Species richness of vascular plants and bryophytes in nine grassland sites (Europe and California collected in 2013-2016)

We sampled vascular plants (VP) and bryophytes (non-vascular plant; NVP) 1×1 m experimental plots in nine sites belonging to the Nutrient Network. Three sites were in California, two in Finland and UK and one in Germany and Switzerland. The data were collected to compare the responses of NVPs and VPs to nutrient addition and grazing exclusion treatments. The NVP and VP cover sampling was conducted in March-August 2016, except for heron.uk and rook.uk, which had been sampled for VPs in 2013. NVPs were mostly identified to species, but in absence of necessary diagnostic characters (capsules, other reproductive organs, distinctive gametophytic features), some specimens were identified at morphospecies group, subgenus, or genus level. We calculated three plant diversity indices for NVPs, VPs and total (NVPs and VPs combined) in each plot. First, species richness (S) is the number of species per 1 m2 for NVPs and VPs. For plots having no NVPs, NVP richness is zero. Second, for plots having at least one NVP, we calculated Inverse Simpson’s index of diversity (referred to as species diversity), which is equivalent to the Probability of Interspecific Encounter or Effective Number of Species (ENSPIE). Third, we calculated Simpson’s evenness (E = ENSPIE/S; referred to as evenness), which was expected to reflect changes in species’ dominance. We also sampled aboveground plant biomass at peak biomass of vascular plants (in May- August, depending on local site level characteristics) by clipping at ground level and removing all aboveground vegetation (live and dead) from two 0.1 × 1 m strips, sorting the current year’s VP and NVP biomass from the previous year’s biomass (dead litter), drying the biomass to a constant mass at 60 °C, and weighing it to the nearest 0.01 g. Except for two sites (heron.uk and rook.uk), we also measured photosynthetically active radiation (PAR) at the ground surface and above grassland canopy at time of peak biomass and calculated the proportion of transmitted light to ground level. The Nutrient Network (NutNet) is a distributed experiment designed to address consumer and nutrient controls of grassland productivity and diversity. The network coordinates collection of consistent plant community data from experiments being conducted identically across a broad range of sites spanning continents (http://www.nutnet.umn.edu/). The network’s standard field protocol includes sampling of vascular plant community data at species level from experimental plots every year. Two experiments were established at each site: (1) multiple nutrient addition (9 sites) and (2) herbivore exclusion by nutrient addition (8 sites). In each case, the experimental units were 5×5 m plots, and treatments were assigned as a randomized block design (3–5 blocks per site). The multiple nutrient experiment was composed of controls and factorial combinations of three treatments: nitrogen (N; 10 g N m-2 yr-1), phosphorus (P; 10 g P m-2 yr-1), and potassium with micronutrients (K+µ; 10 g K m-2 yr-1 combined with micronutrients). The N addition rate (10 g m-2) was chosen to overcome N limitation (Seabloom et al., 2021a). The N fertilizer was applied as granular time-released urea, P fertilizer as triple super phosphate and K fertilizer as potassium sulfate. The micronutrient treatment was applied only in the first year of treatments and was composed of boron (B; 0.1 g m-2), calcium (Ca; 6 g m-2), copper (Cu; 1 g m-2), iron (Fe; 17 g m-2), magnesium (Mg; 3 g m-2), manganese (Mn; 2.5 g m-2), molybdenum (Mo; 0.05 g m-2), sulphur (S; 12 g m-2), and zinc (Zn; 1 g m-2). The herbivore exclusion experiment was composed of controls and factorial combinations of two treatments: nutrient addition (fertilized) and herbivore exclusion (fenced). The fertilization treatment was the same as the all-nutrients-added treatment (NPK+µ) in the multiple nutrient experiment. The fences were designed to prevent entry by nonclimbing, medium to large herbivores. They were 110-230 cm tall, with the lower 80-90 cm surrounded by 1 cm mesh (except for site valm.ch where fences had 5 cm mesh size). The fence mesh was bent outward at the base in a 30 cm flange and stapled to the ground to prevent animals from digging under the fence (Borer et al., 2014). Borer, E. T., Harpole, W. S., Adler, P. B., Lind, E. M., Orrock, J. L., Seabloom, E. W., & Smith, M. D. (2014). Finding generality in ecology: a model for globally distributed experiments. Methods in Ecology and Evolution, 5(1), 65-73.