The role of climate in determining range margins is often studied using species distribution models (SDMs), which are easily applied but have well-known limitations, e.g. due to their correlative nature and colonization and extinction time lags. Transplant experiments can give more direct information on environmental effects, but often cover small spatial and temporal scales. We simultaneously applied an SDM using high-resolution spatial predictors and an integral projection (demographic) model based on a transplant experiment at 58 sites to examine the effects of microclimate, light and soil conditions on the distribution and performance of a forest herb, Lathyrus vernus, at its cold range margin in central Sweden. In the SDM, occurrences were strongly associated with warmer climates. In contrast, only weak effects of climate were detected in the transplant experiment, whereas effects of soil conditions and light dominated. The higher contribution of climate in the SDM is likely a result from its correlation with soil quality, forest type, and potentially historic land use, which were unaccounted for in the model. Predicted habitat suitability and population growth rate, yielded by the two approaches, were not correlated across the transplant sites. We argue that the ranking of site habitat suitability is probably more reliable in the transplant experiment than in the SDM because predictors in the former better describe understory conditions, but that ranking might vary among years, e.g. due to differences in climate. Our results suggest that L. vernus is limited by soil and light rather than directly by climate at its northern range edge, where conifers dominate forests and create suboptimal conditions of soil and canopy-penetrating light. A general implication of our study is that to better understand how climate change influences range dynamics, we should not only strive to improve existing approaches but also to use multiple approaches in concert.

We provide plant data and site data from a fully demographic transplant experiment. At the cold range margin of the forest herb Lathyrus vernus, we selected 58 forest sites in an area of 16 x 16 km to cover gradients of microclimate, soil moisture, canopy cover and forest type. At each of the sites, we transplanted six adults, ten juveniles and 60 seeds across an area of 5 x 5 m. Each adult was planted separately, whereas 2-3 juveniles or 15 seeds were planted together. Plantings were approximately 50 cm apart from each other, and the holes used for adults and juveniles were 25 cm deep and 5 - 8 dm³ in volume. Seeds were sown at the soil surface and covered with a thin layer of soil. Adult plants were protected from large herbivores with chicken wire cages.To further examine the effect of soil, half of the plants were placed in original site soil, and the other half in garden soil, the latter was expected to provide the better growing conditions compared to the original site soil.

Adult plants for the transplant experiment were raised from seeds from ten different populations in 2011 and 2012 and thereafter grown in a common garden. Seeds and seedlings originated from seeds collected in 2015 from a population in south-central Sweden (59°14'N, 17°16'E). Part of the intact seeds were used to raise seedlings in the greenhouse in 2016, and the remaining seeds were sown directly at our sites. The transplantation of adults and juveniles was carried out in April and May 2016. Seeds were sown in late July 2016, corresponding to the time of natural seed dispersal.

*** Plant data ***

In June 2016, we recorded size (stem diameter and length of shoots) for all transplanted individuals. In June 2017, we recorded survival, size, flowering status, number of fruits, number of seeds per fruit and number and size the of seedlings that had established from the seeds sown in the first year. Recruitment probability was defined as the proportion of seeds sown in the first year that germinated and survived as seedlings to the following year. Growth was defined as the change in log-transformed size between two years, and size was defined as the sum of the products of basal area and length for each shoot ((diameter)² * π/4 * length). The number of fruits per plant and the number of seeds in each fruit was counted for up to five fruits. If the plant had more than five fruits, the total number of seeds was estimated as the total number of fruits multiplied by the average number of seeds per fruit.

*** Site data ***

At each transplant site we took canopy cover photos after canopy closure (June 2016). Light was estimated by extracting the average percentage of white pixels from five binary canopy cover photos using the software ImageJ 1.50b. Soil moisture was measured in volume percent with a soil moisture meter (Delta-T Devices) at three locations per site during six dry days in September 2017. To adjust for differences among measurement days, moisture was also measured at a reference site each day. We used the adjusted average of the three measurements as a site value. We recorded temperature at each site every 3 hours with two iButton loggers (type DS1921G-F5 and DS1923, Maxim Integrated, San Jose, CA, USA), at 5 cm and 1 m height. Loggers were protected from direct sun and rain. Daily minimum (Tmin) and maximum temperatures (Tmax) were calculated from the average of the two loggers at each site. We extracted two complementary microclimate indices: spring growing degree days (GDD) that describes favourability of growing conditions, and spring freezing degree days (FDD) that describes harmful frost conditions.

The Growing Degree Day index corresponds to the sum of daily mean temperatures (Tmean) above a given threshold (here 5°C) following

GDD = ∑_i Tmean_i                with Tmean_i = 0.5 x (Tmin_i + Tmax_i)

and Tmean_i = 0 if Tmean_i < 5°C

The Freezing Degree Days index was calculated as the sum of daily minimum temperatures below 0°C following

FDD = ∑_i Tmin_i                    with Tmin_i = 0 if Tmin_i > 0°C

Both GDD and FDD were estimated for the period 1^st April – 31^st May when most of the shoot growth takes place.

Two csv-files provide the clean site data (one row = one site) and the plant data (one row = one individual) that were used for the demographic models. A README.txt explains abbreviations and details of the csv-files.