Contributions of a global network of tree diversity experiments to sustainable forest plantations

Although the global forest area declined by ca. 13 million ha per year between 2000 and 2010, the forest plantation area actually increased annually by ca. 5 million ha in the same time period, representing ca. 7 %, i.e., 264 million ha, of the global forest area in 2010 (FAO 2010). Afforestation rates may increase further due to incentives for carbon sequestration and the global pledge to protect the remaining natural forests of the world against degradation, e.g., as part of REDD+. Forest plantations already provide up to 33 % of the total industrial roundwood volume harvested annually in the world, and are projected to make up as much as 50 % of the global industrial roundwood production by 2040 (Kanninen 2010). Beyond wood production, plantations also provide a range of other ecosystem services, including carbon sequestration and water retention (Pawson et al. 2013). Moreover, when incorporated into integrated landscape management, plantations can play a large role in achieving biodiversity conservation objectives by offsetting the need to extract resources from natural forests (Paquette and Messier 2010).

Currently, plantation forests are almost exclusively planted as monocultures (Nichols et al. 2006, Box 1). Yet, several reviews published recently provide evidence, from both natural forests and plantations that biomass production and the delivery of other ecosystem services can improve with tree diversity (Nadrowski et al. 2010; Scherer-Lorenzen 2014). Furthermore, global change may increase disturbance frequencies and intensities in both natural forest (Woods et al. 2005) and plantations (Pawson et al. 2013), significantly affecting wood supply chains with severe economic consequences (Hanewinkel et al. 2012). Forest plantations that are diverse in genotypes, species, structure, and function, should be better able to adapt to changing environmental conditions than monocultures (van Hensbergen 2006; Bauhus et al. 2010). This calls for the development of novel, more diversified forest plantations that can improve plantations’ stability, productivity, and delivery of ecosystem services. Since plantations are often established near human settlements, they are the primary window through which society looks at forest management. Changing the way we manage plantations and setting objectives for them can therefore have profound and rapid impacts on the social acceptance of forestry (Paquette and Messier 2013). It has been noted, however, that foresters currently resist establishing mixed plantations, in large parts because of the perception that mixing genotypes and species reduces yield and complicates forest management operations (Carnol et al. 2014).

TreeDivNet, a new global network of tree diversity experiments, responds to the need for a solid, science-based framework for documenting and understanding the benefits and drawbacks of mixed plantations. In this paper, we explain the need for new afforestation trials and present the TreeDivNet network of experimental plantations. We show some early results from the network and formulate suggestions for additional experimental plantations that may cover existing research gaps.

Multi-species tree plantations are still relatively rare worldwide, but is this topic important within the forest research communities and is there an increasing interest in the last 10 years? We investigated these questions using the software WORDSTAT 6.0 (Péladeau 2003) by comparing the percentage of abstracts containing the word “plantation” that also contained the words “species mixture, mixed system, mixed plantation, mixed-species plantation or multi-species plantation” between the proceedings of the IUFRO World Congresses¹ of 2005 and 2014. In the proceedings of 2014, we found 2426 abstracts of which 267 used the term “plantation”. Of these 267 abstracts, 20 (or 7.5 %) also used at least one of the terms referring to mixed plantation mentioned above. In the proceedings of 2005, we found 1454 abstracts of which 238 used the term “plantation”. Of these 238 abstracts, only 1 (or 0.4 %) used at least one of the terms referring to mixed plantation. This clearly shows that the interest in multi-species tree plantations is increasing, which bodes well for the future of such plantations worldwide.

In the eighteenth and nineteenth century, foresters such as von Carlowitz, Hartig and Cotta developed the concepts of sustainable forest management as a response to the increasing overexploitation of European forests (Morgenstern 2007). To base these concepts upon science, the first long-term silvicultural trials were established to identify the most productive species and provenances to plant in novel forests. The trials were definitely a success for the development of production-oriented management; large-scale forest plantations were established with fast-growing tree species. The trials were often designed as common garden experiments comparing the growth and performance of different species and provenances at one site, i.e., under similar environmental conditions. Despite the lively debate about the advantages and disadvantages of pure versus mixed forests (even in that early era), most of the trials consisted of monocultures or, less frequently, two-species mixtures (Scherer-Lorenzen 2014). Presently, 300 years after von Carlowitz´s proposition of sustainability and given recent advances in biodiversity science (e.g., Cardinale et al. 2012), we need to know which mixtures provide higher levels of biomass production and of other ecosystem services and how environmental conditions affect the relationship between tree diversity and forest functioning, both in space and time.

