What does differential consumption mean?
Second, differential consumption of plant growth forms can enforce changes in the composition of vegetation and thereby alter the type and arrangement of fuel.


This sentence means that herbivores eat more of certain plant growth forms than others. "Differential consumption" just means "different consumption." This can be important in the context of fire risk because herbivores will tend to not eat certain types of plant biomass, such as full-grown trees, but will eat new growth, grasses, etc (therefore the total amount of fuel by type is different).