Very few long-term studies have been made of single plant responses either. The impact of more Carbon dioxide (CO2) on plant growth and community interactions is difficult to assess because of the diverse pathways carbon can take within the plant and the community, as well as the tight coupling of carbon flow to water and nutrient fluxes. A powerful way to conceptualize these interactions is through models. Controls on photosynthetic rate include leaf conductance of CO2, temperature, light, CO2 concentration, and carbohydrate content of leaves and roots. In turn, leaf conductance is a function of light, CO2 levels, humidity, and leaf water potential. Plant productivity is controlled, in large part, by the allocation patterns. A particular allocation pattern is controlled by a combination of environmental and genetic factors. Carbon allocation is controlled by such factors as temperature, light, daylength, water, nutrients, carbohydrate levels, and plant age.