Ehleringer, J.R., and S.L. Phillips. 1996. Ecophysiological factors contributing to the distributions of several Quercus species in the intermountain west. Ann For. Sci. 53:291-302.

Aspects of the water relations of three oak species (Quercus gambelii, Q. turbinella and Q. macrocarpa) and their hybrids (Q. gambelii x turbinella, Q. gambelii x macrocarpa) were observed under common garden conditions in northern Utah, USA. In the absence of summer moisture inputs, Q. macrocarpa and Q. turbinella were unable to maintain active gas exchange through the day; following an early morning peak, leaf conductances to water vapor remained very low through the remainder of the day. In contrast, Q. gambelii and the hybrids were able to maintain high leaf conductances throughout this period. Consistent with these observations, Q. gambelii is thought to have a root system penetrating to the deeper, winter-recharged layers, a feature apparently absent in both Q. macrocarpa or Q. turbinella. Based on current hybrid distributions, both Q. turbinella and Q. macrocarpa once extended into drier more northerly regions than they occupy at present. When these parents retreated, they left behind hybrids with Q. gambelii, which do not depend on monsoonal moisture input. Leaf size, leaf longevity, carbon isotope ration, and minimum winter temperatures appear not to be correlated with the absence of Q. macrocarpa and Q. turbinella from summer-dry habitats. Instead it appears that reliance on summer monsoon events is one of the critical factors influencing loss of these oaks from summer-dry sites in the intermountain west.