The influence of mineral nutrients, light, carbon dioxide, ozone, ammonia, water, temperature, soil texture and soil acidity on carbon allocation in trees is reviewed. The growth rhythms of the different plant parts on a seasonal basis are examined as well as the change in source-sink balance caused by plant age and genetic constitution. The exact outcome of all these factors on plant growth and carbon allocation is difficult to predict. However, one distinct pattern with regard to carbon allocation, and important for plant survival, becomes evident from this evaluation. Root growth is always decreased when plants become carbon limited, independently of whether this situation is caused by reduced photosynthesis (O-3, low light, or shortage of K, Mg or Mn) or competition between root growth and NH4+ as sinks for carbon skeletons (atmospheric NH3 and root uptake of NH4+). Low soil temperatures as well as competition from intensive shoot growth affect root development in a similar way. Inhibition of mycorrhizal development after exposure to O-3, NH3, and low availability of Mg can also be explained by the same mechanisms. Copyright (C) 1996 Elsevier Science Ltd.