Elsevier

Forest Ecology and Management

Volume 312, 15 January 2014, Pages 282-292
Forest Ecology and Management

Review
The spatial and temporal dynamics of species interactions in mixed-species forests: From pattern to process

https://doi.org/10.1016/j.foreco.2013.10.003Get rights and content

Highlights

  • Species interactions in mixed-species forests are dynamic, spatially and temporally.

  • Complementarity increased with decreasing soil N when interactions increased N.

  • Complementarity increased with decreasing water availability when interactions reduced competition for water.

  • In some stands complementarity increased with increasing site quality.

  • Few studies have examine the processes driving these dynamics.

Abstract

Mixed-species forests and plantations sometimes have greater levels of ecosystem functions and services, including productivity, than monocultures. However, this is not always the case and there are many examples where mixtures are not more productive. Whether or not mixtures are more productive depends on the net effects of different types of interactions, and these are dynamic, changing through space and time. Many studies have examined how species interactions influence the growth of mixtures, but few have examined how spatial and temporal differences in resource availability or climatic conditions can influence these interactions. This review examines these spatial and temporal dynamics. The processes driving the dynamics are discussed using the production ecology equation, where plant growth is a function of resource availability, multiplied by the fraction of resources that are captured by the trees, multiplied by the efficiency with which the resources are used. Relative complementary effects depended on the types of species interactions and how resource availability changed. Complementary effects increased as soil nitrogen or water availability decreased when mixtures contained nitrogen fixing species, or when interactions were assumed to reduce competition for water. In contrast, some studies found that complementary effects increased with increasing site qualities, however in those studies there were no measurements of soil resource availability or any complementarity mechanisms. In those studies it was assumed that as growing conditions improved, competition for light increased and complementary effects resulted from interactions that improved light absorption or light-use efficiency. Multiple types of interactions can occur simultaneously in mixtures (e.g. nitrogen fixation, increased light absorption, and increased water-use efficiency) and so different resource availability-complementarity patterns will probably occur for a given pair of species, depending on the resource being examined. Less than half of the studies actually measured variables of the production ecology equation to indicate the processes driving the patterns. Several questions are listed that cannot yet be answered with confidence. Finally, stand structural characteristics, such as density, have also been shown to strongly increase or decrease complementarity effects and these need to be taken into account when interpreting results, but the mechanisms driving these density patterns were rarely quantified.

Introduction

Many studies have shown that given the correct conditions, mixed-species forests and plantations can be more productive than monospecific stands (Assmann, 1970, Binkley, 1992, Kelty, 1992, Forrester et al., 2006b, Kelty, 2006). However, interactions between a given pair of species are often dynamic, changing as resource availability or climatic conditions change, and it is not unusual for net complementary interactions between a given pair of species to transform into net competitive interactions, or vice versa (Binkley, 2003, Boyden et al., 2005, Pretzsch et al., 2010, Forrester et al., 2011, Bouillet et al., 2013). Understanding how these interactions change is necessary when managing mixtures and to determine what is driving productivity–diversity relationships, which are the net effects of many different species interactions. Understanding when and how species interactions will change is also critical given that one of the biggest current challenges in forestry is to adapt forest ecosystems to climate change. Mixed-species stands are viewed as one of the most important adaptation and risk-reduction strategies (Reif et al., 2010) so understanding how short- to long-term climatic variability can influence species interactions is considered a priority (Brooker, 2006).

The literature is reviewed to examine what is known about how complementary effects in mixed-species forests and plantations change spatially or temporally, and the importance of considering stand density when examining mixed-species interactions. The mechanisms that drive these patterns are examined using the production ecology equation (Eq. (1); Monteith, 1977).Gross primary production=resource supply×fraction of resource acquired×resource use efficiency

Wood or above-ground biomass production can be examined using Eq. (1) after subtracting carbon allocation to respiration or non-woody tissues (Binkley et al., 2004). Several reviews have used this equation to understand the mechanisms driving growth responses to a wide range of factors, including geographic gradients, fertiliser application, irrigation, pruning, thinning, spacing, genotypes, species, stand age (Binkley, 2012, Forrester, 2013) and the influence of mixed-species interactions on plant nutrition (Richards et al., 2010).

