The effect of the expansion of the clonal grass Calamagrostis epigejos on the species turnover of a semi-arid grassland

Question: How does the dominance of Calamagrostis epigejos influence species turnover of a grassland? Location: Loess grassland at the foothills of Bukk Mountains, Hungary (47 degrees 54' N, 20 degrees 35' E). Methods: Presence/absence of vascular plants and different performance attributes of C. epigejos were recorded in a plot-subplot system between 2002 and 2005. Appearance and disappearance rates of grassland species were calculated for pairs of consecutive years. 1. Mean appearance and disappearance rates were compared in grassland plots dominated by C. epigejos and in plots free from this species, based on Monte Carlo randomization. 2. Mean appearance rates were assessed for categories of C. epigejos performance and their confidence intervals were calculated via Monte Carlo randomization. For two performance variables (percentage cover and shoot number) analyses were performed at two spatial scales. Results: 1. C. epigejos-dominated plots differed from unaffected ones by significantly lower appearance rates. 2. Change in appearance rates was best explained by differences in percentage cover of C. epigejos. Coarse-scale C. epigejos performance had a closer correspondence with appearance rate change than fine-scale performance. Low level C. epigejos performance enhanced appearance rate compared to intact stands, while high level performance decreased it, regardless of the choice of performance measure. Conclusions: C. epigejos lowers species number by hindering reappearance of species of the original grassland. This is best explained by the increased shading effect at the coarse scale. The marked non-linear initial enhancement in appearance rate, however, can also be taken as an early sign of future species loss.