Fire Risk

Methods

The effect of natural grazing was monitored at two rewilding area in the Greater Côa Valley, Vale Carapito (65 hectares) and Ermo das Águias (300 hectares) (Fig. 1). Both areas are characterised by woodland-shrubland mosaic habitats. In 2021, the grazing regime at Vale Carapito changed from a herd of 25 cattle under rotational grazing to continuous grazing by Sorraia horses (10 horses introduced in April 2021, reduced to 6 horses from 2022 onwards). In Ermo das Águias, in 2022 the grazing regime transitioned from continuous grazing with 20-40 cattle to 16 free-living Sorraia horses.

Figure 1: Location of a) Greater Côa Valley region in Portugal, b) rewilding sites in the Côa Valley: site 1 (Vale Carapito) and site 2 (Ermo das Águias). Sampling plots are shown for c) Vale Carapito and d) Ermo das Águias. Each yellow circle represents a 10 x 10 m sampling plot; filled circles represent ungrazed fenced control plots and open circles represent unfenced grazed plots.

Fenced exclosures were set up in both rewilding areas, to understand the change in vegetation in the presence and absence of semi-natural grazing. Within each rewilding area, six 40 x 40 m survey sites were selected to cover representative habitats and dominant vegetation types (Fig. 1). Each survey site contained four 10 x 10 m sampling plots (Fig. 2). Of these, two control plots were fenced to exclude grazing and two plots were open to natural grazing by horses.

Figure 2. Diagram of a 40 x 40 m survey site (left) and the four 10 x 10 m sampling plots within it (right), two fenced controls (blue with outline) and two unfenced grazing treatments (green, no outline). At all four sampling plots, vegetation structure was surveyed along four 5 m transects (represented by arrows). Stipa gigantea presence was recorded in four 1 x 1 m quadrats in each sampling plot (grey squares).

Baseline data, describing the vegetation under the previous cattle grazing regime, were collected at both sites in 2021. Subsequent monitoring to understand the early effects of natural grazing by horses took place in 2023 and 2024. Within each sampling plot, vegetation structure was assessed along four perpendicular 5 m transects originating from the plot centre (Fig. 2). Vegetation type (grasses, forbs, and shrubs) and height class (0–0.25 m; 0.25–0.50 m; 0.5–1.3 m; 1.3–2 m; 2–4 m; >4 m) was recorded at points at 1 m intervals along each transect. In addition, the presence or absence of Stipa gigantea was recorded in four 1 m × 1 m quadrats, located at the end of each transect (grey squares, Figure 2). Stipa gigantea was of particular interest because its fire-prone traits, including tall flower spikes, a dense and persistent tussock structure, and rapid post-fire regeneration, mean that it makes a particular contribution to fuel loads and fire hazard. Aboveground biomass of forbs and grasses was determined by harvesting all plant material within a 0.4 × 0.4 m quadrat randomly placed within one fenced (control) and one unfenced (grazed) sampling plot at each survey site. Plant material was separated into grasses and forbs, oven-dried at 60°C for 72 hours, and weighed to determine dry biomass.

Due to unforeseen changes at Vale Carapito, only four of the initial six survey sites remained under natural grazing from 2021-2024; results from the remaining four survey sites are therefore presented for this area.

Results

At Vale Carapito, grass height increased significantly over time in the ungrazed plots compared to horse-grazed plots in 2023 and 2024 (Fig. 3). However, shrub cover, shrub height and shrub biomass were not affected by the presence of grazing and did not change significantly with year (Fig. 3).

Figure 3: Change in vegetation structure over time in areas with and without grazing by Sorraia horses, in Vale Carapito.

In Ermo das Águias, like in Vale Carapito, grass height increased significantly more in ungrazed plots than grazed plots. Grass height also showed a significant increase in 2024 (Fig. 4) relative to the 2021 baseline. However, there was no statistically significant effect of grazing treatment on shrub cover, height or biomass over time, although across all plots (both grazed and ungrazed), shrub cover, height and biomass were higher in 2024 than 2021 (Fig. 4).

Figure 4: Change in vegetation structure over time in areas with and without grazing by Sorraia horses, in Ermo das Águias.

The frequency of Stipa gigantea presence increased in the ungrazed areas at Vale Carapito (15 out of 32 plots in 2021, compared to 20 out of 32 plots in 2024) but remained stable in the grazed areas ( 12 out of 32 plots in 2021 compared to 13 out of 32 plots in 2024). In Ermo das Águias, Stipa gigantea was not recorded in any of the 48 grazed and ungrazed sampling plots in 2021, but was found in one ungrazed sampling plot in 2024.

Interpretation

The early results of the effect of natural grazing on vegetation in the Greater Côa Valley showed that grazing by horses has the potential to reduce local fire hazard by controlling the height of fire-prone grasses, even in the short term. In addition, the stability of the occurrence of Stipa gigantea indicated that grazing by horses may help to contain its spread. However, the absence of effects of grazing on shrub height and cover suggests that horse grazing alone may not be sufficient to prevent shrub expansion and maintain open landscapes. The addition of complementary species, such as browsers like red deer and large grazers like Tauros, could exert higher pressure on woody vegetation, contributing to the creation of a more structurally diverse mosaic of habitats. This could enhance ecosystem function and resilience to wildfires in this landscape.

Publications

Ribeiro, I., Aliácar, S., Domingos, T., McCracken, D., & Proença, V. (2025). Semi-wild horse grazing as a rewilding strategy: assessing effects on vegetation structure and composition in the Côa Valley, Portugal. Frontiers in Ecology and Evolution, 13, 1596560.