The 2012 ASHS Annual Conference

Numerous woody genera are distributed in both eastern North America and in portions of California characterized by the dry summers of Mediterranean climates.  As part of a broader survey of how these taxa have evolved under conditions of disparate moisture availability, our objective was to compare the effects of flooding and drought on relative growth rate (RGR), photosynthesis, and measures of biomass among seedlings of Ptelea and Sambucus from the San Francisco Bay Area (Ptelea crenulata and Sambucus nigra ssp. caerulea) and from more mesic deciduous forests in the East (Ptelea trifoliata and Sambucus nigra ssp. canadensis).  Seedlings in pots were subjected to six treatments of root-zone moisture that comprised three extents of inundation and three severities of drought (root zones irrigated when soil moisture by volume decreased to 5%, 10%, and 20%).  After five weeks, Ptelea and Sambucus from western provenances showed more deleterious effects of flooding than did their congeners from the East.  For example, RGR of western and eastern Sambucus was reduced 116% and 25%, respectively, when root zones were completely inundated.  Although all western and eastern Ptelea died when root zones were fully inundated, inundating half the root zone killed all western plants but caused only a reduced RGR among eastern plants.  Photosynthesis of Sambucus from the West was lowest with complete inundation (4 µm·m^-2·s^-1) but was similar across the remaining treatments (11 µm·m^-2·s^-1).  In contrast, photosynthesis of eastern Sambucus was lowest under severe drought (1 µm·m^-2·s^-1) but otherwise similar (4-6 µm·m^-2·s^-1).  Photosynthesis of Ptelea was sensitive to both drought and flooding; both western and eastern plants showed the greatest photosynthesis (14 µm·m^-2·s^-1 and 4-8 µm·m^-2·s^-1, respectively) under moderate root-zone moistures.  For both genera, maximal photosynthesis was greater among western than eastern plants.  Leaf area and biomass of eastern plants exceeded those of western plants regardless of genus and treatment.  Root-to-shoot ratios of western Sambucus were greater than ratios of plants from the East after all treatments, whereas western Ptelea had greater root-to-shoot ratios than eastern Ptelea only under severe drought.  These findings suggest that plants from Mediterranean provenances of taxa distributed across North America are less resistant to flooding but not necessarily more drought resistant than their eastern congeners.