2.1. Life history and maximum plant lifespan

Plant lifespan (usually measured in years) is defined as the time period from establishment until no live part remains of the respective individual. Maximum plant lifespan is an indicator of population persistence and is therefore strongly related to land use and climate change. Lifespan is limited in non-clonal plants but may be apparently nearly unlimited in clonal plants. Maximum lifespan is strongly positively associated with environmental stress regimes, e.g. low temperatures and low nutrient availability. The relationship with disturbance frequency is mostly negative, although long-lived (resprouting) clonal plants may also tolerate frequent disturbance. There may be a trade-off between maximum lifespan and dispersal in time and space. Long-lived species often exhibit a short-lived seed bank and produce seeds or fruits with low dispersal potential, in contrast to short-lived species, which often have a very long-lived seed bank and/or high dispersal potential.

How to assess?

    (A) Life history

This simple classification distinguishes among the common types of timing and duration of survival behaviour of individual plants in the absence of disturbances or catastrophes.

(1) Annual. Plant senesces and dies at the end of its first growing season (from seed), after producing seed, which may propagate a new plant in the future (a winter annual germinates in late summer or autumn, and so has two seasons, although the first may be very short).

(2) Biennial. Plant grows vegetatively the first season, then flowers in the second to produce seed, followed by senescence and death of the shoot and root system.
 
(3) Perennial. The individual survives for at least three growing seasons.
 
(a) Monocarpic perennials. After several to many seasons of vegetative growth, the plant produces seeds, then senesces and dies.
 
(b) Polycarpic perennials. All or much of the stem and root system normally survives the harsh or dormant period between growing seasons; stem has lateral thickening over the years. (i) Herbaceous perennial. Aerial shoots (and sometimes roots) die off as growing season ends; in the next season, new shoots grow from a perennating organ such as a bulb, corm, rhizome or ‘root crown’ (bud-bearing stem base or hemicryptophytes) near or below ground surface. (ii) Woody perennial retains, from one growing season into the next, some living, leaf-bearing shoots, which die by the end of their third season or later.
 
Qualitative distinction between life-history classes

A plant with any perennating organ other than the seed is either a perennial or a biennial (the latter only by a storage taproot). If biennial, there should be individuals with a storage root but not an inflorescence, and others with both. A plant that lacks specialised perennating organs may still be perennial, by resprouting from its root-crown. If so, the crown will normally carry wrinkles or scars from bud outgrowth in previous seasons, and can eventually become quite thick and even woody (a caudex); in contrast, the root of an annual is usually relatively soft and smooth, its thickness extending continuously into the stem. A perennial in its first year of growth may resemble an annual in these respects, except that perennial wild plants usually do not flower in their first year, whereas an annual always does (many horticultural perennials, however, have been selected to do so).

    (B) Maximum plant lifespan quantitative assessment

In gymnosperms and angiosperms, even in some non-woody ones, species maximum lifespan can be estimated by counting the number of annual rings representing annual tissue increments. Recently, a study on 900 temperate herbaceous species revealed annual rings in perennating structures in more than 80% of the species. However, the formation of annual rings can depend on habitat conditions. Annual rings will be found in vegetation zones with clear seasonality (cold (winter) or drought seasons) such as the polar, boreal or austral, temperate and even in Mediterranean-type zones. In the two latter climate zones, annual rings may sometimes be absent. In some cases, annual rings may even be found in tropical species, especially in regions with a distinct dry and wet season. Maximum lifespan within a population is studied in the largest and/or thickest individuals. Data are collected from a minimum of 10, preferably 20 individuals (replicates). In woody species (trees, shrubs, dwarf shrubs), annual rings are determined either by cutting out a whole cross-section or a ‘pie slice’ of the main stem (trunk), or by taking a core with a pole-testing drill (tree corer). It is important to obtain a rather smooth surface for clear observation. The annual rings can usually be counted under a dissection microscope. Often a cross-section of a shoot does not represent the maximum age as precisely as the root collar (root-stem transition zone of primary roots), which is especially true for most shrubs where single shoots have a limited age. We, therefore, recommend digging out woody plants a bit and taking (additional) samples from the root collar. In herbaceous species, annual rings are mostly found at the shoot base or in the root collar, and also in rhizomes. Here, microscopic cross-sections are essential and have to be treated first by ‘eau de javelle’ to remove the cytoplasm and then stained (fuchsin, chryosidine, astrablue (FCA); alternatively, astrablue and safranin) to make the annual rings visible. In some cases, polarised light has proven to be useful to identify the annual rings. Maximum lifespan of a species or population is defined as the largest number of annual rings counted among all samples (although the mean lifespan of all individuals may be informative too).

Special cases or extras

(1) In clonal plants, the identification of (maximum) lifespan is more complicated. If a ramet never becomes independent from the genet and will never be released from the mother plant, annual rings in the tap root (e.g. Armeria maritima, Silene acaulis) or annual morphological markers along the rhizome or stolon (e.g. Lycopodium annotinum, Dictamnus albus) are also a suitable tool to identify maximum lifespan of a genet. In the latter case, maximum lifespan can be higher if part of the rhizome or stolon is already decomposed. However, in clonal plants where the genet consists of more or less independent ramets, genet age can be estimated only indirectly by means of size or diameter of a genet in relation to mean annual size increment.

(2) Geophyte species, especially monocotyledons, may disappear above ground for up to several years before reappearing. In such cases, only permanent-plot research with individually marked individuals will give an idea about the maximum lifespan of those species.
 
(3) Cold-climate dwarf shrubs. In some of these species, e.g. the heather (Cassiope tetragona), lateral annual rings are often very hard to discern, whereas annual shoot-length increments of woody stems can be distinguished under a microscope through the winter-mark septa separating them and through the annual sequence of distances between leaf scars.

(4) Life history and location. Life history varies with location and should preferably be assessed in the field rather than by reference to floras. In particular, many short-lived, faster-growing species fall into different life-history categories in different regions and a few differ among habitats, even within the same region.

References on theory and significance: Rabotnov (1950); Schweingruber (1996); Fischer and Stöcklin (1997); Larson (2001); Schweingruber and Poschlod (2005); De Witte and Stöcklin (2010).

More on methods: Tamm (1972); Gatsuk et al. (1980); Cherubini et al. (2003); Rozema et al. (2009).