Drought resistance of 2-year-old saplings of Mediterranean forest trees in the field: relations between water relations, hydraulics and productivity

Drought resistance was examined for 2-year-old saplings of Ceratonia siliqua L., Olea oleaster Hoffmgg. et Link., Quercus suber L. and Q. pubescens Willd. growing in the field in Sicily, with the aim of testing their possible use in the reforestation of degraded areas. To this purpose, leaf conductance to water vapour (g L), transpiration rate (E L), relative water content (RWC) and water potential ((psi)L) were measured between pre-dawn and sunset, monthly from May to November. Parallel measurements of loss of hydraulic conductance of twigs of the current year (PLC) were made together with an estimate of whole-plant hydraulic conductance (K PLANT) on the basis of the ratio of maximum E L to ((psi)PD-(psi) MIN), where (psi)PD is pre-dawn (psi)L and (psi)MIN is the minimum diurnal (psi)L. C. siliqua saplings maintained high g L throughout the study period with high RWC (over 90%) and (psi)L. They grew rapidly and increased their foliage area (A L) by over 60% from May to winter rest. This was accompanied by low twig PLC (about 20% in September) resulting in high K PLANT all through the study period. In contrast, O. oleaster saplings underwent distinct dehydration in July, i.e. they showed stomatal closure due to a drop in RWC (to 75%) and (psi)L (to the turgor-loss point). This was apparently due to twig cavitation, resulting in a strong decrease of K PLANT. Plants, however, tolerated summer drought, and showed growth (A L increased by 15% from May to November). Saplings of Q. suber and Q. pubescens were much more vulnerable to twig cavitation (PLC was 35–48% from June to November) than the other two species, and their average K PLANT was lower. Saplings of Q. suber and Q. pubescens grew only during the wet spring period, and no new foliage was produced thereafter. Changes in twig hydraulic conductance played a dominant role in determining changes in K PLANT in that the two variables were well correlated to each other (r=0.68 for P=0.001). We conclude that C. siliqua is an ideal candidate for reforestation of Sicilian degraded areas as is O. oleaster that, however, requires some additional water supply in the summer, at least during the early years after plantation. In wetter locations of Sicily, to an altitude between 0 and 500 m, Q. suber and Q. pubescens can be used for reforestation with expected higher competitiveness of the latter over the former species.