Warm-climate grasslands can be degraded by overgrazing and reduced soil fertility. However, legume trees integrated into these systems (silvopasture) can provide long-term marketable wood for sale and add N to the system. In addition, tree legumes can improve livestock diet by providing high crude protein forage. Our research assessed biomass and N accumulation by tree legumes: gliricidia [ Gliricidia sepium (Jacq.) Kunthe] and sabia ( Mimosa caesalpiniifolia Benth.) grown in conventionally grazed signal grass ( Brachiaria decumbens Stapf) pasture. The seedlings were planted in 2008 and growth rates were measured in 2012 and 2013. One year after the seedlings were planted, in July 2009, the pastures were grazed. Aboveground biomass doubled from 25 to 50 Mg ha -1 between February 2012 and August 2013. The thickest branches contributed the most ( p?0.05) biomass: 58% for gliricidia and 54% for sabia. Leaves represented the smallest ( p?0.05) fraction: 7 to 13% for gliricidia and 4 to 14% for sabia. Leaf and branch nutrient concentrations varied little ( p>0.05) between species and sampling periods. Gliricidia leaf N ranged from 33.6 to 38.0 g kg -1, while sabia leaf N ranged from 26.9 to 38.5 g kg -1. Biologically fixed N in leaves ranged from 30 to 121 kg ha -1. Sabia branches had less moisture and greater lignin, density, and gross calorific power than gliricidia. While thicker branches represent most of the aboveground tree biomass, leaves and thin branches have greater N concentration, representing an important return pathway to the soil.
