Mixed species forests have been reported to have benefits compared to monospecific forests in terms of increased growth and reduced risks. This advantage can be essential in an uncertain future with increased damages due to global change. In Norway, only three species are predominantly occurring in natural forests, and only two of them are used in forestry, i.e., Norway spruce and Scots pine. Mixing spruce and pine is therefore of great interest in future forest management in Norway and other Nordic countries. This species mixture has been reported to only produce little more than their monospecific counterparts.
A general feature of growth studies for mixed stands is that they have been done in stands close to maximum stand density. In dense stands, proximity of neighbors causes that individual species benefit from trait differences, e.g., in shade tolerance, and together produce more. However, managed forest stands are often at much lower density, either due to low establishment density or due to thinning. We were therefore interested in studying if mixed stands at low stand density are also growing more than monospecific stands. We measured individual tree growth on 28 sample plots in 7 stands with varying species proportions for a period of about 10 years after thinning. Neighborhood species proportions mostly did not affect tree growth in that period, confirming the hypothesis that increased growth due to species mixture only occurs in dense stands, where species interact with each other much more intensively.
A dry summer in 2006 allowed us to study the effects of drought on growth of both species, and the effect that thinning and species mixture had on growth reactions during and after drought. As expected, the growth of pine was less affected by the drought than the growth of spruce. For spruce, dominant trees reduced growth more than suppressed trees. Species proportions in the neighborhood did not explain any of the variation in growth reduction after drought, neither for the reduced growth in the drought year, nor for the reduced growth in the recovery period after the drought. During drought events, the main advantage of mixing these species is therefore the higher drought tolerance of pine.
To study the mechanisms responsible for growth differences in mixed species stands, we described the size and shape of tree crowns using terrestrial laser scanning. Terrestrial laser scanning has been used for detailed descriptions of branches and tree crowns in deciduous tree species, which allow full visibility during winter. The method is more challenging for conifer species due to visibility problems. We applied a simple sampling design to describe as many tree crowns as possible with little field work. We also designed a new algorithm, which extracts individual tree crown data from the total point cloud of laser hits. With this method we were able to get detailed crown models for about 50% of a total of about 1300 trees on the 24 sample plots. The data indicate similar crown shapes of both species in the young stands, but significantly longer crowns of spruce than of pine. Neighborhood species proportions had only minor effects on the size of the tree crowns. Longer crowns but also slightly lower tree heights of spruce in these mixed stands indicate a stratification. The foliage of the shade-intolerant pine trees is located in the upper layer, while most of the foliage of the shade-tolerant spruce is located in the lower layer. This stand architecture, together with the differences in shade tolerance between these two species, might explain the increased growth of mixed stands. However, more growth only seems to occur in stands of higher density than the thinned stands that we studied.
Mixing spruce and pine will in the future also be a common practice due to the fine-scaled mosaic of microsites with different growth conditions characterizing most forest stands in Norway. Frequent drought episodes of the last years and following attacks of bark beetles on spruce have clearly shown the low drought tolerance of spruce planted on dry microsites. Pine is more able to withstand drought and following insect attacks on those microsites. Knowledge created in this project about growth and drought tolerance of mixed species forests is therefore a valuable basis for forest management in the future.
Forests are increasingly exposed to climate-driven biotic and abiotic disturbances. Climate change could thus jeopardize forests' capacity to deliver ecosystem services. There is therefore an urgent need to adapt forest management so as to promote and improve forest resilience at different spatial and temporal scales. Mixed forests are considered as one of the main options for adapting to and reducing risks of climate change. Higher tree species diversity is expected to provide higher productivity, higher temporal stability, higher resistance and resilience to disturbances and a more diverse portfolio of ecosystem services. However, knowledge about how to design and manage mixed forests to achieve these potential benefits is still lacking. REFORM aims at identifying the most optimal composition and management of mixed forests in order to reduce natural and socio-economic impacts of climate change. REFORM is based on data from observational, experimental and modelling platforms provided by twelve partners from ten countries covering different bioclimatic regions in Europe. It will investigate mixed forest features, like species composition, mixing patterns, stand age and density, that best explain resistance and resilience to biotic and abiotic disturbances. It will define the management options to achieve and maintain these optimal mixed forest features. The impact of these management alternatives on the provision of ecosystem services will be also evaluated. REFORM will provide forest managers with practical tools for increasing resilience of mixed forests using a scenario analysis at different scales, including local-adapted silviculture guidelines, forest models, and transnational training forest networks. The project will make recommendations to forest policy makers for the promotion of resilient mixed forestry.