedling establishment; positive effects usually restricted to early stages of germination and establishment.  Most density responses are negative: reduced plant size or increased death risk.  Regulating properties of increasing mortality risk with increasing density: buffering action against unrestrained population increase.

Death risk often assumes a slope of -1.5 (the "3/2 power law):
·        While the number of individuals is decreasing, the weight of the population as a whole is increasing
·        Rate of growth of individuals more than compensates for the decrease in numbers
·        Risk of mortality does not change with age: constant risk of death; also holds true for plant parts on an individual plant

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 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

Higher plants are plastic in size and form: plasticity deriving from the population-like structure of individuals:
·        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

·        Example: ratio of seed total dry matter changes with density stress: high density results in more reproductively inefficient plants

·        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:
    1] vegetative parts
            -stable: height, leaf width, stem diameter, no. of spikelets/spike
            -plastic: branching (tiller formation)
    2] reproductive parts
            -stable: grains per ear, mean weight per grain
            -plastic: fertile tillers per plant (ears)

No reason to believe density stresses resulting in plastic plant responses act any different than those responses to the shortage of supply factors (moisture, nutrients, light, etc.) that exist independent of population size

Plant form and diversity of a community.  Elements contributing to the diversity of plant populations, community structure.

Harper:  "Growth pattern of a genet can itself impose order of diversity on a plant community"

The most likely contact between plants in a field, in a very local community, in among parts of the same individual.  Take this from an individual plant's point of view:
-the nearest neighbor of a plant part (a leaf) is another such unit on the same plant (another leaf on the same plant)
-most contact and interaction is with other parts of same plant self
-next most likely contact is with other plants of same species
-viney, upright, spreading or prostrate plants may alter this

Examples of plant form diversity, somatic polymorphisms:
EX:  Mulberry trees with two different types of leaves (somatic polymorphism), co-existence and cooperation amongst different plant parts of the same individual.  The upper leaves smaller and lobed: more light penetration.  The lower leaves larger, no lobes: more light capture

EX:  Johnsongrass underground organs:  the rhizomes have buds, which result in shoot axis emergence for lateral exploitation and foraging for space.  These rhizomes also for root system for nutrient and water absorption

EX:  soybean cotyledons, unifoliates, 1st trifoliate

Avoiding contact over time.  Avoidance of intra-specific competition:
EX:  winter annuals form rosettes in autumn, then a leafy upright flowering stalk the next spring; e.g. shepard's purse.  Biennials act similarly.

Plant community patterns are determined by the morphology of branching in plants:
•EX:  Indian cucumber growth and development gives an intraspecific community its distinct structure.  Rhizome buds emerge at certain angles, this leads to characteristic, specific patterns of spatial structure in field.

Phenotypic plasticity and somatic polymorphism.  
Means plants that are stuck in one location have to change their form, size, to fit local conditions (in real time).  Phenotypic plasticity is an immediate, short term response to local conditions.  Somatic polymorphism is constitutive, genetic, hence a longer term adaptation.

phenotypic plasticity
1:  The capacity of an organism to vary morphologically, physiologically or behaviorally as a result of environmental flucuations; reaction type
2: the capacity for marked variation in the phenotype as a result of environmental influences on the genotype during development [during the plants life history]

somatic polymorphism: production of different plant parts, or different plant behaviors, within the same individual plant; the expression of somatic polymorphism traits is not much altered by the environmental conditions it encounters (as opposed to phenotypic plasticity)