Quantifying winter forage resources for reindeer: Developing a method to estimate ground lichen cover and biomass at a local scale

• Maximising objectivity, our method estimates reindeer lichen cover and biomass. • Spatially balanced sampling using drone-collected data secures good estimates. • Choice of method for field-assessment of lichen cover greatly impacts the estimates. • The coefficient 0.0148 kg dm -3 was used to calculate lichen biomass from volume. • Our objective estimates of lichen abundance may aid reindeer land use management. Boreal forests serve as the primary winter range for reindeer ( Rangifer tarandus ) in Sweden, where ground lichens constitute the main food source. Lichen-rich forests have declined drastically, and modern forest practices, along with other land uses, impact both the quantity and availability of lichen. The resulting reduction in lichen has serious consequences for reindeer and Sami reindeer husbandry. Consequently, robust methods for mapping and measuring lichen are highly sought after. We developed and implemented a multistep method for objectively estimating lichen cover and biomass. We collected data at three study sites in lichen-rich pine forests in northern Sweden during July–August 2021. First, we collected data on NDVI and tree cover using a drone. These data informed a spatially balanced sampling approach to provide a distribution of plots for a representative field sample. Following this, we collected field data on lichen cover and height in these plots. In addition, we compared methods for assessing lichen cover in the field and found that visual estimates resulted in lower values than a point-intercept method. We recommend the point-intercept method for its objectivity and consistency between observers. By combining data from a literature review and fitting a linear regression on lichen volume (dm³ m⁻²) and biomass (kg dry weight m⁻²) through the origin, we determined a slope of 0.0148 for estimating biomass from volume measures. Using this relationship with field data on lichen cover and height, we obtained statistically unbiased estimates of lichen cover and biomass. This approach reduces the time required compared to destructive methods involving lichen collection and weighing. While our method provides lichen cover and biomass estimates, we also demonstrate how these biomass estimates can be linked to the number of reindeer grazing days an area can sustain each year, enhancing the usefulness of the results. Our estimates will be valuable in planning and management of reindeer husbandry, as well as for quantifying the loss of the ground lichen resource in forestry, energy and mining industries.