To address these issues, several scientific approaches are available. Given the long lifespan and size of trees, simulation models that predict ecosystem service output along a range of tree diversities and environmental conditions are an obvious approach. However, such models need parameterization, which is an enormous challenge given how poorly we understand biotic interactions among species. Parameters can be estimated based on experiments or observational studies, but both the types and ranges of tree diversities we seek to study are not always present. Still, highly interesting and relevant work has been accomplished with simulation tools (e.g., Morin et al. 2011). Observational studies are invaluable for providing real-world reference data (Baeten et al. 2013), but also have many drawbacks because tree species composition strongly depends on environmental factors or management. Experiments avoid these issues, but there are still relatively few experiments with replicated stands of mixed species (Scherer-Lorenzen 2014), and many of these use only a small number of (nevertheless commercially important) tree species.

In response to the need for in-depth knowledge of the functioning of mixed plantations and the services they provide, tree diversity experiments have been planted worldwide over the past 15 years. These experiments have now been integrated within the global network TreeDivNet (www.​treedivnet.​ugent.​be). The unifying characteristic of TreeDivNet experiments is that tree species are grown in both monoculture and mixtures, and that tree diversity levels are replicated in a randomized design, allowing for the effects of diversity to be tested. Tree diversity experiments can yield reliable estimates of ecosystem functioning as the experimental design controls the levels and range of tree diversity and allows accounting for potentially confounding factors due to site conditions and local environmental gradients. In addition, long-term monitoring of the performance of individual trees and multiple ecosystem processes in experiments will provide a rich record of the development of the forest ecosystem and its overall functioning (see for example Potvin and Gotelli 2008). This will lead to a deeper understanding of the influence of the diversity, composition and structure of a forest on its functioning, and a more complete picture of the relationships between productivity and other ecosystem functions and services. Long-term monitoring will also allow us to better understand how forest diversity, structure, and composition influence forest stability. We will then be able to plant and manage forests in a way that increases their resistance and resilience to, e.g., predicted changes in climate. Different aspects of tree diversity, i.e., species richness, genetic diversity, structural, and functional diversity, will be used as tools to face the key challenges of modern sustainable afforestation.

At present, TreeDivNet consists of 18 experiments, located at 36 sites and in five ecoregions (Fig. 1; Table 1). More than 1 000 000 trees have been planted in the experiments on a total surface area of ca. 800 ha, which makes TreeDivNet one of the largest research infrastructures in ecology worldwide. The oldest experiment (Satakunta, Finland) was planted in 1999. The experiments included in TreeDivNet manipulate woody plant diversity—in terms of species richness (taxonomic diversity), evenness, composition, genetic, and functional diversity—over wide diversity gradients and are designed to allow separation of diversity and identity effects (see Fig. 2 for an example, and Bruelheide et al. 2014). The tree species in the TreeDivNet experiments are both widely planted commercial species, but also many less frequently used species. One important additional component is the inclusion of tree provenances from different regions (e.g., BiodiversiTREE, US; FORBIO, Belgium; and Climate Match, UK), providing a valuable opportunity to test whether assisted migration enhances the services provided by diverse plantations in the face of climate change (Pedlar et al. 2012).

TreeDivNet functions according to the guidelines for globally distributed experiments (cf. Borer et al. 2014). At present, the network has no central funding. Participation is entirely voluntary, but has clear benefits for the participants. TreeDivNet offers unique opportunities for multidisciplinary and multifunctional research on the relationship between tree diversity and ecosystem functioning in major forest types around the world and enables synthesis studies across the globe. Thus, TreeDivNet contributes to the lively field of functional biodiversity research, which has delivered a wealth of knowledge about the biotic control of ecosystem functioning over the last two decades. However, most of this knowledge was gained in smaller-stature, shorter-lived vegetation such as grasslands; forests came into the focus of this research field only recently. Despite the young age of most experiments, TreeDivNet can already provide results relevant for policy and management, as illustrated in the following two examples.

