How the structure and form of vegetation in a black locust (Robinia pseudoacacia L.) silvopastoral system influences tree growth, forage mass and its nutrient content

By using two thinning intensity regimes on a dense 16-year-old black locust (Robinia pseudoacacia L.) stand, two silvopastures were created: a medium density silvopasture (MDS) with 60% mean crown cover and a low density silvopasture (LDS) with 30% mean crown cover. An unthinned section of the stand with 90% mean crown cover was used as control (high density silvopasture: HDS). We measured the diameter at breast height (DBH) and tree height of black locust trees, in all three silvopastures, in 2011 (year of thinning), 2012 and 2013. In addition, we determined the forage production (in 2012 and 2013) and nutritive value (in 2013) of herbaceous vegetation and black locust shoots in the understory. The mean DBH for HDS, MDS and LDS increased from 7.6, 9.5 and 10.9 cm in 2011 to 8.6, 10.7 and 12.1 cm in 2013, respectively. The mean tree height also increased in the same period from 7.9, 9.1 and 9.8 m to 8.9, 10.1 and 10.6 m, for HDS, MDS and LDS, respectively. The mean herbage production was similar (P > 0.05) among silvopastures, although there was a tendency to be higher in the MDS and LDS (1866 and 1957 kg DM/ha, respectively) compared to HDS (1682 kg DM/ha). Additionally, browse from black locust shoots increased forage in MDS and LDS by 751 and 1201 kg DM/ha, respectively. Black locust browse had a higher overall nutritive value than herbage across silvopastures and months. In terms of crude protein, the content in black locust browse was on average twice as high, across sites and years, as that of herbage (239.4 vs. 104.1 g/kg DM; P < 0.05). While neutral detergent fiber and acid detergent fiber content of black locust browse were consistently lower (P < 0.05) than that of herbage, acid detergent lignin content of browse material was higher (P < 0.05) than that of herbage and averaged 94.7 and 75.4 g/kg DM, respectively. Overall, our results show that thinning resulted in higher values for both DBH and height of trees and increased forage in the understory by supplementing herbage with browse of high nutritive value.