Ecological Morphology and Life Forms

Historically the first and perhaps most intuitively understandable way of classifying life forms was to distinguish groups by their overall "physiognomic" appearance (von Humboldt, 1806; Grisebach, 1872). We unerringly distinguish a tree from a shrub, a shrub from a herb, and a herb from a liana. These categories reflect fundamental adaptations to climate and growing conditions.

The most famous and widely used ecological-physiognomic system was proposed by the Danish botanist C. Raunkiaer (Raunkiaer, 1905, 1907; Raunkiaer, 1934). It is based on a simple but biologically profound trait: the position of renewal buds relative to the soil surface during the unfavorable season (cold winter or dry period). This trait integrates adaptation to stress survival. Raunkiaer distinguished five main types of life forms (Fig. 1).

Raunkiaer showed that the percentage of these forms ("biological spectrum") changes regularly from the tropics (where phanerophytes dominate) to deserts (where therophytes prevail) and to high latitudes (where the proportion of hemicryptophytes and chamaephytes increases). For example, for Denmark (temperate climate), the normal spectrum includes 7% phanerophytes, 3% chamaephytes, 50% hemicryptophytes, 22% cryptophytes, and 18% therophytes. For humid tropics (Seychelles) — 61% phanerophytes, 6% chamaephytes, 12% hemicryptophytes, 5% cryptophytes, and 16% therophytes (Raunkiaer, 1934; Yakovlev et al., 2008). This approach proved extremely productive for phytogeography and climatology, although it was later criticized for some schematicity and disregard for flora history (Du Rietz, 1931; Warming, 1908).

The ecological-physiognomic approach, for all its clarity, does not answer the question: "how is a given life form built, what growth and branching processes underlie its appearance?" This gap is filled by the structural-ontogenetic direction. Its fundamental idea is that a plant is a modular organism built from repeating elements — metameres (a shoot with a leaf and axillary bud), and larger units (axes, partial bushes, clones) arise from the repetition of these modules during growth and branching (Serebryakov, 1962; Savinykh, 2015; Barthélémy & Caraglio, 2007).

Key concepts here are:

For example, the Rauh model is characterized by a monopodial trunk with rhythmic growth and lateral flowers (typical for many conifers). The Chamberlain model — sympodial growth, where each module ends with a terminal inflorescence (found in many palms, cycads). The Tomlinson model — vegetative axes are equivalent, and the plant "tallies" due to basal branching (many grasses, sedges, dwarf shrubs) (Barthélémy & Caraglio, 2007; Cremers & Edelin, 1995). Knowledge of the architectural model allows predicting crown shape, response to damage, and wind resistance.

The ecological-physiognomic approach gives us a "portrait" of the plant, its functional mask, reflecting adaptation to the environment. The structural-ontogenetic approach provides a "blueprint" of this plant, revealing the rules of its construction and development dynamics. Both views are necessary for a complete analysis of life form.

For example, from Raunkiaer’s point of view, the lingonberry dwarf shrub (Vaccinium vitis-idaea) is a chamaephyte (renewal buds low above the ground). But a structural-ontogenetic analysis (Serebryakova et al., 2006; Savinykh, 2015) shows that it is a vegetatively mobile dwarf shrub with long underground rhizomes (geoxylic), which during ontogeny forms partial bushes and is capable of cloning. It belongs to the category of explicitly polycentric biomorphs (Smirnova et al., 1976). It is the combination of these characteristics that allows lingonberry to successfully compete and dominate in coniferous forests and tundras.

Thus, the synthesis of these two approaches underlies modern biomorphology — a science that considers the life form as a complex, hierarchically organized, and dynamic system reflecting the evolutionary and ecological strategy of the species.

The concept of "life form" has come a long way before acquiring its modern scientific content. Already in antiquity, Theophrastus (ca. 370–285 BC) distinguished trees, shrubs, subshrubs, and herbs, linking their appearance to climate and soil (Serebryakov, 1962). However, for a long time, these categories were used either in a everyday or purely descriptive (physiognomic) sense.

The key step towards modern understanding was made after the establishment of Charles Darwin’s evolutionary theory. Plants began to be considered as systems adapted to their habitat. One of the first scientific definitions was given by the Danish ecologist E. Warming (1884): "life form is the form in which the vegetative body of the plant (the individual) is in harmony with the external environment throughout its life, from seed to death" (cited by: Serebryakov, 1962, p. 53; Dyachenko, 2012). This definition emphasized functional integrity and adaptiveness, but it did not yet include the historical aspect.

