Will plant vigor and tolerance be genetically correlated? Effects of intrinsic growth rate and self-limitation on regrowth

Plants are known to maintain fitness despite herbivore attack by a variety of damage-induced mechanisms. These mechanisms are said to confer tolerance , which can be measured as the slope of fitness over the proportion of plan t biomass removed by herbivore damage. It was recently supposed by Stowe et al. (2000) that another plant property, general vigor, has little effect o n tolerance. We developed simple models of annual monocarpic plants to dete rmine if a genetic change in components of growth vigor will also change th e fitness reaction to damage. We examined the impact of intrinsic growth ra te on the tolerance reaction norm slope assuming plants grow geometrically, i.e., without self-limitation. In this case an increase in intrinsic growt h rate decreases tolerance (the reaction norm slope becomes more negative). A logistic growth model was used to examine the impact of self-limiting gr owth on the relationship between intrinsic growth rate and the tolerance re action norm slope. With self-limitation, the relationship is sensitive to t he timing of attack. When attack is early and there is time for regrowth, i ncreasing growth rate increases tolerance (slope becomes less negative). Th e time limitations imposed by late attack prevent appreciable regrowth and induce a negative relationship between growth rate and tolerance. In neithe r of these simple cases will the correlation between vigor and tolerance co nstrain selection on either trait. However, a positive correlation between growth rate and self-limitation will favor fast growth/strong self-limitati on in a high-damage environment, but slow growth/weak self-limitation in a low-damage environment. Thus, fundamental growth rules that determine vigor have constitutive effects on tolerance. The net costs and benefits of dama ge-induced tolerance mechanisms will thus be influenced by the background i mposed by fundamental growth rules.