Background/Question/Methods Large-scale tree mortality has the potential to drive rapid and large-scale shifts in vegetation under changing climates. To understand the factors that underlie species differences in canopy dieback under drought, we studied the effect of an extreme drought on temperate deciduous trees and shrubs to an extreme drought in the southeastern USA. We measured leaf desiccation, native embolism, stomatal conductance, photoinhibition, and predawn and midday water potential at four sites with varying drought intensities.
Results/ConclusionsAt the two sites most strongly affected by drought, there was 7% stem death, with remaining stems exhibiting extensive wilting and leaf desiccation, low midday water potential (mean = -3.7 MPa) and extensive embolism of xylem  (mean = 69% loss of conductivity; PLC).  However there were large differences among species. Wood density was strongly and positively correlated with native PLC across species (r² = 0.54, n=20 species), contrary to the known tendency for dense wood to be resistant to cavitation.  This counterintuitive result is best explained by differences in drought avoidance behavior among species.  Drought deciduousness was associated with low wood  density; of the six species with the lowest wood density, five exhibited partial drought deciduousness, while none of the nine species with the densest wood exhibited this behavior.  There was also a strong negative correlation between wood density and the sensitivity of stomata to water potential, whereby species with low wood density exhibited tighter stomatal control.  Consistent with these differences, species with dense wood exhibited the largest declines in water potential during drought, with anisohydric behavior that allowed plant water potential to drop below levels at which embolism reached critical levels. These results indicate that vulnerability to embolism may alone result in misleading predictions of drought response, particularly under conditions of extremely low vapor pressure deficit.