Plant Ecology
Herb-chronology
Herb-chronology –
the analysis of annual rings in perennial forbs
Approaching plant age in ecology
Plant populations and communities are commonly characterized by a variety of parameters
describing their present state (e.g. population or community density and size or stage structure). While these static parameters can be easily determined, their variations over extended
periods of time are generally difficult to obtain, particularly for perennial forbs (e.g. Harper
1977, Menges 2000). Since plant population and community underlie dynamic factors and
processes (e.g. Ehrlen 1995, Peterson et al. 1997, Roach 2004), time- and age-related data
(e.g. rates of birth, growth and death, age structure or age-dependent size structure) is essential for a thorough understanding of their development. To meet this requirement, most of
the ecological studies that explicitly included interactions with plant age have been based on
repeated measurements of the considered parameters, usually during several consecutive
years (e.g. Tamm 1956, Sarukhan & Harper 1973, Ehrlen 1995, Roach & Gampe 2004). This
‘real time’ method yields accurate data for the different responses in individual or population
development or for parameterization of population models or models of growth. However,
this longer-term approach is often not feasible due to the high efforts needed and the limited
duration of most studies to two or three years. In a few cases morphological markers can be
used for determination of age, e.g. leaf or bud-scale scars, rhizome or bulb morphology (Oleson & Ehlers 2001 and references therein). However, anatomical markers in perennial plant
parts, such as main roots or rhizomes, that resemble annual rings in woody plants have been
regarded as exceptional or disregarded (e.g. Harper 1977, Barbour et al. 1998).
Fig. 1: Root cross-section of Sanguisorba minor. Black markers delimitate annual ring borders.
Georg von Arx
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Plant Ecology
Herb-chronology
Annual growth rings in perennial forbs
A recently developed methodological approach to investigate age-related patterns in forbs is
herb-chronology − the ecologically-oriented analysis of annual rings in the secondary root
xylem of perennial forbs (e.g. Dietz & Ullmann 1997, von Arx & Dietz 2006). The anatomical
basis of discernible annual rings in the roots of forbs is the formation of earlywood vessels
with wide lumina in spring, and the development of latewood vessels with narrower lumina in
late summer and autumn (Werner 1978, Boggs & Story 1987, Dietz & Ullmann 1997, von Arx
& Dietz 2006; Fig. 1). In a few forb species, variation in vessel size is not very pronounced
but there is often a decrease in vessel density from earlywood to latewood (diffuse-porous
pattern) that helps to identify annual rings (Carlquist 2001). Similar to the better known tree
rings, discernible annual rings in forbs have proven to be a common phenomenon in many
species with persistent main roots (Zoller 1949, Bakshi & Coupland 1960, Werner 1978,
Boggs & Story 1987, Schweingruber & Dietz 2001, Dietz & Fattorini 2002, Dietz &
Schweingruber 2002, Dietz & von Arx 2005, von Arx & Dietz 2006).
Applications of herb-chronology in plant ecology
In fact, herb-chronology may represent a relatively widely applicable post hoc handle on age
and growth responses of perennial forbs. The number of annual rings in the roots of forbs
has been used for age determination, and patterns of ring width have been analysed to study
growth responses to different environmental conditions and life history adjustments in perennial. There is an extended range of possible applications investigating annual rings in forbs,
such as the following:

Community-level analyses, e.g. comparing age structure distributions among communities developing under different conditions.

Determination of population development in space and over time, e.g. elucidating the pattern and history of spread of an invasive species (Dietz et al. 1999, Dietz 2002, Dietz
2004) or the state of a population (declining, stable, increasing) by studying its age structure (Boggs & Story 1987, Dietz & Ullmann 1998, Dietz 2002, Dietz 2004).

Study of life history patterns, e.g. investigating the duration of the life cycle of biennial
forbs under different growth conditions, comparing annual growth increments of generative versus vegetative plants or estimating the longevity of individuals of a given species
growing under different conditions (Boggs & Story 1987, Kuen & Erschbamer 2002,
Erschbamer & Retter 2004, von Arx et al. 2006).

Responses of plant growth to environmental gradients, e.g. studying the effects of (i) gradients of light supply (Dietz & Ullmann 1998), (ii) duration of snow cover (Dietz, et al.
2004), (iii) varying competition intensity, (iv) altitudinal gradients (von Arx et al. 2006), (v)
different disturbance regimes (Rixen et al. 2004), or (vi) different micro-habitat conditions.

Effects of global climate change on growth in forbs, e.g. detecting coherent signals of extreme climatic fluctuations in the root ring patterns (Dietz & von Arx 2005).

Multivariate reconstructive analyses, e.g. combining data on age, position, size and genetic makeup of individuals to obtain more detailed insights in the processes governing
population development (Dietz et al. 1999).

