Herbchronology
Encyclopedia
Herbchronology is the analysis of annual growth rings (or simply annual rings) in the secondary root xylem
Xylem
Xylem is one of the two types of transport tissue in vascular plants. . The word xylem is derived from the Classical Greek word ξυλον , meaning "wood"; the best-known xylem tissue is wood, though it is found throughout the plant...

 of perennial herbaceous plants. 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
Dicotyledon
The dicotyledons, also known as dicots, are a group of flowering plants whose seed typically has two embryonic leaves or cotyledons. There are around 199,350 species within this group...

 group (also known as perennial forbs) are characterized by secondary growth
Secondary growth
In many vascular plants, secondary growth is the result of the activity of the two lateral meristems, the cork cambium and vascular cambium. Arising from lateral meristems, secondary growth increases the girth of the plant root or stem, rather than its length. As long as the lateral meristems...

, 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
Dendrochronology or tree-ring dating is the scientific method of dating based on the analysis of patterns of tree-rings. Dendrochronology can date the time at which tree rings were formed, in many types of wood, to the exact calendar year...

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

History

The term herb-chronology is referring to dendrochronology
Dendrochronology
Dendrochronology or tree-ring dating is the scientific method of dating based on the analysis of patterns of tree-rings. Dendrochronology can date the time at which tree rings were formed, in many types of wood, to the exact calendar year...

 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
Vascular cambium
The vascular cambium is a part of the morphology of plants. It consists of cells that are partly specialized, for the tissues that transport water solutions, but have not reached any of the final forms that occur in their branch of the specialization graph...

 between the root bark and the root xylem
Xylem
Xylem is one of the two types of transport tissue in vascular plants. . The word xylem is derived from the Classical Greek word ξυλον , meaning "wood"; the best-known xylem tissue is wood, though it is found throughout the plant...

. The vascular cambium
Vascular cambium
The vascular cambium is a part of the morphology of plants. It consists of cells that are partly specialized, for the tissues that transport water solutions, but have not reached any of the final forms that occur in their branch of the specialization graph...

 is active during growing season and produces a new layer of xylem
Xylem
Xylem is one of the two types of transport tissue in vascular plants. . The word xylem is derived from the Classical Greek word ξυλον , meaning "wood"; the best-known xylem tissue is wood, though it is found throughout the plant...

 tissue or growth ring every year. This addition of a new lateral layer each year is called secondary growth
Secondary growth
In many vascular plants, secondary growth is the result of the activity of the two lateral meristems, the cork cambium and vascular cambium. Arising from lateral meristems, secondary growth increases the girth of the plant root or stem, rather than its length. As long as the lateral meristems...

 and is exactly the same as in woody plants. Each individual growth ring consists of earlywood 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 element
A vessel element is one of the cell types found in xylem, the water conducting tissue of plants. Vessel elements are typically found in the angiosperms but absent from most gymnosperms such as the conifers....

 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 roots cross sections by the naked eye may be false rings.

The width of annual growth rings depends on conditions during formation; in a favorable year it is wider and in a less favorable year narrower.

Applications

Herbchronology is used in many fields of ecological and biological research, for instance in community ecology, population biology
Population biology
Population biology is a study of populations of organisms, especially the regulation of population size, life history traits such as clutch size, and extinction...

, plant ecology
Plant ecology
Plant ecology is a subdiscipline of ecology which studies the distribution and abundance of plants, the interactions among and between members of plant species, and their interactions with their environment...

 and invasion biology
Invasion biology terminology
The need for a clearly defined and consistent invasion biology terminology has been acknowledged by many sources. Invasive species, or invasive exotics, is a nomenclature term and categorization phrase used for flora and fauna, and for specific restoration-preservation processes in native habitats...

.

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 plant
    A herbaceous plant is a plant that has leaves and stems that die down at the end of the growing season to the soil level. They have no persistent woody stem above ground...

     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 herbaceous plant
Herbaceous plant
A herbaceous plant is a plant that has leaves and stems that die down at the end of the growing season to the soil level. They have no persistent woody stem above ground...

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.

External links

The source of this article is wikipedia, the free encyclopedia.  The text of this article is licensed under the GFDL.
 
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