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Invasive Weeds' Impact on Ecologically Sensitive Sites

Invasive American Skunk Cabbage in a pond wetland in a forest wood

What are "Ecologically Sensitive Sites"

Sensitive sites often contain protected species or natural habitats which are vulnerable to human activities, for example development, forestry and farming. In addition, many more species and the habitats they depend upon are becoming threatened due to climate change and its knock on impacts of rising sea levels, increasing temperatures and more frequent extreme weather events.

In Scotland and across the wider UK, many of the sensitive or ecologically important habitats which remain in or close to their natural state have been designated Sites of Special Scientific Interest (SSSI) and are now protected. However, there are many more sites that were previously drained and/or cleared for farming and hunting. It is often these degraded sites which are most vulnerable to invasive non-native plants.

Upland and Lowland Peat Bogs

Invasive Sitka spruce affecting a peat bog in Scotland

Peat bogs are wetlands where the soil is made up of partially decayed plant vegetation. The waterlogged conditions and high acidity prevent vegetation from fully decaying, leading to a build up of organic matter over thousands of years. Blanket Bogs, which can cover vast areas, are commonly found in upland areas, such as the Highlands of Scotland, whereas Raised Bogs are often found in low lying basins or former kettle lochs and are particularly concentrated in the Central Belt in Scotland.

In their natural state, the high moisture content and acidic nature of peat bogs makes colonisation by invasive non-native plants difficult. However, once drained, the degraded peatlands are then vulnerable to rapid colonisation by invasive non-native plants.

Of these, Rhododendron ponticum and Sitka spruce commonly cause issues as they are often found nearby in woodland and timber plantations. Rhododendron ponticum stands can block out the light and out-compete native bog vegetation, whilst Sitka spruce and other self-seeding conifers dry out the bog surface and alter the soil conditions to favour their own growth.

Temperate Rainforests

Invasive Rhododendron ponticum affecting  a temperate rainforest woodland in Scotland

Temperate rainforests, also known as Atlantic or Celtic rainforests in Scotland, are native forests located in areas with high annual rainfall and relatively low variation in temperatures. Globally, they are incredibly rare forest habitats where many scarce plants, lichens and fungi thrive in the high humidity.

Invasive non-native plants are some of the biggest threats to the survival of the remaining pockets of temperate rainforest on the west coast of Scotland. Again, Rhododendron ponticum is a threat as it aggressively colonises woodlands, outcompeting native shrubs and forest floor plants.

Another major problem is ash die-back, a non-native fungus which has decimated the native ash tree population across the UK. Some neighbouring areas have also been re-planted with non-native conifers, leading to encroachment.

Sand Dunes

Invasive Japanese rose growing on a coastal sand dune system by a beach

Sand dunes are coastal habitats often found around sandy estuaries or large beaches where exposure to wind allows sand grains to pile up. These ever-shifting ecosystems are then colonised by a range of plant and animal species. Drought tolerant grasses on the higher dunes, through to shrubs on the more stable dunes to the rear, pockets of wetland habitats in the dune slacks.

Being coastal, dune systems are vulnerable to erosion, more frequent storms and rising sea levels caused by climate change. In addition, they are threatened by invasive non-native plants for example Hottentot Fig, a dense mat forming plant native to South Africa, which can take over large areas of the dune system. Japanese Rose, Montbretia and Cotoneaster are other invasive non-native plants that are often found growing on dune systems.

Dune systems are also affected by Sea Buckthorn, which is native to the east coast of England but is considered invasive elsewhere. It grows in large, dense clumps which out-compete many smaller plant species. Sea Buckthorn was previously widely introduced to dune systems across the UK as people thought the roots would help stabilise the sand.

Freshwater Wetlands

Invasive New Zealand pygmyweed or Australian swamp stonecrop at the side of a loch

Freshwater wetlands are often found around the verges of lochs and on river floodplains. They are important habitats for a variety of plants, animals, invertebrates and birds. As a result, a large proportion of SSSI’s are wetland habitats or freshwater bodies. Due to their nature of being in shallow, slow-moving or enclosed areas of water, they are vulnerable to many invasive non-native water plants.

Species already found in Scotland include Canadian Pondweed, Nuttalls Pondweed, and Curly Waterweed. We have a detailed article on these invasive non-native plants and more which can be seen throughout the UK although not yet widespread in Scotland. Article available to read here. Many of these plants grow in dense mats which outcompete native water plants, block out the light and cause deoxygenation of the water which can harm aquatic organisms. They are very difficult to remediate completely as many can regenerate from very small fragments which are easily spread by flooding or human activity (e.g. fishing, boating). For this reason it is very important to practice the Check, Clean, Dry campaign when participating in leisure activities on lochs and rivers, and to report any invasive non-native plants you may come across.

In addition to invasive non-native plants that grow in water, there are several land-based invasive non-native plants that are often found around rivers and lakes. The “big three” Japanese knotweed, Giant Hogweed and Himalayan Balsam all commonly grow on the edges of watercourses. Often, they can take over large stretches of the loch-side or riverbank, leaving bare soil when the above-ground vegetation dies back in the winter. This can leave the banks vulnerable to erosion and affect water quality down-stream.

Salt Marshes

Common cord grass growing on a salt marsh in a river estuary

Salt marshes are low-lying, coastal wetlands, often found around river estuaries. Unlike freshwater wetlands, they are brackish due to the high salt-water table near the coast and flooding during high tides. Similar to peat bogs, the waterlogged soil contains a lot of partially decayed vegetation. In addition, sediment from the estuary accumulates around vegetation leading to low oxygen levels and fluctuating salinity levels. This leads to extreme conditions where only highly specialised plants can thrive. They create a very important niche habitat for many invertebrate species, resident and migratory wading birds which feed on them.

Due to the often high salinity environment, the coastal marshes are not affected by as many invasive non-native plants as freshwater wetlands.  One plant which does cause issues is Common Cordgrass, a hybrid produced from European native cordgrasses and Smooth Cordgrass, which was introduced from America in 1870. As with Sea Buckthorn on sand dunes, it was once widely planted in salt marshes across the UK as its extensive root systems were believed to stabilise the soils. Though its impacts have not been widely studied, it is known to outcompete and cross-pollinate with native grasses, leading to their reduction, which may cause subsequent reduction in habitat area and quality for invertebrate and bird species.

Issues to Consider When Remediating Ecologically Sensitive Sites

A rewetted restored peat bog with raised water table

Some of these sites, such as upland peat bogs or remaining pockets of temperate rainforest, are in very remote areas and access can be an issue. On waterlogged, marshy grounds, Low Ground Pressure (LGP) equipment may be required for access and works. Waterside sites may require specialist aquatic plant and boats in order to gain access for treatment.

Additionally, due to the sensitive nature of the habitats and the native plants within these ecosystems, consideration must be given to treatment methods. Herbicide treatment may be a limited option on many sites due to its potential impact on surrounding plants or its distribution through water bodies. Excavating invasive non-native plants may cause irreparable damage to the habitat and the loss of rare or protected species. Often treatments need to be manual, such as hand pulling, cutting or stem injection, which can add to treatment costs and timescales.

ERS’ Invasive Weeds team have experience on treating a wide range of invasive non-native plants in urban, rural and remote habitats across Scotland. If you’re planning or currently working on a rewetting, rewilding or other sensitive habitat restoration programme and would like an invasive weeds survey, management plan or a treatment programme implementing, contact us today and ask to speak to a member of our Invasive Weeds team.

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