In brief
Wild plants whose presence reveals the moisture, acidity, nutrient supply, light or salt of a site, so that the vegetation can be read like a soil report.
What are indicator plants?
Every plant species needs certain conditions to hold its own against its neighbours. Some need wet feet, others tolerate months of drought. Some only grow on acid ground, others on lime. Some follow nitrogen wherever animals rest or dung accumulates. Where a species occurs in numbers, it therefore says something about the site, and a botanist can read a meadow, a field margin or a slope like a first soil report. Plants used in this way are called indicator plants.
A single plant is weak evidence. Seeds arrive by chance, many species tolerate a wide range of conditions, and a plant may survive where it was once favoured long after conditions have changed. The reading becomes reliable when the whole community is considered: if several species point in the same direction, the conclusion holds.
Ellenberg values, explained briefly
The best known system comes from the German botanist Heinz Ellenberg, who first published indicator values for the vascular plants of Central Europe in 1974. Each species receives a number for each of several site factors:
- L for light, from deep shade to full sun
- T for temperature, from alpine cold to warm lowlands
- K for continentality, from oceanic to continental climate
- F for moisture, from very dry to permanently submerged
- R for soil reaction, from strongly acid to calcareous
- N for nutrients, originally read as nitrogen, from very poor to very rich
- S for salt tolerance
Most scales run from 1 to 9, the moisture scale up to 12. An x marks species that are indifferent to a factor. Ellenberg stressed that the numbers describe where a species actually occurs under competition, not where it would grow best in a garden without rivals. The usual practice is to average the values of all species found on a plot, which smooths out the oddities of single plants.
The values were calibrated on Central European vegetation, and under a Mediterranean climate many species behave differently. For Italy, Sandro Pignatti and colleagues published indicator values for the entire Italian flora in 2005, following the same principle but based on how species behave on the peninsula and the islands. Because conditions in the south and on the islands go beyond those of Central Europe, they extended the scales for light and temperature from 9 to 12 steps. Since 2023 a Europe-wide system, EIVE, has combined 31 existing systems on a common scale of 0 to 10, for moisture, nutrients, soil reaction, light and temperature. It has no axis for salt. For Sardinia the Italian values are the more fitting reference, and EIVE serves as a second opinion.
What plants reveal in Sardinia
The list below is deliberately short. A species appears only where at least two sources agree on what it indicates: the Italian values of Pignatti and colleagues (2005) and the European EIVE dataset (2023), and, for roles that no number describes, such as grazing, fire or salt, the specialist literature. Numbers are given as Pignatti and EIVE. The two scales differ, so only the direction counts: low means acid, dry or poor, high means the opposite. All species are widespread in the western Mediterranean.
- Acid soils: tree heath, French lavender, bracken and cork oak have low reaction values in both systems (Pignatti 1 to 3, EIVE 3.3 to 3.8). The strawberry tree lies only a little above (4 and 4.3), so it counts as mildly acid. They typically grow on soils over granite or schist. Rosemary is the opposite case: neutral to slightly basic (6 and 7.0).
- Dry, sunny, poor: sage-leaved rock rose, Montpellier cistus, conehead thyme, rosemary and French lavender rate 2 for moisture in the Italian system and 1.9 to 2.6 in EIVE. Their nutrient values are low as well (Pignatti 1 to 2, EIVE 1.2 to 2.7). Mastic tree, tree spurge and asphodel are just as dry. These are plants of the macchia and its degraded stages.
- Fire: in laboratory trials, rock-rose seeds germinate better after a short burst of heat (Scuderi et al. 2010), and rock roses count among the early colonists of burnt ground. A dense stand of them can therefore point to an earlier fire, without proving it.
- Grazing and fire: asphodel is most abundant in degraded stages of Mediterranean vegetation that have been heavily grazed and repeatedly burnt. Most livestock leave it alone and fire favours it (Terzi 2023). Where it dominates, the palatable plants have long been eaten down. It does not reveal how long ago.
- Rich in nitrogen, often disturbed: stinging nettle (Pignatti 8, EIVE 8.7), small nettle (8 and 8.5), annual mercury (8 and 8.1), false yellowhead (9 and 7.1) and blackberry (8 and 6.5) gather where animals rest, where dung or ash has accumulated, around old stalls and ruins. Not every nitrogen-rich soil carries nettles, so their absence proves little.
- Moisture: soft rush (Pignatti 9, EIVE 6.2), compact rush (8 and 6.2) and common reed (10 and 7.5) mark ground that stays wet for long periods. Pennyroyal (7 and 6.0) grows in hollows that flood in winter and dry out in summer. Oleander (7 and 5.5) lines watercourses, and the European habitat list names it in the type of southern riparian thickets.
- Salt: glassworts, their shrubby relative Sarcocornia, sharp rush, sea rush and sea lavenders mark saline ground on coasts and around lagoons. Pignatti rates them 3 for salt, the highest level, for plants that grow in high salt concentration. European habitat types for salt marshes and coastal cliffs are named after them.
- Trampling: greater plantain, knotgrass and annual meadow grass are core species of trampled ground on paths and around gateways. They show that ground is trodden, not that it is dense. The spade test shows the density.
Limits of the method
Indicator plants describe the conditions their roots experience, which is not always what a laboratory would measure in the topsoil. Many Mediterranean species are annuals that appear only between autumn and spring and vanish in summer, so a reading taken in August misses half the picture. And the vegetation reflects history as much as the present: a stand of nettles may record a stall that disappeared decades ago. That is why indicator plants belong together with texture by feel, the spade test and, where needed, a laboratory analysis.
