Responses of cork oak physiology to manipulated water

Responses of cork oak physiology
availability in a Mediterranean woodland
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2
to
manipulated
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water
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Cathy Béatrice Kurz Besson , Raquel Lobo-do-Vale , Maria Lucília Rodrigues , Pedro Almeida , Alastair Herd ,
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Olga Mary Grant , Teresa Soares David , Markus Schmidt , Denis Otieno , Trevor F. Keenan , Rolf Siegwolf ,
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Célia Gouveia , Catherine Mériaux , Maria Manuela Chaves , João Santos Pereira
1. Centro de Geofísica da Universidade de Lisboa, Instituto Dom Luiz, Lisboa, Portugal
2. Instituto Superior de Agronomia, Tapada da Ajuda, 1349-017 Lisboa, Portugal
3. University College Dublin, Belfield, Dublin 4, Ireland
4. Instituto Nacional de Recursos Biológicos, Quinta do Marquês, Av. da República, 2784-505 Oeiras, Portugal
5. Universität Bayreuth, 95440 Bayreuth, Germany
6. Harvard University, 22 Divinity Avenue, Cambridge, MA 02138, USA
7. Paul Scherrer Institut, 5232 Villigen PSI, Switzerland
8. Instituto de Technologia Química e Biológica, Av. da República, 2780-157 Oeiras,Portugal
Abstract
This study details the physiological responses of cork oaks (Quercus suber L.) to manipulated water inputs. Dry,
ambient and wet irrigation treatments with 80, 100 and 120% of the total annual precipitation, respectively, were
applied to an evergreen oak woodland in south Portugal. Tree water status, water and carbon balance were
monitored from 2003 to 2005.
The impacts of water manipulation were mainly observed on tree transpiration during summer drought
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periods. While xylem δ O showed that trees from the dry and ambient treatments relied on groundwater, while
trees from the wet treatment did not. In spite of significant differences in soil water content and tree transpiration,
hardly any treatment effects could be detected in leaf water potential or gas exchange which included carbon
assimilation, stomatal conductance, and intrinsic water use efficiency, except for a single event. A week after an
isolated late spring irrigation pulse, trees from the wet treatment showed increases of 231% in carbon assimilation,
342% in stomatal conductance, 50% in tree transpiration, and 33% in the annual increment of trunk diameter. This
was not observed for the trees of the two other treatments. A lower competition for water from the reduced grass
understory biomass and vegetation index in 2005 can explain this response.
The extreme drought of winter 2004/2005 had a strong negative impact on tree growth and carbon
balance. The annual increment of tree trunk diameter in the ambient and dry treatments was reduced by 30%,
while it slightly increased for trees from the wet treatment. Precipitation reduction also significantly altered leaf
surface area in trees from the dry treatment, with a significant decrease of the leaf area index (-11%), leaf area
density (-50%) and specific leaf area (-16%). Surprisingly, the annual cumulated tree transpiration and minimum
predawn leaf water potential during this particularly dry year were not affected. While a higher production of litter
was observed in the dry treatment area, the total amount of litterfall measured in 2004/2005 suggests that the
extreme drought of these two consecutive years did not affect the production of dead overstory biomass.
Our results support the eco-hydrological equilibrium theory by which cork oak species show
complementary protection mechanisms such as leaf surface reduction, osmostic adjustment and groundwater use.
In optimizing their biomass increase, trees succeed to restrict water losses to maintain a leaf minimum water
potential above the threshold of -3MPa at which critical xylem embolism may occur.
But our findings also clearly points out a cork oak physiological sensitivity to late spring precipitation, which
could be critical when climate models predict a combined higher severity of droughts and reduction of spring
precipitation in the Iberian Peninsula by the end of the 21rst century. In causing water stress before the onset of
summer droughts, the predicted spring precipitation pattern could drive the species closer to the threshold of
catastrophic xylem embolism at the peak of the drought period.