The most in-depth survey of the British and Irish flora ever undertaken has found that over half of Ireland's native plant species are in decline with native grassland plants suffering the most.
The Botanical Society of Britain and Ireland, which conducted the study, said declines are being driven by habitat loss, mineral enrichment and altered grazing pressure rather than climate change.
It said human activity is the cause of the decline and that human activity must be recruited to restore it.
This is only the third time since 1962 that the Botanical Society of Britain and Ireland has completed a Plant Atlas mapping the distribution of wild plants in Ireland.
It is the culmination of 20 years of field work during which 2,500 people in Ireland made almost 3 million plant records that contributed to the atlas showing what plants grow where and how their distributions have changed.
In all 1,939 plant species were recorded, just under half of which were native to Ireland. The study found that 56% of Ireland's native plants have declined, either in range or abundance, or both.
Native grassland plants have suffered the most, but many plants of lakes and wetlands have also declined.
In contrast, 80% of non-native plants introduced into Ireland since 1500 have increased.
Most of these non-native species are benign but some, such as Himalayan Balsam and Rhododendron, have become invasive, with a negative impact on the native flora.
Another non-native plant which has shown a very marked increase is American Willow herb, a North American plant species, common as a weed on waste ground, walls, and in gardens.
It was first recorded in Ireland in 1958, but has spread explosively since 1980.
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Native woodland plants, such as Bluebells, have fared relatively well, with many holding their own or gaining slightly, even though Ireland is one of the least wooded countries in Europe.
In recent years the extent of woodland has been gradually increasing, most notably through the planting of conifers, and to a lesser extent of broadleaved trees.
However, many other Irish native species have suffered. Plants of grassland, such as Field Gentian, have fared worst according to the findings.
Other plants that grow in fen, marsh, and swamp habitats, such as Marsh Lousewort and Corn Mint, have also showed a marked decline over the past 30 years.
Many peatbogs have been planted with conifers or converted to agriculture, excluding the native bog plants such as heathers and sundew.
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Peatland habitats are important for carbon storage, and their restoration is essential as part of our efforts to combat climate change.
The Botanical Society of Britain and Ireland said the loss of wet habitat plants is a major issue for biodiversity conservation in Ireland because these species make up a substantial proportion of the Irish flora.
There are signs too of continued disruption and loss in aquatic plant communities.
Some lakes are now dominated by the few aquatic plants favoured by nutrient enrichment, such as the introduced Nuttall's Pondweed.
Overall, the main causes of biodiversity loss according to the Plant Atlas is habitat loss and degradation rather than climate change, which it says has only had minor effects so far.
It notes that Ireland’s climate has been warming in line with global trends. Average temperatures have risen by nearly one degree in a century, and the distribution of rainfall through the year is shifting.
It warns if changes like these continue, there will inevitably be consequences for wild plants, with species more at home further south are likely to expand their range northwards, and that those more typical of northern climes will retreat away from the south of the country.
In the meantime, it points out that Ireland’s native flora is under such pressure from other factors, such as habitat loss, mineral enrichment and altered grazing pressure that disentangling the causes of decline is problematic.
Many of the habitats Irish wild plants depend on have been destroyed or altered by farming and forestry since the 1950s.
Re-seeding, over-fertilising, nitrogen deposition, herbicides, soil drainage, and changes in grazing pressure have all contributed to the decline of species like Agrimony, Field Gentian, Marsh Lousewort, and many others.
The Botanical Society of Britain and Ireland said the Plant Atlas 2020 provides a powerful and concerning insight into the changing distributions of our wild plants. Human activity, it said, is the cause of the decline and that human activity must be recruited to restore it.
BSBI President Dr Micheline Sheehy Skeffington said the Plant Atlas 2020 must be seen as a wake-up call for action as plants are the basis of all biodiversity.
Dr Kevin Walker, BSBI Head of Science and Plant Atlas 2020 co-author, said there is a lot that can be done to reverse these declines, but the most important are to increase the protection plants receive, extend the habitat available to them, and to place their needs at the very heart of nature conservation.
He said we also need to ensure that our land, water, and soil are managed more sustainably so that plants, and the species which rely upon them for food and shelter, can thrive.
