Species-rich monoculture?

Interview

With curiosity to the research award

Sarah Baum (© Thünen-Institut/Helge Meyer-Borstel)

For her work on biodiversity in short rotation coppice plantations (SRC), Sarah Baum received the Thünen Research Award 2012. The award is bestowed every two years by the Friends of the Thünen Institute Association. In an EU-funded project she analyzed how many plant species existed within the plantations and how many in the surrounding of 15 by 15 kilometers. The results were remarkable: not only arable fields were species-poor, but also mixed and deciduous forests in Northern Germany hosted only half the number of plant species occurring in the SRCs.


Willow plantation (© Thünen-Institut/Sarah Baum)

Monocultures – for instance of spruces – were always regarded as a counterpart to species-rich forests. Is this no longer true for poplar or willow plantations?

These are of course monocultures but their value for biodiversity depends on where and how they are established. In an agricultural landscape dominated by annual crops they represent enrichment. By introducing a perennial crop, the structures change. Different animals are now able to live in the wooded and sylvan structures. Especially the edge zones are of high value because, for instance, birds can nest there. The multi-year crops also bring more tranquility to the soil, which leads to a higher number of plant species than in annual plantations and a changing species composition during the seasons of the short rotation coppice plantation. The plants in turn nourish other organisms, which again serve others as food. Not much grows in, for example, a dense spruce population because over time – and a spruce population can easily exist for 80 years – it becomes increasingly dark. With the transformation of open country to forest-type structures in a plantation, the remaining vegetation and fauna will transform as well.

Is the plantation an oasis in the rape and corn desert?

Absolutely, if the plantation is an additional structural element and fosters the growth of other plant and animal species. But the plants that can be found are nevertheless often common species, not rare or worthy of protection. These plants need to grow naturally in the area surrounding of the SRC before the cultivation and they also need to be able to reach the designated area. SRCs, if not all harvested in the same year, create more structural variety than areas with a large number of hectares planted with willows or poplars. If harvest is delayed in time every three to five years, the different stages between open country and near-forest can not only be populated by a variety of plant and animal species: the diversified population can as well wander back and forth. It’s even better for the biodiversity if intermediate spaces remain open and edges are kept clear, because more species can be found in the marginal zones, forming transition areas, than in the inside of the SRC.

Popular research object: particularly fast-growing poplars (© Thünen-Institut/Sarah Baum)

How much greater is the variety of species in SRCs?

Other projects dealt with this question – also such concerning animals. They are of course interesting, especially for birds that can nest there. We wanted to observe the vegetation within the plantation. At the beginning, the cuttings – sprout pieces of about 20 centimeters that are simply bedded in the ground – struggled to prevail over the uncontrolled growth of all sorts of herbage. Without pesticides or mechanical reduction of the rank growth they would have almost no chance against grasses and herbaceous plants that grow faster. But the trees become stronger with increasing height; they reach the light earlier and eliminate competitors.

What exactly were you looking for?

I wanted to know which variety of soil vegetation will evolve, how it will change over time and which factors influence it. Also, how are the populations behaving towards the surrounding agricultural landscape? I addressed all that within the European research project. Other partners examined soil ecology, soil water balance and animals (including also spiders, ground beetles and birds) in this context. What is influenced in which way? What guidance can be deduced for farmers who want to cultivate SRC?

Were some of your expectations left unsatisfied?

Many assumptions on correlations that were already known from other habitats but also from other projects on SRC proved to be right. New about our studies was the fact that we examined SRCs in two different European regions, namely Germany and Sweden, and in doing so analyzed not only factors within the SRCs, but also their significance for the close surroundings and for large landscape sections.

Planted in strips: typical SRC (© Thünen-Institut/Sarah Baum)

Will short rotation coppice plantations expand rapidly?

Only about 5,000 hectares have been cultivated in Germany so far because farmers face a number of barriers in this regard. SRC are furthermore not advantageous for all landscapes and animal species. Skylarks are a common example that open landscapes are just as important as habitats. Nevertheless a lot of research is undertaken – for instance on how to achieve good timber growth; which plant variety best suits a particular location. Considering the research projects in progress, the meetings and conferences, the practice lags behind. A lot of people don’t even know what this means – short rotation coppice plantations. It’s something I didn’t know either, until I answered the employment announcement for my position.

What barriers exist for farmers?

A farmer ties himself down for 20 years with a SRC but has often a lease contract for a shorter period. He can no longer react to annual price changes. In Sweden, 15,000 hectares are cultivated as SRC overall, which is, related to the size of the country, not that much more than here but there are some key points. In Central Sweden, for instance, a combined heat and power station solely provides energy from wood pellets from SRCs and other fuels for a municipality of 20,000 inhabitants. The 80 ha of cultivated SRC are sprinkled with preliminarily treated wastewaters – an added benefit because the SRCs absorb a lot of pollutants.

Meanwhile the space required here is quite large...

It’s simply a case of agricultural use. Not everyone will like their appearance – especially when they block the view after a few years, but that’s not much better with corn either.

So will this remain as one of many ways to balance ecological and economical demands?

Yes. But for biological diversity, a big advantage results from the perennial compared to the annual cultivations. We were able to verify that for cornfields. The plant species diversity was higher in SRCs than in coniferous forests, even higher than in German mixed forests. The diversity in grassland was equal to SRC, whereas the open Swedish mixed forests, however, hosted even greater plant richness.