Braun-Blanquet Relevé Method

Braun-Blanquet Relevé Method (also termed the Zurich–Montpellier floristic-sociological approach or sigmatist phytosociology, after the Station Internationale de Géobotanique Méditerranéenne et Alpine [SIGMA] founded in Montpellier in 1930) is the foundational field-sampling, tabular sorting, and hierarchical classification methodology of modern vegetation science (phytosociology). Codified by the Swiss phytogeographer and plant ecologist Josias Braun-Blanquet (1884–1980) in his treatise Pflanzensoziologie: Grundzüge der Vegetationskunde (1st German edition 1928; English translation Plant Sociology: The Study of Plant Communities by George D. Fuller and Henry S. Conard, 1932; 3rd expanded edition 1964) building upon earlier concepts of Charles Flahault (1852–1935) and Carl Schröter (1855–1939), the method centers on the relevé (French: relevé floristique; German: Vegetationsaufnahme): a standardized, exhaustive floristic and structural inventory of a single, ecologically and physiognomically homogeneous stand of vegetation whose plot area meets or exceeds the community's minimal area (Minimalareal).

Within each relevé plot, the investigator records environmental and structural header metadata (geographic coordinates, elevation, slope aspect and inclination, soil profile, and vertical vegetation stratification across tree, shrub, herb, and bryophyte/lichen layers) and assigns every vascular plant, bryophyte, and terricolous lichen species present a combined score on the seven-degree Braun-Blanquet cover–abundance scale (r, +, 1, 2, 3, 4, 5) paired with a five-degree sociability index (Soziabilität, 1 to 5) describing horizontal spatial clumping. By compiling tens to thousands of individual relevés into a raw species-by-sample matrix and iteratively rearranging both rows (species) and columns (relevés) into a differentiated phytosociological table (Differentialtabelle), the method identifies diagnostic character species (Charakterarten / Kennarten exhibiting high ecological fidelity [Treue]), differential species (Differentialarten / Trennarten), and constant companion species (Begleiter). These floristically defined abstract community units (syntaxa) are then named and arranged into a formal, internationally regulated hierarchy—Association (suffix -etum), Alliance (-ion), Order (-etalia), and Class (-etea)—governed by the International Code of Phytosociological Nomenclature (ICPN) and synthesized in global repositories such as the European Vegetation Archive (EVA) and sPlot.

From Flahault and Schröter's 1910 Brussels Congress to SIGMA (1930)

During the early twentieth century, plant ecology split into three major traditions regarding how plant communities (Pflanzengesellschaften) should be defined and sampled:

1. The Anglo-American Dynamic/Climax School (Clements, 1916; Tansley, 1920): Emphasized broad physiognomic formations and successional development toward a regional monoclimax or polyclimax, relying on dominant canopy species.
2. The Uppsala (Nordic) School (Du Rietz, 1921, 1930): Focused on species-poor boreal and alpine vegetation where small-scale sociations were defined by the quantitative dominance of species across individual vertical strata (synusiae).
3. The Zurich–Montpellier (Franco-Swiss) School (Flahault, Schröter, Rübel, Pavillard & Braun-Blanquet): Working in the floristically rich landscapes of the Swiss Alps and the French Mediterranean basin—where canopy dominance alone fails to distinguish communities on calcareous vs. siliceous soils—Charles Flahault and Carl Schröter proposed at the Third International Botanical Congress in Brussels (1910) that the fundamental unit of vegetation classification, the plant association (assoziation), is "a plant community of definite floristic composition, presenting a uniform physiognomy, and growing in uniform habitat conditions."

Trained under Schröter and Eduard Rübel in Zurich and Flahault and Jules Pavillard in Montpellier, Josias Braun-Blanquet synthesized their concepts into an operational protocol between 1913 and 1928. In 1930, Braun-Blanquet founded SIGMA in Montpellier and later launched the journal Vegetatio (now Plant Ecology) in 1948 alongside Reinhold Tüxen (1899–1980), who organized systematic phytosociological mapping across Central Europe.

Core Epistemological Tenets of the Floristic-Sociological Synthesis

As formalized by Braun-Blanquet (1928, 1964), Dieter Mueller-Dombois & Heinz Ellenberg (1974), and Victor Westhoff & Eddy van der Maarel (1973, 1978), the Zurich–Montpellier approach rests on three empirical premises:

• Complete Floristic Composition Overrides Single-Species Dominance: Every plant species occupies a multidimensional ecological niche (Grime's CSR plant strategy theory; niche complementarity hypothesis). Whereas a single dominant canopy species (such as Fagus sylvatica or Pinus sylvestris) often spans acidic podzols to calcareous rendzinas, recording the full assemblage of vascular plants, bryophytes, and lichens—especially stenotopic understory herbs and cryptogams—provides a sensitive biological integrator of site conditions.
• Concrete Stands (Phytocoenoses) vs. Abstract Community Types (Syntaxa): Braun-Blanquet distinguished between the concrete vegetation stand sampled in the field by a single relevé and the abstract vegetation unit (association or syntaxon) derived inductively by comparing multiple concrete stands in a phytosociological table.
• Diagnostic Species Fidelity (Treue) as the Basis of Classification: Abstract vegetation units are circumscribed by groups of diagnostic species (character species and differential species) that show high ecological fidelity (Treue)—strong preference or restriction to a specific syntaxon relative to all other regional communities.

