The reference value ranges for sustainable supply capacity presented in this article depict a wide range of estimates. This reflects challenges related in particular to data availability and methodological robustness. It means that the results should be interpreted with caution and seen as a first step toward developing a method to, e.g., incorporate safe operating space concerns regarding land use into estimations of future sustainable timber supply capacities, and to, e.g., consider how such supply constraints may be translated into reference values (and eventually toward targets) for benchmarking sustainable levels of consumption. Section 4.1
discusses some of the key challenges for strengthening the reliability and applicability of results. Section 4.2
discusses the results in more detail and considers implications with regard to appropriateness for the EU policy context. Finally, key future research needs are highlighted (Section 4.3
4.1. Challenges Relating to the Methodological Approach and Data
The basic approach to aggregate national data on sustainable supply seems appropriate for estimating sustainable supply capacity, in particular due to major differences in forest types and management across the world. However, better and more harmonized national data is needed for this approach to produce more reliable results.
With regards to forest measurement, better Earth observation data combined with harmonized surveys would greatly enhance knowledge on forest area and forest growth. This article relied primarily on source (e.g., FAO, International Tropical Timber Organization (ITTO), etc.) reporting data based on surveys of countries. In some cases this means that estimates given by countries on, for instance, their forest area, have been generated using different methods and with different levels of reliability. Some countries also opt out of reporting or may significantly change their reported data between years. With regards to data sources specifically dedicated to fast-growing plantations, this article heavily relied on the FAO Planted Forests Database, which is subject to similar problems (consistency, reliability, and lack of harmonization due to the survey method of data generation). Along with other sources on fast-growing plantations, the data is also somewhat outdated. Outdated data, or data from different years, has been a challenge for all types of forests. Efforts were taken to use the most recent and reliable sources for both area and productivity at the time of analysis, but as this was not always possible, older data was sometimes relied on. This also means that data may not always be 100% comparable. Advanced remote sensing processes would dramatically improve data quality, not only for forests as a whole, but also for plantations. This is critical to generating reliable scientific evidence on limits. Progress to this end is being made. For example, [47
] was the first study to use Landsat Earth observation data to map global forest loss.
A key challenge is estimating how sustainable forest management could be reflected in the sustainable supply capacity estimates of global forest use. By its definition, the concept of sustainable forest management generally includes maintaining multiple functions of forest systems across social, economic, and environmental dimensions. However, to estimate sustainable supply, an indication of timber removal only (i.e., harvesting below NAI as a maximum threshold) is relied on. How other functions of the forest can be better taken into account is an open question for future research. In particular, research should consider the issue of scale, asking what scale is appropriate for estimating a balance between the rate of timber removals and, e.g., maintaining other ecosystem services. The indicator “share of NAI” has the benefit of being a transparent and relatively simple indicator that can be applied at multiple scales of analysis. However, as an indication of sustainable harvest rates, it also depends on the structure and age of the forests, as well as the period assessed. This is because even-aged stands reach a point of no or even negative growth, implying that in countries with a high share of old forests, harvest rates could be above NAI for a short period of time. On the other hand, countries with a high share of young forests may keep harvests well under NAI to allow the forest resource time to grow. This means that the “share of NAI” may be most appropriate to countries, regions, or forests with an even distribution of trees in each age class. Related economic and environmental dimensions should also be further explored in future research—e.g., considering price shocks associated with periods of intense harvesting as well as the ecosystem functions associated with older stands in particular. Additionally, future research may explore alternatives, such as ways to incorporate the structure and quality of standing stock, looking at actual harvests, or aggregating local data on sustainable forest management to national and global levels. Approaches to incorporate residues as well as thinnings more comprehensively, in light of environmental constraints related to, e.g., soil fertility, and also timber demands related to, e.g., energy, should be assessed. Altogether this implies that a future indicator of sustainable supply may become more comprehensive and dynamic, considering harvest rates under criteria of sustainable forest management as well as timber availability (stocks and usable growth). If NAI is judged as the most reliable and feasible metric for estimating sustainable supply capacities, research is needed to consider how much of NAI can and should be removed globally under different conditions of sustainability (considering multi-functional forest uses) for different types of forests (e.g., plantations, semi-natural, managed, etc.) and in different regions of the world.
With regards to future trends, changes in the root causes of deforestation (including population, economic growth, and national and international demand for agricultural products, wood products, and minerals [48
]) make it difficult to extrapolate past trends as indicative of future developments. Forest productivity and management assumptions are also prone to uncertainty, especially over the long term. With regards to productivity, climate change could dramatically change forest structure and growth, as well as increase the risk of disturbances such as fire and pests. It is difficult to foresee such impacts and better modeling of potential impacts on the global level especially is needed to provide a more realistic picture of potential future supply capacities. At the same time, forests are also crucial to efforts to mitigate climate change. Strategies to raise carbon storage by adjusting management practices in forests would certainly have an impact on sustainable supply capacities [2
Strengthened data is needed regarding historical developments, the current situation, and potential future developments. For example, there is a lack of data on the historical development of the area of forest available for wood supply. While this is dependent on regional elements like terrain, infrastructure development, zoning, and conservation, which may be difficult to aggregate to a global level or transfer to other countries, the large differences in forest available for wood supply (e.g., around 50% globally compared to 85% in the EU) suggest that some indications of historical developments may be useful to set the context for potential future developments.
