Data_Hartmann_Metz_2025_Productivity_moisture_competition_affecting_viability_of_Dactylorhiza_majalis_under_conservation

We studied 95 wet grassland sites across the Federal State of Brandenburg in NE-Germany . All sites harbored extant populations of D. majalis in our study years (86 sites) or at least until 5 years ago (9 sites). In order to characterize the vegetation and derive abiotic parameters, we conducted vegetation relevés in 3 – 6 plots of 2m × 2m in all sites during the main flowering period of D. majalis (mid-May – June). Across all 95 sites, we sampled in total 355 plots (3.7 plots per site on average), spread over three years for feasibility (2020 – 2022). In each 2m × 2m plot, all vascular plant species were identified to species level and their abundance was estimated using the scale of Braun-Blanquet (1964). As indicators of vegetation structure, we measured the average height of the vegetation (hereafter vegetation height), estimated the percent of ground covered by bryophytes (hereafter moss cover) and summed up the percent of all Poaceae species per 2m × 2m plot (hereafter grass cover). Moreover, we derived abiotic parameters from the vegetation relevés by calculating mean Ellenberg indicator values (EIV; weighted by the log-transformed cover per species) for moisture (F), nitrogen (N), and soil reaction (R) for each 2m × 2m plot (Ellenberg et al., 1992). Four proxies for population viability (hereafter viability proxy) were measured within the above described 2m × 2m plots: As abundance-based viability proxies, we counted the total number of i) juvenile and ii) flowering individuals of D. majalis per plot. As individual-based proxies of flowering individuals, we iii) counted their total number of flowers and iv) quantified their plant size. For the latter, we counted the leaves, measured the width and length of the first (subscript 1) and second (subscript 2) leaf closest to the ground, and quantified plant size as: leaf number × [(0.5 × leaf width1 × leaf length1) + (0.5 × leaf width2 × leaf length2)] (similar to Kindlmann and Balounová, 1999). Both individual-based proxies were quantified for and subsequently averaged across 5 (2021, 2022) or 10 (2020) randomly chosen flowering individuals per 2m x 2m plot, if available. If fewer flowering individuals occurred, all were measured. This resulted in a total of 1715 measured flowering individuals across 214 plots in 74 sites. As a fifth viability proxy, describing the long-term performance of D. majalis at the population level, we categorized whether population trends were positive, negative or had no trend (stable). For 60 sites, trend categories were calculated from long-term monitoring data of practitioners (1993 – 2020) with yearly counts of flowering D. majalis: (details in Hartmann and Metz, 2023). For 35 sites, the population trends (positive, negative, no trend) were based on expert judgements of practitioners that are familiar with the respective site for several years.