Dataset for manuscript: Invasive tree species affect terricolous bryophytes biomass and biodiversity in nutrient-poor but not nutrient-rich temperate forests

We conducted the study in managed forests of Western Poland (Fig. S1), characterized by the climate typical of a temperate zone, transitional between maritime and continental. The mean annual temperature in the study area was 8.5°C while mean annual precipitation ranged between 500 and 550 mm. To cover two environmental contexts we selected study plots (500 m2) on two forest types: nutrient-poor sites with Pinus sylvestris and nutrient-rich with Quercus spp. (Q. robur and Q. petraea; Fig. 4a). Nutrient-poor sites usually grew on rustic soils or podzols, and were covered by suboceanic coniferous forest Leucobryo-Pinetum or by secondary communities with P. sylvestris. Nutrient-rich sites usually occurred on luvisols, leptosols, or cambic soils, covered by oak-hornbeam forest Galio sylvatici-Caprinetum or secondary oak forests 45. We selected study plots in two age classes: near to rotation age (80-120 years for P. sylvestris and 100-140 years for Quercus spp.) and in the middle of rotation age (40-80 years). The stand age on our plots varied from 42 to 139 years old for Quercus spp. stands and from 42 to 117 years old for P. sylvestris stands. We aimed to cover the gradient of invasive species biomass. We decided to designate control plots per habitat type and age class (n=8 per variant, in total n=32; Fig. 4a), without studied invasive trees in shrub and stand layers. For each studied invasive tree species, age class, and habitat we searched eight plots with medium invader abundance (initially assessed by canopy cover &lt;30%), and eight with a medium abundance (&gt;50%). We selected only plots with homogenous understory vegetation and canopy cover, maintaining a distance of 5 km between plots of the same variant, to avoid spatial autocorrelation. After a field check of all criteria, we finally obtained a set of 160 study plots (Fig. 4b).Within each study plot, we measured the diameter at breast height of all tree species over 1.3 m height. We used these measurements for calculating the aboveground biomass of studied invasive tree species, using published and unpublished allometric models (<b>Table S4</b>) ^46–48. The invasion gradient in the stand with <i>P. serotina </i>on nutrient-poor sites ranged from 0.18 to 47.11 Mg ha^-1, with an average of 7.34±1.55 Mg ha^-1 (<b>Fig. 5</b>). After excluding the last observation it ranged from 0.18 to 15.43 Mg ha^-1, with an average of 6.05±0.89 Mg ha^-1. On nutrient-rich sites it ranged from 0.18 to 27.39 Mg ha^-1, with an average of 6.68±1.28 Mg ha^-1. Invasion gradient in the stand with <i>R. pseudoacacia </i>on nutrient-poor sites ranged from 0.22 to 153.00 Mg ha^-1, with an average of 20.91±5.60 Mg ha^-1, while on nutrient-rich sites it ranged from 0.82 to 278.24 Mg ha^-1, with an average of 50.77±12.44 Mg ha^-1.In each plot, we established four square subplots (25 m2 each), randomly distributed within a plot (Fig. 4b). Within subplots we surveyed terricolous bryophytes using a modified, nine-degree Braun-Blanquet’s scale. We surveyed only terricolous bryophytes, i.e. only those occurring on soil, excluding epiphytic, epilithic, and epixylic bryophytes to compare our results with previous studies e.g. 21,40. We surveyed bryophyte species composition in summer 2021, 2022, and 2023. In the same years, we studied species composition in summer, in spring, and in summer we collected four samples of bryophyte aboveground biomass. Within each plot, we systematically established four subplots, according to the cardinal direction in spring (north, east, south, and west), and we rotated this by 45° in summer (northeast, southeast, southwest, and northwest; Fig. 4b). We set subplots 6.31 m from the plot center (midpoint of the main plot radius), using a compass and measuring tape. In each of them, we placed a circular frame (d=58 cm, i.e., 0.264 m2) and we collected all plants within a frame, separating it into herbaceous plants and bryophytes. After that we transported material to the lab and using tweezers we cleaned bryophyte samples from vascular plants, animals, and litter. Then, we sieved samples to separate sand and small particles of litter. After cleaning we dried the material up to a constant mass in the oven (65°C) and we weighed it with an accuracy of 0.001 g. We decided to sample on two dates, as we simultaneously sampled understory herbaceous plants that differed in biomass production patterns between spring and summer 49. We decided to include data from both seasons and average it, as our sampling unit for biomass (0.264 m2) was smaller than vegetation survey plots (25 m2), to increase the stability of the results.