Adaptations and responses to drought in Quercus species of North America.

Most North American oaks (Quercus spp.) are adapted to drought-prone sites by an ability either to avoid, or to tolerate, water stress, or both. Generally, they have deep-penetrating root systems, enabling them to maintain relatively high predawn water potentials during drought. Oaks have thick leaves and some have relatively small stomata, both characteristics that favor high water use efficiency. However, some species, from warm regions, have large stomatal pores. The rapid evaporative cooling made possible by large stomata, may be an adaptation to high temperature. Some southeastern species display leaf curling during drought, and Q. douglasii a native of California is drought deciduous. Oaks have a ring-porous xylem anatomy, allowing rapid sap movement in large diameter, early-wood vessels when soil water is plentiful, and slower, but sustained, water movement in narrower, late-wood vessels, which are more resistant to cavitation, during drought. Oaks frequently maintain a higher rate of photosynthesis at low leaf water potentials and high vapor pressure deficits than co-occurring species of other genera. An exception is Quercus rubra, which is generally restricted to relatively mesic sites. During drought, many oak species, especially those native to arid regions, undergo changes in tissue osmotic potential. However, it remains to be shown whether such changes are phenological or drought induced. Reported values for bulk modulus of elasticity vary widely among species and studies, and have been observed to both increase and decrease during drought in a way that is unrelated to region or to changes in predawn water potential or osmotic potential. Diurnal leaf water potential during drought is probably a poor indicator of differences among oak species in gas exchange rate, because of interspecific variation in desiccation avoidance and tolerance.