Plant Life Forms (Growth forms)

Plants are classified taxonomically into families, genera, species, varieties, etc. This, however, is not the only way to classify plants. Species and individuals can be grouped into life form or growth form classes on the basis of their similarities in structure and function.

A plant life form is usually understood to be a growth form which displays an obvious relationship to important environmental factors.

E.g. a deciduous tree is a plant life form that responds to an unfavorable season by shedding its leaves.

A geophyte is an herbaceous plant form that responds to an unfavorable season by dying back to an underground structure, be it a root or modified underground stem.

The life form of a plant species is usually a constant characteristic. The same species, however, may assume a different life when growing under very different environmental conditions.

E.g certain high altitude trees species assume a creeping growth habit, called “krummholz”, near the upper limit of its distribution, while they grow as perfectly normal trees below.

Plants of the same life form growing together are likely to compete directly for the same space or niche (ecological role of a species in an envir.).

Their similarity in structure and form indicates a similarity in adaptation to the utilization of the environmental resources offered in a given space.

(The most extreme form of life form similarity is shown among individuals of the same species.

Wherever they grow close together, they are also the strongest competitors, because they are adapted to use the environmental resources in the same manner.

Species of very unrelated families may also be of the same life form. E.g. similar stem-succulents evolved in the families Cactaceae, Euphorbiaceae, Asclepidiaceae, and Liliaceae (or Agavaceae) – convergent evolution.

Plant life forms, or growth forms, can tell you a lot about the forces shaping an ecological community.

An analysis of the types and relative proportions of life forms in vegetation provides a basis for vegetation comparisons among different places, even when species composition is unknown.

Life form analysis lends itself to a rapid assessment because it doesn’t rely on prior identification of species.

Environment is the summation of all biotic (living) and abiotic (nonliving) factors that surround and potentially influence an organism; it is the organism’s habitat.

Biotic factors include competition, mutualism, allelopathy, and other interactions between organisms.

Abiotic factors include all chemical and physical aspects of the environment that influence a plant’s growth and distribution. These may include temperature, precipitation, light, substrate, etc.

The environment can be divided up into two parts: the macroenvironment and the microenvironment.

The macroenvironment is the prevailing regional environment.

The microenvironment is the environment close enough to an object to be influenced by it. It may be quite different from the macroenvironment.

The microenvironment beneath a forest canopy is different from the macroenvironment above it in such traits as humidity, wind speed, and light intensity.

The microenvironment beneath a rock in desert soil may be cooler and moister than other parts of the macroenvironment.

The microenvironment just 1 mm above a leaf surface may differ in wind speed, humidity, temperature from the macroenvironment 10 mm away.

Vegetation consists of all the plant species in a region (flora) and the way those species are spatially or temporally distributed.

If the region is large, its vegetation will consist of several prominent plant communities.

Each vegetation type is characterized by a growth form of its dominant plants (the largest, most abundant, characteristic plants).

Examples of growth forms are: annual herbs, broadleaf evergreen trees, drought-deciduous shrubs, plants with bulbs or rhizomes, needle-leaf evergreen trees, perennial bunchgrass, dwarf shrubs.

Growth form may include life history strategy of a plant, size, life-span, woodiness of a taxon (herb, annual, perennial, herbaceous perennial, woody perennial, tree, or vine).

Growth form may include the degree of independence of a taxon: green and rooted in the ground, parasitic, saprophytic, epiphytic.

Growth from may include the morphology of a taxon: stem succulent, leaf succulent, rosette form, spinescent, pubescent.

Growth from may include the leaf traits: large, small, sclerophyllous, simple, compound, evergreen, winter-deciduous, drought-deciduous, needleleaf, or broadleaf.

Growth from may include the location of the perennating bud, as defined by Raunkiaer (1934).

A vegetation type that extends over a large region is called a formation.

Example: a tropical rainforest is a formation dominated by broadleaf evergreen trees and is characteristic of thousands of square miles in humid tropical regions on several continents.

Formations may be subdivided into associations.

An association is the collection of all plant populations coexisting in a given habitat.

Associations are usually named for their dominant or most characteristic taxa: the red fir forest in California’s Sierra Nevada, which occurs on well-drained, undisturbed sites between 1800-2400 m, is a red fir (Abies magnifica) association in a conifer forest formation.

A population is a group of individuals of the same species occupying a habitat small enough to permit interbreeding among all members of the group.

A fascinating ecological phenomenon is the similarity of vegetation types in similar macroenvironments scattered around the world. It is as though a particular physiognomy has been selected for in similar, but isolated, habitats. Evidently there has been convergent evolution among vegetation types.

