Assessing the field protocol implementation for understory plant functional groups monitoring within the Italian National Forest Inventory

Understory biodiversity monitoring has been included for the first time in the Italian National Forest Inventory (IFNI 2025), the national monitoring system aimed at supporting sustainable forest management planning. The system integrates field observations with remote sensing data to improve forest assessment and orient management and policy decisions. This study provides a preliminary assessment about the application of a novel protocol for understory plant functional groups monitoring during the 2025 survey campaign. The survey has been conducted by personnel of the Carabinieri Command of Forestry, Environmental and Agri-Food Unit (CUFAA) across the whole national territory. After personnel training, data for Panel 1 (year 2025) are collected at 1370 sampling points distributed across all administrative regions. At each point, a standardized 10 m linear transect is established. The total length (in cm) of contacts or overlaps between living plant parts and the string within the observed meter segment for six morpho-functional plant groups (lichens, bryophytes, pteridophytes, forbs, grasses, and woody species) is recorded. Systematic photographic sampling is also performed, with one image taken at each meter. Approximately 13,700 images (10 per transect across all sampling points) were reviewed to evaluate compliance with the protocol, length measurement and to identify recurrent sources of error that could affect ecological interpretation during the panel 2 of survey (year 2026). The assessment revealed considerable variability among regions in data quality and protocol application. Concerning images check, common issues included poor visibility of the measuring tape used as a reference and differences in image sharpness, both potentially affecting cover estimation of the morpho-functional groups. At the protocol level, inconsistencies were observed in the classification of functional groups, particularly for graminoids, bryophytes, and pteridophytes, as well as in distinguishing woody from non-woody species, including juvenile woody individuals, lianas, and small chamaephytes. Additional inconsistencies concerned the inclusion of dead wood and mosses and lichens growing on specific substrates. Quantitative estimates of these errors will be carried out in a subsequent phase of the analysis. Overall, the results are promising but highlight substantial variability in field implementation within a large-scale standardized monitoring system undergoing methodological transition, emphasizing the need for improved training and the development of an integrated checklist of the Italian forest flora.