Jacobaea paludosa is what ecologists call a river corridor plant, a species found almost exclusively along the valleys of large European rivers, where the ground alternates between inundation and drying. Across central Europe many of these plants have become vanishingly rare, and in the Po Plain of northern Italy the species sits at the very edge of its range, cut off from the main continental populations by the Alps. Decades of agricultural expansion, industrial development and hydraulic engineering have obliterated the vast marshes and floodplain forests that once covered the region, leaving the plant clinging to artificial refuges such as drainage canals, quarry basins and man-made wetlands. Water pollution and eutrophication continue to squeeze these last outposts, and the species now appears on several regional red lists.

The research team focused on a natural population at Piuda, within the protected Mincio river area, where many hundreds of flowering scapes grow across roughly 6.8 hectares of peaty wet grassland. They compared it with a second, much smaller population along the Collettore delle Acque Basse Reggiane canal, part of which descends from a reinforcement intervention carried out in 2006 using seeds from the local natural nucleus. The central question was deceptively simple: is this large, apparently thriving population actually reproducing itself, or is it a demographic illusion sustained only by vegetative spread?

Jacobaea paludosa propagates mainly by breaking apart its rhizomes, underground stems that sprout new shoots, so a field full of flowering scapes may in fact be dominated by a few genetically identical individuals. Sexual reproduction through seeds is the process that generates genetic novelty and allows a population to adapt, colonise new ground and recover from disturbance. To probe how well this works, the researchers ran a series of field pollination experiments. Some flower heads were bagged at bud stage to test whether the plant can self-pollinate spontaneously without insects. Others were hand-pollinated with their own pollen. A third group was left open to natural pollination, and additional heads received supplemental pollen, either from other plants within the same population or from plants of the distant reinforced population.

The results of the breeding system tests were unambiguous. Flower heads that were bagged and left to self-pollinate spontaneously produced no well-developed, viable fruits at all, only shrunken or empty cypselae, the characteristic one-seeded fruits of the daisy family. Hand self-pollination did produce some well-formed fruits, demonstrating that the species is self-compatible, meaning its flowers can be fertilised by their own pollen, but the yield was dramatically lower than under open, insect-mediated pollination. The researchers also confirmed that the outward appearance of a fruit reliably predicts its contents: plump, pale brown cypselae carried living embryos, while dark, shrivelled ones were sterile, a finding verified with a biochemical tetrazolium test that stains living tissue.

These findings overturn earlier suggestions that the species might reproduce without insect pollinators or even asexually through seeds. Instead, the study shows that Jacobaea paludosa depends essentially on insect pollen vectors to set viable seed, making it vulnerable to any decline in pollinator service on top of its habitat problems. Natural pollination produced the highest proportion of well-developed fruits, roughly three quarters of the fruit output in the large natural population, indicating that insects are visiting flowers and delivering pollen in adequate quantities. Pollination, in other words, is not the problem.

The problem lies deeper, in what happens after fertilisation. When seeds from naturally pollinated flower heads were brought into the laboratory and given favourable germination conditions, only 1.48 percent of the viable-looking seeds from the large natural population actually sprouted. By contrast, seeds from the reinforced population germinated at a rate of 75 percent, a difference so stark that it dominated the study’s statistical analysis. Adding pollen from other plants within the same natural population barely improved matters, with germination still below three percent. The population was producing fruits in respectable numbers, but those fruits were, for practical purposes, reproductively dead.

The most intriguing result emerged when the researchers crossed the natural population with pollen from the distant reinforced population. Inter-population crosses produced fewer well-developed fruits than within-population crosses, a pattern the authors interpret as a possible sign of outbreeding depression at the fruit-set stage, yet the seeds that did form germinated at 10.37 percent, more than three times the rate achieved with pollen from the natural population’s own plants. Since previous work had shown that pollen vitality in the study population never falls below about 50 percent, poor pollen quality cannot explain the failure. The most plausible explanation, the researchers argue, is inbreeding depression: because the plant spreads clonally over short distances, neighbouring flowering scapes are likely to be close relatives, and the resulting embryos, though morphologically normal, lack the vitality to germinate. Post-zygotic barriers of this kind, which allow embryo development but cripple seed viability, are a classic signature of a genetically depauperate population.

For conservation planners, the implications are significant and somewhat counterintuitive. The reinforcement carried out in 2006, which boosted seed germination in the recipient population, appears to have worked, but the new study cautions that reinforcing a population with material drawn from itself, as was done then, may add little or no genetic variability. For the large natural population, a similar intervention using its own stock would likely fail; the authors conclude that effective reinforcement would require sourcing plants or seeds from a genetically distinct population. Just as importantly, the study demonstrates that population size is a poor proxy for long-term viability. A population of many hundreds of flowering individuals can still be sliding toward extinction if its seeds cannot germinate, a lesson already learned with the endangered water-clover Marsilea quadrifolia in similar wetland habitats.

The authors also stress that genetic rescue alone cannot save the species if its habitat keeps degrading. Jacobaea paludosa evolved on unstable floodplains where erosion and deposition constantly create the bare, open regeneration niches it needs, and traditional wetland management such as harvesting, grazing and controlled burning once mimicked those natural dynamics. That management is becoming rarer just as wetland deterioration and eutrophication accelerate. The study’s broader message resonates far beyond one Italian marsh: conservation assessments that count individuals or flowering stems may miss the reproductive constraints that actually determine whether a population persists. Identifying such constraints requires targeted, population-level experiments of exactly the kind performed here, and the researchers argue that treating populations as the key management units is the most efficient way to allocate scarce conservation resources before species like this silent casualty of the Po Plain slip away for good.

Subject of Research: Reproductive constraints and inbreeding depression in isolated populations of the threatened wetland plant Jacobaea paludosa

Article Title: An isolated, yet extensive population of the threatened species Jacobaea paludosa ( Asteraceae ) shows alarming reproductive constraints

Article References: Barberis, M., Buldrini, F., Bogo, G., Severi, R., & Galloni, M. (2026). An isolated, yet extensive population of the threatened species Jacobaea paludosa (Asteraceae) shows alarming reproductive constraints. Italian Botanist, 22, 1-16. https://doi.org/10.3897/italianbotanist.22.183002

Image Credits: AI Generated

DOI: 10.3897/italianbotanist.22.183002

Keywords: Jacobaea paludosa, plant conservation, inbreeding depression, pollination ecology, seed germination, wetland habitats, Po Plain, river corridor plants, population reinforcement, genetic diversity, Asteraceae, generation turnover