Vascular Flora on Croatian Historic Structures: Drivers of Biodeterioration and Conservation Implications

Biodeterioration, the alteration of materials by living organisms, affects approximately two-thirds of the world's cultural heritage. When organisms colonize the surfaces, they induce physical and chemical changes that can lead to significant damage. Despite its relevance, this phenomenon remains understudied in Croatia. This study aims to assess the deteriogenic vascular flora colonizing Croatian historical structures, including castles, towers, and archaeological remains, in relation to several environmental and anthropogenic factors: distance from the sea (0-1, 1-10, 10-65, and 65-165 km), elevation (0-50, 50-150, 150-300, and 300-600 m a.s.l.), exposure (north, south, east, west), and the state of conservation of the site (absent, low, good, excellent). Vegetation cover and floristic diversity, assessed using the Shannon Index, were primarily influenced by elevation and conservation status. As expected, vegetation cover decreased significantly, by 67.75%, from sites classified as 'Absent' to those with 'Excellent' conservation status (p < 0.001). To explain the observed differences in vegetation cover across the four altitudinal ranges, an analysis of plant life forms was carried out, revealing a wide variability and statistically significant patterns also related to the type and frequency of maintenance interventions. The potential risk posed by vascular plants was evaluated using the Hazard Index (HI), which revealed significant differences only for elevation and distance from the sea. The highest risk levels were recorded at mid-elevations (150-300 m), where the Hazard Index reached its maximum value (HI = 158). Exposure did not show a significant effect on biodeterioration processes. These findings provide new insights into plant-driven biodeterioration in Croatia and highlight the need for targeted conservation strategies to protect the country's cultural heritage.