Tropical forests are essential components of the earth system and play a critical role for land-surface feed backs to climate change. These forests are currently experiencing large-scale structural changes of which the most apparent may be the increase in liana abundance and biomass. The first study that documented liana proliferation in the Neotropics (1) was initially debated but later confirmed by multiple other studies (2). The consensus on liana proliferation led to speculations that this phenomenon could potentially have large impacts on the carbon cycle of tropical forests. But experimental proof for such speculations was lacking until recently.In their recent study van der Heijden et al. (3) show for the first time that lianas substantially reduce tropical forest carbon storage. The study is based on a well-developed liana-removal field experiment.The experiment was performed over a limited period(3 y) in an area with particular high liana abundance in Panama and therefore still needs to be confirmed by long-term studies elsewhere in the tropics. Nevertheless the results are striking. By comparing natural forests with forests where lianas are artificially removedthe authors found that forests with lianas reached only 24% of their carbon sink potential compared with liana-free forests. Lianas were shown to reduce tree growth and recruitment to increase tree mortalityand to shift carbon allocation to leaves rather than woody tissue. Based on this study (3) it is now clear that liana proliferation has a potential high impact on the future carbon cycle of tropical forests. We speculate that this impact might even be strengthened by the long-term impact lianas have on forest species composition and demography. In this context it is striking that currently no single global vegetation model accounts for lianas. State-of-the-art global vegetation models do have problems to realistically simulate the carbon cycle of tropical forest (4). A major source of uncertainty in global vegetation models is their poor representation of demographic processes (5). We are convinced that modeling the ecosystem demography in tropical forests is only possible by accounting for lianas. We therefore suggest that the next generation of global vegetation models should include lianas at least in an implicit way for example by including liana load as a climate- and disturbance-dependent property of a tropical forest plant functional type. Moreover for models aiming to simulate vegetation demography in detail we suggest explicitly including lianas as a functional group. We are convinced that as long as lianas are neglected vegetation models will not be able to simulate the future of the tropical forest carbon cycle in a realistic way. We admit that comprehensive data sets available for tropical lianas are currently restricted to a few areas and that this hampers model development. Nevertheless we think it is the right time now to start developing the concepts of vegetation models that do account for lianas. The process of developing such models will help us to improve our understanding of tropical forest functioning and it will guide the design of future field and experimental studies on lianas.