McNaughton SJ. Grazing as an Optimization Process: Grass-Ungulate Relationships in the Serengeti. The American Naturalist. 1979 May 1;113(5):691–703.
McNaughton lists 9 possible mechanisms by which productivity of herbivore affected plant tissues may be compensated or stimulated:
1. Increased photosynthetic rates in residual tissue;
2. Reallocation of substrates from elsewhere in the plant;
3. Mechanical removal of older tissues functioning at less than a maximum
4. Consequent increased light intensities upon potentially more active underlying
5. Reduction of the rate of leaf senescence, thus prolonging the active photosynthetic period of residual tissue;
6. Hormonal redistributions promoting cell division and elongation and activation of remaining meristems, thus resulting in more rapid leaf growth and promotion
7. Enhanced conservation of soil moisture by reduction of the transpiration
surface and reduction of mesophyll resistance relative to stomatal resistance;
8. Nutrient recycling from dung and urine;
9. Direct effects from growth promoting substrates in ruminant saliva
Theoretical work by Claire Mazancourt (1998 and 1999):
But how much difference can herbivory make when ecosystem ecologists so routinely ignore it when calculating NPP?
Relationship between average aboveground annual NPP and average annual precipitation for 100 ecological regions within the central grassland region of the USA . Sala,O. E., Parton, W. J., Joyce, L. A. and Lauenroth, W. K. (1988b) Primary production of the central grassland region of the United States. Ecology 69, 40-45. More info, including soil carbon relationships.
One possibility is that there isn't much wiggle-room, and even that is dependent on nutrient concentrations:
Cebrian J, Williams M, McClelland J, Valiela I. The dependence of heterotrophic consumption and C accumulation on autotrophic nutrient content in ecosystems. Ecology Letters. 1998;1(3):165–170.