From species traits to community gradients: Validation of Borhidi-like naturalness indicators in steppe vegetation

Naturalness is a key ecological concept reflecting the extent to which plant communities correspond to their natural state under minimal anthropogenic influence. The Borhidi system provides a widely used framework for assigning species-level naturalness indicator values based on ecological behaviour; however, its application at the community level remains insufficiently validated. This study aimed to assess whether species-level naturalness indicators correspond to empirically observed patterns in vegetation and to develop a reproducible, community-calibrated naturalness scale for the steppe ecosystems of Ukraine. The analysis was based on a large vegetation dataset encompassing a broad gradient of naturalness and anthropogenic transformation, including natural, semi-natural, urban, agricultural, and technogenic habitats. A modified Borhidi-like classification was developed by integrating functional traits, ecological niche breadth, synanthropic status, and conservation information into a unified species-level index. Relationships between species traits, environmental gradients, and community composition were analysed using RLQ ordination. A curvilinear naturalness trajectory was derived from the ordination space and used to construct a continuous community-level indicator scale. The results show that species-level naturalness categories exhibit non-random patterns in community composition and are related to environmental gradients. They do not form a single linear gradient; instead, naturalness emerges as a multidimensional property structured by multiple ecological processes. The most consistent dimension corresponds to the contrast between stress-tolerant and ruderal strategies, which approximates a functional naturalness gradient. Functional traits, particularly stress tolerance and ecological niche breadth, significantly contribute to explaining variation along this gradient, although they do not fully determine species positions. The proposed approach allows the transformation of discrete Borhidi-like categories into a continuous, empirically calibrated naturalness scale. This scale reflects community-level organisation and provides a more robust basis for ecological interpretation than purely expert-based classifications. At the same time, the results highlight that the transfer of species-level indicators to the community level is only partially valid, as many categories exhibit context-dependent behaviour. The study demonstrates that naturalness can be operationalised as a continuous, community-derived indicator when species’ ecological properties are integrated with empirical vegetation data. The proposed framework offers a reproducible tool for assessing vegetation naturalness and analysing ecosystem transformation across heterogeneous landscapes.