We are now entering the second decade of experimental manipulations of tree diversity. The TreeDivNet experiments have been designed to understand mechanisms and to quantify a large suite of ecosystem functions and services relevant to twenty-first century forest plantations. Gaps remain, however, in both the scale and scope of the existing experiments. We outline some important aspects here to guide future tree diversity experiments (see also Bruelheide et al. 2014).

First, while biodiversity research has made considerable advances on theoretical grounds, there is still a lack of linkages to applied sciences and industrial practices, even though it has been shown that different management types and intensities affect diversity–function relationships (e.g., Weigelt et al. 2009). In addition, the provision of wood is always listed among the ecosystem services a forest, planted or not, can provide. The outreach of next-generation experiments would be tremendously increased if practical issues were added already during the design phase, for example treatment testing and costing different planting patterns, maintenance methods, and harvesting techniques in a multi-species context, both in plantations and in naturally regenerated forests (see also Nichols et al. 2006). There is hence an important need for mixed species demonstration experiments, set-up in collaboration with forest managers and industries, and established at operational scales using available equipment and techniques. This could apply to both forestry and agroforestry systems, including short-rotation coppices and all variations of selection and multi-cohort stands. Moreover, to be practically relevant, future experiments may need to focus more strongly on testing or finding well-functioning genotypic and species compositions.

A second big issue in the design of tree diversity experiments is the scale, both temporal and spatial. Because of the high costs of large plots and the long-term time commitments, most plots in TreeDivNet experiments are, with a few exceptions, ¼ hectare or smaller (Table 1). Many processes affecting forest dynamics, e.g., competition and mortality, are scale dependent, and many of the forest ecosystem services, including the provision of timber, biodiversity, water purification, carbon storage, and recreational opportunities, are supplied at different spatial and temporal scales. Hence, there is an urgent need for tree diversity experiments that capture these larger-scale processes, similar to seminal watershed-level studies such as Hubbard Brook (www.​hubbardbrook.​org). Studies spanning multiple scales could provide pivotal information regarding the spatial and temporal scales at which forest biodiversity influences ecosystem functions and services. Comparing watersheds with different manipulated tree diversities would be a truly important step forward. Such large-scale experiments could be inspired by a land-sharing vs. land-sparing approach, such as the functional zoning in forestry (e.g., Messier et al. 2009). Furthermore, as effects of biodiversity on ecosystem functioning appear to be time dependent and to grow larger with time (Reich et al. 2012), longer-term studies are also required. While some of our experiments are planned with such long-term temporal perspective, others focus on early phases of establishment. Still missing are experiments where species are planted at different points in time, with pioneer and mid- to late-successional species, which without doubt would enhance our predictive capabilities of diversity effects along successional trajectories.

Third, theory and empirical evidence suggests that biodiversity is particularly important to buffer ecosystems against stressors and to increase their stability (Loreau and de Mazancourt 2013), but to date few TreeDivNet experiments explicitly incorporate stress as an experimental factor. The ORPHEE (France) and IDENT (Canada, Italy) experiments have incorporated a water availability treatment, and the IDENT site in Germany and Ridgefield (Australia) incorporate nutrient addition treatments, but the inclusion of other stressors would clearly broaden the inferences of TreeDivNet experiments. For example, results from smaller-scale experiments have shown that including factors such as mammalian herbivory (Cook-Patton et al. 2014) and fire (Adair et al. 2009) can influence the direction and magnitude of diversity effects.

Fourth and finally, although TreeDivNet includes experiments in tropical, temperate, and boreal systems, the distribution of experiments is skewed as relatively few are located in other important biomes/climate regions. For example, only two experiments lie in Central/South America and one in Africa, but these are not located in the largest forested areas and biodiversity hotspots on either continents (i.e., in the Amazon or Congo Basin). In addition, despite covering large areas on the globe, shrublands are also underrepresented.

The foresters of the nineteenth century demonstrated an impressive long-term perspective when they established the first forestry trials to find answers to the pressing questions of that time. Globally distributed experiments, such as TreeDivNet, could become new important research pillars to face the great challenges that global changes will put on forest ecosystems and to deliver highly relevant guidelines for forest policy and management worldwide. This is particularly important since plantations are likely to increase tremendously in area worldwide in the next decades.