Interactions between species growing in mixtures are often described in terms of competition, competitive reduction and facilitation (Vandermeer, 1989). Competition occurs when different plants or populations interact such that one exerts a negative effect on the growth or survival of the other. Competitive reduction occurs when inter-specific competition is less than intra-specific competition because of an inter-specific differentiation in resource use. Facilitation occurs when plants interact such that at least one species positively influences the growth or survival of another. The stress-gradient hypothesis (SGH) is often referred to when examining the dynamics of species interactions, and suggests that facilitation will increase, and competition decrease, as conditions become harsher (Bertness and Callaway, 1994). The SGH usually fits with observations in forests, however it is worth noting that it was largely developed outside of forest ecosystems, and definitions of facilitation usually differ between forest and SGH studies. In SGH studies facilitation is usually quantified by comparing the survival, growth or fitness of a plant growing with neighbours vs. without neighbours. This “without neighbours” situation is rare in forests, which have higher stand densities, and instead facilitation or competitive reduction are often assumed to have occurred when growth (or survival) is greater with inter- than with intra-specific competition. Consistent with this contrast, a recent SGH meta-analysis and review by He et al. (2013) did not include any studies of tree-tree interactions unless the trees were in woodlands (not forests) or seedlings of tree species were examined.

Section snippets

Complementarity calculations and a conceptual complementarity model

Competitive reduction and facilitation often occur simultaneously and their effects can be difficult to separate, so in this review each of these are collectively described as complementarity. Many studies have examined the interactions that occur between trees in forests and these have been reviewed previously (Assmann, 1970, Binkley, 1992, Kelty, 1992, Forrester et al., 2006b, Kelty, 2006, Richards et al., 2010), however far fewer studies have examined how these interactions are influenced by

Spatial effects of interactions that influence nutrient availability

Mechanisms that improve nutrient availability, uptake and use efficiency often result in greater complementary effects on sites where that nutrient is limiting, consistent with the conceptual model of Fig. 1. Several examples of this trend are shown in Fig. 2. A classic example is the facilitative effect of nitrogen(N)-fixing species on the growth of non-N-fixing species, which increases as N becomes more limiting (Binkley, 2003, Forrester et al., 2006b, Forrester et al., 2006c, Bouillet et

Spatial effects of interactions that influence water availability

Following the model in Fig. 1, interactions or mechanisms that influence water availability and uptake will be more useful where water is limiting (Fig. 4). For example, Abies alba was found to be less sensitive to summer droughts when growing in mixtures with Picea abies or Fagus sylvatica than when growing in monospecific neighbourhoods and this complementary effect was only found at dry sites and not at mesic or humid sites (Fig. 4, Study 5, Lebourgeois et al., 2013). It was suggested that

Spatial effects of interactions that influence light absorption and use

Increasing complementary effects with increasing site quality, or as climatic conditions improve, have been observed in several studies (Fig. 3, Fig. 5). While none of those studies quantified the mechanisms behind the response by measuring the variables on the right-hand side of the production ecology equation, they are useful for developing hypotheses that can be tested in new experiments. For example, one of those (Study 6) showed that for a given tree diameter, the crown projection area of

Temporal effects and interactions between resources

Temporal changes in species interactions result from abiotic factors, such as climatic conditions and stand disturbances, or stands developing and potentially influencing the availability of light and soil resources. Considering all of these factors, and the fact that different species often have contrasting growth dynamics, it is not surprising that as stands develop there are often changes in the relative dominance of species or complementarity effects, in terms of the relative growth of

Stand structure

Stand density may also influence species interactions and growth in mixed-species forests. Stand density is important for at least two reasons when analysing species interactions. Firstly, stand growth usually increases with density initially before levelling off at higher densities, and stand density can be positively correlated with species diversity (e.g. Chisholm et al., 2013, Vilà et al., 2013). Therefore greater productivity in mixtures may sometimes result from higher densities or

Conclusions and implications for future research

The literature reviewed above indicates that for a given species combination, there are probably several different processes that lead to the net complementary (or competitive) effects. For example in mixtures that contain N-fixing species, there are also processes that reduce competition for light and accelerate rates of P cycling (Binkley et al., 1992a, Forrester et al., 2005, Richards et al., 2010, Hinsinger et al., 2011). The contribution of these processes to the net complementary effects

Acknowledgements

This study was part of the Lin2Value project (project number 033L049) supported by the Federal Ministry of Education and Research (BMBF, Bundesministerium für Bildung und Forschung). Dan Binkley and Damien Bonal provided comments that improved the manuscript and these are gratefully acknowledged. Thank you also to two anonymous reviewers who also provided comments that improved the manuscript.

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