1. Do species and functional trait diversity respond similarly to deterministic and random processes? Theory predicts that the contributions of random and deterministic processes to species diversity depend on patch size. Smaller patches are more strongly influenced by random sampling effects, by having fewer individuals, as well as ecological drift, which propagates the effects of small samples through stochastic birth and death processes. These random processes decrease species richness and increase compositional differences among small patches. Larger patches are predicted to be more deterministically assembled, with greater species richness and greater predictability of composition for a particular environment. The consequences of patch size for the diversity of functional traits, however, are poorly understood. Species diversity may be a poor proxy for functional diversity due to trait redundancies among species, making it unclear how random and deterministic processes alter functional diversity within patches of differing size, and how these differences scale up to determine among-patch functional diversity. 2. We report a novel experimental study of species and functional diversity across spatial scales. We manipulated patch area in an experimental plant metacommunity and used a nested sampling design to distinguish the effects of deterministic processes, ecological drift, and sampling effects on species and functional trait diversity. 3. Our study revealed a pervasive influence of ecological drift and sampling effects on diversity, with distinct influences on functional traits and species composition within and among patches. Overall, drift and sampling effects caused a 2- to 3-fold decrease in the importance of deterministic processes in small fragments. Species and functional diversity showed similar patterns with patch size: larger patches had greater within-patch (alpha) diversity and lower among-patch (beta) diversity, consistent with theory. However, our nested sampling design revealed that sampling effects (i.e. the size of the sample area) largely determined alpha species diversity and beta functional diversity, while ecological drift had a stronger influence on alpha functional diversity and beta species diversity. 4. Synthesis. The compositions of species and functional diversity in a community are influenced by distinct processes, resulting in divergent spatial scaling of species and their traits within patches and across landscapes.