Task Force T47
Stand Complexity in Planted Forests

By planted forest, according to FAO definition, we understand stands established by plantation or sowing, for timber production but also for ecosystem restoration/protection and or Carbon sequestration.

Global demand for wood products is set to triple by 2050, driven by human population growth and the growing need for sustainable raw materials. Plantations, which make up about half of the total planted forest area, produced in 2012 about 35 percent of the world's total round-wood in predominantly intensively managed industrial plantations, that are made of pure, even-aged stands. 

Climate change risks and impacts are increasing and there are significant concerns about, and evidence of, decline in the resilience of forests. Pests, diseases, fire, wind and storm impacts are five key challenges we face. Industrial plantations are quite ecologically simplified ecosystems in comparison to natural forests and increasing complexity has been identified to potentially enhance forest resilience. Additionally, at the landscape level, single-species even-aged plantations, properly governed and well located, can play a significant role in landscape sustainability and biodiversity conservation.

However, single-species, even-aged plantations (and the clearfell and restock method of their management) tend to be viewed negatively by local communities and wider society, and this perception exerts strong pressure on policies that affect intensively managed plantations. Therefore increasing diversity and complexity, at the stand and landscape levels, could increase the social acceptability and amenity of plantations, as well as their resilience – which will be important in securing the future supply of raw materials, and other ecosystem services, and maintaining the forest industry's social 'licence to operate'.

By complexity, we mean both the diversity of tree species and structural heterogeneity (e.g. diversity of age or diameter classes) of the stand as well as a diversity created by mosaics at the landscape level. More specifically, forest stand complexity encompasses the structural, compositional, and spatial attributes of a forest stand, influencing its biodiversity, productivity, and resilience. Structural complexity refers to variations in tree sizes, canopy layers, and the presence of features such as snags and coarse woody debris, which often correlate with increased habitat diversity and ecosystem stability (Seidl et al., 2014). Compositional complexity pertains to the diversity of tree species within a stand. Greater species richness can enhance stand productivity and carbon storage, although it may not always increase leaf area (Forrester & Bauhus, 2016). Spatial complexity involves the arrangement and distribution patterns of trees, such as clustering or randomness, which affect light availability, competition, and overall stand dynamics (Pretzsch et al., 2017)

There are large gaps in knowledge about how and why more complex planted forests will be more resistant and resilient to climate change impacts and will better provide ecosystem services in this context. Another important question is how complex planted forest can be managed to ensure that economic, environmental, and social goals are met and appropriately balanced. Questions related to this topic include: Should the diversity be created at planting? Or should we promote management that converts even-aged mono-specific forests into more complex stands? Should diversity be obtained at tree level or at landscape level?

The Task Force will act as a focal point for developing and communicating scientific evidences on complexity and resilience in planted forests – testing the hypothesis that increased complexity increases resilience while preserving performance, which should then lead to greater public acceptance, and enhances the wider contribution that plantations can make, while ensuring productivity and the sustainable supply of timber and wood products now and in the future.

The Task Force will also serve as a platform for linking the research activities of the working groups of the eight IUFRO Divisions that are interested in mitigating the risks and impacts of climate change on planted forests, which differentiates it from the IUFRO Silviculture Unit, which focuses on comparing silvicultural systems.

The Task Force is in line with the conclusions of the fifth International Congress on Planted Forests (2023) and the setup of a new definition of plantation by FAO for the Global Forest Resources Assessment 2020. The conference highlighted the fact that planted forests are still just as important for timber production as they are for their protective functions and the provision of ecosystem services, combining the two objectives in most cases, but that they are increasingly exposed to multiple risks and that their management must therefore adapt to them.

The Task Force will study the consequences of making planted forests more complex, and the associated impacts on timber supply chains, exploring the trade-offs between resilience and wood production by:

1. Understand how diversity in composition, structure and functional traits determine the functioning and resilience of planted forests.
2. Comparing the ecosystem services provided by pure even-aged plantations with those from more complex plantations, with a particular attention to wood production and Carbon sequestration and other products9
3. Analysing the positive and negative impact of complexity from the diversity created by mosaics at the landscape level
4. Proposing and exchanging best management and genetic practices to improve the resilience of plantation forests
5. Proposing management methods towards the conversion of pure monoculture plantations into more complex stands