In addition to the provisioning functions for wood and non-wood forest products, plantations have important regulating functions for water, nutrient and carbon cycles as well as for ecosystem resiliency. Although the role of plantations for hydrological cycles and carbon storage and sequestration have been highlighted previously in Chapters 3 and 4, we will here return to the question of how biodiversity, with a focus on tree species richness, might influence the ecosystem functioning related to these services.
Water
Despite the fact that water is a fundamental and critical resource economically, environmentally and socially (Calder, 2005), the effect of tree species mixtures on forest hydrological cycles has been largely ignored. This is surprising, since as we have seen above, mixed-species forests are often more productive than their mono-specific counterparts and forest water use does normally increase with productivity (Law et al, 2002). Therefore it should be asked whether mixtures use significantly more water than monocultures on the same site. However, there are only a few studies that have compared the water use of trees in mixtures to monocultures (Schume et al, 2004; Anders et al, 2006; Forrester et al, 2010). Forrester et al (2010) found that water use in mixtures was higher than in monocultures, although the water-use efficiency of the companion species had also increased. Similarly, water use in mixed stands of Norway spruce and European beech was substantially higher than in pure Norway spruce stands (Schume et al, 2004). However, in mixtures and monocultures of Pinus sylvestris and Fagus sylvatica, groundwater recharge was higher under mixtures (Anders et al, 2006) than under pine, despite a higher water use of trees in the mixture. The improved soil moisture and drainage in the mixed stand was the result of suppression by the shade-casting beech of a dwarf-shrub and grassy understorey with high transpiration. This study shows that interactions in mixed stands can be complex and go beyond the direct influence of one tree species on the other. However, these findings demonstrate that there may be trade-offs between the use of tree species diversity for other functions and the provision of water or the susceptibility to drought stress.
Carbon sequestration and storage
A functional relationship between tree species diversity and C storage and sequestration in plantation forests would have important implications for the management of the C-sinks in reforestation and afforestation projects (UNFCCC, 2005). The effects of tree species diversity on above ground
sequestration of C in plantations is closely related to that of ecosystem productivity, which has been discussed above (see also Chapter 3). In addition, tree species diversity can have effects on below ground and soil carbon through effects on litter quality and decomposition (Giardina et al, 2001; Binkley and Menyailo, 2005) and also the diverse rooting patterns of trees leading to the deposition of organic material at various soil depths (Johnson, 1992). There are some indications of the positive effects of plant species richness on soil organic carbon storage in forest or agroforest ecosystems (Chen, 2006; Saha et al, 2009). These observational studies cannot clearly separate the effects of species diversity from that of all the other environmental influences. However, Gleixner et al (2005) suggested that these are likely to be indirect effects of plant diversity mediated by feedbacks between above ground and below ground diversity, and effects on water cycles and decomposition processes. Thus species-specific influences on these processes or those that are related to functional types are likely to have a large influence on soil C storage. One example for such an effect is the inclusion of N-fixing species leading to increased soil organic carbon sequestration, presumably through reduced mineralization of N-enriched soil organic C under N-fixing species when compared to non-N-fixing species (Resh et al, 2002).
New insights in the effects of biodiversity on soil C sequestration and nutrient cycling can be expected from controlled manipulative experiments of tree species diversity (e.g. Scherer-Lorenzen et al, 2007). In addition, an indirect effect of tree diversity on forest C storage may be through increased resistance against disturbances (see also Chapter 3).
Nutrient cycling
Mixing tree species may lead to increased availability and more efficient use of nutrients through accelerated cycling or through increased capture of nutrients. The question of nutrient cycling is of paramount importance in high- yielding plantations, where the export of nutrients with harvested products may be very high (Gonçalves et al, 1997). Any processes and mechanisms that lead to reduced nutrient losses per unit of exported biomass and to reduced fertilizer inputs, in particular of nutrients with limited supply such as P, will help to improve the sustainability of plantations. How changes in tree diversity, from monocultures to mixtures of varying species richness and composition, affect nutrient cycling is still rather equivocal (Scherer-Lorenzen et al, 2005). Interspecific differences in resource capture can be attributed to different resource requirements, uptake abilities and niches occupied (Rothe and Binkley, 2001). Hence, mixing species with such different traits and resource niches may lead to niche differentiation and resource partitioning, resulting in increased resource use complementarity. A recent meta-analysis showed that in the majority of cases the above ground nutrient content and nutrient use efficiency (N and P) of species grown in mixtures were higher than in monocultures, indicating an increase in the proportion of resources captured from a site (Richards et al, 2010). However, this study also indicated that in a
substantial number of cases there is either no effect or a negative effect. This again points to the need for careful selection of companion species to obtain the desired influence of mixtures of ecosystem functioning, here the maintenance of site fertility.