Population dynamics of adventitious roots, e.g. determining root turnover rates and reconstructing chronologies of root population development. Several of the previous studies
using herb-chronology indicated that its applicability may depend on (i) the taxonomic affiliation of the species, (ii) the type and persistency of belowground plant parts (main
roots, adventitious roots and rhizomes), (iii) the climatic zone and (iv) the type of habitat.
Georg von Arx
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Plant Ecology
Herb-chronology
Keep up-to-date with herb-chronology
If you want to know more about herb-chronology, its applications, methods, developments or
a comprehensive list of published scientific projects using herb-chronology, please visit:
http://www.plantecology.ethz.ch/spotlights/herbchronology
Literature cited
Bakshi TS, RT Coupland. 1960. Vegetative propagation in Linaria vulgaris. Canadian Journal of Botany 38:243-249.
Barbour MG, JH Burk, WD Pitts. 1998. Terrestrial plant ecology. Addison Wesley Longman, Menlo
Park. 649 pp.
Boggs KW, JM Story. 1987. The population age structure of spotted knapweed (Centaurea maculosa)
in Montana. Weed Science 35:194-198.
Carlquist S. 2001. Comparative wood anatomy: systematic, ecological, and evolutionary aspects of
dicotyledon wood. Springer, Berlin. 448 pp.
Dietz H. 2002. Plant invasion patches − reconstructing pattern and process by means of herbchronology. Biological Invasions 4:211-222.
--- 2004 Plant invasion patches revisited − Changes in development. Weed Technology 18:1381-1385.
Dietz H, M Fattorini. 2002. Comparative analysis of growth rings in perennial forbs grown in an alpine
restoration experiment. Annals of Botany 90:663-668.
Dietz H, M Fischer, B Schmid. 1999. Demographic and genetic invasion history of a 9-year-old roadside population of Bunias orientalis L. (Brassicaceae). Oecologia 120:225-234.
Dietz H, FH Schweingruber. 2002. Annual rings in native and introduced forbs of lower Michigan,
USA. Canadian Journal of Botany 80:642-649.
Dietz H, I Ullmann. 1997. Age-determination of dicotyledonous herbaceous perennials by means of
annual rings: Exception or rule? Annals of Botany 80:377-379.
--- 1998 Ecological application of 'Herbchronology': Comparative stand age structure analyses of the
invasive plant Bunias orientalis L. Annals of Botany 82:471-480.
Dietz H, G von Arx. 2005. Climatic fluctuation causes large-scale synchronous variation in radial increments of the main roots of northern hemisphere forbs. Ecology 86:327-333.
Dietz H, G von Arx, S Dietz. 2004. Growth patterns in two alpine forbs as preserved in annual rings of
the roots: the influence of a snowbank gradient. Arctic, Antarctic, and Alpine Research 36:591597.
Ehrlen J. 1995. Demography of the perennial herb Lathyrus vernus 2. Herbivory and populationdynamics. Journal of Ecology 83:297-308.
Erschbamer B, V Retter. 2004. How long can glacier foreland species live? Flora 199:500-504.
Harper JL. 1977. Population biology of plants. Academic Press, London. 892 pp.
Kuen V, B Erschbamer. 2002. Comparative study between morphology and age of Trifolium pallescens in a glacier foreland of the Central Alps. Flora 197:379-384.
Menges ES. 2000. Population viability analyses in plants: challenges and opportunities. Trends in
Ecology & Evolution 15:51-56.
Oleson JM, BK Ehlers. 2001. Age determination of individuals of Corydalis species and other perennial herbs. Nordic Journal of Botany 21:187-193.
Peterson DL, EG Schreiner, NM Buckingham. 1997. Gradients, vegetation and climate: spatial and
temporal dynamics in the Olympic mountains, USA. Global Ecology and Biogeography Letters
6:7-17.
Rixen C, A Casteller, FH Schweingruber, V Stoeckli. 2004. Age analysis helps to estimate plant performance on ski pistes. Botanica Helvetica 114:127-138.
Roach DA, J Gampe. 2004. Age-specific demography in Plantago: Uncovering age-dependent mortality in a natural population. American Naturalist 164:60-69.
Sarukhan J, JL Harper. 1973. Studies on plant demography − Ranunculus repens L, R. bulbosus L
and R. acris L. 1. Population flux and survivorship. Journal of Ecology 61:675-716.
Georg von Arx
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Plant Ecology
Herb-chronology
Schweingruber FH, H Dietz. 2001. Annual rings in the xylem of dwarf shrubs and perennial dicotyledonous herbs. Dendrochronologia 19:115-126.
Tamm CO. 1956. Further observations on the survival and flowering of some perennial herbs. Oikos
7:274-292.
von Arx G, H Dietz. 2006. Growth rings in the roots of temperate forbs are robust annual markers.
Plant Biology 8:224-233.
von Arx G, PJ Edwards, H Dietz. 2006. Evidence for life history changes in high altitude populations of
three perennial forbs. Ecology 87: 665-674.
Werner PA .1978. On the determination of age in Liatris aspera using cross-sections of corms: implications for past demographic studies. The American Naturalist 112:1113-1120.
Zoller H. 1949. Beitrag zur Altersbestimmung von Pflanzen aus der Walliser Felsensteppe. Bericht
über das Geobotanische Forschungsinstitut Rübel in Zürich 61-68.
Georg von Arx
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Plant Ecology
Herb-chronology
Materials necessary for herb-chronological analysis
Root collection and storage
1 spate
2 small shovels
1 sharp knife
1 pair of hedge shears
200 zip-bags for root storage
2 permanent markers (labelling bags)
1 l Ethanol 50 % (for conservation of roots)
Making of cross-sections
Sledge microtome, with thickness 20-60 m, in 5-m-steps or smaller: e.g. Euromex
10-20 rasor blades
Dissecting microscope: continuous zoom 6-50, e.g. Leica MZ8
Digital camera: e.g. Nikon CoolPix 990 & adapter
Object slide, 50 pcs.
Covering glasses 1818 mm, 200 pcs.
Covering glasses 2126 mm, 100 pcs.
50 ml-dropper bottle HCl. conc. (32%)
50 ml-dropper bottle Phloroglucinol (20 ml): 1 spattle of Phloroglucinol in 50 ml of 75%-ethanol
2 brushes for cutting with microtome (ethanol): intermediate and thin
2 brushes for cutting with microtome (acid): intermediate and thin
1 dl Ethanol 100% (for cuttings with microtome)
1 little bowl for Ethanol 100%
Copper wire as a scale reference: e.g. 50 m diameter
1 roll of paper towels
1 chopping board
1 laboratory overcoat
1 small bottle mineral oil for gliding surface of sledge microtome: e.g. Motorex
Georg von Arx
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