Effects of grazing abandonment and climate change on mountain summits flora: a case study in the Tatra Mts

Changes in the local flora of mountains are often explained by climate warming, but changes in grazing regimes may also be important. The aim of this study was to evaluate whether the alpine flora on summits in the Tatra Mts, Poland and Slovakia, has changed over the last 100 years, and if the observed changes are better explained by changes in sheep grazing or climate. We resurveyed the flora of 14 mountain summits initially investigated in the years 1878–1948. We used ordination methods to quantify changes in species composition. We tested whether changes in plant species composition could be explained by cessation of grazing and climate change, and whether these factors have influenced shifts in Ellenberg’s plant ecological indicator values and Raunkiaer’s life forms. Changes in alpine flora were greater on lower elevation summits, and lower on summits less accessible for sheep. More accessible summits were associated with a decrease in mean values of plant species’ light ecological indicator values over time, and a concurrent increase in temperature and nitrogen ecological indicator values. No significant relationships were found between accessibility for sheep and changes in Raunkiaer’s life-forms. Greater accessibility for sheep (meaning high historical grazing pressure) led to greater compositional changes of mountain summits compared with summits with low accessibility. Our results suggest that cessation of sheep grazing was the main factor causing changes in the species composition of resurveyed mountain summits in the Tatra Mts, while climate change played a more minor role.