Contrasting responses facing peak drought in seedlings of two co-occurring oak species
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To evaluate leaf physiological mechanisms involved in plant tolerance to water deficit between sites of varying overstorey density, we investigated leaf water relations of two broadleaved oaks (Quercus petraea (Matt.) Liebl. and Quercus pyrenaica Willd.) planted within a dense Scots pine stand, in a thinned adjacent area and in a nearby gap at a Mediterranean, mid-mountain field site. Leaf water parameters were estimated in established seedlings at the end of June and August over two consecutive years by measuring pressure-volume relationships with the pressure chamber technique. Plant water status was always similar in both species, and it was lower in August than June due to lower soil moisture at the end of summer. Higher light and diurnal water deficits in the gap were associated to more negative osmotic potentials. Impacts of overstorey density on some leaf water parameters depended on the species. While Q. pyrenaica showed active osmotic adjustment across sites, seedlings of Q. petraea were unable to cope with increased water deficit by osmotic adjustment in the unthinned dense area. While Q. pyrenaica showed consistent increases in the maximum bulk modulus of elasticity (epsilon(max)) from June to August across sites, epsilon(max) decreased in the unthinned dense area for seedlings of Q. petraea. These results could reflect distinct species strategies to cope with water deficit under the low-light conditions created by a dense overstorey. Higher leaf osmotic adjustment and bulk modulus of elasticity in Q. pyrenaica seedlings at peak summer drought might confer on them a competitive advantage during establishment in dry sub-Mediterranean understories. Opening of moderate canopy gaps in dense Scots pine stands improves some leaf mechanisms involved in drought tolerance in oak seedlings.
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