Carbon isotope discrimination and water relations of oak hybrid populations in southwestern Utah

The evergreen oak Quercus turbinella and the deciduous Q. gambelii form nat ural hybrids in southwestern Utah and northern Arizona. Hybrid individuals also are found in northern Utah in a region where only Q. gambelii currentl y exists, indicating that Q. turbinella has recently retreated southward. O ur objectives were to (1) examine the ecophysiology of parental taxa and hy brids under natural conditions in southeastern Utah, and (2) investigate th e level of integration between leaf carbon isotope discrimination (a synthe tic gas exchange trait) and structural and chemical traits of leaves in mor phologically variable hybrid populations. Leaf length, width, mass-to-area ratio (LMA, g m(-2)), and nitrogen concentration (N, g g(-1)) within 2 hybr id populations near New Harmony, Utah, were highly intercorrelated. Variati on within the hybrid populations spanned mean values for these traits obser ved in parental taxa from adjacent "pure populations of each species. Carbo n isotope discrimination (Delta), an integrated measure of the ratio of int ercellular to ambient CO2 concentration, ranged from 16.1 parts per thousan d to 19.6 parts per thousand within the 2 hybrid populations and was positi vely correlated with leaf nitrogen concentration and negatively correlated with LMA; individuals in hybrid populations with leaves resembling Q. gambe lii had the highest leaf Delta and N concentrations and lowest LMA compared with leaves from plants that resembled Q. turbinella. CO2 uptake is limite d by stomatal conductance and possibly by mesophyll resistance to a greater extent in Q. turbinella phenotypes than in intermediate or Q. gambelii phe notypes. delta D of stem xylem water (an indication of active rooting depth ) and predawn water potential during the peak monsoon period in August were not correlated to leaf Delta values within the hybrid populations. Several individuals that were morphologically similar to Q. turbinella in the hybr id populations maintained high predawn water potentials and derived moistur e from winter recharge that presumably was taken from deep soil layers. App arently, a few adult individuals of the Q. turbinella phenotype in hybrid p opulations accessed water from deep in the soil profile, which enabled them to avoid summer drought. Reduced monsoonal activity may have been an impor tant, but not the single, determinant of Q. turbinella's retreat from north ern Utah during the recent Holocene.