David I. Forrester

Multiple factors modulate tree growth complementarity in Central European mixed forests

Summary 1.Mixed species forests can often be more productive and deliver higher levels of ecosystem services and functions than monocultures. However, complementarity effects for any given tree species are difficult to generalize because they can vary greatly along gradients of climatic conditions and resource availability. Identifying the conditions where species diversity can positively influence productivity is crucial. To date, few studies have examined how growth complementarity across species and mixture types is modulated by stand and environmental factors, and fewer have considered more than one or two factors. 2.We investigated how complementarity effects for several major central European tree species change with climatic and edaphic conditions, and with stand structural characteristics, including species composition. We used data from the Swiss National Forest Inventory, which is based on 3231 plots of pure and mixed stands (19 mixture types) across a broad environmental gradient, to test (i) how mixing effects change depending on the identity of the admixed species and (ii) if complementarity consistently increases when environmental conditions become harsher. 3.The magnitude, whether positive or negative, of complementarity increased with increasing stand density and stand developmental stage, but no general pattern could be identified across mixture types. Complementarity for many species increased as drought intensity and temperature increased, but not for all species and mixture types. While soil conditions, nitrogen and site topography influenced complementarity for many species, there was no general pattern (increases and decreases were observed). 4.Synthesis. Our study indicates that complementarity varies strongly with stand density and stand development as well as with topographic, climatic and soil conditions. This emphasizes the need to account for site-dependent conditions when exploring mixture effects in relation to forest productivity. We found that under certain conditions (i.e., increasing drought, higher temperature), mixed forests can promote individual-tree growth in central European temperate forests. However, careful assessments depending on the species composing the stands are required under changing resource availability as well as under different levels of stand density and development. This article is protected by copyright. All rights reserved.

Ecological Stability of Mixed-Species Forests

In many parts of the world, forests are likely to face novel disturbance regimes as a result of global change processes, and there is concern that the capacity of forest ecosystems to withstand, recover from, or adapt to these novel disturbance regimes may decline. Creation and maintenance of species-diverse forests is seen as an important option to adapt forests to uncertain future disturbances. However, it is not known whether benefits of mixed-species forests consist mainly of risk spreading among tree species that have different susceptibility to various stressors and disturbance agents or whether they also have emergent properties resulting from interactions among species, which increase the resistance and resilience of participating species or the entire ecosystem. Here we review the evidence for the effects of tree diversity on the resistance and resilience of forests in relation to a number of abiotic (drought, wind, fire) and biotic (insect herbivores, pathogens) stress and disturbance factors. For the abiotic disturbances, damage or reduction in ecosystem function can be reduced, compared with monocultures of susceptible or less resilient species, when more resistant or resilient species are mixed with less susceptible and less resilient species. However, storm, fire, or drought damage to individual species may not be reduced in mixtures when compared to monocultures. The stress or disturbance impacts may even be aggravated for one or more species in some mixtures, as is shown for drought. There is more evidence for beneficial diversity effects in relation to biotic disturbance agents. Mixing tree species reduces the impact of insect herbivores on individual susceptible tree species in the majority of cases, where the community is dominated by specialist herbivores. However, the opposite effect may occur with generalist herbivores, which can be promoted by tree diversity. Similarly, tree diversity can reduce the impact of specialist pathogens on host tree species, whereas there is little evidence for positive influences in the case of generalist pathogens. In most cases, tree species diversity dilutes the impact of disturbance agents and, owing to different susceptibility of species to specific disturbances, insures against a complete damage or loss. In addition, mixing tree species can reduce temporal variation in growth and stabilise productivity. However, there is little evidence for true, positive diversity effects, where diversity leads to an increase in the resistance and resilience of component species in mixed-species communities. From an economic point of view, mixing might help to reduce risk for a more vulnerable and valuable species, even if there are no benefits for the admixed species. However, forest managers should be aware that mixtures do not provide universally higher resistance or resilience in relation to disturbances than monocultures. In most cases, it depends to a large extent on the attributes of the species in mixture in relation to the specific disturbances.

Ecological and Physiological Processes in Mixed Versus Monospecific Stands

Tree growth depends on the resource availability, the proportion of resources acquired, and the efficiency with which those resources are used. Each of these variables can be influenced by species interactions. These interactions are dynamic and change spatially and temporally as resource availability and climatic conditions change. It is important to understand these processes when designing and managing mixed-species stands and also when modelling these processes. These interactions and their dynamics are the focus of this chapter. To begin with, the production ecology equation is described because it provides a useful framework to quantify the types of processes that influence the growth of forests and how these are influenced by species interactions. This equation describes growth as a function of resource availability, resource acquisition, and resource-use efficiency. Then, while referring to this equation, some of the main types of processes are described in terms of how they influence these variables and hence the productivity of mixtures. This is done for nutrients, then light, and then water. The influence of a given type of interaction on growth is not static. Instead, it changes with spatial and temporal variability in resource availability and climatic conditions and as a stand develops. Therefore, the next section describes a framework that explains these spatial and temporal dynamics and indicates when different types of interactions are important. Finally, stand density can influence the effect of these interactions. As stand density increases, interactions may become more favourable or less favourable, depending on how, and which, resources are influenced by the change in density. The final section therefore shows why stand density needs to be taken into account when examining how species interact.

