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The effect of tree density on competition between Leucaena leucocephala and Chloris gayana using a Nelder wheel trial: I. Above-ground interactions
Silvopastoral systems with leucaena (Leucaena leucocephala) and grass pastures are widely used for ruminant feeding in subtropical and tropical regions. Different densities and planting configurations of leucaena will influence relative yields of both species due to intra- and interspecific competition. With the aim to describe the effects of competition between leucaena and Rhodes grass (Chloris gayana), a Nelder wheel trial with 10 different leucaena tree densities (from 100 to 80,000 trees ha-1) growing with and without Rhodes grass was established in a subtropical environment at Gatton in southeast Queensland in November 2013. From 2014 to 2016, the biomass of leucaena (6 harvests) and Rhodes grass (7 harvests) was measured using allometric equations and the BOTANAL sampling procedure over 742 and 721 days respectively. At each harvest, all vegetative material of leucaena and Rhodes grass, over 1 and 0.1 m height respectively, were cut and removed from the experiment. No complementary or facilitative above-ground interactions were observed between the leucaena or Rhodes grass components of the pasture system. Increasing leucaena tree density resulted in greater above-ground intra- and interspecific competition. Individual leucaena tree yield was negatively related to the log10 of tree density (R2=0.99). Average maximum individual tree yield (38.9 kg DM tree-1 year-1) was reached at 100 trees ha-1 without grass competition, and was reduced by 60% with grass competition. Rhodes grass biomass yield was negatively affected by shading from the leucaena canopy, with negligible grass yield at tree densities ≥8,618 trees ha-1. Therefore, there was effectively no grass competition on individual tree yield at higher leucaena densities. Accordingly, edible leucaena biomass per unit area was positively related to the log10 of leucaena density (R2=0.99) regardless of grass competition, reaching 21.7 t DM ha-1 year-1 (2014-2015) and 27 t DM ha-1 year-1 (2015-2016) at the highest leucaena density of 80,000 trees ha-1. In contrast, the yield of Rhodes grass was linearly and inversely correlated with the log10 of tree density (R2=0.99). The leucaena component constituted 8.2% (1.4 t DM ha-1 year-1), 64% (8.4 t DM ha-1 year-1) and 94% (15.8 t ha-1 year-1) of total (leucaena + grass) edible biomass yield at tree densities of 210, 1,951 and 8,618 trees ha-1, respectively. Practical implications for the design and management of commercial leucaena/grass pastures are discussed.
CP17311 Accepted 21 December 2017
© CSIRO 2017