Competitive and facilitative effects of shrubs on grass in forests and grasslands of Dry Chaco, Argentine

We performed a 4-year field experiment to determine the type of interaction between grasses and shrubs in two ecological sites from the Chaco region: forests, in the highland, and savannas, in the lowland. A removal experiment was used with the treatments: a) “only-shrubs”, all the grasses were cut; b) “only-grasses”, all the shrubs were cut and c) “control”, shrubs and grasses were maintained. 9 plots of 20 x 20m were placed in each ecological site (n = 3). Soil moisture (SM) was recorded (n = 2), at three depths and at the growth period end. Three 0,25 m ² subplots in “control” and in “only grasses” treatments were cut to determine ANPP, number of clumps (Cl) height (H), and basal area (BA) of the grasses considering year, site, and removal treatments. Relative Interaction Index RII was compared by site and year. Year (p < 0,0001), treatment (p = 0,0261), Year*Site (p = 0,0012) and Site*Treatment (p = 0,0265) had significant effects on grass aboveground net primary productivity (ANPP). In the forest, shrub exclusion had no modified ANPP and grass BA, diminished Cl, increased H and did not modify SM. In the grassland, the shrub exclusion increased grass ANPP, Cl, H and SM decreased from the only-grass plots to the only-shrub plots by approximately 19% (p < 0,016). Ecological site influenced RII (p < 0,0017); in the grassland RII =-0,26, indicating competition, while in the forest RII  = 0,07, indicating a facilitative or neutral effect of shrubs on grasses. These results could contribute to comprehending the changes in the dominance of different plant functional types under anthropic perturbation.