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A novel comparative research platform designed to determine the functional significance of tree species diversity in European forests

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Abstract

One of the current advances in functional biodiversity research is the move away from short-lived test systems towards the exploration of diversity-ecosystem functioning relationships in structurally more complex ecosystems. In forests, assumptions about the functional significance of tree species diversity have only recently produced a new generation of research on ecosystem processes and services. Novel experimental designs have now replaced traditional forestry trials, but these comparatively young experimental plots suffer from specific difficulties that are mainly related to the tree size and longevity. Tree species diversity experiments therefore need to be complemented with comparative observational studies in existing forests. Here we present the design and implementation of a new network of forest plots along tree species diversity gradients in six major European forest types: the FunDivEUROPE Exploratory Platform. Based on a review of the deficiencies of existing observational approaches and of unresolved research questions and hypotheses, we discuss the fundamental criteria that shaped the design of our platform. Key features include the extent of the species diversity gradient with mixtures up to five species, strict avoidance of a dilution gradient, special attention to community evenness and minimal covariation with other environmental factors. The new European research platform permits the most comprehensive assessment of tree species diversity effects on forest ecosystem functioning to date since it offers a common set of research plots to groups of researchers from very different disciplines and uses the same methodological approach in contrasting forest types along an extensive environmental gradient.

Introduction

The past two decades of research on biodiversity and ecosystem functioning (BDEF) have shown that biodiversity is not only a passive consequence of the environmental conditions and interactions between species (Naeem, 2002, Loreau, 2010), but that differences in biodiversity can largely affect ecosystem functioning. Several hundreds of published effects have provided ample evidence that plant biodiversity can influence key ecosystem processes such as biomass production, nutrient cycling and pest regulation (Cardinale et al., 2012). While it is now generally accepted that plant diversity can affect function, the importance of BDEF relationships in complex natural ecosystems and their relevance to ecosystem management and conservation have been debated (Srivastava and Vellend, 2005, Duffy, 2009, Hillebrand and Matthiessen, 2009). Many of these concerns have arisen because early experiments and models on functional biodiversity lacked environmental and biological complexity and did not consider ecosystem multifunctionality, i.e., the maintenance of multiple functions (Hillebrand and Matthiessen, 2009). A new generation of studies therefore attempts to investigate BDEF relationships in real-world settings (Solan et al., 2009). Short-lived test systems such as microcosms, mesocosms and grasslands have dominated previous research (Balvanera et al., 2006), but much greater emphasis is now being placed on structurally more complex systems containing long-lived plants, including forests (Scherer-Lorenzen et al., 2005a).

Trees are text book examples of ecosystem engineers capable of modifying aspects of their environment, yet surprisingly little is known about the functional significance of tree species diversity in forests (Nadrowski et al., 2010). Experimental forestry trials comparing monocultures with two-species mixtures have existed for many decades (Pretzsch, 2005), but large-scale experiments with more species richness levels were only installed during the past decade (e.g., Scherer-Lorenzen et al., 2005b; www.treedivnet.ugent.be), meaning that these systems are still far from maturity. While important diversity-functioning patterns have already been reported from these experiments (Vehviläinen and Koricheva, 2006, Potvin et al., 2011, Lei et al., 2012), the results might differ considerably from those found in mature forests, when trees are older and the species diversity affected the environment for several decades. Furthermore, experimental tree communities face several specific limitations including small plot size, simplified age distributions and simplified stand structure compared with mature forest (reviews: Scherer-Lorenzen et al., 2005b, Leuschner et al., 2009). Studying existing, mature forests in real landscapes would therefore complement the results from these experiments. To this end, two approaches seem to be promising. First, regional or national forest inventory databases are ready-to-use and have now also been explored to examine BDEF relationships in mature forest (Caspersen and Pacala, 2001, Vilà et al., 2005, Paquette and Messier, 2011, Gamfeldt et al., 2013). However, they suffer from the disadvantages that tree diversity is generally confounded with many environmental variables and that usually only a few functions are measured. The second approach, comparative observational plots set up along tree diversity gradients in mature forests while maximally controlling other environmental factors, would allow to better isolate potential diversity effects from confounding environmental conditions. This approach would complement the existing networks of experimental research sites and inventories.

