Author: Alex Bush (Lancaster University)
A majority of European biodiversity is associated with forests, and biodiversity within them has generally fared better than in other land cover types like agriculture (EFI, 2021). Although European forests are improving to some degree, many remain at risk due to issues like pollution in the air and water, loss of habitats and diverse wildlife, and the growth of cities into forested areas. Forestry itself is reported as a threat to 11% of assessed species (EEA, 2020).
With growing global and European commitments to ecosystem restoration, such as the EU Biodiversity Strategy for 2030 and the UN Decade on Ecosystem Restoration, structured monitoring is essential not only to demonstrate restoration success but to guide and improve ongoing efforts. For example, we need to ensure biodiversity benefits are achieved alongside parallel objectives to mitigate climate change, secure sustainable biomass and improve societal health. An evidence-based and transparent reporting framework is therefore key to recognising and rewarding effective restoration.
Foundational principles: Objectives and reference ecosystems
Successful biodiversity monitoring begins with clear, explicit, and measurable objectives. These should be time-bound and based on a thorough baseline assessment that includes current site conditions (e.g. soil chemistry, drainage, topography) and threats (e.g. fire, erosion, acid deposition). A fundamental concept is the use of reference ecosystems—models of what ecosystems could look like following restoration (e.g. Gann et al. 2019; Nelson et al 2024). These models form the benchmarks against which we can measure recovery and set realistic targets, aiming to capture the variability of an ecosystem across time and space. Naturally it can take many decades before forests resemble mature reference ecosystems, and therefore models that include the expected trajectory of ecosystem recovery help set realistic expectations and guide adaptive management during long-term restoration.
Selecting indicators: multidimensional and temporal considerations
Given the complexity of forest ecosystems, monitoring should target a diverse set of ecosystem attributes. These span both biotic and abiotic components and collectively provide a comprehensive view of ecosystem condition and function. The Society for Ecological Restoration (Gann et al. 2019) recommends including indicators across six categories:
- Absence of threats – e.g., reduced presence of invasive species or disease.
- Physical conditions – e.g., soil chemistry, hydrology, and terrain.
- Species composition – presence and diversity of native species characteristic of a healthy reference ecosystem.
- Structural diversity – variability in vegetation layers, canopy complexity, deadwood, and age structure.
- Ecosystem function – processes like nutrient cycling, productivity, decomposition, and disturbance regimes.
- External exchanges – connectivity with surrounding habitats to support species movement and ecological flows.
Using a combination of these attributes provides a robust, multidimensional understanding of how restoration efforts are progressing. Not all attributes respond at the same rate - for instance canopy structure may change quickly, while understorey composition or ecosystem functions may take much longer to change - so monitoring should be designed to capture these different temporal dynamics.
As the reference ecosystems represent the natural range of variation in the target ecosystem for each attribute, including multiple attribute dimensions reduces the chance for projects to be unfairly judged by a single threshold, or the presence or absence of a handful of species, that are rarely within the specific control of forest managers.
Image: Muys et al. 2022
From inputs to outcomes: measuring real impact
Historically, restoration projects have leaned heavily on reporting input-based metrics—resources invested (e.g., money, staff, number of trees planted) or basic actions (e.g., hectares fenced, stakeholder events held). While these measures are simple, and relevant to documenting project delivery, they provide weak evidence of ecological success (Pressey et al. 2021). This reliance on inputs has obscured the true effectiveness of restoration work and wasted opportunities for learning (Lindenmayer 2020). The perennial problem for ecosystem restoration has been the recognition to implement and follow through with a long-term monitoring plan. Conservation outcomes are not proportional to effort unless results are monitored and verified. Without monitoring, restoration is an act of faith, not a scientific intervention.
Moving forward, restoration projects must transition to tracking these ecological outcomes as the core measure of success and with advances in ecological monitoring tools and technologies, from drones to acoustic recorders to eDNA, collecting outcome-focused data is now more feasible and cost-effective than ever. Further efficiencies are likely to be possible when multiple techniques are applied in parallel, thereby identifying how congruent responses are between attributes, and the level at which implementing both would be redundant. Finally, the greatest savings for restoration will occur when data is shared, particularly from reference models and re-used for multiple projects, and this can be facilitated by collaboration of environmental regulators and funding bodies.
Conclusion: making monitoring matter
Monitoring should be planned from the outset and scaled to available resources, but without compromising its core function: providing actionable insights. Adaptive management depends on feedback from monitoring to refine restoration strategies, justify continued funding, and sustain biodiversity outcomes over time. To ensure forest restoration achieves lasting ecological impact, monitoring must focus on what changes, not just what’s done. By prioritizing outcome-based measures, selecting indicators that span key ecosystem attributes, investing in strong reference models, and designing efficient sampling strategies, projects will deliver outcomes that stakeholders trust.
Long-term investment in monitoring is not optional - it is essential. The historical underinvestment in biodiversity monitoring has led to data gaps and ineffective management. Today, with expanding technological capabilities and increasing policy emphasis on ecological outcomes, robust monitoring is both more feasible and more urgently needed than ever.
References
EEA 2020. State of nature in the EU. Results from reporting under the nature directives 2013-2018. Report no. 10/2020. https://www.eea.europa.eu/en/analysis/publications/state-of-nature-in-the-eu-2020
EFI 2021. What do we know about the status of forest biodiversity? https://efi.int/forestbiodiversity/status
Gann, G.D., McDonald, T., Walder, B., Aronson, J., Nelson, C.R., Jonson, J., Hallett, J.G., Eisenberg, C., Guariguata, M.R., Liu, J., Hua, F., Echeverría, C., Gonzales, E., Shaw, N., Decleer, K. and Dixon, K.W. 2019. International principles and standards for the practice of ecological restoration. Second edition. Restor Ecol, 27: S1-S46. https://doi.org/10.1111/rec.13035
Muys, B., Angelstam, P., Bauhus, J., Bouriaud, L., Jactel, H., Kraigher, H., Müller, J., Pettorelli, N., Pötzelsberger, E., Primmer, E., Svoboda, M., Thorsen, J.B., Van Meerbeek, K. 2022. Forest Biodiversity in Europe. From Science to Policy 13. European Forest Institute. https://doi.org/10.36333/fs13
Nelson, C.R., Hallett, J.G., Romero Montoya, A.E., Andrade, A., Besacier, C., Boerger, V., Bouazza, K., Chazdon, R., Cohen-Shacham, E., Danano, D., Diederichsen, A., Fernandez, Y., Gann, G.D., Gonzales, E.K., Gruca, M., Guariguata, M.R., Gutierrez, V., Hancock, B., Innecken, P., Katz, S.M., McCormick, R., Moraes, L.F.D., Murcia, C., Nagabhatla, N., Pouaty Nzembialela, D., Rosado-May, F.J., Shaw, K., Swiderska, K., Vasseur, L., Venkataraman, R., Walder, B., Wang, Z., & Weidlich, E.W.A. 2024. Standards of practice to guide ecosystem restoration – A contribution to the United Nations Decade on Ecosystem Restoration 2021–2030. Rome, FAO, Washington, DC, SER & Gland, Switzerland, IUCN CEM. https://doi.org/10.4060/cc9106en
