Tree Autecology: The Species As An Ecological Unit

II.A. TREE AUTECOLOGY: THE SPECIES AS AN ECOLOGICAL UNIT

OBJECTIVE: To recognize the fact that a "species" is not genetically homogeneous and to consider the influence of that fact on the ecology of a species.

DEFINITIONS OF A SPECIES

1. A morphological definition: A species is a population of organisms whose members, within a considerable range of natural variations are structurally different from all other organisms.

2. A biosystematic definition: The members of a species can interbreed and produce fertile offspring, but cannot do so with members of a different species.

3. An ecological definition: A species consists of groups of morphologically and ecologically similar natural populations that may or may not be interbreeding but that are reproductively isolated from other such groups.

Three key elements to this definition:

a. morphology (similarity of appearance)

b. breeding behavior (generally are not interbreeding with other species)

c. ecologically similar (e.g. members of the same species tend to occur in the same type of habitat)

GENOTYPE AND PHENOTYPE

Genotype is the genetic constitution of an individual.

Phenotype refers to the observable properties of an organism produced by the genotype interacting with the environment.

To what degree is a phenotypic character controlled by the genotype and by the environment?

Traits of trees that show a high degree of heritability include stem straightness, stemwood specific gravity, susceptibility to fungal infections of leaves/needles (rusts), and date of bud burst in spring.

Traits that have lower heritability (i.e. more strongly influenced by the environment) include tree height (strongly influenced by soil water and fertility) and diameter (strongly influenced by stand density).

Plasticity of the phenotype is the degree to which a character of a given genotype can be modified (a non-genetic change) by environmental conditions.

For herbaceous plants, plastic characters include size of vegetative parts; numbers of shoots, leaves, and flowers; and elongation rate of stems. Non-plastic characters include leaf shape, serration of leaf margin, and floral characteristics.

In general, characters formed over a long time period (e.g. growth of stems) are more plastic because they are more subject to environmental conditions. Traits that are formed rapidly (e.g. reproductive structures or leaf shape) are less influenced by the environment and are less plastic.

SOURCES OF VARIATION IN PHENOTYPES:

1. Environmental variation (i.e. plasticity), including both the external and internal environment (e.g. juvenile vs. adult phases) of the plant.

2. Genetic variation:

Major sources of genetic variation are mutation and recombination of genes

a. mutation -- both changes in the molecular structure of genes at individual loci and chromosomal aberrations such as duplications, deletions, translocations; net result is to add to the pool of genetic variability by increasing the number of alleles (different forms of a gene) available for recombination at each locus.

b. recombination of genes in sexual reproduction.

Gene flow plus selection results in genetic differentiation of populations.

GENECOLOGY AND THE ECOTYPE CONCEPT

G. Turreson in the 1920s defined genecology as the study of the adaptive properties of a population in relation to its environment.

An ecotype is a population distinguished by morphological and/or physiological characterisitics, interfertile with other ecotypes of the same species, but usually prevented from naturally interbreeding by ecological barriers. It is a subpopulation of a species which is genetically differentiated so that its survival in a particular habitat is enhanced.

Key aspects of the ecotype concept:

a. differences are genetically based;

b. differences may be morphological, physiological or phenological;

c. ecotypes occur in distinct habitats;

d. the genetic differences are "adaptive" (i.e. enhance survival and reproduction in that particular habitat);

e. ecotypes are potentially interfertile;

How distinct are ecotypes? Turreson stressed that they are discrete entities but often the variation is gradual along environmental gradients forming clines or ecoclines.

The much older concept of "provenance" in forestry refers to a local population of tree species distinguished by its growth and survival from other such populations.

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What is the significance of the ecotype concept in the contexts of understanding the distribution of a tree species, variation in its regeneration ecology, and variation in its ability to tolerate different environments? What are the implications for forest management?

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Brief overview of vegetation patterns in the northern Front Range

(from Kaufmann, M.R., T. T. Veblen, and W. H. Romme. 2006. Historical fire regimes in ponderosa pine forests of the Colorado Front Range, and recommendations for ecological restoration and fuels management. Colorado Forest Research Institute Report)