Plant growth model for respiration. The two state variables are substrate mass, M_CS, and structural mass, M_CX. There are four key parameters: Y_G, growth efficiency – fraction of substrate C utilized for growth which appears in structure – the remaining fraction (1 – Y_G) is respired; f_rec, fraction of senescing/degrading structural C recycled to substrate, the remainder becoming litter; k_S, rate constant for substrate utilization; and k_X senescence rate constant.