Topographic indices predict the diversity of Red List and non-native plant species in human-altered riparian ecosystems

Disturbance regime and site productivity are considered the main factors shaping plant species assemblages and community diversity in riparian ecosystems, but whether and how topographic indices correlate with these two predictors remains largely unexplored. In this study, the stream power index (SPI) and topographic wetness index (TWI) were calculated as surrogates of flooding-induced disturbances and site productivity, respectively. We used a robust dataset of 879 vegetation plots sampled in temperate floodplain forests of Slovakia with the goal of testing the predictive power of interactions between the SPI and TWI on the diversity patterns of all vascular plants, ferns, woody species, aliens, neophytes and Red List species. Generalized additive models (GAMs) revealed the complementary importance of both predictors for explaining the species richness of different plant guilds in floodplain forests, including non-native and Red List species. The SPI and TWI accounted for 4.0-42.0% of the variation in species richness data, depending on species group, with the highest variation explained for ferns and the lowest variation explained for Red List species. Our results indicated that higher flooding-induced disturbances (SPI) associated with light availability increased the number of all species. Their richness declined towards less disturbed (SPI) sites with higher productivity (TWI) levels, where a higher rate of interspecific competitive exclusions is expected. In contrast, the species-reducing effect of frequent flooding (SPI) was recorded for aliens, neophytes and woody richness, with maximum richness values found at more productive sites. These trends differed from the patterns recognized for Red List species richness, which peaked at the intermediate disturbance (SPI) and productivity (TWI) levels. Our findings suggest that river networks with natural flow regimes and recurring episodes of floods facilitate species richness and simultaneously make riparian forests more resistant to plant invasions than regulated or channelized rivers. We confirmed that topographic indices represent tools for estimating the biodiversity status of floodplain forests, which could be useful in regions with a low rate of vegetation sampling or with insufficient cost and personnel capacities for vegetation monitoring. The TWI and SPI, as easily calculated measures, could be effectively applied to identify areas with various conservation and management targets.

Automated summary

This study investigates the relationships between disturbance regime, site productivity, and plant species richness in temperate floodplain forests. The results show that flooding-induced disturbance and site productivity are complementary predictors for explaining the diversity patterns of different plant guilds. Higher disturbance levels increase species richness, while less disturbed sites with higher productivity lead to higher rates of interspecific competitive exclusions. However, frequent flooding may reduce species richness for certain groups. The richness of Red List species peaks at intermediate disturbance and productivity levels.