The production functions (or provisioning services) comprise the processes that combine and change organic and inorganic substances through primary and secondary production into goods that can be directly used by mankind (Chapter 2; de Groot et al, 2002). In the context of forest plantations, these are primarily the production of woody biomass, commonly of defined qualities, and the production of non-timber goods such as herbs, mushrooms, huntable animals, etc. Whereas the availability of information considering the effects of tree species diversity on production of woody biomass is reasonable (but see Scherer-Lorenzen et al, 2005), the information about non-wood products is scant. Here, we will initially focus on the effect of tree diversity on the production function, because it relates to many of the regulating functions such as carbon, nutrient and water cycling, and because the productive capacity of plantations is the principle motivation for their establishment in most situations (see Chapter 1). Where plantations are not established primarily for the production of woody biomass but for other purposes such as the conservation or restoration of soils, the achievement of these aims will also be influenced by the basic ecosystem functions listed above.
The information about diversity effects on forest ecosystem productivity comes mostly from the analyses of large-scale permanent forest inventory plots representing gradients in tree species richness (Caspersen and Pacala, 2001)
and from controlled experiments or forest yield plots comparing monocultures and tree species mixtures, mostly, however, two-species mixtures (e.g. Pretzsch, 2005; Forrester et al, 2006; Piotto, 2008). Whereas the first source of data stems mostly from all forms of forests (native, semi-natural and plantations), the latter stems mostly from plantations or semi-natural forests.
So far, there have been only a few studies utilizing an inventory-based analysis of the relationships between tree species diversity and productivity. In two of these studies (Caspersen and Pacala, 2001; Vilà et al, 2007) positive relationships between diversity and productivity have been observed for North American forests and in typical early successional Mediterranean-type forests of Catalonia, Spain, respectively. In the study by Caspersen and Pacala (2001) it was not possible to untangle the confounding effects of site quality on productivity and diversity. Here, cause and effect may actually be reversed, namely that more productive stands simply permit the coexistence of more species. However, Vilà et al (2007) could demonstrate that the positive relationship between tree species diversity and forest productivity was maintained when considering the effects of forest structure, environmental variables and management practices.
In another study by Vilà et al (2003), no effect of species richness was observed in Pinus sylvestris-dominated forests, but a positive effect was detected in P. halepensis stands of Catalonia. In the latter case, however, tree species richness was no longer a significant factor, when the climate, bedrock types, radiation and successional stage of inventory plots were included in the analysis. The latter study points to the problem of these analyses: the large number of co-variables that may influence the relationship between diversity and productivity (or other function), and the difficulty of accounting for these in a statistically sound way. In addition, forest inventory plots are usually not selected to represent a diversity gradient, and thus most inventory plots are located at the lower end of tree species diversity (Vilà et al, 2007). Furthermore, these types of analyses are not suited to explaining the underlying mechanisms of any diversity–ecosystem function relationship.
In terms of a second source of information about diversity effects on forest ecosystem productivity, most experiments with mixed-species stands also cover only the low end of tree species diversity. They are, however, in contrast to inventory-based analysis, well suited to exploring the underlying mechanisms. Positive effects of tree diversity on productivity occur through competitive reduction, also described as complementarity above, and facilitation (Vandermeer, 1989; Kelty and Cameron, 1995). Productivity in mixtures will out-yield monocultures, when the positive interactions of facilitation and competitive reduction dominate the competitive interactions. Bauhus et al (2006) and Piotto (2008) have shown that in most reported cases, where a statistically valid comparison between mixed- and single-species stands could be made, the productivity was equal or greater in mixed stands, in particular, if nitrogen-fixing species were admixed (see also Forrester et al, 2006) (Figure 5.1). However, it has to be kept in mind that cases where mixtures are inferior
in productivity to monocultures of the respective companion species, are probably less likely to be reported in the literature.
The most important mechanisms that lead to increased productivity in tree mixtures comprised the following processes and patterns.
