Biomass allocation is a concept in plant biology which indicates the relative proportion of plant biomass present in the different organs of a plant. It can also be used for whole plant communities. Rationale Different organs of plants serve different functions. Leaves generally intercept light and fix carbon, roots take up water and nutrients, and stems and petioles display the leaves in a favourable position and transport various compounds within the plant. Depending on environmental conditions, plants may change their investment scheme, to make plants with relatively bigger root systems, or more leaves. This balance has been suggested to be a ‘functional equilibrium’, with plants that experience low water or nutrient supply investing more in roots, and plants growing under low light or CO2 conditions investing more in leaves or stems. Alternatively, it is also known as the 'balanced growth hypothesis', or the 'optimal partitioning theory'. Next to environmentally-induced changes, there are also inherent differences in biomass allocation between species, and changes that depend on the age or size of plants. Related concepts Biomass allocation is the end result of a number of processes which take place in the plant. It starts with the way how sugars are allocated over the different organs after having been fixed by the leaves in the process of photosynthesis (sugar allocation). Conceptually this is simple to envisage, but to quantify the flow of sugars is challenging and requires sophisticated machinery. For plants growing under steady state conditions, it is feasible to determine sugar-allocation by constructing a C-budget. This requires determination of the C-uptake by the whole plant during photosynthesis, and the C-losses of shoots and roots during respiration. Further C-losses may occur when sugars and other C-based compounds are exuded by the roots, or disappear as volatiles in the leaves. When these measurements are combined with growth measurements and the C-concentrations present in the biomass of leaves, stems and roots, C-budgets can be constructed from which sugar allocation is derived. These C-budgets are instructive, but require extensive measurements. A next level of analysis is to measure the growth allocation: what is the increase in total biomass of a plant, and to what extent is the increase due to growth of leaves, of stems and of roots. In young plants, growth allocation is often quite similar to the actual biomass allocation. But especially in trees, there may be a high yearly turnover in leaves and