Article: Compensatory plant growth as a response to hervibory
Author: McNaughton, S.J.
Journal: Oikos 40: 329-336 1983
The relationship between grasses and grazers is a complex one influenced by a number of biotic and abiotic factors. Some researchers and theorists have argued that grazing, on the whole, has a negative influence on plant biomass as a result of repeated defoliation.
Advocates of the grazing optimization hypothesis have argued for the opposite. One of the most prolific and well respected proponents of this hypothesis is behavioral ecologist S.J. McNaughton. He summarizes a principle tenet of the grazing optimization hypothesis thusly:
Providing there is an intervening period of growth, removal of vegetative tissues to a certain proportion of their initial level is rarely translated into a commensurate proportional reduction in the final yield of those or other plant tissues.
McNaughton does not base this assertion on simple conjecture, but on a wide body of empirical evidence, both from his own research and the research of others. Here are some excerpts from the research literature that support his claims:
…a 50% defoliation at the 2nd-4th leaf stage [in radishes] resulted in only an 8% reduction in final leaf area and that a 100% defoliation at this stage resulted in only a 42% reduction in final leaf area.
In an experiment in which cattle were stocked on Cynodon spp. at 7.5, 10, and 15 animals per hectare over a two year period, maximum yield of both forage and animal biomass occurred at the intermediate animal density.
Plant responses to herbivory and defoliation are certainly species specific, as some species have developed stronger compensatory mechanisms in the face of constant evolutionary pressure.
Additionally, plant responses may depend “on plant developmental stage at the time of defoliation.” Research on the effect of the Colorado potato beetle (Leptinotarsa decemlineata) on tuber yields demonstrates this fact:
…tuber yield is unaffected when defoliation occurs between the fourth and sixth week of growth. Prior to and after this, there are escape windows in time, and even defoliation levels approaching 100% have little effect of final yield.
Likewise, the defoliation of soybean had less effect on the yields of seed,
if it occurred during vegetative stages of growth than if it occurred during seed filling…Removal of half the foliage during vegetative stages reduced yield only about 10%, and 100% defoliation resulted in a yield reduction of less than 40%.
According to McNaughton, the mechanisms responsible for plant compensatory growth as a response to herbivory are manifold:
- Cytokinins promote cell division and the activation of meristems, promoting tillering in grasses as a response to herbivory
- Prevention of shading of leaves lower in the canopy can extend the lifetime of productive tissue
- Increased root-shoot ratio can improve plant water status
- Reduction of the transpiration surface conserves soil moisture and extends the growing season
- Reduced competition for substrates between reproductive tissues leads to larger seed size and nutrient content
- Hormones present in animal saliva may promote plant growth
- Associations between herbivores and mycorrhizae improve plant nutrient uptake
Finally, McNaughton concludes:
I do not contend that herbivory maximizes plant fitness, but that plants have the capacity to compensate for herbivory and may, at low levels of herbivory, overcompensate for damage so that fitness may be increased.