Genetic variation of drought-induced cavitation resistance among Pinus hartwegii populations from an altitudinal gradient
Résumé
Hydraulic failure can cause massive die-back of forest trees during drought. With extreme climatic events set to become more frequent and severe due to climatic change, it is essential to study resistance to water stress-induced cavitation. We investigated the genetic differentiation for cavitation resistance among Pinus hartwegii populations, the pine species growing at the treeline in M,xico. Open-pollinated seeds were collected from seven natural populations along an altitudinal gradient (3,150-3,650 masl) from Pico de Tancitaro, Michoacan, western M,xico. Seedlings were raised in a nursery and then established in a randomized complete block design in a common garden experiment. Resistance to cavitation (P (50), xylem pressure inducing 50 % loss of hydraulic conductance and S, slope of the vulnerability curve) and specific hydraulic conductivity (k (s)), were evaluated on branches of 5-year-old seedlings using the Cavitron technique. Mean P (50) was -3.42 +/- A 0.05 MPa, indicating that Pinus hartwegii is one of the more vulnerable pine species to cavitation. No significant genetic differentiation was detected between populations for cavitation resistance traits (P (50) and S), but a significant altitudinal cline was found for S. In contrast, k (s) exhibited a significant differentiation among populations and a significant decline with increasing altitude. The lack of genetic differentiation among P. hartwegii populations for cavitation resistance is likely to represent a limitation for adapting to the warmer and drier climates that are expected to occur in M,xico under climatic change. Finally, a worldwide comparison within the Pinus genus showed that pines growing at the treeline were on average more vulnerable to cavitation than those from lowland. This might reflect an adaptation to dry environmental conditions at low elevation.