“Water use and woody crops” workshop
20-21th May, Ilhavo, Portugal
ARE CORK OAK AND HOLM AOK USING THE SAME WATER SOURCES IN THE MONTADO? INDICATIONS FROM DEUTERIUM NATURAL ABUNDANCE.
Cathy Kurz-Besson 1, Rolf Siegwolf 2, Cécile Barbaroux 1, Teresa Soares David 3, Jorge Soares David 1, João Santos Pereira 1, Manuela Chaves 1
1. Instituto Superior de Agronomia, Tapada da Ajuda, 1349-017 Lisboa, Portugal, firstname.lastname@example.org
2. Laboratory of Atmospheric Chemistry, Paul Scherrer Institute, CH-5232 Villigen-PSI, Switzerland, email@example.com
Estação Florestal Nacional, 2780-159 Oeiras, Portugal, firstname.lastname@example.org
Keywords: Quercus suber, Quercus rotundifolia, D, water source, hydraulic lift
In the future climate water deficits may increase, as arid and semi-arid climate will probably expand throughout southern Portugal, due to global warming. A dramatic increase in the number of dry months is expected to occur and the ecophysiological limits for water stress tolerance may be overdue, leading to large-scale tree mortality. In order to improve our understanding on plant adaptation to summer drought and specific differences in water use, we studied the most important ligneous plants (Quercus suber and Q. rotundifolia) of an evergreen oak woodland ecosystem (Montado).
The aim of the study was to identify the depth of root water uptake in Q. suber and Q. rotundifolia, located in a Montado agroforestry system near Evora (38º32'N, 8º00'W) within a Mediterranean mesothermic humid climate. To characterize seasonal changes in water sources for both species, soil samples were collected in May and October 2001, and June and August 2002 at 0.2m intervals till 1m depth. Samples of twigs, precipitation and ground water (GW) were collected monthly in 2001, 2002, and 2003. Water was extracted from samples by cryodistillation and analysed by mass spectrometry to determine the hydrogen/deuterium ratio (D) relative to VSMOV standard.
There was no significant interspecific difference in xylem D signature in 2001, 2002 and 2003, except in June 2002 and March 2003. Water uptake patterns were similar for both species, indicating that Q. suber and Q. rotundifolia were using the same water sources. The significant difference observed in June 2002 suggests that Q. suber was using soil moisture at 1m depth whereas Q. rotundifolia seemed to withdraw water from deeper soil layers or even from GW.
The results also show that in Spring and in Fall both tree species were using topsoil water, at 0.6m depth. During Summer, xylem water signature for both tree species was more negative (-35 to –38°/oo) than that of GW (-30°/oo), but generally less negative than soil water signature at the estimated maximum root depth (-41 to –45°/oo). It is unlikely that trees were taking water from the top 0.2m soil layer during summer since its soil water content was then only 2% in volume. Thus three hypothesis may explain these results:
1/ Trees are performing hydraulic lift, redistributing water during the night from the GW to the top soil at about 0.6m depth, through the shallower root system. Due to the low water content, water diffuses until morning, leading to a more negative signature in the soil and an intermediate signature in the roots and twigs compared to GW. In this last hypothesis, trees would withdraw water from GW only during the night due to an inversion of water potential after stomata closure, and use soil redistributed water during the day.
2/ Trees are not performing hydraulic lift but have access to GW trough vertical root system. They are using soil water and GW at the same time, leading to an intermediate twig isotopic signature.
3/ Trees are not taking water directly from the GW, but using water from the unsaturated zone immediately above, that should have a more negative signature compared to GW, due to H2O migration through a diffusion process.
Further investigation through isotopical labelling is needed for more conclusive results on water sources for oak species in the Montado during the summer drought period.