Shrubs
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Increase vegetation diversity
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Cairngorms Connect
Scotland
Problem: Historic land management in much of the British uplands, including grazing by high densities of deer, has led to a dominance of heather, which limits tree regeneration in formerly wooded areas by forming a dense and shaded field layer.
Action: Cairngorms Connect is reducing grazing and browsing pressure by actively managing deer densities and also promoting trees to recolonise areas of open heath within the project area, by targeted planting and vegetation management.
Indicator: Increased species diversity of dwarf shrubs, which dominate vegetation in this landscape and respond quickly to changes in light and disturbance regime, indicate the recovery of this upland habitat. As the native vegetation recovers, the shrub layer will become more diverse, characterised by an increase in species of conservation importance like blaeberry and a reduction in the dominance of heather.
Cairngorms Connect staff go out in all weather to collect vegetation survey data. Photo: Cairngorms Connect.
Vegetation height being measured. Photo: Pip Gullett.
Methods
Two complementary approaches are being used to monitor ericaceous shrubs across the project area. The first is a “longitudinal” study (measuring change through time) in which four sites at a range of altitudes (300, 500, 700, 900 m above sea level) were monitored twice, in 2020 and 2024, to track changes in the vegetation community as a result of restoration. The second is a “chronosequence” approach (simultaneously comparing different areas that are at different stages of restoration), where the following areas are being monitored:
- 24 sites in the Eastern Cairngorms – representing an early stage of restoration, with variable land management practices and generally low levels of natural tree regeneration
- 17 sites in Cairngorms Connect with high deer densities – representing an early stage of restoration, with deer numbers still too high to enable successful natural tree regeneration
- 19 sites in Cairngorms Connect with low deer densities – representing a moderate stage of restoration, where deer management has enabled some natural tree regeneration
- 20 sites in Norway – representing a late stage of restoration, where extensive natural tree regeneration has taken place over the past 100 years
Results from these sites show how species assemblages in the Cairngorms Connect area compare to areas of geographically and climatically similar habitat under contrasting management regimes. These sites were each monitored once during the ELSP funding period, with a resurvey planned for ~20 years later.
At each site, vegetation is surveyed in July or August in 25 1 x 1 m quadrats within a randomly located 200 x 200 m square. Percentage cover is estimated for each ericaceous shrub species, and other cover types grouped into categories (e.g. grasses, Sphagnum mosses, bare ground). The same longitudinal and chronosequence sites are also being used to monitor moth and songbird diversity.
Results
Vegetation surveys were completed at all four longitudinal sites in summer 2020 and 2024. Changes in vegetation, including coverage of ericaceous shrubs and heather, between the two surveys were small and the direction of changes was not consistent across sites. There was some evidence of a positive change in ericaceous shrub diversity, which showed a consistent, but non-significant, increase over the four-year period at all four sites (Table 1).
Table 1. Percentage cover of target ericaceous species, common heather, and diversity of ericaceous species recorded at each longitudinal site in 2020 and 2024. Percentage cover given as mean (±s.d.) across all 25 quadrats at a site, diversity calculated once per site. There is some evidence of positive change in ericaceous shrub diversity, which showed a consistent, but non-significant, increase over the four-year period at all four sites (2-tailed t-test: t4.8=2.3, P=0.07).
The chronosequence sites, at different stages of the restoration trajectory, showed considerable variation in the cover of heather and the Simpson’s diversity of ericaceous shrubs between sites. Heather cover was significantly lower and ericaceous shrub diversity was significantly higher in the Norway sites compared to the other three areas (Figures 1 & 2).
Fig 1. Percentage cover of heather at each of the four chronosequence sites at different stages of the restoration trajectory (from left to right). Heather cover was significantly lower (2-way Anova: F3,76=16.8, P<0.0001; Tukey HSD: P<0.0001 for all three pairwise comparisons) in the Norway sites compared to the other three areas.
Fig 2. Simpson’s diversity of ericaceous shrubs at each of the four chronosequence areas at different stages of the restoration trajectory (from left to right). Ericaceous shrub diversity was significantly higher (2-way Anova: F3,76=14.8, P<0.0001; Tukey HSD: P<0.0001 for all three pairwise comparisons) in the Norway sites compared to the other three areas.
Interpretation
There was some evidence of an increase in ericaceous shrub diversity through time, with increases at all four sites between 2020 – 2024. Although these increases were not statistically significant, this is probably related to the small number of sites that were surveyed. This is a promising sign that the diversity of the main ground layer vegetation is gradually recovering across the Cairngorms Connect area, likely due to the reduced impacts of grazing and trampling through increased deer control. This result was supported by the findings from the chronosequence study, which found significantly lower cover of heather and higher ericaceous shrub diversity in the late-restoration stage Norway sites compared to all three Cairngorms areas, which are at earlier stages in the recovery of native woodland ecosystems.
Overall, the results from the first five years of monitoring three indicator species groups have provided evidence that ericaceous shrub diversity has already started to increase, and that diversity of all three species groups (moths, songbirds and ericaceous shrubs) is increasing in areas where browsing/grazing levels have been reduced. Given the relatively short time period of monitoring so far, this is an encouraging sign that restoration management is helping the recovery of species groups associated with restored habitat types. Future monitoring will help to build on and extend these findings and identify sites or habitats where further restoration management intervention may be required.
