Every day is like Christmas for some declining farmland birds

Farmland habitat is changing across Europe as farms industrialise. For birds that could previously live a life on a farm, this intensification of farming is turning a previously benign environment hostile. However, farming is also producing some unusual crops. Demand for Christmas trees has risen in the 21st century. Steffen Kämpfer and colleagues at Osnabrück University have examined how the woodlark and other declining farmland birds have used Christmas tree plantations as a new habitat. Their work is to be published in The Annals of Applied Biology.

In the past, farms have provided a rich and varied environment for wildlife. Indeed, much work has been done on how farmland diversity aids biodiversity. This is important work as farms move to bigger areas of monocultures. However, not so much work has been done on some of the newer crops that farmers can grow. Dr Thomas Fartmann has led a project, “Biodiversity of Christmas tree cultures in Central Europe” to plug this gap in knowledge. It’s where you might expect to find more biodiversity than you might expect, given the plantations tend to be monocultures of Caucasian fir, Abies nordmanniana, as the authors explain in their article.

“The cultivation of Christmas trees is generally associated with intensive management. The plantations are usually characterised by short rotation cycles, a moderate input of fertiliser and the application of herbicides, especially during the first growing years of the trees… However, they provide open to semi-open and heterogeneous habitat structures, which are known to be beneficial for the majority of farmland specialists… Accordingly, recent studies found that Christmas-trees plantations can harbour a rich biodiversity, regardless of the rather intensive farming practice,” write Kämpfer and colleagues.

Dr Fartmann’s team went to the Sauerland, a region of Germany to the east of Düsseldorf, where the Christmas tree industry is particularly strong. “With around 18,000 hectares of trees, the Sauerland is now the most important European cultivation area for Christmas trees. The relatively nutrient-poor soils in combination with the cool low mountain range climate and a short vegetation period prevent excessive height growth and favour the stepped structure of the trees with not too large whorl distances desired by the customers.” Dr Fartman wrote for the Osnabrück University website.

Among the trees, the team examined territory selection of four declining farmland birds – Common Linnet (Linaria cannabina), Tree pipit (Anthus trivialis), Woodlark (Lullula arborea) and Yellowhammer (Emberiza citronella).

The management of the study sites wasn’t perfect for the birds, but not too bad. The owners fenced the sites off to prevent deer from attacking the young tips of the trees. These same fences kept out humans and any dogs they would like to walk. The large mesh of the fence didn’t prevent foxes from entering, though.

Weed control varied with the trees that were growing. For Christmas trees, there was plenty of herbicide used for the first six years. In the parts of the plantation used for growing brushwood, there was less herbicide and more weeds, as tree quality is less important for brushwood.

The result is that while the trees in the plantations might be repetitive, the soil cover is not. Instead, there is a mix of bare ground, stones and weeds.

The team found that territories for the four birds all contained higher shares of young Christmas-tree plantations. Young plantations were particularly popular with Tree Pipits and Woodlarks. One possible reason for the preference is the herbicide.

“Due to a lower tree cover and regular weed control…, young Christmas-tree plantations are characterised by open habitat structures with a heterogeneous mosaic of bare ground and herbaceous vegetation between the tree rows…. On the one hand, these conditions ensure a high availability and accessibility of invertebrate prey (i.e. ground-dwelling arthropods, such as carabids and spiders…) for the insectivorous model organisms… On the other hand, they also provide suitable nesting sites and sufficient shelter against predators for ground-nesting species.” write Kämpfer and colleagues.

Older plantations had more of a place in the territories of Common linnets and Yellowhammers. The scientists argue that these birds eat more seeds and would benefit from areas with more weed cover.

The authors close their paper with a discussion of weed management. They’re certainly not arguing for a herbicide extravaganza. As they write in their article, herbicide use is known to “cause detrimental effects on biodiversity and ecosystem services.” So are there alternatives that foresters could use? Yes, but each has its problems.

Sheep could be used to graze the weeds. This is a natural method of control. Unfortunately, sheep don’t cause patches of bare earth regularly, instead creating very trimmed and homogenous grasslands. Grazing also prevents flowering in many species, reducing the seeds for the seed-eating birds.

Another method is milling, mowing or mulching. But these plantations are not gardens, so this needs to be done rapidly and on a large scale. The kind of action necessary means you risk damaging or destroying the nests of ground-nesting birds.

For these reasons, herbicide might have the best benefits for birds, but the authors insist this is within limits. “[H]erbicide use should be carefully applied and restricted to the absolute minimum necessary rate. In particular, we suggest restricting herbicide use to inter- rows between the trees while avoiding it along uncultivated areas, such as fences and tramlines, of all types of Christmas-tree plantations.”

Something plantation owners are doing that helps aid biodiversity is creating a patchwork of habitats with crop rotation, with the relatively rapid turnover of trees. Kämpfer and colleagues suggest adding some solitary trees or hedgerows to help boost this diversity.

One of the most striking results of the paper is how much birds can gain from Christmas tree plantations when managed well. Fartmann’s team found that the plantations boosted Woodlark territory density by 20% over their preferred natural habitat, the heathlands in the North German Plain. For some birds, new approaches in farming may prove a welcome respite from losses elsewhere.

READ THE ARTICLE

Kämpfer, S., Löffler, F., Brüggeshemke, J. and Fartmann, T. (2022) “Untangling the role of a novel agro‐ecosystem as a habitat for declining farmland birds,” The Annals of Applied Biology, https://doi.org/10.1111/aab.12789