Later, with the development of comparative morphology and phylogenetics, a more complete understanding emerged. The most concise and widely recognized definition belongs to I. G. Serebryakov (1962, 1964). As already cited in section 0, he defined life form as "the distinctive general appearance (habitus) of a particular group of plants, arising during their ontogeny as a result of growth and development in specific environmental conditions. This habitus historically arose in these soil-climatic and coenotic conditions as an expression of the plants' adaptation to these conditions" (Serebryakov, 1964, p. 146). Later he gave a shorter formula: "a plant’s life form is its habitus, linked to the rhythm of development and adapted to present and past environmental conditions" (cited by: Serebryakova et al., 2006, p. 86). Thus, Serebryakov combined three inseparable aspects in the concept of "life form": morphological (habitus), ecological (adaptation to environment), and historical (result of evolution).

It is important to distinguish a life form from an ecological group. Ecological groups (xerophytes, hygrophytes, helophytes, etc.) are distinguished by their relationship to a single factor (moisture, light, salinity). A life form reflects adaptation to the entire complex of conditions and includes the whole set of morphological and biological features (Serebryakov, 1962; Poplavskaya, 1948). For example, among xerophytes (plants of dry places), one can find trees (saxaul), thorny shrubs, cushion-like subshrubs, succulents, and ephemerals — all different life forms that evolved under the pressure of an arid climate, but by different paths.

The study of life forms has a rich history, in which several stages can be distinguished (according to: Du Rietz, 1931; Meusel, 1951; Serebryakov, 1962).

The early (physiognomic) stage is associated with the works of A. von Humboldt (1806) and A. Grisebach (1872). Humboldt, traveling through tropical America, was the first to note that the appearance of a landscape is determined by the dominance of certain "basic forms" of plants — palms, bananas, cacti, orchids, etc. He identified 19 such forms, laying the foundation for the physiognomic approach. Grisebach developed a more detailed system of "plant forms" (54 forms) grouped into 7 categories and attempted to link them to climatic conditions (Serebryakov, 1962; Niklas, 2008). The main drawback of this stage was formalism, since not all identified groups had real adaptive significance.

The ecological-adaptive stage began with the works of E. Warming (1884, 1908) and especially with the publications of C. Raunkiaer (1905, 1907). Raunkiaer proposed a coherent and easy-to-apply system based on the position of renewal buds (see section 1.1). He introduced the statistical method ("biological spectra") for comparing floras of different climatic zones, showing, for example, that the dominance of hemicryptophytes is a sign of a temperate-cold climate, phanerophytes — of humid tropics, and therophytes — of deserts (Raunkiaer, 1934). Despite criticism (Warming, 1908; Scottsberg, 1914; Du Rietz, 1931), Raunkiaer’s system became classic and is still widely used, especially in geographical research.

However, as early as the end of the 19th and beginning of the 20th centuries, data accumulated showing that life forms depend not only on climate but also on the historical development of flora, on the internal organization of plants. The comparative-morphological direction emerged, associated with the names of Th. Irmisch, A. Braun, and later I. G. Serebryakov and his school (Serebryakov, 1962; Savinykh & Cheremushkina, 2015). Irmisch in the mid-19th century studied in detail the morphogenesis of perennial herbs, identifying types of rhizomes, tubers, and bulbs. These studies showed that life form is determined not only by external appearance but also by the structure of underground organs, the method of renewal and shoot growth.

In Russia (USSR), the development of the study of life forms is associated with the names of G. N. Vysotsky, B. A. Keller, V. V. Alekhin, A. P. Shennikov, and, of course, I. G. Serebryakov. Vysotsky (1915) developed a classification of steppe plants according to their ability for vegetative reproduction (taprooted, tussock, rhizomatous, root-sprouting, etc.). Keller (1933, 1938) emphasized that life forms are closely linked to the taxonomic position of plants, i.e., not every form can arise in any group — there is a certain "mode of adaptability."

The pinnacle of synthesis was the system of I. G. Serebryakov (1962, 1964), which combined the physiognomic approach (trees, shrubs, dwarf shrubs, herbs) with a deep morphological-biological analysis (types of root systems, lifespan of axes, ability for vegetative renewal, types of inflorescences, etc.). He distinguished 4 divisions (woody plants, semi-woody plants, terrestrial herbs, and aquatic herbs), within which — types, classes, subclasses, groups, sections, and the life forms themselves. Unlike Raunkiaer, Serebryakov considered the lifespan of above-ground skeletal axes as the main trait, which allowed building an evolutionary series: trees → shrubs → dwarf shrubs → subshrubs → perennial herbs → annuals (Serebryakov, 1962). He convincingly showed that within any taxonomic genus, parallel series of life forms can be observed, similar to N. I. Vavilov’s homologous series.