The numbers have limits of their own. EIVE merges many existing systems, and for Italian species an Italian system is probably among them, so the agreement of the two datasets is not entirely independent. Some well-known plants are missing from the list because the sources do not support them clearly enough. For giant fennel the two systems disagree about nutrients, and on Sardinia a genetic study found poisonous and non-poisonous forms (Marchi et al. 2003), which makes it a poor grazing signal. Bermuda grass as a trampling plant and milk thistle as a nitrogen indicator each rest on a single source.
How Terra Humana understands them
For Terra Humana, indicator plants are the first conversation with a piece of land, before anything is planted. They tell us where water collects, where the ground is tired or trodden, where nitrogen is already present. Plants that appear on their own are not dismissed as weeds: as the article on pioneer plants puts it, they show up where there is work to do. Reading them is also a way of following ecological succession: as a site recovers, the plant community changes, and with it what the vegetation says about the soil.
Sa Medada
What this looks like at Sa Medada
The LiDAR wetness map of Sa Medada shows a comparatively dry main area and moister channels along the edges. Indicator plants could help to check such a map on the ground: the expectation is that species which need moisture gather in the channels and species of dry ground on the main area. Whether that holds can only be seen in the vegetation itself. Project Eden begins with pioneer plants, ground cover and holding water, and what grows there of its own accord helps to show where each of these is needed most.
More about Project EdenPlants in this entry
Strawberry treeArbutus unedo RockroseCistus Mastic treePistacia lentiscus
Frequently asked
What are indicator plants?
Indicator plants are wild species whose presence says something about the conditions of the place where they grow: how moist or dry the soil is, how acid or calcareous, how rich in nutrients, how much light reaches the ground, whether there is salt. They work because every species can only hold its own against competitors within a certain range of conditions. Where it thrives in numbers, the site is likely to lie within that range. A single plant proves little, but the combination of all species growing together gives a reliable first picture of a site.
What are Ellenberg indicator values?
They are numbers that the German botanist Heinz Ellenberg assigned to the vascular plants of Central Europe, first published in 1974. Each species receives a value for light, temperature, continentality, moisture, soil reaction, nutrients and salt, mostly on a scale from 1 to 9, for moisture up to 12. An x marks species that are indifferent to a factor. The values describe where a species occurs under competition in the wild, not where it would grow best on its own. Averaged over all species of a stand, they give a robust estimate of the site.
Do Ellenberg values work in the Mediterranean?
Only with caution, because they were calibrated on Central European vegetation, and many species behave differently under a Mediterranean climate. For Italy, Sandro Pignatti and colleagues published indicator values for the whole Italian flora in 2005, following the same principle but based on the species' behaviour on the peninsula and the islands. Since 2023 there is also a Europe-wide system, EIVE, which combines 31 existing systems for moisture, nutrients, soil reaction, light and temperature. For Sardinia the Italian values are the better starting point, and EIVE a useful second opinion.
Which indicator plants grow in Sardinia?
Several widespread species of the western Mediterranean give a clear signal in both the Italian and the European system. Tree heath, French lavender, bracken and cork oak point to acid soils. Rock roses, conehead thyme and rosemary mark dry, sunny, nutrient-poor ground. Stinging nettle, annual mercury and false yellowhead point to nitrogen, rushes and common reed to wet soil, glassworts and sea lavenders to salt. Which of them grows on a given site has to be found out on the ground, and a single plant says little without the community around it.
Which plants indicate trampled or compacted soil?
Plants cannot measure compaction, but they do show trampling. Greater plantain, knotgrass and annual meadow grass are core species of trampled ground on paths and around gateways, because they tolerate being trodden on. Where such ground also floods in winter, pennyroyal may join them, although pennyroyal itself points to alternating wet and dry conditions rather than to compaction. A spade test on the same spot shows whether the soil beneath is really dense, and it is the better evidence.
Is asphodel a sign of overgrazing?
Often, but not on its own. Asphodel is most abundant in degraded stages of Mediterranean vegetation that have been heavily grazed and repeatedly burnt, because most livestock leave it largely alone and fire favours it. A pasture dominated by asphodel therefore tells of grazing and fire pressure over a long time, but not how long ago it was or whether it still goes on. Giant fennel is often named alongside it. We leave it out, because the indicator values disagree and none of the sources we read shows it as a grazing indicator.
Related terms
See this in practice
Sources
- Heinz Ellenberg et al., Zeigerwerte von Pflanzen in Mitteleuropa, Scripta Geobotanica 18, 3rd edition (2001)
- Sandro Pignatti, Patrizia Menegoni and Silvio Pietrosanti, Valori di bioindicazione delle piante vascolari della Flora d'Italia, Braun-Blanquetia 39 (2005)
- Jürgen Dengler et al., Ecological Indicator Values for Europe (EIVE) 1.0, Vegetation Classification and Survey 4 (2023)
- Massimo Terzi, A new Asphodelus ramosus-dominated association from the Murge Plateau (SE Italy), Hacquetia 22(2) (2023)
- Domenica Scuderi et al., Germination behaviour of four Mediterranean Cistus L. species in relation to high temperature, Ecological Questions 12 (2010)
- Annalisa Marchi, Giovanni Appendino, Ivo Pirisi, Mauro Ballero and Maria Cecilia Loi, Genetic differentiation of two distinct chemotypes of Ferula communis (Apiaceae) in Sardinia (Italy), Biochemical Systematics and Ecology 31 (2003)
- Council Directive 92/43/EEC on the conservation of natural habitats and of wild fauna and flora, Annex I (1992)
Last updated: 2026-10-12