Water stress results in a reduction of the metabolism of plants and in a reorganization of their use of resources geared to survival. In the Mediterranean region, periods of drought accompanied by high temperatures and high irradiance, occur in summer. Plants have developed various mechanisms to survive in these conditions by resisting, tolerating or preventing stress. We used three typical Mediterranean tree species in Israel, Pinus halepensis L., Quercus calliprinos and Quercus ithaburensis Webb, as models for studying some of these adaptive mechanisms. We measured their photosynthetic rates (A), stomatal conductance (gs), and terpene emission rates during spring and summer in a geophysical gradient from extremely dry to mesic from Yatir (south, arid) to Birya (north, moist) with intermediate conditions in Solelim. A and gs of P. halepensis were three fold higher in Birya than in Yatir where they remained very low both seasons. Quercus species presented 2-3 fold higher A and gs but with much more variability between seasons, especially for Q. ithaburensis with A and gs that decreased 10-30 fold from spring to summer. Terpene emission rates for pine were not different regionally in spring but they were 5-8 fold higher in Manuscript Click here to download Manuscript: Israel A gs Emiss spring summer 2013 rev 19 may 2015 Trees JP 10 aug 015-jp 13 aug.docx Click here to view linked References Birya than in Yatir in summer (P < 0.05). Higher emissions were also observed in Solelim for the drought resistant Q. ithaburensis (P < 0.001) but not for Q. calliprinos. -Pinene followed by limonene and 3carene were the dominant terpenes. Warmer summer conditions result in increased Terpene emission rates except under severe drought, in which case they strongly decrease.