Species mixing effects on forest productivity in the Netherlands

Many monoculture forests (dominated by a single tree species) have been converted to mixed-species forests (dominated by more than one tree species) in Europe over the last decades. The main reason for this conversion was to increase productivity, including timber production, and enhance other ecosystem services, such as conservation of biodiversity and other nature values. In addition, it has been suggested that mixed-species forests are more resistant, resilient and stable to disturbances. In line with the <em>niche</em> <em>complementarity hypothesis</em>, inter-specific differences in crown architecture, leaf phenology, shade tolerance and root distribution may allow tree species to partition resources in mixed forests. Such mechanisms may lead to a higher productivity of mixed forests versus monoculture forests, a phenomenon often referred to as overyielding. Interestingly, the <em>stress-gradient hypothesis</em> and the <em>resource-ratio hypothesis</em> suggests that such inter-specific interactions vary along a soil fertility gradient, but in different ways. The <em>stress-gradient hypothesis</em> emphasizes that more efficient partitioning increases overyielding at low fertility soils, whereas the <em>resource ratio hypothesis</em> considers that the denser packing of crowns on fertile soils allows for partitioning of light and overyielding on high fertility soils. Several studies have been carried out about species mixing effects on forest productivity, but so far their findings are ambiguous. Probably, this ambiguity comes from the sites that they studied, which differ in species, age, management history, and/or environmental conditions. This thesis analyses the mixing effect on productivity in relation to the combination of species, stand age and soil fertility, and discusses possible consequences of forest management, for five two-species mixtures in the Netherlands: Douglas-fir (<em>Pseudotsuga menziesii</em> (Mirb.) Franco)<em>–</em>beech (<em>Fagus sylvatica</em> L.), pine (<em>Pinus sylvestris </em>L.)<em>–</em>oak (<em>Quercus robur</em> L.), oak<em>–</em>beech, oak<em>–</em>birch (<em>Betula pendula </em>Roth) and pine<em>–</em>birch. These mixtures and their corresponding monoculture stands were studied using long-term permanent forest plots over multiple decades, but also using two inventories (around 2003 and 2013) across the entire Netherlands. These forest plots data were used together with empirical models at total stand level (chapter 2), species level (chapter 3) and tree level (chapter 4) to evaluate the mixing effect on forest productivity. In <em>chapter 2</em>, four two-species mixtures and their corresponding monospecific stands were compared for productivity (volume stem wood in m3 ha-1 year-1). It was explored whether mixing species differing in leaf phenology and shade tolerance would lead to overyielding of mixed forest stands, and whether overyielding changes with stand development. In line with the niche complementarity hypothesis, the two evergreen–deciduous species mixtures (Douglas-fir–beech and pine–oak) showed overyielding whereas deciduous–deciduous species mixtures (oak<em>–</em>beech and oak<em>–</em>birch) did not. The overyielding was strongest for the Douglas-fir–beech mixture than the pine–oak mixture, which can be attributed to the greater difference in shade tolerance in the former mixture. Overyielding did not significantly change with stand development. It is argued that the regular thinning maintained the ability of species to partition resources, i.e. the complementary resource use in those mixed stands over all stand ages. In <em>chapter 3</em>, it was analysed which of the two species in these four mixtures contributed to overyielding, and whether this overyielding changed along a soil fertility gradient. It was discovered that both the fast-growing and the slow-growing species could contribute to overyielding. Yet, it was mainly the fast-growing Douglas-fir that contributed to higher productivity in the Douglas-fir–beech mixtures, and the slow-growing oak that did so in the pine–oak mixtures. For both mixtures, the greatest relative productivity gain was achieved by mixtures on the poorer soils. At first sight, these results seem in line with the stress-gradient hypothesis and not the resource-ratio hypothesis. Yet, it was argued that not only complementary use of soil resources, but also use of light, may contribute to the higher productivity of mixed stands on the poorer soils. In <em>chapter 4</em>, it was assessed how the growth of individual trees in mixtures was influenced by inter- and intra-specific competition, and whether this competition was mainly size-symmetric for soil resources or size-asymmetric for light on soils differing in fertility. This chapter focussed on three mixtures, i.e. oak–birch, pine–oak and pine–birch, which were available at sufficient numbers in the Dutch national forest inventory data. It was concluded that intra-specific competition was not necessarily stronger than inter-specific competition and this competitive reduction was less seen at lower soil fertility and dependent on species mixtures, which is not in line with the stress-gradient hypothesis. Moreover, size-asymmetric competition for light was more associated with tree basal area growth than size-symmetric competition for soil resources, suggesting that light is the most limiting resource. Competition for light was generally much stronger at high fertility soils, supporting the resource-ratio hypothesis. These results suggest that light is the most limiting resource for tree basal area growth and that reduced competition for light can be explained to some degree by complementarity in light use to increase tree growth in mixed forests. This thesis thus described the productivity patterns when mixing tree species and explored possible mechanisms of higher productivity in mixed stands compared with monoculture stands in the Netherlands. Complementary use of aboveground and belowground resources probably contributes to the higher productivity in mixed stands, but other factors including pathogens, nutrient cycling and litter decomposition were not addressed but cannot be excluded. Overyielding in Douglas-fir–beech and pine–oak mixtures was maintained over time, probably owing to the intensive thinning in Dutch forests. The results shed new light on the stress-gradient and resource-ratio hypotheses. For mixtures in Dutch forest, the greatest productivity gain in Douglas-fir–beech and pine–oak mixtures was achieved on the poorer soils, and it was argued that this is at least partially driven by complementary use of light, while the role of complementarity in use of soil resources is more obscure. Overall, this thesis suggest a substantial potential of species mixing for increasing productivity, which may run in parallel with enhancing other ecosystem services such as conservation of diversity and other nature values. Yet, more <em>experimental</em> studies on productivity in mixed stands are required to better unravel alternative mechanisms. Such understanding is required to manage the forests effectively in a century of unpreceded human driven changes in environmental conditions.