As reported previously, during the year 2000 (a drought year following a dry winter) the leaves of post oak (Quercus stellata) suffered little herbivory, and had high levels of toxicity (as determined by a bioassay with brine shrimp Artemia salina) that declined through the summer. During the year 2001 (a relatively moist growing season following a wet winter), however, leaves of the same trees experienced greater herbivory and had relatively low levels of toxicity throughout the summer. These results indicate that plant-herbivore relations can differ markedly between years. During the moist year, the trees had sufficient resources for growth and presumably would not have benefited from allocation to herbivore defense. At the same time, the leaves, with presumably greater water content, would have been more attractive to herbivores.

Key words: bioassay, Herbivory, Quercus stellata, temporal variability