Drought response of two Mexican oak species, Quercus laceyi and Q. sideroxyla (Fagaceae), in relation to elevational position

To investigate the relationship between the altitudinal distribution of Quercus laceyi and Q. sideroxyla and their physiological responses to drought, we measured relative water content (RWC), water potentials (ψ _predawn and ψ_midday), photosynthesis (A _max), stomatal conductance (g), chlorophyll fluorescence (F _v/F_m), and spectral reflectance (400-1100 nm) five times during a 7 wk acute drought. Quercus laceyi was drought tolerant, while Q. sideroxyla was a drought avoider; Q. laceyi tolerated lower RWC (Q. sideroxyla=54%, Q. laceyi=44%), ψ_pd (Q. sideroxyla=-2.6 MPa, Q. laceyi=-3.3 MPa), and ψ_md (Q. sideroxyla=-4.5 MPa, Q. laceyi=-6.6 MPa). The F_v/F_m also declined first in Q. sideroxyla in wk 6, whereas F_v/F_m did not decline in Q. laceyi until wk 7. A_max and g fell in wk 4, 6, and 7 in drought seedlings of both species, suggesting a decline in CO₂ assimilation during the drought. Leaf spectral reflectance increased with time in response to decreases in leaf photosynthetic pigment concentrations in latter weeks of the drought. The results suggest a close association between the altitudinal distributions of these species and their adaptation to water stress.