Questions: What is the significance of climate and land-use management asdeterminants of plant species composition of Scotland’s soft coasts, and how arethesedeterminantsspatiallyscaled?Whatistherelativecontributionofdifferentcommunity assembly processes in governing the coastal plant communities ofanimportantnationalresource?Location:Scotland,UK.Methods: We used national-scale survey data of Scotland’s soft coasts, and asubset representativeof machair grassland,a conservation priority habitat. Prin-cipal coordinates of neighbour matrices (PCNM), an eigenvector-based method,wasusedtoassessthespatialcomponentofenvironmentaldeterminantsatmul-tiple scales. The variation-partitioning framework was applied to unravel thescale-specific importance, relativetothe study design (Broad > 50 km, Meso 10–50 km,small + fine1.5–10.0 km)ofeachenvironmentalpredictorset.Results: Modelled environmental and spatial predictors captured ca. 20% ofthe variation for both response matrices. Management predictors captured sig-nificant proportions, identifying vegetation structure, proxies for grazing inten-sity and disturbance as important descriptors of patterns in species compositionfor both data sets. The spatial scale of management predictors was poorly cap-turedbymodelledPCNMvariables,suggestingspatiallydependentmanagementvariablesoperateatfinerspatialscalesthanthisstudydetected.Climatealsocap-tured significant, yet smaller fractions of variation compared to management.Potential evapotranspiration (PET) and humidity were identified as importantclimaticdeterminantsofspeciescompositionoperatingentirelyatthebroadspa-tial scale. Pure spatial fractions across all scales were significant (P 0.001) forbothdatasets, alluding tounmeasured,spatially structured environmental vari-ables suchassoilchemistryand/orexposureatlargerscales,andpotentiallybio-ticprocesssuchasseeddispersalatthefinestdetectablescale.Conclusions: The use of spatial PCNM variables within the variation-partition-ing framework is a valuable tool for dissecting scale-specific importance of envi-ronmental determinants of species composition. This study reveals importantclimatic and management determinants of Scotland’s soft coast and machairvegetation, and will help to better understand the relative scale at which theyoperate.IntroductionDefinedasthevariabilityinspeciescompositionfromplaceto place (Whittaker 1960, 1972; Legendre et al. 2005),interest in beta-diversity has increased with increasingrecognition of its pivotal role in identifying and under-standing diversity patterns at multiple scales (Soinenenet al. 2007). Mechanisms responsible for driving beta-diversity can be seen as either deterministic or stochasticprocesses; understanding the relative contributions of
