Herbivory resistance traits in populations of Poa ligularis subjected to historically different sheep grazing pressure in Patagonia

Seeding selected populations with high grazing resistance may foster recovery of plant populations threatened by overgrazing. Resistance to grazing depends on grazing avoidance (escape from grazers) and grazing tolerance (ability to growth after defoliation). Many studies of grazing tolerance defoliate plants at a fixed height instead of removing the same proportion of biomass and therefore confound tolerance with avoidance. For this reason, the information on evolution of tolerance to defoliation at the intraspecific level is remarkably scarce despite the abundance of papers published that evaluate responses to defoliation. The estimation of the cost of tolerance is also troublesome because current methods usually include spurious correlations due to correlation between variables that share common terms. The objectives of this paper were to assess the intraspecific variation in tolerance and in traits associated with avoidance and growth in populations with different sheep grazing histories. We also estimated the percentage of biomass removed when the defoliation treatment was imposed at fixed height in order to separate tolerance and avoidance. Finally, we estimated the cost of tolerance using a new method proposed for spurious correlations. Results of a greenhouse experiment indicated no difference in tolerance among the three compared populations. However, the populations from overgrazed fields had more prostrate growth form, higher specific leaf area, and higher tillering rate (when no defoliated) than populations from exclosures. We confirmed that fixed height defoliation would have removed a higher proportion of shoot biomass from taller than from shorter individual plants, confounding grazing tolerance and avoidance. Regarding the cost of tolerance, we found no differences from a null model of no cost, indicating that the evolution (or future breeding) of more tolerant genotypes would not be constrained by this cost.