The relationships of plant species occupancy to niches and traits vary with spatial scale

Aim Support for different underlying mechanisms of species occupancy is inconsistent, yet this could be related to spatial scale. Since abiotic filtering typically acts at broader scales than biotic interactions, we hypothesise that occupancy could be more driven by species' abiotic niche (i.e. tolerance and preference of abiotic conditions) at broad scales, whereas species' traits affecting competitive ability could be more important at fine scales. Here, we test these hypotheses by assessing relationships of occupancy to niche and trait metrics across spatial scales. Location Four study areas located north of Arctic Circle. Taxon Vascular plants. Methods We derived occupancy for 106 species at four spatial scales (micro-scale with plot size of 0.04 m2 and extent of 2 km, local-scale with plot size of 4 m2 and extent of 40 km, regional-scale with plot size of 4 ha and extent of 800 km, and polar-scale with plot size of 4 km2 and extent of 5200 km). We then assessed using generalized additive models whether the relationships between occupancy and species' niche breadth, niche marginality, intraspecific trait variability (ITV) and trait distinctiveness vary across the scales. Results At the finer scales, ITV (especially of specific leaf area) had the highest contribution with positive relationship in explaining occupancy. At the broader scales, occupancy was better explained by niche metrics. Especially at the broadest scale, the occupancy had a positive relationship with species' climatic tolerance. Main Conclusions Abiotic filtering, especially related to macro-climate, drives species occupancy at broader spatial scales while biotic interactions are relatively more important at local scales. This scale-dependency of factors behind species occupancy should be accounted for when, for example, planning conservation of rare species, forecasting invasions or anticipating the effects of changing climate on biota at local versus global scales.