Hedgerows act as wildlife corridors for the wildlife that cannot walk

We’re used to the idea that hedgerows can act as wildlife corridors, for animals to move along. But it’s not just animals that can benefit from shade. Thomas Vanneste and colleagues in Belgium and Sweden have examined how plants could use hedgerows to migrate as the climate warms. The work, published in Annals of Botany, investigated how two common forest flowers coped with rising temperatures and transplantation in hedgerows and forests. Their study of Anemone nemorosa and Geum urbanum shows that forest herbs will have mixed fortunes in the future.

Plants can occupy a location for many reasons, such as rainfall, temperature or suitable neighbours. Tweaking some of these parameters, like increasing the temperature, can make a place less welcoming. It’s no surprise that climate change is driving some plants uphill or further north. A problem for plants is how they move their range. Animals can walk and fly, so they can pass through hostile terrain if their trip is brief. Plants, in contrast, have to grow from seed and survive to scatter their seeds to move their offspring north.

For forest plants like Anemone nemorosa, Wood anemone, and Geum urbanum, Wood avens, travel is a problem. Historically in Europe, there was more forest and better-connected forest so that plants could colonise woodlands. Urbanisation and agriculture have cut back forests to a patchwork of islands. In their paper, Vanneste and colleagues highlight specific habitat requirements, low long-distance dispersal abilities and slow colonisation rates as challenges for many forest herbs.

While forests have diminished, the botanists argue another feature of agriculture could help forest plants. Hedgerows are lines of shrubs and trees. They run marking boundaries and can act as links between habitats for wildlife. While they’re not forests, the authors argue they might be close enough for forest plants to travel along between habitats.

The scientists note two crucial reasons why a hedge isn’t simply a mini forest. A hedge, being smaller, doesn’t protect its understory from external conditions as well as a forest. There’ll be less shade to protect from heat and less moisture as water evaporates from the soil. They also add that fields surround hedges. That means hedges have higher inputs of nutrients from farming. A plant in the understory of a hedge will get much more phosphorus and nitrogen than in a forest. So a hedge habitat might be dramatically more affected by climate change than a forest environment.

“In spite of the importance of hedgerows as potential biological conduits in a changing world, we are unaware of any other study that has assessed the effect of temperature changes on plant species in these wooded corridors. This severely hampers our ability to predict how the utility of hedgerows to conserve forest species in agricultural landscapes will alter in the face of climate warming,” write Vanneste and colleagues.

“We specifically tested the following hypotheses:
1) The two model species respond differently to temperature changes owing to their differences in phenology and life history. We expect the spring flowering species to respond more strongly to warming than the summer flowering species.
2) The impact of experimental warming on plant performance and survival is larger in hedgerows than in forests owing to the less buffered microclimate in the linear elements.
3) Plant individuals transplanted at their home site perform better than those transplanted further away owing to long-term adaptation to the local environment.”

Some of the results were a surprise.

The team found both species grew taller and produced heavier seeds with warming, though the anemones also suffered higher mortality. They also found that warming effects did not significantly differ between forests and hedgerows. The herbs both performed better in forests than hedgerows. The scientists believe this may be due to increased canopy cover in forests keeping out competitors like nettles.

Where there were differences were in transplantation. The anemones did best when transplanted at their home site. Vanneste and colleagues interpret this as confirming that anemones were locally adapted. The avens, in contrast, did well when transplanted north, and this might be due to their lifecycle.

Avens grow in the spring, before the forest canopy fills out, to take advantage of the light. But there was a difference between the Swedish and Belgian forests in the experiment. “In our experiment, the total canopy cover was c. 18 % lower in Swedish than in Belgian common gardens (more specifically, 60% lower in the Swedish vs. Belgian hedgerows but c. 27% higher in in the Swedish vs. Belgian forests). Additionally, light availability is expected to be greater in the northern common gardens owing to an increase in photoperiod during the growing season towards the north…,” write Vanneste and colleagues. After the spring equinox, the further north you go, the earlier the sunrise and the later the sunset. The botanists argue that the performance of G. urbanum shows it is light-limited. The northern avens have more light.

They also found that there was a difference between the plants in the forest and the hedgerows. The forest plants were taller and produced more seeds. Again, the team argues that this could be due to lesser competition on the forest floor than in hedgerows. But they also note another possible explanation.

The year they did the experiment, 2018, coincided with a drought in northwest Europe. “The adverse impact of this extreme drought event could have been larger on plants in hedgerows than in forest interiors because the linear structure of the former makes them more prone to the effects of high solar radiation and desiccation by wind…,” write Vanneste and colleagues.

There are limitations to the study. The team state this is an early study manipulating just two species. If competition in hedgerows is a problem, it is critical to understand how those competitors will be affected by warming. If warming makes nettles and brambles more formidable opponents for resources, then hedgerows might offer less hope for forest herbs.

But if these plants can travel along hedgerows, then they could be lifelines. Vanneste and colleagues conclude: “…management strategies aimed at restoring and maintaining networks of hedgerows and remnant forest patches in agricultural landscapes are of paramount importance, given that these complexes may literally serve as a “hedge” against biodiversity loss under global environmental change by alleviating the isolation of natural habitats within and beyond species’ current distribution ranges.”