A first case study in a central European beech forest compared forest patches on similar soils, in which past ownership and forest use had created a small-scale mosaic of tree species diversity (Leuschner et al., 2009). However, this study used a complete dilution design (Nadrowski et al., 2010; see Glossary) where the only monocultures were the ones of Fagus sylvatica and this tree species was present in all mixtures. This does not allow for separating effects of increasing species diversity from the confounding effects of decreasing dominance of F. sylvatica. At present few other local-scale comparative research platforms exist and most do not allow for disentangling the diversity signal from confounding factors such as environmental gradients and species identity (see review by Nadrowski et al., 2010). A more extensive comparative approach, which goes considerably beyond typical observational studies, was recently adopted in the German Biodiversity Exploratories, which focus on the effects of land-use change on biodiversity and ecosystem processes in forest and grassland (Fischer et al., 2010). A similar comparative approach could be used to allow for comparisons of stands of different tree species diversity in mature forest, in analogy to the land-use comparisons in the German Biodiversity Exploratories.

A comparative approach with study sites varying in tree species diversity and replicated at the continental scale would address many of the deficiencies in forest functional biodiversity research outlined above. Here we present the design and implementation of the FunDivEUROPE Exploratory Platform, a network of forest plots along tree species diversity gradients in six major European forest types (FunDivEUROPE: Functional significance of forest biodiversity; www.fundiveurope.eu). With the additional asset of studying forest types of very different European regions, the platform will contribute to answering some of the most important unsolved questions in BDEF research: does tree species diversity affect ecosystem functioning and the provisioning of ecosystem services? Do the effects of differences in resource use and facilitation among species (i.e., complementarity effects) vary along broad environmental (climatic, soil) gradients? Using forest inventory data bases, Paquette and Messier (2011) showed that complementarity may be more important for tree productivity in boreal compared with temperate forests, while Zhang et al. (2012) found similar complementarity effects across biomes. Furthermore, there are virtually no data available on ecosystem processes other than productivity. In the FunDivEUROPE project we will therefore measure a large and comprehensive spectrum of different functions and related services in each plot of our platform. The Exploratory Platform provides further added value because it is combined with two complementary platforms that are studied in parallel (Fig. 1): (i) a network of European tree species diversity experiments (Experimental Platform) and (ii) a compilation of national and regional forest inventories (Inventory Platform). As the Exploratory Platform was set up as a hypothesis-driven network of plots, we begin this contribution with an overview of the specific research questions and hypotheses we are aiming to test. The rationale of the platform and the design are subsequently discussed.

Section snippets

Guiding research questions and hypotheses

The research platform was designed to test five general hypotheses that have been highlighted as unresolved in recent review and opinion papers (e.g., Hillebrand and Matthiessen, 2009, Nadrowski et al., 2010, Cardinale et al., 2012). The first four hypotheses have been explored extensively in other systems but need further attention in forest ecosystems, while the fifth is especially relevant to trees and forests:

  • (1)

    Tree species mixtures outperform ecosystem functioning of monocultures, including

Rationale: maximizing three fundamental design criteria

A research platform designed to answer the questions above should comprise a systematic network of research plots in existing forests that maximizes three fundamental criteria: comprehensiveness, representativeness and orthogonality (Nadrowski et al., 2010; see Glossary). First, comprehensiveness refers to the spectrum of ecosystem functions and services that can be quantified. Ecosystems inherently exhibit a multitude of functions, so increasing the relevance of BDEF studies requires a

Six European forest types

The Exploratory Platform covers the major forest regions of Europe that extend from southern Mediterranean Europe (‘Alto Tajo’ in Spain and ‘Colline Metallifere’ in Italy) to the northern Boreal (subarctic) region ‘North Karelia’ in Finland (Fig. 1). The European Environment Agency classification, which is based on the distribution and classification of the natural vegetation and ecological site conditions, was used to select the different types (EEA, 2007; Table 1). The selected focal regions

Outlook for scientists and managers

With this Exploratory Platform we move away from earlier research on the effects of environmental factors on forest biodiversity to a new research paradigm focussing on the effects of biodiversity on ecosystem functioning. Together with the experiments and inventory data, the Exploratory Platform provides an important new European research infrastructure for long-term monitoring of the effects of tree species diversity on forest ecosystem functions and the ecosystem services provided by

Acknowledgements

The research leading to these results has received funding from the European Union Seventh Framework Programme (FP7/2007–2013) under grant agreement n° 265171. The first three and last author coordinated the development of this research platform, the last author is also coordinator of the whole project. The site managers responsible for each of the six study regions are authors four to nine. All subsequent authors have been involved in the design and implementation of this platform, either by

Glossary

Complementarity effect
The complementarity effect quantifies the combined effects of species interactions on mixture performance after accounting for changes in the relative abundance of species (selection effect – see below). Positive values result when mixtures do better than expected based on the relative abundance of species and their monoculture performances and are consistent with ‘niche differentiation’ in terms of resource partitioning, facilitation or diversity-dependent effects of

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