Traditional plant functional groups explain variation in economic but not size-related traits across the tundra biome

Aim: Plant functional groups are widely used in community ecologyand earth systemmodelling to describe traitvariation within and across plant communities. However, this approach rests on the assumption that functional groups explain a large proportion of traitvariation among species. We test whether four commonly used plantfunctional groups represent variation in six ecologically important plant traits. Location: Tundra biome. Time period: Data collected between 1964 and 2016. Major taxa studied: 295 tundravascular plant species. Methods: We compiled a database of six plant traits(plant height, leaf area, specific leaf area, leaf dry matter content, leaf nitrogen, seed mass) for tundraspecies. We examined the variation in species-level traitexpression explained by four traditional functional groups (evergreen shrubs, deciduous shrubs, graminoids, forbs), and whether variation explained was dependent upon the traitsincluded in analysis. We further compared the explanatory power and species composition of functional groups to alternativeclassifications generatedusing post hoc clustering of species-level traits. Results: Traditional functional groups explained significant differences in traitexpression, particularly amongst traitsassociated with resource economics, which were consistent across sites and at the biomescale. However, functional groups explained 19% of overall traitvariation and poorly represented differences in traitsassociated with plant size. Post hoc classification of species did not correspond well with traditional functional groups, and explained twice as much variation in species-level traitexpression. Main conclusions: Traditional functional groups only coarsely represent variation in well-measured traitswithin tundraplant communities, and better explain resource economic traitsthan size-related traits. We recommend caution when using functional group approaches to predict tundraecosystem change, or ecosystem functions relating to plant size, such as albedo or carbon storage. We argue that alternative classifications or direct use of specific plant traitscould provide new insight into ecological prediction and modelling.