With a fantastic history of recording plants and significant contributions from volunteers of the Botanical Society of Britain and Ireland, Scotland is able to identify long-term and short-term trends for our plant populations, writes NatureScot’s Biodiversity and Geodiversity Adviser, Sarah Smyth.

Scotland supports 75 percent of all British species recorded in the BSBI Plant Atlas 2020, which provides a comprehensive UK atlas of the distribution of vascular plants. The records are summarised as 10×10 km² grid distribution maps for 3,495 species. 

Using the data for Scotland, we identified long-term (1939-2019) and short-term (1987-2019) trends for Scottish plant populations. We used a modelling approach, corrected to reflect the smaller population and recording community in Scotland, and excluded very rare species which were not statistically significant.

BSBI Volunteers recording lady’s mantle. © Botanical Society of Britain and Ireland.

We also considered the possible causes for changes in plant populations. These align to the IPBES drivers of biodiversity loss and include changing land use, invasive non-native species and the impacts of climate change.

The trends we see in the data show there is a clear split between native plant species and archaeophytes versus recently introduced species (neophytes). Since 1950, 48 percent of native species have declined in range while 56 percent of recently introduced species have increased in range.

Native plants are plants that were already here during the last Ice Age whereas archaeophytes are plants that have been found in Scotland for a long time. Introduced and naturalised before 1500AD, many of these can be associated with early farmers or even Romans. 

Neophytes are plants ‘recently introduced’ after 1500. Around this time there was a change in plant collecting when “plant explorers” had both new and easier access to the Americas and other parts of the world. This was also a time when international trade increased, with some plants “hitching a lift” on wool or hidden amongst other goods.

Many neophytes are familiar to us as ornamental garden plants. Those that are well suited to life in the UK become naturalised and thrive without our help. A very small number of these are so well suited, with little to keep them in check, that they become a threat to native plants. These are known as invasive non-native species, or INNS for short.

Gunnera, along with other INNS, is a threat to our native plants.
© Stan Whitaker / NatureScot.

Plants that have increased most notably are those that have been introduced or have naturalised by escaping from gardens and establishing wild populations such as montbretia and Spanish or hybrid bluebells. Rhododendron, American skunk cabbage and gunnera were once confined to gardens but found the Scottish climate to suit them and can now be recorded in a range of locations; an expensive and challenging legacy of Victorian garden fashions!

Across Scotland, plants of use to people show increases, such as commercial forestry species. Sitka spruce leads the way for short and long-term trends, showing the greatest increase of any plant in Scotland.

Sitka spruce – Scotland’s most increased plant over the short and long term.
© GBNNSS.

Native species that have shown increases, such as deergrass and sheep’s-fescue, are likely to have benefited from nitrogen deposits as a result of human activities. This build-up of nutrients provides an advantage to ‘weedy’ species that can tolerate the conditions at the expense of more sensitive plants, reducing the diversity of the area.

Some trends reflect human actions such as recording bias, favouring recording only native species and overlooking garden escapees, while dedicated national aquatic surveys can also skew the data.

For the species that declined, the largest group were associated with arable land. Changes in land use have seen a move away from traditional and small-scale cultivation, meaning arable weeds that might have been common post-war have since been eliminated. Linked to changing agricultural practices, the species associated with nutrient poor grasslands and heathlands, such as corn marigold and wild pansy, have significantly declined as soil is ‘improved’ with fertiliser or developed. 

Wild pansy, along with other arable plants, has declined significantly in recent decades due to changes in farm practices and the use of fertiliser.
©Andrew MacGregor/NatureScot

The second group of declining species are those also associated with human use, but falling from favour. These include plants grown for animal feed as well as species with a long medical or culinary association such as charlock and good King Henry. Plants such as these would have been used to supplement diet or in times of famine.

It might seem that there is no issue; as we lose some plants, other new species take their place. But our native flora has a vital role in supporting a diverse range of associated species. Recent arrivals might not provide nectar or pollen at the right time of year for our insects or there may be risks to human health from toxic sap. Negative impacts can also be indirect. For example, dense swathes of Himalayan balsam can outcompete native species, die-back suddenly after flowering and then leave riverbanks more exposed to erosion during floods than they would have been with a native flora. 

The significant rise in non-native plants since 1950 is having a negative impact on Scotland. In 2021 it was estimated that INNS cost Scotland £499 million per year, while putting our native biodiversity at risk. That’s why we now have a Scottish Invasive Non-Native Species Action Plan, to help drive forward action to reduce and prevent further spread of priority invasive species.

The trend analysis was made possible by the outstanding contribution of volunteer plant recorders, who take the time to observe and record what they see. Anyone can make a valuable contribution; even something as abundant as a daisy needs to be recorded. If you’d like to help Scotland’s plants, why not get out in nature, see what you can spot and record your findings on iRecord.