Infer what a negative value of change in shoot biomass indicates

The correct answer would be that the mass of the shoot is reduced. This might be the consequence of predation (if there is an increase in predator population) or death (caused by decreases in food and other life resources) which reduce the population of the shoot more than its growing rate.

Biomass is the total mass of a specific organism in the area and it is a renewable source of energy (contains stored energy from the sun).