Agroscope, ESA, Angers

Green Alder Encroachment Control: Combining Mechanical Clearing with Goat Grazing

The green alder encroachment of summer pastures reduces biodiversity, forage quality, and landscape attractiveness. Goat grazing can help limit this process and restore pasture vegetation.

The decline in agro-pastoral activities has led to an increase in the cover of green alder (Alnus viridis (Chaix) DC.) in the Alps. It is one of the most expanding shrub species in Central Europe, with negative agro-environmental impacts, such as eutrophication, greenhouse gas emissions, soil acidification, and a subsequent reduction in forage quality and pasture biodiversity. Historically, small ruminants have played a crucial role in limiting woody species expansion, but the number of goats has dropped by 80% in Switzerland since the late 19th century. In this context, cattle could help fulfil this role through trampling and consumption of shrub species.

In this study, we examined mechanical clearing combined with grazing by goats and heifers in Swiss mountain pastures to limit green alder encroachment. In three summer pastures invaded by green alder, 28 goats (Capra grigia and chamoisée breeds) and 21 heifers (Holstein, Simmental, and Swiss Fleckvieh breeds) were allowed to graze in four plots per pasture. Each pasture included two plots for goats and two plots for heifers. For both animal types, one plot had uncut green alder shrubs, while the other had shrubs cut at the start of the season and grazed at the end of the season. Direct observations of livestock foraging behaviour were conducted to assess their preferences and the composition of their diets. Moreover, the percentage of debarking, defoliation, and resprout consumption was reassessed on alder shrubs after grazing.

Heifers preferred herbaceous species and avoided green alder

Herbaceous species constituted 87% of the heifers’ diets, while green alder represented less than 1% and was avoided even when highly abundant. However, heifers consumed other shrubs, such as rose, raspberry, and blueberry. The resprouts of cut green alder shrubs were consumed or trampled by heifers, but most of them were only slightly affected (less than 30% of the resprouts per shrub were affected).

Goats fed on green alder, and they intensively debarked, defoliated, and consumed the alder resprouts

Woody species constitute a significant part of the goats’ diets. In uncut plots, green alder accounted for up to 57% of their diet. Goats also consumed understorey vegetation in green alder stands (e.g. ferns). Unlike heifers, goats explored alder stands extensively and adjusted their diets based on available plants. Notably, 70% of the green alder branches were defoliated by goats, and 18% were debarked (with significant variation among the summer pastures). The goats also consumed almost all the resprouts of cut green alder shrubs.

Conclusions

  • The high level of goat bark-stripping activity and the strong impact on alder resprouts suggest that repeated goat grazing, combined with mechanical clearing, could be an effective tool to limit green alder encroachment and restore pasture vegetation.
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