Root cross-section of Penstemon venustus

Herbchronology is the analysis of annual

growth rings Dendrochronology (or tree-ring dating) is the scientific method of dating tree rings (also called growth rings) to the exact year they were formed. As well as dating them, this can give data for dendroclimatology, the study of climate and atmos ...

(or simply annual rings) in the secondary root xylem of

perennial A perennial plant or simply perennial is a plant that lives more than two years. The term ('' per-'' + '' -ennial'', "through the years") is often used to differentiate a plant from shorter-lived annuals and biennials. The term is also wide ...

herbaceous plants Herbaceous plants are vascular plants that have no persistent woody stems above ground. This broad category of plants includes many perennials, and nearly all annuals and biennials. Definitions of "herb" and "herbaceous" The fourth edition of t ...

. While leaves and stems of perennial herbs die down at the end of the growing season the root often persists for many years or even the entire life. Perennial herb species belonging to the

dicotyledon The dicotyledons, also known as dicots (or, more rarely, dicotyls), are one of the two groups into which all the flowering plants (angiosperms) were formerly divided. The name refers to one of the typical characteristics of the group: namely, t ...

group (also known as perennial

forbs A forb or phorb is an herbaceous flowering plant that is not a graminoid (grass, sedge, or rush). The term is used in biology and in vegetation ecology, especially in relation to grasslands and understory. Typically these are dicots without woo ...

) are characterized by

secondary growth In botany, secondary growth is the growth that results from cell division in the cambia or lateral meristems and that causes the stems and roots to thicken, while primary growth is growth that occurs as a result of cell division at the tips of ...

, which shows as a new growth ring added each year to persistent roots. About two thirds of all perennial dicotyledonous herb species with a persistent root that grow in the strongly seasonal zone of the northern hemisphere show at least fairly clear annual growth rings. Counting of annual growth rings can be used to determine the age of a perennial herb similarly as it is done in trees using

dendrochronology Dendrochronology (or tree-ring dating) is the scientific method of dating tree rings (also called growth rings) to the exact year they were formed. As well as dating them, this can give data for dendroclimatology, the study of climate and atmos ...

. This way it was found that some perennial herbs live up to 50 years and more.

History

The term herb-chronology is referring to

because of the similarity of the structures investigated. The term was introduced in the late 1990s, however, the existence of annual rings in perennial herbs was already observed in earlier times by several researchers.

Annual growth rings

Like trees and woody plants, perennial herbs have a growth zone called

vascular cambium The vascular cambium is the main growth tissue in the stems and roots of many plants, specifically in dicots such as buttercups and oak trees, gymnosperms such as pine trees, as well as in certain other vascular plants. It produces secondary xyle ...

between the root bark and the root xylem. The vascular cambium ring is active during growing season and produces a new layer of xylem tissue or growth ring every year. This addition of a new lateral layer each year is called

and is exactly the same as in woody plants. Each individual growth ring consists of

earlywood Wood is a porous and fibrous structural tissue found in the stems and roots of trees and other woody plants. It is an organic materiala natural composite of cellulose fibers that are strong in tension and embedded in a matrix of lignin tha ...

tissue that is formed at the beginning of the growing season and latewood tissue formed in summer and fall. Earlywood tissue is characterized by wide

vessels Vessel(s) or The Vessel may refer to: Biology *Blood vessel, a part of the circulatory system and function to transport blood throughout the body *Lymphatic vessel, a thin walled, valved structure that carries lymph *Vessel element, a narrow wat ...

or denser arrangement of vessels, whereas latewood tissue shows narrower vessels and/or lower vessel density. Annual growth rings in herbs are usually only visible by means of a microscope and a specific staining method. Ring-like patterns visible in root cross-sections by the naked eye may be "false rings". The width of an annual growth ring depends on conditions during its formation: in a favorable year, a ring is wider, and in a less favorable year it is narrower.

Applications

Herbchronology is used in many fields of ecological and biological research, for instance in community ecology,

population biology The term population biology has been used with different meanings. In 1971 Edward O. Wilson ''et al''. used the term in the sense of applying mathematical models to population genetics, community ecology, and population dynamics. Alan Hastings us ...

, plant ecology and invasion biology. Herbchronology is used as a tool to estimate plant age. This may be relevant information to determine : *… the longevity of a perennial

herbaceous plant Herbaceous plants are vascular plants that have no persistent woody stems above ground. This broad category of plants includes many perennials, and nearly all annuals and biennials. Definitions of "herb" and "herbaceous" The fourth edition of t ...

species; this is valuable information for flora books and an important aspect of life history strategy of a species. *… the population age structure; this is valuable for questions into population dynamics or the spreading patterns of invasive species. Herbchronology allows to assess long-term annual growth rates of a perennial

without having to monitor it. This may be relevant information to assess … *… the impact of fluctuations in specific climatic conditions on growth. *… the impact of changes in site conditions such as competition. *… lifetime growth patterns.

References

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External links

Herb-chronology web pagesThe Xylem Database
Dating methodologies in archaeology Botany Incremental dating