The Colchis lowland occupies the eastern shore of the Black Sea like an amphitheater of mountains, stretching from Tuapse in Russia to Trabzon in Turkey, with its heart in Georgia. Its moderately warm, near-subtropical climate receives up to 2,500 millimeters of annual rainfall at Batumi, feeding sphagnum peatlands, relict peat-covered forests, coastal dunes, and wetland meadows that exist nowhere else on Earth. The regional flora counts 3,596 species of higher plants across 932 genera and 169 families, a richness shaped by thousands of years of human cultivation and by vegetation zones that climb from swamp forests at sea level to beech and hornbeam stands in the foothills.

Between 2021 and 2023, researchers conducted botanical expeditions across natural, semi-natural, and agricultural ecosystems in central and southern Colchis, including coastal Adjara. They documented thirty alien species that have become widely distributed in the region, belonging to seventeen families and twenty-nine genera. Nearly half of these newcomers, fourteen species, trace their origins to East Asia, while ten come from North America, three from South America, two from tropical and subtropical America, and one from Western Europe. The life-form spectrum is equally telling: twelve species are woody phanerophytes and eighteen are herbaceous plants, including ten fast-cycling annuals, a combination that lets invaders exploit both forest gaps and disturbed ground.

To quantify how aggressively the worst offenders spread, the team applied the classic Braun-Blanquet quadrat method along marked transects in Kolkheti National Park and the Chorokhi Delta. For each of six focal invasive species, they laid out forty transects, each two meters wide and fifty meters long, and sampled ten one-square-meter plots spaced five meters apart. Every plot was assessed twice during the growing season, first early in the vegetative period and again at flowering and fruiting, generating 1,600 records over three years. From these data the researchers calculated mean density, frequency, and percentage cover for each invader, the standard metrics ecologists use to gauge invasive potential.

The numbers reveal sharply different invasion strategies. Common ragweed, Ambrosia artemisiifolia, an annual from North America, proved the densest occupant, averaging fifty-seven individuals per square meter and appearing in one hundred percent of quadrats. Because so many seedlings crowd together, many never flower, and average cover settled at forty-four percent, though individual robust specimens can reach ninety to one hundred percent coverage through intensive growth and branching. Ragweed completes its life cycle by maturing and dispersing enormous quantities of highly germinable seeds, a strategy that overwhelms competition both within and between species.

Brazilian vervain, Verbena brasiliensis, plays a longer game. This perennial keeps growing through mild winters and resprouts in early spring from old stems and root sprouts, so individual plants are fewer, averaging sixteen per quadrat. Yet its abundant fruiting and effective seed dispersal push its frequency to ninety-four percent, and its sheer size lifts average cover to eighty-three percent. Bur cucumber, Sicyos angulatus, is the sprinter of the group: an annual creeping vine whose seeds germinate poorly but whose stems race six to eight meters on average, with recorded individuals reaching twelve to sixteen meters or more. Climbing over and almost entirely smothering supporting vegetation, it achieves ninety percent frequency and eighty-five percent cover despite averaging only twelve plants per quadrat.

Canadian goldenrod, Solidago canadensis, first recorded in Georgia in the 1920s, now blankets the entire Colchis lowland with an average density of forty-four individuals per square meter, sixty-four percent frequency, and seventy-four percent cover. Its success rests on a double reproductive engine: prolific seeds plus rhizome buds that generate new shoots nearly throughout the growing season. Honey locust, Gleditsia triacanthos, introduced in the early twentieth century for forest plantations and windbreaks, is escaping cultivation at an accelerating rate. In the Katsoburi Forest Reserve, transects held everything from ten to fifteen centimeter seedlings to mature trees and stump resprouts, driving cover to nearly one hundred percent in some plots and averaging forty percent where only young shoots persist, with a frequency of forty-eight percent.

False indigo, Amorpha fruticosa, tells a similar story of delayed explosion. Present in Georgia since the early twentieth century and showing signs of naturalization by the 1940s, it began spreading widely across western Georgia only in recent decades. It reproduces both by seed and by numerous lateral root shoots; in seed-invaded quadrats it averages thirteen individuals, but over time many die off while survivors grow into large bushes whose lateral shoots can fill an entire square-meter plot alone. The result is eighty-eight percent frequency and eighty percent cover in the surveyed transects, making it one of the most structurally dominant invaders measured.

The study places these findings in a deep historical context. Alien species first arrived in Colchis with crop introductions: rice cultivation brought sweet flag, wavyleaf basketgrass, Indian goosegrass, and small-flower umbrella sedge, while the early twentieth-century replacement of local forests with tea, citrus, bamboo, and tung plantations opened the door for many more. Widespread distribution of today’s invaders began around the 1930s, and species such as Solidago canadensis, Verbena brasiliensis, and Amorpha fruticosa, first recorded in the 1930s and 1940s, exploded across the lowland only in the last two to three decades, a timing the researchers attribute to intensifying human pressure combined with global climate change. Georgia still lacks a unified national list of non-native organisms; the best current estimate, based largely on literature, counts roughly 450 alien plants in the country, with up to 500 species of foreign origin in the Adjara floristic region alone.

The consequences for Colchis’s unique habitats are concrete. Invasive species compete with native plants, alter soil composition, and disrupt ecosystems that support globally rare sphagnum peatlands, relict forests, and endemic wetland communities, while also invading agroecosystems and reducing crop yields. The authors assess Amorpha fruticosa, Ambrosia artemisiifolia, Gleditsia triacanthos, Sicyos angulatus, Solidago canadensis, and Verbena brasiliensis as dangerous invasive species based on their growth, fruiting, reproduction, distribution, frequency, cover, and occurrence. They caution that the list of thirty widespread species is not exhaustive and plan to study the invaders’ impacts on ecosystem services next. Their conclusion is unambiguous: constant monitoring of alien species, and of both their negative and positive effects on ecosystem services, is essential if the legendary landscapes of Colchis are to keep their character in a warming, increasingly connected world.

Subject of Research: Distribution and invasiveness of alien plant species in the Colchis lowland of western Georgia

Article Title: Distribution and invasiveness of some invasive alien species ( IAS ) of plants in ecosystems of Colchis lowland (W Georgia / Sakartvelo)

Article References: Mikeladze, I., Tsiskaridze, D., Manvelidze, Z., & Shainidze, G. (2025). Distribution and invasiveness of some invasive alien species (IAS) of plants in ecosystems of Colchis lowland (W Georgia / Sakartvelo). Scientific Annals of the Danube Delta Institute, 30, 91-106. https://doi.org/10.3897/saddi.30.165717

Image Credits: AI Generated

DOI: 10.3897/saddi.30.165717

Keywords: invasive alien species, Colchis lowland, Georgia, plant invasion, Kolkheti National Park, Chorokhi Delta, Braun-Blanquet method, biodiversity, common ragweed, Canadian goldenrod, ecosystem services, climate change