At the present stage, biomorphology (ecological morphology) integrates ideas of modular plant organization (Barthélémy & Caraglio, 2007), architectural models (Hallé & Oldeman, 1970), ontomorphogenesis (Savinykh, 2015), and actively uses quantitative methods to analyze life form spectra in relation to climate, soils, and anthropogenic impacts (Ciaccia et al., 2020; Belousova et al., 2015). Thus, the history of the study of life forms is a path from external description to a deep understanding of the plant as an integral, historically formed, and dynamic system reflecting its life strategy in a changing world.

Main page: System of life forms by C. Raunkiaer

As already noted (section 1.1), Raunkiaer’s classification is based on the position of renewal buds relative to the soil surface (Raunkiaer, 1905, 1907; Raunkiaer, 1934). The advantage of this system for the agronomist is its simplicity and standardization. By compiling a "biological spectrum" (percentage of phanerophytes, chamaephytes, hemicryptophytes, cryptophytes, and therophytes) for the weed flora of a field, important conclusions can be drawn:

Raunkiaer’s system is indispensable for rapid comparative assessment of floras in different agroclimatic zones. It is actively used in Europe and the USA for climate change monitoring and assessing the sustainability of agricultural landscapes (Bloch-Petersen et al., 2006; Ciaccia et al., 2020). However, its drawback is the "lumping" of groups: for example, among hemicryptophytes are both loose-tussock grasses, rosette dicots, and taprooted perennials — with completely different biology and responses to treatments.

Main page: Ecological-morphological classification by I.G. Serebryakov

In Serebryakov’s system (1962), all plants are divided into divisions (woody, semi-woody, herbaceous), within which types are distinguished (trees, shrubs, dwarf shrubs, subshrubs, polycarpic herbs, monocarpic herbs, annuals). For the agronomist, the most valuable are the subdivisions of herbaceous polycarpics (perennials) by type of underground organs:

Raunkiaer’s system provides a strategic, macro-assessment: it allows predicting which life forms will dominate in a given climate and how climate change (warming, aridization) will shift the spectrum towards therophytes or, conversely, hemicryptophytes. This is the basis for long-term forecasting of the weed component (Ciaccia et al., 2020). In Germany and the Netherlands, Raunkiaer’s methods are used to assess changes in the climatic niches of agricultural crops and invasive weeds (Bloch-Petersen et al., 2006).

Serebryakov’s system provides a tactical, local assessment: it directly indicates which specific perennial weed we are facing — rhizomatous (couch grass) or taprooted (dandelion) — and which tillage system should be applied. In the USA and Canada, for example, to control common couch grass (Elymus repens) — a classic long-rhizomatous grass — a combination of deep plowing with fallow and herbicides is used. To control root-sprouting weeds (Canada thistle, field bindweed), repeated cutting at different depths is required to deplete reserves in the roots. In Russia and post-Soviet countries, Serebryakov’s classification is basic in botany and agriculture courses (Belousova et al., 2015; Serebryakova et al., 2006).

The synthesis of the two systems in agronomic practice looks as follows:

Thus, an agronomist armed with knowledge of both systems is able not only to identify a weed but also to predict its behavior, choose the optimal control strategy, and even forecast changes in the weed flora composition with changes in technology or climatic conditions. This is the applied power of the study of life forms.

The process of habitus change during ontogeny is called ontomorphogenesis (Serebryakov, 1962; Savinykh, 2015). Each phase of ontomorphogenesis is not just an increase in size, but a qualitative change in structure, the appearance of new modules and shoot systems.

A similar change of forms occurs in trees. A young tree (spruce, oak) has a bush-like form for the first few years — the primary bush phase. Only after decades does a single trunk form, and the plant transitions to the tree phase (Serebryakov, 1962; Barthélémy & Caraglio, 2007). Importantly, at each phase, the plant uses different ecological niches and affects the environment differently.

Knowledge of weed ontomorphogenesis allows the agronomist to select a vulnerable phase. For example, couch grass is most sensitive to cutting at the 2–3 leaf stage (before powerful rhizomes form), while in the explicitly polycentric phase, repeated impact is required (Ciaccia et al., 2020).

For coenopopulation studies, a detailed periodization of ontogeny has been developed (Rabotnov, 1950; Uranov, 1975; Gatsuk et al., 1980). The following age states are distinguished (according to: Serebryakova et al., 2006):

The duration of these periods varies greatly. Ephemeral annuals (therophytes) complete their entire ontogeny in 4–6 weeks. Many perennial herbs enter the generative phase in the 3rd–10th year, and some (e.g., spreading millet Milium effusum) may flower only in the 7th–9th year (Serebryakov, 1962). Long-lived trees (sequoia, oak) remain in the virginile state for decades, and the generative phase can last hundreds of years. This extension of ontogeny is an important mechanism for species preservation in the community: juvenile and virginile individuals form a "reserve" that occupies the vacated space after the death of old plants (Rabotnov, 1950; Belousova et al., 2015).

