Ability to avoid water stress in seedlings of two oak species is lower in a dense forest understory than in a medium canopy gap

The ecophysiological response to summer drought in seedlings of two co-occurring oak species (Quercus petraea and Quercus pyrenaica) outplanted below a dense Pinus sylvestris stand and a medium canopy gap was examined in two experiments carried out in central mountains of the Iberian Peninsula (Spain). Leaf water relations and gas exchange were studied in the first experiment. For both species, lower pre-dawn leaf water potential (Ψpd) evidenced a higher degree of water stress in the understory seedlings, even though soil moisture was similar in both sites. Rates of photosynthesis (Asat) and stomatal conductance (gsat) at saturating light were higher in the gap seedlings in all measuring dates, but the earlier and more pronounced stress imposed in the understory precluded the comparison of tolerance responses between sites. Q. pyrenaica showed always higher Asat and gsat than Q. petraea, independently of Ψpd, which did not differ significantly between species. At the end of summer, values of Asat/gsat and leaf carbon isotopic composition reflected a less efficient water use in Q. pyrenaica. In the second experiment, growth and root development were examined in a different set of seedlings planted in the same sites. Q. petraea allocated less biomass to roots, and attained 20% higher total plant dry mass than Q. pyrenaica at the end of the second growing season in the field. For both species total plant dry mass and coarse plus fine root dry mass reached at this time were approximately three-fold higher in the gap than in the understory. Poor root development could explain the more limited access to water of seedlings outplanted in the understory. This study reveals that seedlings of Q. petraea and Q. pyrenaica planted under a dense, mature pine stand are more susceptible to summer drought and present a delay in growth with respect to a medium canopy gap.