The Sámi reindeer pastoralist system and climate change: the effects of weather, forage and insect harassment on calf autumn body mass and summer growth

In northern Europe, the indigenous Sámi reindeer herders rely on natural pastures for their reindeer (Rangifer tarandus tarandus), continuously adapting to the changing environment and available resources. Long-term studies are essential to understand the effects of climate change on these systems. In the 1980s, the Ruvhten sijte herding community in Sweden began implementing selective slaughter strategies focused on female reproduction and calf condition, documenting reproductive success and calf body mass. This dataset (1986-2021) provides a unique resource for examining how changing environmental conditions during the vegetation growing season influence calves' condition in autumn. Using autumn body mass and daily growth from July to November as response variables, we fitted two separate linear mixed-effects models to understand the influence of maximum temperature, consecutive days with temperatures above 20 degrees C, precipitation, insect harassment, start of the vegetation growing season, and forage availability, throughout this period with known climate warming. To control potential cofounding effects, we included the impact of the mother having a calf recorded in the preceding year at calf marking, the winter herd size, and the mother's identity and year as random effects. Results showed that a 2.5-degree increase in maximum temperature decreases calf body mass by 1 kg. At the same time, number of consecutive days with temperatures above 20 degrees C and total precipitation in summer had a positive impact on body mass, possibly mediated through improved forage production. Other environmental variables and herd size were non-significant. Moreover, our results suggested a negative effect of the mother's condition on calf autumn body mass, reflecting the consequences of the environmental conditions experienced by the mother the preceding summer and winter, while the effect on summer growth was not significant. Our analysis reveals the effects of the summer conditions on the S & aacute;mi reindeer husbandry system and its vulnerability under steadily increasing temperatures.