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How it all began: macrovegetation

The travertine-building process is a distinctive feature of karst rivers. Plants, especially mosses, serve as the substrate upon which calcite can settle once it dissolves out of the water.

Though this process is fundamentally a physicochemical process, travertine building is largely dependent also on cyanobacteria, diatomaceous algae, and other algae groups, as previously explained. Mosses are the ideal substrate for this process, due to their numerous, branching stems covered with tiny leaves, creating a large surface area with a multitude of micro-spaces. Later, when the entire stem is covered in travertine, this serves as a reinforced structure upon which new travertine can grow in three dimensions. Travertine can also form on vascular plants (those with leaves, stems and roots), creating specific plant communities of the travertine barriers.

During the 20th century, many researchers recognised the important role of mosses and algae in the travertine-building process and the creation of travertine formations. The vegetation at the travertine barriers is the most visible part of the living world on these barriers, and in terms of biomass makes up its greatest part. The last systematic research on the macrophyte vegetation (mosses, vascular plants and algae) at the barriers was conducted in the mid-1950s. Zlatko Pavletić was the first to start systematic research on the flora, vegetation and ecology of mosses on travertine barriers. He performed detailed field studies on the moss flora, but also examined other vegetation and their ecological relationships. More than half a century later, the Croatian Association of Freshwater Ecologists conducted a new study in 2018, and gave the first comprehensive overview of the flora and vegetation of the travertine barriers, at all seven waterfalls on the Krka.

The results of this recent study showed that the flora and vegetation of the travertine barriers of the Krka River are well structured and have high species richness, indicating their natural state and high level of preservation. In comparison with the results of Pavletić’s research, it is evident that the waterfalls are more shaded by woody vegetation now, and that the surface area of the open waterfalls has been reduced.

The overgrowth of the barriers with microvegetation is a problem: vegetation changes the direction of river flow, slowing or halting the travertine building process, until the flow is completely stopped in certain areas. The roots of microvegetation also negatively affect the survival and development of the travertine barriers, as they erode the stability of the barriers themselves and cut off the flow of water as channels are closed up. This negative influence of microvegetation is particularly evident when invasive plant species appear, such as the Tree of Heaven (Ailanthus altissima). With the aim of ensuring the long-term, sustainable management of the travertine barriers at Skradinski Buk, in 2017, the Public Institute of Krka National Park started to implement the interdisciplinary scientific project “Managing and maintaining the macrovegetation at Skradinski buk – developing a multi-criteria model of sustainable management”.