Importance of life forms

1. The shift from grasses to shrubs, such as mesquite and creosote, can alter carbon and nitrogen accumulation in the soil.
2. The shift from grasses to shrubs may alter water cycling in semiarid systems (root distribution patterns and aboveground plant structure).
3. The expansion of mesquite in both Texas and New Mexico shifts rooting patterns from fine, surface roots to one dominated by coarse roots concentrated deeper below shrub canopies.
4. Above ground structure may affect airflow, albedo and water percolation patterns.
5. Abiotic conditions differ sufficiently under shrub canopies relative to beneath grass canopies.
6. On the other hand, the increase in woody vegetation in grasslands and savannas has been implicated in desertification and decreased economic productivity of southwestern rangelands.
7. The invasion of Bromus tectorum (annual grass) to the Great Basin (Bromus madritensis rubens in the Mojave Desert) has reduced forage quality for livestock and altered fire frequency.
8. It may give information on the response of a community to particular environmental factors, on the utilization of space (if combined with cover values), and the probable competitive relations within a community.
9. Valuable to the student who cannot visualize from the name how a species looks, especially with respect to functional life forms like deciduousness or evergreenness in woody plant species.
10. Community descriptions based on physiognomy, life form, and other functional traits (as opposed to taxonomic traits such as the identity of species in the community) are useful because they permit comparison of widely disjunct stands that have little or no floristic similarity. These comparisons often show a convergence of vegetation types, given a similar macroenvironment.

Expected life forms in different areas.

Alpine areas - Chamaephytes (small, woody or herbaceous perennial, having resting buds not more than 25 cm above soil surface) and hemicryptophytes (plant with resting buds at or near the level of the surface) dominated throughout the alpine areas.

Life Form Classification – according to Raunkiaer, the range of adaptation of plants relates to the mode of shoot withdrawal in the unfavorable season.

Perennial plants range from no withdrawal in trees, shrubs, and herbs in humid tropics, to foliage withdrawal in deciduous woody plants and shoot-reducing shrubs and herbs (chamaephytes and hemicryptophytes), to total shoot reduction in geophytes. The most complete form of shoot reduction is represented by the annuals or therophytes, which survive only in the form of seeds.

1. Phanerophytes
1. the surviving buds or shoot apices are borne on shoots which project into the air.

i. evergreen trees

ii. decidiuous trees

iii. resting buds more than 25 cm from soil surface

2. Chamaephytes
1. the surviving buds or shoot apices are borne on shoots very close to the ground.

i. those bearing erect shoots which die back to the portion that bears the surviving buds.

ii. resting buds < 25 sm from soil surface

iii. passive chamaephytes – persistent weak shoots that trail on or near the ground.

iv. active chamaephytes that trail on or near the ground because they are persistent and have horizontally directed growth.

v. cushion plants.

3. Hemicryptophytes
1. the surviving buds or shoot apices are situated in the soil surface.

i. rosette plants bearing all their foliage in basal rosette.

ii. Partial rosette plants bearing most of their foliage (and the largest) on short internodes near ground level.

4. Cryptophytes
1. the surviving buds or shoot apices are buried in the ground (or under water).

i. geocryptophytes or geophytes which include forms with: rhizomes, bulbs, stem tubers, root tubers.

ii. marsh plants (helophytes)

iii. aquatic plants (hydrophytes)

5. Therophytes

a. plants that complete their life cycle from seed to seed and die within a season (this group also includes species that germinate in autumn and flower and die in the spring of the following year).

Leaf categories

1. simple – margin entire, serrate, dentate, etc.

2. lobed – incisions at least halfway to center of blade

3. compound – with separate leaflets

Leaf size

1. absent

2. linear/filiferous

3. microphyllous – size of a dime or less

4. macrophyllous – size of a quarter to a dollar bill

5. megaphyllous – larger than a dollar bill

Glossary

1. herbaceous stem – non-woody; a stem that does not grow in diameter, and forms neither hard interior tissue (wood) or bark.
2. Woody stem – a stem that grows in diameter from a cambial meristem (secondary growth). Tissue towards the inside of the stem become hard, or lignified, to form wood. The outside of the stem usually has bark.
3. Annual plant– a plant that cannot survive longer than one growing season. There is no woody stem tissue and no remains of growth from the previous year.
4. Perennial plant – a plant that can survive for several growing seasons. Perennials will have woody stems, underground storage organs, or remains of stems from previous years at the base of living stems.
5. Cushion herb – (caespitose) – a plant with short, highly branched stems densely interlaced to form a mat or cushion.
6. Rosette – a cluster of leaves arranged in a circular pattern on the soil surface, like the petals of a double rose.
7. Simple leaf – a leaf with one continuous blade, either attached directly to the stem of held away from the stem on a stalk-like petiole.
8. Compound leaf – a leaf with more than one blade.
9. Population - a group of individuals of a single species living in close proximity and capable of interbreeding.
1. Populations within a relatively small geographic area (e.g. whose pollinators have ready access to all individual members) are called local populations
2. Population density – the number of individuals in a given area (e.g. 5 blackberry bushes per square meter).
10. Communities consist of populations of different (and interdependent) species living in the same location.
11. Ecosystem consists of living organisms that interact with one another and with the nonliving (abiotic) environment.
12. Accuracy: close agreement of sample means with actual parameter means.
13. Precision: close agreement of sample means to each other, without reference to the true mean.Precision can be measured without knowing the true mean; accuracy can only be measured when the actual parameters are known, and in vegetation sampling this is never the case.
14. Cover is the percentage of plot area beneath the canopy of a given species.

a. The canopy of an overstory species creates a microenvironment that smaller, associated species must contend with. The overstory canopy, therefore, exerts a biotic control over the microclimate of the site. It is assumed that a comparison of cover for each species or life form in a given canopy layer will reveal the relative control or dominance that each species exerts on the community as a whole, such as relative amounts of nutrients or other resources each species commands.

b. Canopy coverage, then, can be expressed as the percentage of ground covered by canopy, when the edges of the canopy are mentally projected down to the surface.