Canopy stratification
Successful (meaning more productive than respective monocultures) mixed- species plantations often have stratified canopies with a fast-growing shade- intolerant species forming the upper canopy and a more shade-tolerant species forming the lower canopy (Assmann, 1970; Bauhus et al, 2004; Pretzsch, 2005). Shade-intolerant species are capable of higher maximum rates of photosynthesis than more shade-tolerant species. Therefore more efficient use will be made of the higher light intensities in the upper canopy of a mixture than in the upper canopy of a more shade-tolerant species in monoculture. In contrast, shade-tolerant species are capable of maintaining foliage and assimilating at higher rates than shade-intolerant species at lower light intensities (Kelty, 1992). When grown in mixed stands with shade-intolerant species, shade-tolerant species increase the amount of light intercepted compared to monocultures of the shade-intolerant species (e.g. Binkley, 1992; Bauhus et al, 2004). When shade-intolerant species do not form the main canopy, productivity in mixtures may not be higher in mixtures than in
Figure 5.1 Frequency of relative yield totals of mixed-species plantations
reported in the literature
Note: The relative yield total (RYT), provides the factor by which the productivity in a mixture is superior or inferior
when compared to a monoculture; e.g. an RYT of 1.4 indicates that 1.4ha of monocultures (0.7ha of each species) would have to be planted to obtain the same yield as in 1ha of a 50:50 mix of the same species.
monocultures (Parrotta, 1999; Redondo-Brenes and Montagnini, 2006). Canopy stratification alone is not the only factor responsible for the success or failure of these mixed stands, but it is a key factor to ensure the long-term coexistence of species (Forrester et al, 2006). Stratified canopies also offer different habitat niches with consequent effects on dependent faunistic diversity (see below).
Below ground stratification
Competition may be reduced also below ground through physical or chemical stratification of roots. Physical stratification comprises differences in fine-root distribution that affect exploitation strategies. This can be in the form of the layering of root systems (e.g. Schmid and Kazda, 2002) or different fine-root architectures that can be associated with different soil exploration and exploitation strategies (e.g. Bauhus and Messier, 1999). Chemical stratification may occur when co-occurring species take up different forms of nutrients, for example ammonium vs. nitrate or organic N, or employ different acquisition strategies for nutrient uptake through different types of mycorrhiza (Ewel, 1986; Schulze et al, 1994).
Facilitation
Facilitation among tree species may occur through direct amelioration of harsh environmental conditions or increasing the resource availability, or indirectly through the introduction of beneficial organisms (mycorrhizae and other soil microbes) or protection from herbivores (Callaway, 1995; Forrester et al, 2006). Increased resource availability in mixed-species stands may result from accelerated cycling of nutrients, for example in the forest floor, the mineral soil or in biomass. This has been observed for example for N in mixed stands of Sitka spruce (Picea sitchensis) and Scots pine (Pinus sylvestris) in Scotland (Millar et al, 1986 in Williams, 1996) or P in mixtures of Eucalyptus globulus with Acacia mearnsii (Forrester et al, 2005). The increase in the pool of available nutrients or in the rate of its cycling may result from stratification of the root systems, or the accelerated decomposition of organic material in a mixture, which has been observed in the majority of cases, when litter of different species was mixed (Gartner and Cardon, 2004; Hättenschwiler, 2005). As indicated in Figure 5.1, fixation of atmospheric N by one of the co- occurring species often has a large impact on N availability and thus productivity. The greatest increases in productivity in mixed stands when compared to mono-specific stands have been found in mixtures with N-fixing species (see also Kelty and Cameron, 1995; Forrester et al, 2006). Once fixed
through symbiotic bacteria or actinomycetes, N can be transferred from N2-
fixing species to the non-N2-fixing species via litter decomposition and
subsequent mineralization of organic N.
