Among-population variation and plasticity to drought of Atlantic,Mediterranean, and interprovenance hybrid populations of maritime pine

Maritime pine grows naturally under a wide range of climatic conditions, from strongly Atlantic to strongly Mediterranean. Aiming to improve our understanding of the genetic structure and inheritance of drought resistance strategies in the species, we conducted an environmentally controlled experiment to assess the genetic variation and plasticity to drought of Atlantic and Mediterranean populations, and the interprovenance hybrids between them. Hybridization could also help to provide new genetic material for use in transitional areas between the two regions, for which reproductive materials of good quality are generally lacking. Plastic responses to water stress appeared to be highly conserved among populations, with a common conservative isohydric strategy based on promoting growth when water was abundant, and stopping it when water became limiting. We found, however, a strong intraspecific variation in biomass allocation patterns. The Atlantic populations showed a risky growth-based strategy with a larger amount of juvenile needles, whereas Mediterranean populations showed a more conservative strategy, minimizing aerial growth and increasing the proportion of adult needles that is more resistant to water loss. Hybrid populations performed more similarly to the Mediterranean parent, suggesting a dominance of the Mediterranean-like characteristics. Some of the tested hybrid populations, however, combined high growth with traits of drought adaptation, and thus represent potentially interesting materials for use in transitional regions between the two climate zones.