Is shade beneficial for mediterranean shrubs experiencing periods of extreme drought and late-winter frosts?
OBJECTIVE
Plants are naturally exposed to multiple, frequently interactive stress factors, most of which are becoming more severe due to global change. Established plants have been reported to facilitate the establishment of juvenile plants, but net effects of plant-plant interactions are difficult to assess due to complex interactions among environmental factors. An investigation was carried out in order to determine how two dominant evergreen shrubs (Quercus ilex and Arctostaphylos uva-ursi) co-occurring in continental, Mediterranean habitats respond to multiple abiotic stresses and whether the shaded understorey conditions ameliorate the negative effects of drought and winter frosts on the physiology of leaves.


METHODS
Microclimate and ecophysiology of sun and shade plants were studied at a continental plateau in central Spain during 2004-2005, with 2005 being one of the driest and hottest years on record; several late-winter frosts also occurred in 2005.


RESULTS
Daytime air temperature and vapour pressure deficit were lower in the shade than in the sun, but soil moisture was also lower in the shade during the spring and summer of 2005, and night-time temperatures were higher in the shade. Water potential, photochemical efficiency, light-saturated photosynthesis, stomatal conductance and leaf 13C composition differed between sun and shade individuals throughout the seasons, but differences were species specific. Shade was beneficial for leaf-level physiology in Q. ilex during winter, detrimental during spring for both species, and of little consequence in summer.


CONCLUSIONS
The results suggest that beneficial effects of shade can be eclipsed by reduced soil moisture during dry years, which are expected to be more frequent in the most likely climate change scenarios for the Mediterranean region.
Valladares
Fernando
F
Laboratorio Internacional de Cambio Global (LINC-Global), Instituto de Recursos Naturales, CCMA, CSIC, Serrano 115, E-28006 Madrid, Spain. valladares@ccma.csic.es
Zaragoza-Castells
Joana
J
Sánchez-Gómez
David
D
Matesanz
Silvia
S
Alonso
Beatriz
B
Portsmuth
Angelika
A
Delgado
Antonio
A
Atkin
Owen K
OK
D000704:Analysis of Variance; D029792:Arctostaphylos / Q000502:physiology*; D002247:Carbon Isotopes; D055864:Droughts; D017753:Ecosystem; D005615:Freezing; D019083:Mediterranean Region; D008834:Microclimate; D010777:Photochemistry; D010788:Photosynthesis; D029963:Quercus / Q000502:physiology*; D011891:Rain; D012621:Seasons; D012987:Soil; D013472:Sunlight; D013696:Temperature; D013997:Time Factors; D014867:Water / Q000502:physiology
D002247:Carbon Isotopes; D012987:Soil; D014867:Water