Resiliency
Resiliency, as a regulating function of ecosystems, is related to the concept of ecological insurance, which hypothesizes that more diverse communities are more likely to cope with stress or disturbance (Yachi and Loreau, 1999). Tree species diversity may influence both the resistance as well as the resilience in relation to specific disturbance agents, which may be of biotic as well as abiotic nature. Resistance can be defined as the ability of the system to withstand changes of the current state, whereas resilience relates to the capacity of the system to recover from disturbance and to regain the pre-disturbance condition (Attiwill, 1994). For forest ecosystems, the living biomass stock is often used to define this pre-disturbance reference condition.
The most important abiotic disturbances affecting plantations are wind storms and fire. While there are many publications that have analysed the storm resistance of individual tree species and certain stand structures (e.g. Foster, 1988; Schütz et al, 2006), few have considered the question of mixed- species stands (e.g. Lüpke and Spellmann, 1997; Dhôte, 2005), and none have analysed the question of diversity. The resistance to storm is rather species specific and determined by traits such as tree height, rooting patterns, deciduousness vs. evergreen, foliage density, etc. For example, Lüpke and Spellmann (1997) found that Norway spruce when mixed with European beech is no less susceptible to storm damage than in mono-specific spruce stands. However, the mixed stands are more stable, since the beech component is less affected. This implies that these mixtures are not more stable than mono-specific beech stands. Unless the traits conferring wind firmness of the different tree species participating in mixed stands are not influenced by the stand composition, similar results may be expected for other species combinations. The persistence of at least a partial cover of trees following catastrophic storms may, however, be an important advantage for the recovery of stands (Dhôte, 2005).
The issue is much more complicated for fire disturbance, since there are interactions between species traits and disturbance frequency and intensity. Important tree species traits related to fire resistance are: bark thickness, shedding of dead branches, etc. (e.g. Fernandes et al, 2008). In addition, there are species traits promoting fire such as the flammability of leaves and needles as well as litter production (Facelli and Pickett, 1991). Some widely distributed plantation genera such as Pinus and Eucalyptus are known for their fire promoting traits (Scarff and Westoby, 2006; Ormeno et al, 2009). Thus tree species with low flammability of litter have been commonly used as green fire breaks in plantations (Johnson, 1975). Mixing such species into plantations to accelerate the decomposition of flammable litter, to change the fuel bed
properties or to shade a flammable understorey such as grasses may be additional options to reduce fire risks. However, these benefits regarding increased resistance to fire are the result of specific combinations of species, not diversity per se.
In relation to biotic disturbances through pest or pathogen species, there is evidence that lower tree species diversity is related to greater pest insect abundance, density or damage (Jactel et al, 2005). The mechanisms leading to a greater resistance of mixed-species stands comprise reduced accessibility of host trees to pests, a greater impact by natural enemies and the diversion of pests from less susceptible to more susceptible tree species (Jactel et al, 2005). Further, Jactel and Brockerhoff (2007) and Koricheva et al (2006) found that the composition of tree mixtures was likely to be more important than species richness per se. This is because the diversity effects on herbivory were greater when mixed forests comprised taxonomically more distant tree species, and when the proportion of non-host trees was greater than that of host trees. In addition, the effect of tree species diversity is more pronounced for specialist herbivores than for generalists (Koricheva et al, 2006; Jactel and Brockerhoff, 2007). However, there are many situations where the direct comparison of monocultures with polycultures or plantations with native forests has failed to demonstrate that mono-specific stands are more susceptible to diseases (Nair, 2001).
The habitat functions of ecosystems relate to the importance of ecosystems to provide habitat for various stages in the life cycles of wild plants and animals, which, in turn, maintain biological and genetic diversity and evolutionary processes (Chapter 2). Since more tree species can support more dependent species such as micro-organisms and invertebrates, the benefits for biodiversity are obvious, in particular, if native species are components of mixed-species plantations (Hartley, 2002). This other potentially important regulating function of plantations is dealt with below, when we discuss the silvicultural options to maintain or enhance biodiversity in plantation forests, since the effects of plantation management on biodiversity are of particular public concern (e.g. Brown et al, 2006).