Differences in seedling field performance, water use efficiency, and root structure and function explains the distribution of four Iberian pines

Pedro Villar-Salvador, Robert Savé, Jorge Castro, Andrei Toca, Judit Maroto, Beatriz Grau, Paolo Zuccarini, Felicidad de Herralde, Juan Oliet

Background: The spatial segregation of pine species in the Iberian Peninsula is mainly related to the distribution and intensity of summer drought and low winter temperatures. Pinus sylvestris and P. uncinata, the mountain pines, concentrate their natural populations in the highest parts of the largest Iberian mountain ranges. In contrast, P. pinaster and especially P. halepensis, the low-land pines, tend to concentrate at lower altitudes than mountain pines. These differences in the distribution of natural populations of pines might be attributed to differences in their capacity to withstand drought and water use efficiency.

Objectives: We compared the seedling field survival and growth of Pinus sylvestris, P. uncinata, P. halepensis and P. pinaster). Moreover, we assessed the root growth and hydraulics and the water use efficiency (WUE) of pine seedlings under contrasting environmental conditions.

Methods: Field survival and shoot growth was compared in a common garden experiment in a lowland site with pronounced summer drought stress. Root growth and structure was measured in a rhizotron under two levels of water availability for 1 year. WUE was measured as the plant growth per unit of water transpired during a 2 month period and plants were subjected to two level of water availability by controlling soil water content.

Results: After the first summer, field survival (66%) and shoot growth of P. uncinata was the lowest while low-land pines, especially P. halepensis had the highest survival (>97%) and growth. Survival in P. sylvestris was 94% and its growth was higher than P. uncinata growth but lower than growth in remainder pines.P. pinaster and P. halepensis had similar WUE, which was significantly higher than P. uncinata and P. sylvestris. Differences among species occurred at both low and high water availability, but differences were greater under drought conditions.Maximum depth of root system, root growth was higher in Pinus halepensis and P. pinaster than in both mountain pines. In contrast, root hydraulic resistance measured at 20 and 10 ºC was 2-4 times lower in low-land pines than in the mountain pines.

Conclusions: This study suggests that the natural distribution of four Iberian pine species is related to their ecophysiological patterns at seedling stage, mainly to water use efficiency under drought conditions.