1. Stand Homogeneity and Minimal Area (Minimalareal)

Unlike mechanical grid sampling—where plots placed at random coordinates may straddle an ecotone between a sedge fen and an adjacent dry grassland—the Braun-Blanquet relevé uses preferential stratified sampling (Mueller-Dombois & Ellenberg, 1974) governed by two field criteria:

1. Floristic, Structural, and Edaphic Homogeneity: The sample plot must lie within a stand exhibiting uniform floristic composition, uniform vertical layering, and uniform habitat conditions (slope, aspect, soil moisture, and bedrock). Ecotones, disturbed edges, and atypical rock outcrops are excluded or sampled separately.
2. Satisfaction of the Minimal Area (Minimalareal) Curve: The plot must be large enough to capture the characteristic species complement of the stand. By constructing a nested species–area curve (doubling quadrat size from 1 → 2 → 4 → 8 → 16 → 32 → 64 → 128 m² and plotting cumulative species richness S against plot area A), the minimal area is identified at the asymptote inflection where a 10% area increase adds fewer than 5% to 10% more species (Cain, 1938; Chytrý & Otýpková, 2003):

2. Header Metadata and Vertical Stratification Layers

Every relevé begins with standardized header data: date, observer, GPS coordinates, elevation (m), slope inclination (°), aspect azimuth, plot area (m²), bedrock, soil type, humus form (mull, moder, mor), soil pH, percentage of bare soil/rock, and land-use regime. Because a tree species plays a different ecological role as a 28-meter canopy tree than as a 15-centimeter seedling, species are recorded separately across four vertical strata (Schichten):

1. Tree layer (Baumschicht, E₃ or T): Woody phanerophytes >5 m (often split into upper canopy T₁ and subcanopy T₂).
2. Shrub layer (Strauchschicht, E₂ or S): Woody shrubs and saplings 0.5–5 m tall.
3. Herb layer (Krautschicht, E₁ or H): Herbaceous vascular plants, dwarf shrubs, and tree seedlings <0.5–1.0 m tall.
4. Moss and Lichen layer (Moosschicht, E₀ or M): Terricolous bryophytes and lichens on the soil surface.

3. Cover–Abundance Scale, Barkman Refinement, and Ordinal Transformations

For each species in each stratum, the investigator estimates vertical projection cover (% of plot area shaded by foliage) combined with shoot density (abundance) at low cover. For numerical ordination and clustering, ecologists combine the classical 7-degree Braun-Blanquet (1928, 1964) scale with the Barkman, Doing & Segal (1964) subdivision of class 2 (2m, 2a, 2b) and Eddy van der Maarel's (1979) 1–9 ordinal transformation or geometric class midpoints:

4. The Five-Degree Sociability Scale (Soziabilität)

Separated from the cover–abundance code by a dot (e.g., 3.4 or +.1), the second digit (1 to 5) records sociability (Soziabilität)—horizontal spatial aggregation (Braun-Blanquet, 1932, 1964):

• Sociability 1: Growing singly as isolated stems (e.g., solitary orchids like Orchis mascula).
• Sociability 2: Growing in small groups or tufts (e.g., tussock grasses like Deschampsia cespitosa).
• Sociability 3: Growing in small patches or cushions (e.g., Silene acaulis or Anemone nemorosa).
• Sociability 4: Growing in extensive colonies or broken mats (e.g., Vaccinium myrtillus heaths).
• Sociability 5: Growing in dense, continuous pure carpets across the stand (e.g., Phragmites australis reedbeds scored as 5.5).

Differentiated Tables (Differentialtabellen) and Fidelity (Treue)

Once N relevés are compiled into a raw table (Rohtabelle) of species rows (i = 1 … S) by relevé columns (j = 1 … N), iterative matrix block-diagonalization—automated by Mark O. Hill's (1979) TWINSPAN algorithm and the JUICE software (Tichý, 2002)—reorders rows and columns into a differentiated table (Differentialtabelle) and a synoptic constancy table (Stetigkeitstabelle, scored in Roman constancy classes I [1–20%], II [20.1–40%], III [40.1–60%], IV [60.1–80%], and V [80.1–100%]).

Braun-Blanquet (1928, 1964) categorized species into five fidelity classes (Treuegrade): Fidelity 5 (Exclusive / treu), Fidelity 4 (Selective / fest), and Fidelity 3 (Preferential / hold)—which together form the diagnostic Character Species (Charakterarten / Kennarten)—above Fidelity 2 (Indifferent companions / Begleiter) and Fidelity 1 (Accidental intruders / Zufällige) (Connell–Slatyer succession models; Odum's strategy of ecosystem development). In statistical phytosociology, Chytrý et al. (2002) quantified fidelity via the Pearson phi (Φ) coefficient of association:

Φ = (N · n_p − n · N_p) / √[ n · N_p · (N − n) · (N − N_p) ]

where N is the total number of relevés in the dataset, N_p is the number of relevés in the target vegetation unit, n is the total occurrences of the species in the dataset, and n_p is its occurrences inside the target unit (with Φ × 100 ≥ 25 to 50 defining diagnostic character species).

