Short-rotation biomass trial of mixed and pure stands of nitrogen-fixing trees and<i>Eucalyptus grandis</i>

Abstract

Summary Mixtures with nitrogen-fixing tree species may alleviate the need for N fertilizer in Eucalyptus plantations, but questions remain as to how growth and yield are affected. In this five-year trial grown at sea level, Eucalyptus grandis was planted alone and in combination with an N-fixing species; either a Leucaena hybrid, Paraserianthes falcataria, or Enterolobium cyclocarpum, by alternating tree types within rows. The N-fixing species were also grown in pure stands. All trees were initially coppiced after the first year and then harvested on one, two and four-year rotations. Mean annual increments (MAI) at one, two and four-year rotations averaged 8.2, 15.6, and 15.5 Mg ha−1 yr−1 total biomass dry matter, respectively. Several mixed plantings equalled or exceeded the pure Eucalyptus plots in yield. Two-year old P. falcataria plots had the highest MAI at 21.6 Mg ha−1 yr−1. Stem diameter (DBH) was the best predictor of tree biomass, and height explained significant additional variation only for leucaena. Separate allometric equations were developed for each Eucalyptus treatment to account for differences in height, DBH, leaf biomass, and leaf+stem biomass. Separate equations to predict biomass were required because of differences in Eucalyptus morphology related to inter-specific competition with E. cyclocarpum. It is concluded that mixtures of Eucalyptus with N-fixing trees can equal biomass yields of pure Eucalyptus.