Annual nitrophilous and subnitrophilous vegetation of (semi)arid regions of the Southern Iberian Peninsula and Canary Islands
Plant species display a range of structural, physiological and morphological adaptations to environmental gradients. Various authors proposed different categories of plant life forms (e.g., Du Rietz 1931; Raunkiær 1934; Ellenberg & Mueller-Dombois 1967). The categories of life forms defined here are based on a combination of traits important for vegetation structure in Europe, including plant height, woodiness, branching type of woody plants (trees vs shrubs), leaf type, life span (annual vs perennial), and habitus (e.g., graminoid, non-graminoid, fern, succulent, aquatic plant). Broad taxa of cryptogams (ferns, bryophytes and lichens) are also considered here as separate life forms based on their specific morphological and ecophysiological features (except for aquatic cryptogams, which are grouped with other aquatic plants).
The dominant life form is the plant life form that usually attains the highest cover in at least one of the four main vegetation layers (tree, shrub, herb or moss layer). More than one dominant life form is given for vegetation types with well-differentiated vegetation layers. For example, conifer trees, dwarf shrubs and bryophytes can be given for a boreal forest alliance, each life form being dominant in a different vegetation layer. However, layers that have, on average, much lower cover than other layers in the given alliance are not considered. We do not report the dominant life form for the layers with a mean cover lower than 20% of the mean cover of the layer with the highest cover. For example, in a grassland vegetation type with a mean herb layer cover of 80%, shrub layer of 5% and moss layer of 10%, we only include the dominant life form of the herb layer. More than one life form may be reported for the same layer if different life forms dominate this layer at different sites (e.g., perennial graminoids and perennial non-graminoid herbs in a meadow alliance).
Some species may have different life forms depending on the environment or region (e.g., tree in lowlands vs shrub near the timberline). In such cases, the life form prevailing in the given alliance is reported. Tree saplings in the shrub layer are considered trees.
The phenological optimum is defined here as a period when most of the species in the community (or the dominant species in species-poor communities) are flowering. This period usually corresponds to the peak of aboveground biomass in the community. In vegetation dominated by ferns, the phenological optimum corresponds to the period of highest biomass. In vegetation types that have shifted phenological phases for different species groups, more than one phenological optimum is given. For example, temperate deciduous floodplain forests have two phenological optima associated with different life forms: early spring (flowering of spring geophytes and trees) and summer (maximum biomass of tree leaves and flowering and maximum biomass of shade-adapted hemicryptophytes such as Urtica dioica). Some types of annual weed or ruderal vegetation may comprise two or three groups of species with the same life form but different phenological optima, resulting in different spring, summer and autumn aspects. Some vegetation types have a long phenological optimum spanning at least two periods defined here. More than one phenological optimum is given in such cases. If spring flowering of deciduous trees or shrubs precedes leaf development, and no other life form is flowering, the time of fully developed foliage (the highest biomass), which usually corresponds with the flowering of other plants, is considered the phenological optimum. The phenological optimum of bryophytes and lichens is not considered.
Phenological optimum categories were defined primarily based on phenological patterns in the Central European lowlands, which roughly correspond to the mean phenological stages in vegetation types across Europe. However, the assessment considers that spring phases start earlier in the Southern European lowlands and later in the Central European mountains and Northern Europe. Because there is little information on the phenological optimum in the phytosociological literature, category assignments were based largely on field experience and the phenology of dominant or other common species in each vegetation type.
Substrate moisture reflects the availability of water to plants during the growing season.
Substrate reaction is measured as the pH of soil solution or water. Reaction categories are defined by pH thresholds following USDA (2017). However, only three categories are used because the exact ranges of pH values are unknown for many vegetation types, and the reaction category had to be estimated based on bedrock type and reaction indicator values (Dengler et al., 2023; Tichý et al., 2023) of diagnostic, constant or dominant species for each vegetation type.
Salinity refers to the concentrations of soluble salts (especially carbonates, chlorides and sulphates of calcium, magnesium, sodium and potassium) in soil or water. In coastal areas, salinity can be caused by sea surges, tides, storms, salt spray from the sea and the mixing of freshwater and seawater in estuaries. In inland areas, salinity can be high due to mineral-rich springs or salt accumulation in the soil from salt-rich sediments in dry climates with high evaporation. Measurements of salinity are rare; therefore, salinity is usually estimated from the occurrence of plants along a soil-salinity gradient.
Nutrient status refers to the concentration of available nitrogen, phosphorus and potassium in soil or water.
Soil organic matter consists of remains of dead plants and animals at various stages of decomposition. The following categories are distinguished according to the relative proportion of mineral or organic components. Aquatic vegetation types that do not root in soil are not classified.
Vegetation regions defined here are large areas with relatively uniform climates and vegetation. These regions partially correspond to other land classification units such as biogeographical regions, ecoregions or biomes (Rivas-Martínez et al., 2004a; Schultz 2005; EEA 2016; Mucina et al., 2016; Dinerstein et al., 2017; Bruelheide et al., 2018). However, these classification systems are inconsistent across Europe, differ in the number of units and the location of boundaries between them, and some of them do not consider vegetation as the main classification criterion. Here, we propose a system based mainly on a combination of the Biogeographic and Bioclimatic Maps of Europe by Rivas-Martínez (2004a, 2004b), European Biogeographical Regions (EEA 2016) and Ecoregions by Dinerstein et al. (2017).
The assignment of vegetation types to vegetation regions is partly based on the distribution maps of European alliances (Preislerová et al., 2022). Since these maps do not provide sufficient details, such as point distribution, we also used information from various literature and expert knowledge.
The elevational vegetation belts reflect the vertical zonation of vegetation in relation to the changing climate with increasing elevation. In the Arctic and Boreal vegetation regions, we distinguish only the Boreo-Arctic lowland and the Boreo-Arctic mountain belts because the distinction between multiple belts is often unclear due to the low timberline in maritime and northern areas. In the Hemiboreal, Nemoral, Forest-steppic, Steppic and Semidesertic vegetation regions, we distinguish lowland, submontane, montane, subalpine, alpine and subnival belts. In the Mediterranean and Macaronesian vegetation regions, we use the division into Inframediterranean, Thermomediterranean, Mesomediterranean, Supramediterranean, Oromediterranean and Cryomediterranean belts. Each elevational belt may shift up or down in any mountain range or region, depending on local environmental conditions.
Boreo-Arctic vegetation belts
Temperate vegetation belts
Vegetation belts of the Mediterranean and Macaronesian vegetation regions
The successional status reflects the position of vegetation types in the successional series from early successional stages that occur after the emergence of new habitats (primary succession) or after a major stand-replacing disturbance (secondary succession). Some vegetation types are assigned more than one successional stage because they may have different successional statuses in different climatic zones (e.g., some types of scrub can be late successional under dry climates and mid-successional in wet climates).
Naturalness expresses the degree to which vegetation has been formed by natural processes or under human influence. Forest vegetation types are considered natural here, although they have been under human influence for centuries or millennia, and specific forest stands can be considered semi-natural or even anthropogenic. Some types of grassland or scrub may occur naturally at some sites, while their formation at other sites has been caused by human management. Such vegetation types are classified into more than one category.