Are plant growth-form-based classifications useful in predicting northern ecosystem carbon cycling feedbacks to climate change?

1. Plant species affect ecosystem carbon uptake via biomass production and carbon release via decomposition processes. Differences in their responses to climate change and effects on ecosystem carbon cycling processes may thus feedback to the atmospheric carbon balance and the climate at a global scale. Hierarchical species classifications based on plant growth forms are widely used in cold, northern biomes to generalize and predict these differences. This review investigates the usefulness of broad (vascular, non-vascular), intermediate (woody, non-woody) and narrow (evergreen shrubs, deciduous shrubs, graminoids, forbs) plant growth-form-based groups in these biomes for predicting plant responses to climate change and effects on the main processes of the full carbon cycle by looking at the similarity of species within growth-form groups and the consistency of differences among groups under changing environmental conditions. 2. Production responses to climate change differ between broad growth-form groups, but their opposite responses do not imply that the responses of non-vascular plants are consistently negative. Within vascular growth forms, production responses to climate change are not always similar among species under identical conditions, and average differences among narrow vascular growth forms are usually small. Moreover, differences in production responses among growth forms strongly depend upon the duration of the study, the region and the ecosystem type. 3. Species within narrow growth forms show a high similarity for a range of leaf litter chemistry variables and differences among narrow growth forms are often large and consistent. However, differences in leaf litter decomposability are large between broad growth-form groups, but small and environment-dependent among important narrow vascular growth forms. Litter feedback effects to plant production vary among narrow vascular growth forms, but the differences strongly depend on the study duration. Data on the climate dependence of growth-form differences regarding this aspect of the carbon cycle are currently lacking. 4. Synthesis: Overall, the usefulness of growth-form-based groups clearly differs between carbon cycling processes. Different aggregation levels are therefore needed for different processes. For most processes there is evidence that the differences among growth forms depend on environmental conditions, which hampers their use for generalizations and modelling. Future studies should therefore explicitly test for differences among growth-form groups and aim to unravel the dependence of growth-form differences on environmental conditions.