Hedgerow age affects the species richness of herbaceous forest plants

Kathrin Litza & Martin Diekmann

Hedgerows of different ages in the agricultural landscape of Schleswig-Holstein, Germany. Photo credit: Kathrin Litza.

Hedgerows are narrow strips of wooded habitats that are known to provide refuge habitat for forest species, such as herbaceous forest plants, in agricultural landscapes. We analysed the importance of hedgerow age for their species composition with a focus on the colonisation by forest plants. Many herbaceous forest plants lack efficient dispersal mechanisms. This can be challenging for conservation measures because the colonisation of new habitat patches takes time.

In this study, we compared hedgerows in pairs that each consisted of one ancient hedgerow (at least 140 years old) and one recent hedgerow (between 14 and 80 years old) that otherwise were similar in terms of structure, location and management. This way, we could trace our found patterns back to the age difference.

We found more forest plants in the ancient hedgerows than in the recent ones. But also the recent hedgerows were more species rich than we had anticipated. One factor we identified as beneficial for a fast colonisation was close proximity to species-rich ancient hedgerows and ancient forests. Also, hedgerow width was important, with wide hedgerows containing more forest plants than narrow ones. Hedgerows get more forest-like the wider they are because solar radiation is reduced and the interior conditions like humidity and temperature are more stable. We also found an influence of the soil properties. A base-rich soil and low nutrient content in the form of phosphorus were beneficial for forest specialists.

Additionally, we analysed which form of dispersal was represented more often in recent hedgerows than in the ancient ones and found that species which were dispersed externally by animals (i.e. not eaten but transported, e.g. stuck to the fur) were the ones most dominant in the recent hedgerows. This dispersal mode, called epizoochory, can be very efficient especially as hedgerows are used as migration corridors by many animals.

One of our main results is that the species composition of the recent hedgerows (including not only specialists but all plants) of the pairs got more similar to the ancient hedgerows with increasing age of the recent ones. Hedgerows are regularly cut back to keep them in a dense and healthy state, and therefore they never reach the natural climax state which would be a forest. But this result shows that they develop an established state with a typical species composition. We conclude that ancient hedgerows are very important for the regional biodiversity and therefore need to be protected. But also the importance of recent hedgerows should not be neglected. If they are managed traditionally, they can develop into valuable habitats similar to ancient hedgerows.

This is a plain language summary for the paper of Litza & Diekmann published in the Journal of Vegetation Science.