Silvicultural Options for Mixed-Species Stands

Silvicultural approaches for mixed-species stands typically aim at creating and maintaining stand compositional and structural diversity to provide the desired ecosystem function. An important aim of regeneration and tending is to develop stable mixtures that facilitate the anticipated interactions among the participating tree species and at the same time require little silvicultural input to maintain the composition of compatible tree species. To ensure survival and vigorous growth of trees in mixture in the long term, it is often necessary to separate species in space and time during the regeneration phase. Weaker competitors can be given a head start through advance regeneration or may be regenerated in patches that protect the majority of individuals from interspecific competition. In managed forests, tree species in mixture may also have a service function. For example, nurse crops may be used to facilitate establishment of sensitive tree species and to accelerate ecosystem recovery following disturbances. Trainer species are typically used to improve the quality development of stems of economically important species. Whereas the regeneration of mixtures has received considerable attention in research, there is little specific information regarding the thinning of mixed-species stands, which are often treated like an agglomerate of monospecific stands of the companion species. In the absence of quantitative information on optimal tree spacing, stand density, or the growth response to thinning in mixtures, crop tree thinning approaches are recommended. These facilitate the maintenance of desired species proportions as well as stand stability and development of quality attributes in crop trees through simple indicators. Although it is often assumed that the development of stem quality in mixtures is inferior to that in monospecific stands, there is so far little solid evidence to support this assumption. Rather, it appears that stem quality depends on specific interactions between species and the structural heterogeneity of stands.

Diversity-dependent temporal divergence of ecosystem functioning in experimental ecosystems.

The effects of biodiversity on ecosystem functioning generally increase over time, but the underlying processes remain unclear. Using 26 long-term grassland and forest experimental ecosystems, we demonstrate that biodiversity–ecosystem functioning relationships strengthen mainly by greater increases in functioning in high-diversity communities in grasslands and forests. In grasslands, biodiversity effects also strengthen due to decreases in functioning in low-diversity communities. Contrasting trends across grasslands are associated with differences in soil characteristics.Long-term grassland and forest experiments reveal that temporal strengthening in biodiversity functioning relationships is mainly driven by increases in functioning in high-diversity communities.

Stand Dynamics of Mixed-Species Stands Compared with Monocultures

Based on well-established principles and using reference values for monocultures, this chapter examines how mixed-species stands differ from monocultures in terms of stand growth and yield, self-thinning, alien thinning, stand density, and density-growth relationships. This chapter also considers how the species interactions and the resulting mixing effects on growth can vary spatially and temporarily in response to the prevailing environmental conditions.

EuMIXFOR empirical forest mensuration and ring width data from pure and mixed stands of Scots pine (Pinus sylvestris L.) and European beech (Fagus sylvatica L.) through Europe

Key messageThis data set provides unique empirical data from triplets of Scots pine (Pinus sylvestrisL.) and European beech (Fagus sylvaticaL.) across Europe. Dendrometric variables are provided for 32 triplets, 96 plots, 7555 treesand 4695 core samples. These data contribute to our understanding of mixed stand dynamics.Dataset access athttp://dx.doi.org/10.5061/dryad.8v04m. Associated metadata available athttps://metadata-afs.nancy.inra.fr/geonetwork/apps/georchestra/?uuid=b3e098ca-e681-4910-9099-0e25d3b4cd52&hl=eng.

Modelling Mixed-Species Forest Stands

The chapter first describes common models for monospecific stands and then the environmental conditions, processes, and structures that need to be included in forest growth models that are to be applied to mixed-species forests, how these different processes are incorporated into models, and the strengths and weaknesses of tree-level and stand-level approaches. The chapter gives an introduction to empirical models, process-based models, and hybrid models, which are a combination of the former two groups. Empirical models describe the system behaviour statistically, not the structure and mechanistic functioning of the system. Process-based models describe the trees and stand development on the basis of the underlying structure, within-stand environment, and functioning. Hybrid models represent a compromise between empirical and process-based models; they may bridge knowledge gaps of processes using statistical relationships. The chapter focuses on models that start at the individual tree level and scale up to the stand level or models that start and finish at the stand level.

Positive biodiversity–productivity relationships in forests: climate matters

While it is widely acknowledged that forest biodiversity contributes to climate change mitigation through improved carbon sequestration, conversely how climate affects tree species diversity–forest productivity relationships is still poorly understood. We combined the results of long-term experiments where forest mixtures and corresponding monocultures were compared on the same site to estimate the yield of mixed-species stands at a global scale, and its response to climatic factors. We found positive mixture effects on productivity using a meta-analysis of 126 case studies established at 60 sites spread across five continents. Overall, the productivity of mixed-species forests was 15% greater than the average of their component monocultures, and not statistically lower than the productivity of the best component monoculture. Productivity gains in mixed-species stands were not affected by tree age or stand species composition but significantly increased with local precipitation. The results should guide better use of tree species combinations in managed forests and suggest that increased drought severity under climate change might reduce the atmospheric carbon sequestration capacity of natural forests.

From Observations to Evidence About Effects of Mixed-Species Stands

A critical understanding of the scientific evidence that we have about the effects of tree species diversity on ecosystem properties and processes is required to guide practical forest management as well as future research. However, current understanding is limited by the lack of an appropriate framework for evaluating the reported evidence. In this chapter we outline how research on mixed-species forests may fit into concepts of ecosystem hierarchy and how previous studies may be ranked regarding their level of evidence. We introduce the most important hypotheses and theories underpinning research on the relationship between tree diversity and ecosystem functioning and illustrate how these may be tested by analyses of forest inventories, experiments, and exploratory research platforms or a combination of these.