Forest ecosystems: does tree diversity matter?

Submitted by editor on 5 May 2015.Get the paper!

Forests provide a large collection of ecosystem functions and services. They produce wood for pulp and timber, and contribute to the regulation of the carbon and water cycles. Furthermore, they host a major part of the world’s biodiversity, and provide numerous cultural services and possibilities for recreation activities. Several decades of biodiversity research in grasslands suggest that plant diversity is a key driver of ecosystem functioning; it enhances primary productivity and increases resistance against herbivores. We tested whether tree diversity can simultaneously provide such functions in forest ecosystems. Results are presented in our paper “Contrasting effects of tree diversity on young tree growth and resistance to insect herbivores across three biodiversity experiments”.

Testing tree diversity effects on ecosystem functioning is not straightforward, especially in forests. Most forests have been managed for a long time to enhance the delivery of goods and services and to make the most of local conditions. As a result, the current diversity of any forest stand keeps the footprint of previous management and site history. This may bias the diversity – functioning relationship. To avoid spurious confounding factors, we used three tree diversity experiments established along a latitudinal gradient in Europe. These experiments are part of a global network, called TreeDivNet. In our paper, we gathered information from three sites: the French (Orphee), the German (Biotree) and the Finnish (Satakunta) experiments (Figure 1).

Figure 1: The Tree Diversity Network, TreeDivNet. Red dots locate the experiments on which we collected data. Source: http://www.treedivnet.ugent.be/

What do these experiments have in common? Experimental plots establishing a gradient of tree diversity from monocultures (one single species per plot) to mixtures of five species (Figure 2). Species pools differed among the three experiments, but all included both conifers and broadleaves, fast and slow growing species, pioneer and late succession species… The common features further allowed ranking plots along comparable gradients of functional diversity. We went beyond previous species-specific approaches by analyzing the response of both individual tree species but also the ‘tree community’ to tree diversity.

Figure 2: The Satakunta (A), Orphee (B) and Biotree (C) experiments. Photo credits: Kalle Rainio (A), Bastien Castagneyrol (B), Charlotte Klank (C).

 

In 2010, we measured tree height. We found that, on average, trees tended to grow taller in mixtures than in monocultures. This occurred as soon as two species were associated, and there were no great benefits of adding extra species. Tree growth was, however, favoured in mixtures of species with overall larger specific leaf areas (plots with high CWMSLA).

We also quantified damage caused by different insect feeding guilds (Figure 3). Instead of focusing on guild specific responses to tree diversity as commonly reported, we were interested in the overall response of insect herbivory to tree diversity which is more important from a forest management perspective. We therefore combined all damage types into a single herbivory index and found that, on average, insect damage tended to be higher in mixed plots than in monocultures.

Figure 3: Example of inset damage found on the tree Orphee experiment. All damage types were aggregated into a single ‘total herbivory’ variable, comparable among tree species and experimental sites. Photos by Bastien Castagneyrol.

The overall response of tree growth and insect herbivory to tree diversity hides large species-specific differences between trees that may be of practical interest. Among the 11 species included in our study, none benefitted from being planted in mixture in terms of both growth and resistance to herbivores. Achieving both increased growth and reduced herbivory in tree species mixtures may be challenging. However, our findings may help to carefully select species that combined could help to maximize ecosystem services in forests.

Interested in learning more about tree diversity effects at the stand and individual species level? Then why not read this paper: Haase, J., Castagneyrol, B., Cornelissen, J. H. C., Ghazoul, J., Kattge, J., Koricheva, J., Scherer-Lorenzen, M., Morath, S. and Jactel, H. (2015), Contrasting effects of tree diversity on young tree growth and resistance to insect herbivores across three biodiversity experiments?

The authors through Josephine Haase

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