Syntaxonomy of steppe depression vegetation of Ukraine

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The post provided by Viktor Shapoval & Anna Kuzemko

Phytocoenoses of the association Lathyro nissoliae-Phalacrachenetum inuloidis on the bottom of the Ahaimansky pid, aspect of Phalacrachena inuloides, single shoots of Rumex crispus and Beckmannia eruciformis visible in the background, June 6, 2008. Photo credit: V. Shapoval.

This post refers to the article Syntaxonomy of steppe depression vegetation of Ukraine by Shapoval & Kuzemko, published in Vegetation Classification and Survey (https://doi.org/10.3897/VCS/2021/62825).

Steppe depressions or pody are a unique habitat type, which are large drainless depressions of elliptical or round shape with gentle slopes and flat bottoms, periodically flooded by meltwater and characterized by Luvic Planosol soils and original ephemeral grassland and wetland vegetation. The term pody includes large depressions, with a bottom area of more than 500–1000 ha, rather than small saucers, which are often generally called pody, thus erasing their identity. Previously, such depressions were much more widespread in Europe and were particularly known from the Pannonian Plain, but almost all of them were plowed and now survive only in Ukraine and the Russian Federation.

Despite the long history of their studies, the problem of the origin of depression still has no clear solution. There are many hypotheses and theories of pody genesis, but it seems most likely that they represent a polygenetic group, and their development is caused by a complex of subsidence-suffusion, fluvial and aeolian transformations.

There are two seasonal types of flooding of depressions: in winter and spring, caused by melting snow during thaws, and extremely rare in summer and autumn, by heavy rains. Interestingly, according to existing literature during the 20th and 21st centuries, summer floods were observed only three times, but the last such phenomenon occurred in July – August 2021 in the Chorna Dolyna (Black Valley) depression, coinciding with the publication of our paper. According to the materials of the retrospective analysis of the known dates of pody floods, the average duration of the period between strong floods is 7-12 years. Occasionally, flooding is observed for 2 or 3 years in a row, but much more often at intervals of 15-17 years or more. In the past, the flooding of the steppe depressions was much larger, so on old maps, they were marked as lakes. Currently, due to the over-regulation of the catchment area, their total plowing, and crossing of most catchment basins by various communications (like water supply canals or highways), the frequency and duration of flooding has decreased significantly, leading to xerophytization of these habitats. Usually, modern heavy flooding begins in February and lasts until the beginning of June (the last small puddles may last until the end of July). The area of flooding can reach 3-4 thousand hectares, but the water depth in the center of a bottom is only 20-40 cm.

Phytocoenoses of the subassociation Elatino-Butometum umbellati damasonietosum alismae in the central part of the bottom of the Great Chapelsky pid during flooding, flowering individuals of Damasonium alisma among vegetative shoots of Butomus umbellatus and Elytrigia repens subsp. pseudocaesia, May 17, 2010. Photo credit: V. Shapoval.

Polygenetics, different sizes, differentiation of microrelief and soil cover of depressions together with sporadic hydrogenic fluctuations, historical and current management determine the heterogeneous nature and pulsating dynamics of their vegetation. It is a unique dynamic complex of hydro-, meso- and xeromorphic communities, which complicates its study. In fact, the existing vegetation types (dry, mesic, wet grasslands, and wetlands) in the pody are scattered territorially (some are confined to the deepest, moist areas of the bottom, others tend to its raised dry periphery), and on the other hand – delimited in time (initially the deepest areas with open water mirror overgrown with wetland vegetation, which is later replaced by mesic grasslands and an extensive list of steppic communities). At the same time, the boundaries between these phytocenoses are often blurred.

Too irregular and short-term flooding of pody, as well as the shallow water of these temporary standing water bodies, do not have a detrimental effect on perennial mesic as well as some xerophytic components. Such hydrogenic fluctuations can withstand even perennial turf of the xeromorphic plant Festuca valesiaca s.l.

Unfortunately, most steppe depressions in Ukraine are plowed, and the surviving remains are exploited, mainly as meadows and pastures, often without compliance with rational norms of management. The only depression that has a conservation status is the Great Chapelsky Pid, as part of the natural core of the Askania-Nova Biosphere Reserve. In fact, this depression is the most studied, and our long-term research, which is summarized in this paper, originates from it.

Considering the floristic, coenotic, and habitat specifics of the steppe depressions, as well as the absence of such units in the EUNIS system, and accordingly in Resolution 4 of the Berne Convention, which makes it impossible to protect such habitats in the Emerald Network of Ukraine, the authors of this paper prepared proposals to include them in the Convention. In 2018, these proposals were adopted by the Executive Committee of the Berne Convention, and the depression of the steppe Ukraine was included as a complex type X36 in Resolution 4 and the EUNIS (https://eunis.eea.europa.eu/habitats/8009), which requires a comprehensive study and protection of this habitat type.

Due to the exceptional rarity of floods, it is possible to observe and describe these original phytocoenoses only for very limited periods of time, and the interval between the growing seasons favorable for the mentioned ephemeral vegetation can reach decades. At the same time, it is very difficult to predict such a possibility of contemplation and research. Floods depend on the coincidence of a number of factors, so the probability of flood forecasting is possible only on the eve of the flood – in 1-1.5 months. The largest dataset of such unique data was collected in 2010 during the last major flood.

For the first time, the idea of the need for a syntaxonomic revision of the pody vegetation arose in 2011 during the excursion of the EDGG conference participants in Askania-Nova. Since then, repeated meetings of authors and discussions have outlined a common range of problems, developed their common vision, and ultimately led to a comprehensive revision of pody vegetation. The situation was complicated by the fact that no syntaxonomic unit of pody vegetation has been validly described in accordance with ICPN.

Ecological and coenotic profile of model steppe depressions of the Left Bank of the Lower Dnieper. The central part of the bottom is occupied by wetland communities, which change along the slopes by wet, mesic, and xero-mesic phytocoenoses. The transect shows the difference in absolute height between the bottom of the depression and its slope, the length and asymmetry of the «body» of the depression along the line: slope-bottom. Figure by V. Shapoval.

According to the research results, an updated and consistent syntaxonomic system of mesic and wet grasslands vegetation of the pody was obtained, which reflects their ecological and territorial differentiation, the syntaxonomic status of a number of syntaxa, which were considered doubtful, was restored, and a proper place of pody vegetation in the system of syntaxonomic units of Europe according to EuroVegChecklist was found. We sincerely hope that our study will help to properly organize the conservation of these unique habitats.