Testing the Plant Growth-Defense Hypothesis Belowground: Do Faster-Growing Herbaceous Plant Species Suffer More Negative Effects from Soil Biota than Slower-Growing Ones?

According to the growth-defense hypothesis in ecology, faster-growing plant species should suffer more from herbivores and pathogens than slower-growing species. Tests of this hypothesis have focused on aboveground plant tissues, herbivores, and pathogens; however, it should also apply to root defense. To test whether faster-growing species suffer more negatively from soil biota than slower-growing species, we estimated first-season growth rates of 34 herbaceous plant species and used weighted linear regressions to assess the relationship between growth rates and responses to being grown in sterilized versus unsterilized soil (biotic soil effects) and to growing in soil previously occupied by conspecifics versus a mixture of species (conspecific soil effects). We found a negative relationship between relative growth rate and biotic soil effects, with slower-growing species tending to suffer less or even benefit from the presence of soil biota, while faster-growing species were more negatively affected. Biotic soil effects were also negatively related to size-corrected growth rates. These relationships remained negative after accounting for influential species, but a large amount of variation remained unexplained. Moreover, there was no clear relationship between growth rates and conspecific soil effects. A simple relationship between growth and defense aboveground may not be so clearly reflected belowground because of the many interacting antagonistic and mutualistic organisms likely involved.