The Syntaxonomic Hierarchy and the International Code (ICPN)

Regulated by the International Code of Phytosociological Nomenclature (ICPN; Theurillat et al., 2021), syntaxon names are formed from one or two diagnostic plant taxa combined with a rank-specific Latin suffix:

1. Class (Klasse) — Suffix -etea: Biome-scale complexes (e.g., Querco-Fagetea Braun-Blanquet & Vlieger 1937, Eurasian deciduous broadleaf forests).
2. Order (Ordnung) — Suffix -etalia: Regional alliance groups (e.g., Fagetalia sylvaticae Pawłowski 1928, European mesophytic beech and mixed forests).
3. Alliance (Verband) — Suffix -ion: Ecologically allied associations (e.g., Fagion sylvaticae Luquet 1926, Central European beech forests).
4. Association (Assoziation) — Suffix -etum: The basic unit typified by a holotype relevé (e.g., Galio odorati-Fagetum Sougnez & Thill 1959, sweet-woodruff beech forest).
5. Subordinate Ranks: Subassociation (suffix -etosum, e.g., Galio odorati-Fagetum caricetosum pilosae), Variant, and Facies.

Gleason and Whittaker's Individualistic Continuum Critique

During the 1950s and 1960s, Robert H. Whittaker (1956, 1962, 1967) and John T. Curtis (1959), building on Henry Allan Gleason's (1926) individualistic concept, demonstrated via gradient analysis that species response curves (Gaussian coenoclines) peak independently along continuous environmental gradients rather than forming discrete "superorganisms." Eddy van der Maarel (1975, 1979), Victor Westhoff, and Dieter Mueller-Dombois & Heinz Ellenberg (1974) reconciled the two perspectives: while species distributions are continuous in abstract niche space, recurring geological, topographic, and land-use configurations generate reproducible floristic nodes that can be classified into standardized Braun-Blanquet syntaxa for EU Habitats Directive (Natura 2000) conservation mapping (Mucina et al., 2016).

Big Data Phytosociology: Turboveg, EVA, and sPlot

In the twenty-first century, digitized Braun-Blanquet relevés power global macroecology: Turboveg (Hennekens & Schaminée, 2001), the EuroVegChecklist (Mucina et al., 2016, codifying 109 classes, 300 orders, and 1,108 alliances across Europe), the European Vegetation Archive (EVA), and the global plot database sPlot (Bruelheide et al., 2019), which links over 2.0 million vegetation relevés across all seven continents with the TRY Plant Trait Database.

Why does the Braun-Blanquet scale combine abundance and cover into a single symbol?

Above 12.5% ground projection (2b, 3, 4, 5), light interception and transpiration are governed by canopy projection area (% cover) regardless of stem count. Below 5% cover (r, +, 1, 2m), visual percentage estimation is imprecise, whereas shoot density (abundance) separates a solitary seedling (r) from hundreds of tiny spring ephemerals (2m), allowing rapid field recording of 40 to 80 species per plot.

How can qualitative Braun-Blanquet codes (r, +, 1–5) be used in numerical ordination (PCA, NMDS, DCA)?

Raw alphanumeric codes are transformed prior to multivariate analysis using van der Maarel's (1979) 1–9 ordinal transformation (r → 1, + → 2, 1 → 3, 2m → 4, 2a → 5, 2b → 6, 3 → 7, 4 → 8, 5 → 9) or replaced by the geometric midpoint percentage of each cover class (0.05%, 0.5%, 2.5%, 4.0%, 8.75%, 18.75%, 37.5%, 62.5%, 87.5%).

What is the difference between a character species (Charakterart) and a differential species (Differentialart)?

A character species (Charakterart) shows high statistical fidelity (Φ) to a specific syntaxon compared against all other communities in the regional flora. A differential species (Differentialart) may occur across several unrelated classes, but within a given association it consistently separates one subassociation or variant from its sister subunit along a local soil moisture or pH gradient.

How are formal phytosociological names constructed from plant names?

Under the ICPN (Theurillat et al., 2021), a syntaxon name attaches the rank suffix (-etum, -ion, -etalia, -etea) to the Latin stem of the primary genus name with its species epithet in the genitive case (e.g., Galium odoratum + Fagus sylvatica → Galio odorati-Fagetum).

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2. Van der Maarel, E. (1979). "Transformation of cover-abundance values in phytosociology and its effects on community similarity." Vegetatio, 39(2), 97–114. https://doi.org/10.1007/BF00052021
3. Chytrý, M., Tichý, L., Holt, J., & Botta-Dukát, Z. (2002). "Determination of diagnostic species with statistical fidelity measures." Journal of Vegetation Science, 13(1), 79–90. https://doi.org/10.1111/j.1654-1103.2002.tb02025.x
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