Tundra shrub expansion in a warming climate and the influence of data type on models of habitat suitability

Warming across the low Arctic is increasing tundra vegetation productivity and facilitating the expansion of upright shrubs. We modeled the effects of warming on habitat suitability in green alder, dwarf birch, Labrador tea, bog bilberry, and lingonberry and assessed the influence of data type (true absence or pseudo-absence) on species distribution models (SDMs). We generated SDMs using the two absence data types under current (1970-2000) and future (2061-2080) climate projections. Our results show that warming leads to range expansion of all shrubs, though responses vary in magnitude and extent, with mean increases in suitability ranging from 0.080 (Labrador tea) to 0.369 (lingonberry) with true absences. Differences in driving variables and suitability projections suggest that physiological and ecological variability between species mediate responses to warming. Between data types, we observed inconsistencies in model performance, suitability projections, and variable importance. Bog bilberry and lingonberry exhibited larger differences in suitability (0.201 and 0.288, respectively), whereas alder showed similar responses (difference of 0.01). These results are important to consider when assessing changes in habitat suitability or identifying environmental or climatic determinants of species' distributions. We suggest further development of open data repositories, facilitating access to true absence data to support conservation and land use planning.