With regards to the SOS scenario, the method to define the boundaries of such a scenario should be strengthened, in particular through multi-stakeholder discussions on the amount of forest area change that could be considered within an acceptable degree of risk, taking into account the precautionary principle. To this end, existing targets could help set the context, such as the Aichi Biodiversity Targets with the aim to at least halve the rate of loss of all natural habitats, including forests, by 2020 [29
]. To enable such a stakeholder process for delineating the safe operating space, more robust knowledge on the dynamic processes of land use change for forestry and its relation to other planetary boundaries would be beneficial. In particular, the potential for afforestation on degraded land and the relationship between forestry and agriculture land use change should be explored.
Finally, with regards to the derivation of reference values to translate the safe operating space into benchmarks for consumption, challenges relate in particular to population growth. Uncertainty regarding assumptions about population growth make long-term per capita reference values subject to higher uncertainty. Especially long-term variation in population prognoses may have a significant impact on long-term reference values. This may make a range more appropriate, but would also make benchmarks less suitable for possibly developing into targets that are easy to communicate. The political implications of data uncertainty and related challenges for developing per capita benchmarks, and eventually targets, should be addressed by future research.
4.2. Challenges for Interpretation: Is a Global or EU Reference Value Appropriate for EU Policy Orientation?
This article has presented both global and EU current and future reference values for benchmarking consumption. It has been shown that the EU-27 is estimated to have a per capita sustainable supply potential that is around 2.3 times higher than the global average in 2010 (realistic potential estimates), between 2.6 and 3.1 times higher than the global average in 2030 and between 2.9 and 3.5 times higher than the global average in 2050. The question of which reference value the EU should orientate towards, or if alternatives are needed, is an ethical discussion that should take social, legal, ethical, and political considerations into account.
On the one hand, countries with a large forest resource seem to have built up a larger cultural significance to both the forest and forest products. In terms of renewable energy, it seems reasonable for countries rich in forests to use a higher share of timber on a per capita basis (e.g., in local supply and demand chains) than countries with no local resources (e.g., Qatar and Egypt, which seem to have a greater opportunity to use, e.g., solar power, also considering that the heating demand is not as high compared to some forest-rich countries such as Sweden and Finland). This would imply that timber is not a global resource to which every global citizen has the same right. On the other hand, global forests are a common concern of humanity. They are a carbon sink and strongholds of biodiversity. Overuse of the global timber resource surpasses a planetary boundary with universal consequences. Moreover, the concept of a ‘safe and just operating space’ [49
] suggests that limited natural resources critical to meeting basic human needs (including shelter and energy) must be shared in a humane way. In other words, a highly disproportional distribution of use is not sustainable, if it means that a portion of the global population consumes at levels under the minimal conditions for a dignified life. Both environmental and social concerns imply that a global reference value is appropriate, with equal distribution being the most transparent and fair method.
Taken together, these perspectives highlight that the challenge for reference values in relation to forestry seems to be more nuanced than comparable efforts toward global targets or reference values, such as for cropland. For example, one could argue that cropland for food production relates to more of a basic human need than forests; in other words, the right to food and freedom from hunger. This is a discussion on the concept of “fair shares” for different types of resources that must take place at an international level in a multi-stakeholder context. At its core, the challenge for forestry is how to take regional variability into account when considering global capacities. All in all, it is about smarter consumption (balanced with regional and global capacities) with the aim of preventing problem shifting. Keeping the climate challenge in mind and the need to reduce fossil fuel use in a smart way, the question is about finding a middle space between two extremes: (1) problem shifting induced by too high of demand and (2) a protectionist type of market with no trade.
Both global and EU reference values could be valuable as an orientation for European policy makers. In particular, a reference value on global sustainable supply can be used to prevent problem shifting—for instance striving to meet other global targets (like climate targets) in a way that crosses other planetary boundaries (like land-system change). Alternative approaches to develop reference values could be explored by future research. One option could be to distinguish among “levels of sustainability” within a reference value range. Another option could be to develop adjusted reference values, for example developing a reference value for the “sustainable supply capacity for the rest of the world” (subtracting the EU’s share from the global sustainable supply capacity) and applying this to net imports. Such alternatives should be tested against social and environmental sustainability criteria, in particular considering global equity and global risks of deforestation, and should be cautiously explored in an interdisciplinary and multi-stakeholder context. Finally, the rather broad reference value range developed by this article, in particular with the perspective to 2050, could be adjusted over time and as better research becomes available.