A biomass/yield model for high-density Acacia nilotica plantations in Sind, Pakistan

Abstract

A yield model for irrigated, high-density plantations of Acacia nilotica in Pakistan was constructed from data collected on 65 temporary plots, distributed across the existing range in age and stand density. Plantation development is simulated by projecting changes in quadratic mean diameter and number of stems ha −1. Stands are assumed to asymptotically approach an empirical size/density limit by a self-thinning process. Rates of self-thinning and change in mean diameter are controlled by rate of height growth. Parameters of the Weibull probability density function are recovered from whole-stand attributes to facilitate estimation of diameter distributions. Model output includes the biomass of four tree fractions, defined by diameter outside bark ( D ob): fodder (leaves and succulent twigs), fine woody material (<2.0 cm D ob), fuelwood (2.0–6.5 cm D ob), and pit props (6.5–17.0 cm D ob). Pit-prop-grade distribution is estimated from stem diameter distribution and a rule defining the grades of pit props expected from a tree of given diameter. Simulation results and plot data indicate that these Acacia plantations can produce up to 40 t dry-weight of total above-ground biomass ha −1 year −1, rivaling some of the most productive forest systems in the world.