Consistency among forest structure and biodiversity potential index (IBP): an assessment of stand structural complexity for floodplain poplar woodlands

Forest structural complexity arises from several attributes whose combined effects make it difficult to disentangle their single contributions. Such contributions may vary substantially across different ecosystems and management types, requiring the development of fast and effective forest structural evaluation indices for simplified, highly managed systems, such as poplar plantations in riparian areas. With this aim, the suitability of the Index of Biodiversity Potential (IBP) to catch the stand structural variations was tested in three different types of poplar forests (cultivated, semi-natural and natural) along the Po river (Northen Italy) in contrast with quantitative structural parameters (e.g., deadwood biomass, diameter diversity) and synthetic structural complexity indices (Index of Structural Heterogeneity - SHI, Forest Structure Index - FSI). The IBP consistently assessed stand structure complexity using structural parameters and synthetic indices. The Gini coefficient for diameter was generally low among sites, but the IBP identified slight variations in vertical structure enabling differentiation between cultivated and non-cultivated stands. The SHI showed the strongest correlation with IBP, likely due to its context-related features and its ability to describe structural differences within a relatively homogeneous forest stand. Although data on the presence of large woody elements in IBP allow distinctions among stand types, the lack of their quantification may lead to an overestimation of their actual role in forest biogeochemical cycles. In conclusion, the IBP, with some site-specific refinements, could represent a suitable and rapid method for implementing management plans to enhance woodland stand complexity in riparian areas.