Delving into “The Corridors of Time”: ancient hedgerows connected to woodlands are living museums of forest biodiversity

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The post is provided by Jonathan Lenoir & Guillaume Decocq

Figure 1: A dense network of hedgerows in the so-called “bocage” landscape in the Thiérache region in northern France (Picardie, Hauts-de-France). Photo by Guillaume Decocq.

This post refers to the article “Historical continuity and spatial connectivity ensure hedgerows are effective corridors for forest plants: evidence from the species-time-area relationship” by Jonathan Lenoir et al. published in the Journal of Vegetation Science (https://doi.org/10.1111/jvs.12845).

Hedgerows are linear woody elements heavily debated for their effectiveness as biological corridors for forest species in agricultural landscapes, especially so under the current context of climate and land-use changes. During the Middle Ages in Europe, hedgerows were chiefly used to delineate land ownerships, to protect houses from wind and as natural enclosures for cattle confinement. Spiny shrub and tree species like common hawthorn (Crataegus monogyna) were typically used for that purpose. In some regions, hedgerows have been continuously present in the landscape, at least since the oldest available maps like the map of Cassini for the Kingdom of France (1756-1815). Today, these hedgerows are several centuries old. Yet, even if individual trees and shrubs constituting the hedgerows can be younger than that, today’s hedgerows may have existed since the Middle Ages, either as well-identified hedgerows already present at that time or as ancient woodlands reduced to linear woody elements in the landscape.

Ancient hedgerows still standing today bear the imprints of past land uses with the potential to accumulate forest biodiversity over time, especially so if the network of hedgerows is dense and well connected to ancient woodlands (Fig. 1). The older the hedgerow, the richer in forest species it should be according to the species-time relationship. Similarly, the longer and wider the hedgerow, the richer in forest biodiversity it should be according to the species-area relationship. Whether hedgerows’ age interacts positively with hedgerows’ surface area (length or width), in such a manner that ancient and long/large hedgerows host even more forest biodiversity than the sum of its part, remains unknown. Yet, such a species-time-area relationship would be striking evidence that ancient hedgerows are living museums of forest biodiversity and may thus act as effective biological corridors to connect woodlands in the long term.

Hedgerows dating is not an easy task. Here, we literally travelled “The Corridors of Time” by delving into historical maps and archives. In some areas in France, thank God for our administrative madness, several historical maps and archives go back to the 18th century and even before in northern France. For instance, in 1656, the French cartographer Nicolas Sanson published a map of the Vermandois area in the Aisnes department (Picardie, Hauts-de-France) which is available at the National Library of France (BnF) (CPL GE DD-2987, 299). This historical treasure, as well as more recent aerial photographs, are now searchable online thanks to the “Time Machine” provided by the French National Institute of Geographic and Forest Information (IGN) (Fig. 2). This is a great tool to confirm the historical continuity of hedgerows in the landscape.

Figure 2: Comparisons between satellite images from 2017 and several historical maps available around Contay in the Somme department (Picardie, Hauts-de-France). Pictures are a mosaic of screenshots taken from the online portal of the French National Institute of Geographic and Forest Information (IGN) (https://remonterletemps.ign.fr/).

Using the IGN’s “Time Machine” and older maps from the BnF, we built a unique chronosequence ranging from very recent (ca. 10 yrs old) to very ancient (ca. 300 yrs old) hedgerows, either attached or not to a woodland and of varying width and length. Surveying forest plant biodiversity along this chronosequence, we found a crystal clear species-time-area relationship for forest understorey plants. Not only did we find a synergistic interaction between hedgerow age and length, but we also demonstrated that hedgerows attached to a woodland host even more forest plant species in the understorey of hedgerows. In addition, we showed that large and tall hedgerows are richer in forest plant species than narrow and short ones while intensive adjacent land uses have a negative impact on species richness in the understorey of hedgerows. Last but not least, the distribution profile of forest specialists (e.g. Anemone nemorosa, Stachys sylvatica, Viola reichenbachiana) along ancient hedgerows attached to a woodland is showing a curvilinear decreasing pattern starting from the connecting point and away into the hedgerow. Altogether, this suggests that ancient, long, large and tall hedgerows connected to woodlands are, without any doubt, the safest biological corridors for forest plants within intensive agricultural landscapes. Given the species-time-area relationship we found for understorey forest plants in hedgerows, it is clear that ancient hedgerows connected to woodlands are both refugia and biological corridors for the survival of forest plant diversity within agricultural landscapes. Hence, it is of utmost importance to protect the most ancient hedgerows in order to safeguard forest biodiversity and thus ensure a sustainable delivery of ecosystem services in agricultural landscapes.

Brief personal summary: Jonathan Lenoir is a CNRS researcher in Ecology and Guillaume Decocq is a Professor in Vegetation and Fungal Sciences as well as the head of the research unit « Ecologie et Dynamique des Systèmes Anthjropisés » (EDYSAN, UMR 7058 CNRS, Université de Picardie Jules Verne, Amiens, France; https://www.u-picardie.fr/edysan/). The research unit EDYSAN focuses on the impact of global change drivers (climate change, land-use changes and biological invasions) on forest ecosystems and agricultural landscapes.