15. Relative cover is the cover of a particular species or life form as a percentage of total plant cover. Relative cover will always tally up to 100%, even when absolute cover is quite low.

16. Density is the number of plants rooted inside the plot.

17. Relative density is the density of one species or life form as a percent of total plant density.

18. Frequency is the percentage of total plots that contains at least one rooted individual of a given species or life form. (Sociability). [Frequency is an artifact of plot size, and so is a more artificial statistic than either cover or density].

19. Relative frequency is the frequency of one species or life form as a percentage of total plant frequency.

20. Dominance - the dominant species is that species that contributes the most cover or basal area to the community, compared with other overstory species.

21. Species richness – the number of species in some area within a community.

Each species is not likely to have the same number of individuals. One species may be represented by 1,000 plants, another by 200, and a third by a single plant.

22. The distribution of individuals among the species is referred to as species evenness, or species equitability.

23. Species diversity is a combination of richness and evenness; it is species richness weighted by species evenness, and there are formulae that permit the diversity of a community to be expressed in a single index number.

Plots and Data Therein

Methods of sampling the plant community

In order to understand plant distribution, whether as species (taxonomic) or as life forms, we have to measure that distribution (quantitatively) and summarize the data in some effective way before these questions can be understood.

If one wishes to describe a particular plant community or distribution, the best way is to make a complete census of that community or distribution. This isn’t usually practical, so we measure a representative sample of those plants.

If the samples are chosen carefully, investigators feel confident in extrapolating from their sample data to estimate the true values of the parameters of the entire community. If the samples are not chosen carefully, the samples will not be representative of the true community parameters, and they are said to be biased.

There are a number of approaches to locating representative samples.

1. Complete subjectivity: Releve method – a seasoned field worker very familiar with a particular region formulates a concept of a particular plant distribution, finds a few representative stands within those distributions, and sets up a few sample plots within those stands.

2. Combination of subjective selection of stands with an objective (random or regular) placement of plots within them.

3. Random selection of stands and random placement of plots (completely randomized sampling is time-consuming and inevitably undersamples rare, but interesting and ecologically informative, kinds of vegetation).

We are going to select stands of vegetation subjectively (or regularly) and various elevations along a gradient. Those elevations have been predetermined. We will then set up plots randomly at each elevation (according to a random numbers table or the toss of a rock). Stratified: restricted random fashion.

The best sampling methods will be both accurate and precise.

Accuracy: close agreement of sample means with actual parameter means. (see p.189, Barbour)

Precision: close agreement of sample means to each other, without reference to the true mean.

Precision can be measured without knowing the true mean; accuracy can only be measured when the actual parameters are known, and in vegetation sampling this is never the case.

Cover is the percentage of plot area beneath the canopy of a given species.

The canopy of an overstory species creates a microenvironment that smaller, associated species must contend with. The overstory canopy, therefore, exerts a biotic control over the microclimate of the site. It is assumed that a comparison of cover for each species or life form in a given canopy layer will reveal the relative control or dominance that each species exerts on the community as a whole, such as relative amounts of nutrients or other resources each species commands.

Canopy coverage, tehn, can be expressed as the percentage of ground covered by canopy, when the edges of the canopy are mentally projected down to the surface.

Relative cover is the cover of a particular species or life form as a percentage of total plant cover. Relative cover will always tally up to 100%, even when absolute cover is quite low.

Density is the number of plants rooted inside the plot.

Relative density is the density of one species or life form as a percent of total plant density.

Frequency is the percentage of total plots that contains at least one rooted individual of a given species or life form. (Sociability). [Frequency is an artifact of plot size, and so is a more artificial statistic than either cover or density].

Relative frequency is the frequency of one species or life form as a percentage of total plant frequency.

Dominance the dominant species is that species that contributes the most cover or basal area to the community, compared with other overstory species.

Species richness – the number of species in some area within a community.

Each species is not likely to have the same number of individuals. One species may be represented by 1,000 plants, another by 200, and a third by a single plant.

The distribution of individuals among the species is referred to as species evenness, or species equitability.

Species diversity is a combination of richness and evenness; it is species richness weighted by species evenness, and there are formulae that permit the diversity of a community to be expressed in a single index number.

Richness and diversity are quite different, even though they are often positively correlated. Environmental gradients exist in which you can have a decrease in richness accompanied by an increase in diversity.

Community A, with 5 species but uneven numbers of individuals in each species, has a lower diversity than Community B, with four species that have a very similar number of individuals in each. Community A has higher species richness, however. Biologically, diversity is the measure of population heterogeneity of a community.