The relationship between tree species diversity and productivity as well as between diversity and some of the other ecosystem processes such as decomposition and nutrient cycling is, however, idiosyncratic. The performance
of the mixtures in comparison to respective monocultures is often more strongly influenced by the specific species combinations and site conditions than by the diversity (Forrester et al, 2005; Pretzsch, 2005; Redondo-Brenes and Montagnini, 2006). It is therefore difficult to forecast the outcome of the various interactions in mixed stands, also because these interactions may change as the stands develop (e.g. Forrester et al, 2004). In relation to site conditions, the success of mixtures seems to depend on whether nutrient or water limitations of the companion species can be reduced in the mixture (e.g. Laclau et al, 2008; Forrester et al, 2010). However, there is some indication that mixtures of species representing complementary functional types (e.g. shade-
tolerant with shade-intolerant, early- and late-successional, or N2-fixing and
non-fixing species) result in most cases in higher productivity when compared to monocultures (Kelty, 1992). The uncertainties regarding the expected outcome in terms of productivity of mixed-species plantations are certainly one of the reasons for the low uptake of mixed plantations in practice. This suggests that to reduce this uncertainty, the relationships between functional traits of tree species in mixtures in relation to ecosystem functioning should be the focus of future research.
In most types of plantations, however, the goal is not only to produce a certain amount of woody biomass but also to produce a certain quality of wood for industrial end-uses. This is a very important aspect of the production function of species-diverse plantations, since most forest owners will only adopt higher levels of fine-scaled tree species diversity, if mixtures produce at least the same quality of timber as mono-specific stands. Properties such as wood density, lignin content and fibre length are unlikely to be influenced by tree species mixtures. The quality of trees for solid wood products depends, in addition to year-ring structure and internal rot, most importantly on the criteria dimension, taper, straightness and proportion of branch-free wood (e.g. Cutter et al, 2004). These determinants of tree quality are mainly driven by crown dynamics, which may be strongly influenced by species mixtures. For example, branchiness, which is the most important criterion for downgrading of timber (Montagu et al, 2003), is closely related to crown size, which is commonly closely related to the competition experienced by subject trees (e.g. Alcorn et al, 2007). Thus tree quality parameters of subject trees are dependent on interactions with neighbouring trees, which may be highly influenced by the tree species composition and diversity of the neighbourhood (Sumida et al, 2002). However, very little is known about this, since most studies examining timber quality have focused on mono-specific stands (e.g. Alcorn et al, 2007; Hein et al, 2008).
Previous studies on tree species diversity have focused on stand-level productivity (Pretzsch, 2005; Forrester et al, 2007), but not on the quality of trees grown in mixtures. However, the interactions observed between trees in these studies already point to possible crown interactions. Hetero-specific trees may pose less, equal or more competition than neighbouring con-specific trees. This will depend on species traits influencing competition such as height growth
rates, crown expansion rates, shade tolerance, stiffness of branches, protection of buds against abrasion, timing of leaf flush, etc. (e.g. Sumida et al, 2002). Differences in growth rates, where subject trees grow slower than hetero- specific neighbours, may affect the dimension and straightness of stems, in particular in phototrophic species with low apical dominance. As a result of spatially heterogeneous competition, crowns may become one-sided (or imbalanced) (Jones and Harper, 1987) leading to tension and compression wood (Bowyer at al, 2003). In situations where neighbouring species are weak competitors, the green crown of subject trees may rise more slowly leading to larger, lower branches and delayed self-pruning. In contrast, improved self- pruning and reduced development of epicormic branches may result where neighbours are shade-tolerant species of lower height. Often mixed-species stands result from the unplanned colonization of monocultures through voluntary regeneration of native species. These spontaneous mixtures, which may only be temporary, can have beneficial effects on form and quality, where they increase stand density without outcompeting the crop tree species (Valkonen and Ruuska, 2003). This may not be an effect of diversity but of stand density. However, spontaneous mixtures, in particular with native species, are likely to also have beneficial effects on biodiversity (see below).
Given the interest in mixed-species forests in many regions, as can be seen in efforts to convert coniferous monocultures into mixed deciduous–coniferous forests in Europe (Spiecker et al, 2004), this knowledge gap on the influence of tree species diversity on timber quality is surprising. The available information suggests that the quality of trees may not have to be compromised in mixed stands, if the species composition is judiciously chosen and managed. This points also to the need for information about the traits of companion tree species to make the outcomes of mixed-species plantations more predictable.