If life form answers the question "what does the plant look like?", then life strategy answers "how does it survive and reproduce in the community?" The most well-known classification of strategies was proposed by L. G. Ramensky (1938) for plants, and later developed by J. Grime (Grime, 1979; Grime, 2001). Three primary types are distinguished (the so-called CSR theory):

In practice, species with mixed strategies (CS, CR, SR) are often found. For example, long-rhizomatous grasses (couch grass) are violents with features of explerents (high vegetative spread). Rosette perennials (dandelion) are an SR strategy: stress-tolerance (withstands trampling) and ruderality (abundant seed reproduction).

The relationship between life form and strategy is obvious but not unambiguous. Therophytes are almost always explerents. Long-rhizomatous herbs are usually violents or CR-strategists. Succulents and cushions are patients. Knowledge of the strategy allows the agronomist to predict how a species will respond to changes in technology: tillage suppresses perennial rhizomatous violents but provokes mass germination of annual explerents from the seed bank. Therefore, weed control must be systemic, including rotation of treatments, application of herbicides with different mechanisms of action, and the use of competitive violents as cover crops (green manures) (Ciaccia et al., 2020).

Thus, the answer to the question "why does a plant grow this way?" consists of three components: (1) the hereditary program of morphogenesis (architectural model), (2) the regular change of ontogenetic phases (ontomorphogenesis), and (3) the evolutionarily developed strategy of resource use (violence, patience, explerence). It is this synthesis that constitutes the essence of ecological morphology as a science of the integral life of a plant in changing conditions.

The most harmful and difficult-to-eradicate weeds in arable land are perennials with a powerful system of vegetative regeneration. According to I. G. Serebryakov’s classification (1962), these include long-rhizomatous (couch grass, Elytrigia repens; coltsfoot, Tussilago farfarus), root-sprouting (Canada thistle, Cirsium arvense; field bindweed, Convolvulus arvensis), and stoloniferous (field mint, Mentha arvensis) plants.

Each of these groups requires a specific approach:

Key conclusion: By identifying the life form of a malicious perennial weed by the structure of its underground organs, the agronomist chooses the type, depth, and frequency of treatments, as well as the herbicide system. There is no universal recipe — an "individual approach" to each biomorph is needed.

The type of life form of perennial grasses determines their resistance to grazing, ability to regrow, and value in fodder production. The classic classification of grasses by tillering type (V. R. Williams, 1922; A. M. Dmitriev, 1947) is essentially a detailing of Serebryakov’s ecological-morphological approach.

Similarly, among leguminous herbs, an important economic distinction is related to root system type: taprooted (alfalfa, sainfoin, red clover) are more drought-resistant but sensitive to grazing (the root collar suffers); cluster-rooted and rhizomatous (white clover, bird’s-foot trefoil) tolerate trampling and defoliation better, but are less productive.

Creating shelterbelts, stabilizing ravines, afforesting sands — these are tasks where knowledge of the life forms of woody plants comes to the fore (Serebryakov, 1962; Hallé & Oldeman, 1970; Barthélémy & Caraglio, 2007).

In modern forest reclamation, knowledge of architectural models (Hallé & Oldeman, 1970; Barthélémy & Caraglio, 2007) is increasingly used to predict the crown shape of an adult tree while still in the nursery. For example, the Rauh model (spruce, pine) gives a narrow pyramidal crown, the Massart model (araucaria, some oaks) gives a broad spherical crown. For shelterbelts, models with sympodial branching and a compact crown are preferred to avoid excessive shading of adjacent fields (Cremers & Edelin, 1995).

Modern research shows that taking into account the spectrum of life forms of weed vegetation allows optimizing crop rotations and tillage regimes without the use of herbicides. For example, in the work of Navarro-Miró et al. (2019) and Ciaccia et al. (2020) on vegetable crop rotations in Europe, it was shown that the use of cover crops (green manures) and reduced tillage (roller-crimper) changes the spectrum of weed life forms: the proportion of therophyte-explerents decreases, but the proportion of perennial hemicryptophytes and geophytes increases. This requires adaptation of the control system (e.g., including deep loosening once every few years).

In Russia, in the Leningrad region, Belousova et al. (2015) established that perennial hemicryptophytes (couch grass, thistle, dandelion) migrate to fields from roadsides and roads. Therefore, to prevent infestation, it is necessary to treat not only the field but also adjacent territories — mow roadsides before weed seeds mature, and carry out herbicide treatments there.

Thus, the agricultural application of biomorphology — from macro- (landscape ecology, land reclamation) to micro- (cultivation technologies, plant protection) — makes agriculture more environmentally friendly, economical, and scientifically sound. Understanding the life form is the foundation on which rational exploitation and conservation of plant resources are built.