In the Karkonosze Mountains, where tens of thousands of tourists visit annually, common yarrow (Achillea millefolium) demonstrates an environmental dependency that may explain the differences between populations at various altitudes. Rather than simply surviving, the plant can alter its morphology, physiology, and even its genetic structure to adapt to the harsher conditions of the high-altitude environment.
It is well known that yarrow shows distinct variation as elevation increases: the content of photosynthetic pigments in its leaves decreases, ploidy levels (the number of chromosome sets) change, and a reduction in genome size is often observed. These changes occur both in natural populations and in synanthropic ones—those growing along hiking routes and near mountain shelters.
The presence of the plant at high altitudes is closely linked to human infrastructure. Trails and roads serve as corridors through which lowland species migrate upward. As a result, populations appear on slopes where they were previously absent, beginning to compete with the native alpine flora. Research shows that this process is particularly noticeable in the Karkonosze due to its extensive network of tourist routes—the national park features more than 150 hiking trails that are traversed by hundreds of thousands of visitors each year.
At high altitudes, yarrow populations are dominated by plants with a hexaploid set of chromosomes (2n = 54). This elevated ploidy level provides better adaptation to the cold, intense ultraviolet radiation, and the short growing season of high-altitude conditions. Hexaploids, which arose through evolutionary processes of hybridization and polyploidization, historically spread from Eurasia into temperate and even Arctic regions. Simultaneously, genome "downsizing" occurs, which likely reduces the organism's energy expenditure for DNA maintenance and replication under stressful conditions.
The decrease in chlorophyll in the leaves of high-altitude populations also appears to be an adaptive strategy. Fewer photosynthetic pigments mean less intensive light absorption, which can protect the plant from damage by the excessive ultraviolet radiation common at such heights. The plant essentially dampens its photosynthetic activity while maintaining sufficient capacity to survive in the extreme mountain climate.
Monitoring how species adapt to altitude and human-induced landscape changes helps to better plan the conservation of mountain ecosystems and predict which other plants might expand their ranges up mountain trails, displacing rare alpine species that are more vulnerable to competition and climatic shifts.
