Drought stress and fluctuating asymmetry in Quercus undulata leaves: confounding effects of absolute and relative amounts of stress?

One of the most general types of stress experienced by plants is water-limitation, which becomes particularly pronounced during periods of drought. We evaluated fluctuating asymmetry (FA) in Quercus undulata leaves for two subsequent dry years: 2001, when precipitation was 25% below average, and 2002, when precipitation was 65% below average, from a plot receiving ambient water and one in which water was excluded. In the first and less severe drought year, ambient-water trees had a slightly higher index of FA than the water-exclusion trees, contrary to expectations. However, in the second and much more extreme drought year, water-exclusion trees exhibited greater FA as expected, but in additional observations water-supplement trees exhibited by far the greatest amount of FA, contrary to expected. Further data on plant water potential confirmed that degree of plant stress corresponded to plot treatments: water exclusion>ambient water>water supplement. Stable carbon isotope ratios indicated that trees on the water-supplement plots were less stressed than ambient-water and water-exclusion trees, and leaf size was much greater for water-supplement trees than ambient-water or water-exclusion trees. We hypothesize that the complexity of the results could be due to the confounding effects of relative vs. absolute stress.