Limitations and strengths of indicators for evaluating the nitrogen status of forest ecosystems

Anthropogenic nitrogen (N) deposition has increased N availability in many forest ecosystems. In contrast, global changes such as elevated atmospheric CO₂, longer growing season under warming, and increased precipitation may stimulate plant growth, intensifying biotic demand for N. These diverging trends influence the N status of forests, reflecting the balance between N supply and demand and indicating whether a forest is in a state of N limitation, transition phase, or N saturation. Numerous direct and indirect indicators are used to evaluate forest N status. However, an overview of their effectiveness for N status assessments, which require established threshold values between different phases, is limited. Here, we review N status indicators and their threshold values to evaluate their effectiveness in revealing changes in forest N status. In addition, we assess whether indicators provide insights into forest health conditions such as defoliation, nutritional status, species richness of the forest understory, the prevalence of diseases, and forest growth dynamics. The N status indicators included are categorized as tree-related (N content of foliage, foliage or nutrient ratios particularly N:P, natural¹⁵N abundance (δ¹⁵N) of foliage and wood), soil-related (soil C:N, inorganic N concentrations, N transformation rates, δ¹⁵N of soil, enrichment factor), and ecosystem-related (litterfall N flux, nitrate leaching, and N₂O emissions). Our synthesis suggests that while most indicators are useful to assess N availability, the majority lack clearly defined thresholds to identify N saturation and N limitation. Similarly, we found that the majority of indicators cannot be used for assessing forest health conditions. As most indicators assess specific components of the forest N cycle, a comprehensive assessment of forest N status requires combining indicators to evaluate changes in multiple N cycling processes. This review guides the selection of indicators for assessing forest N status by evaluating their effectiveness in detecting N saturation or limitation, reflecting forest health conditions, responsiveness to N deposition, and the required research effort.