Numerous woody genera are distributed in both eastern North America and in portions of California with the dry summers of Mediterranean climates. Such genera may harbor substantial variation among congeners in response to soil water content. During drought, taxa from seasonally xeric habitats may maintain greater photosynthetic rates, allocate proportionally more photosynthates to root foraging, and survive at greater frequencies than their mesic counterparts (Abrams, 1990; Abrams et al., 1990; Baraloto et al., 2007; Chunyang et al., 2000; Kubiske and Abrams, 1992). Conversely, taxa restricted in nature to mesic or seasonally flooded habitats may perform better after being planted at sites prone to flooding because of their capacities to maintain greater photosynthetic rates and to resist physiological stress in hypoxic soils (Baraloto et al., 2007; Keeley, 1979). Knowledge of variation in photosynthetic capacity, growth rate, and biomass accrual between congeners in Mediterranean California and the comparatively mesic eastern United States could enhance our ability to select the most suitable taxa for specific soil-moisture conditions in managed landscapes.
Shrubs and trees in the genera Sambucus and Ptelea are valued in horticultural landscapes. Members of the genus Sambucus produce showy clusters of white flowers followed by attractive fruits that attract wildlife and can be used for jellies and wines, whereas trees of Ptelea are shade-tolerant and display unusual fruits (Dirr, 1998). Congeners of Sambucus and Ptelea exist in eastern North America and in portions of California characterized by the dry summers of Mediterranean climates. Because of morphological diversity across their distributions, taxonomic confusion persists within both genera, and various species and subspecies have been designated (Bailey, 1962; Bolli, 1994; Eriksson and Donoghue, 1997; Greene, 1906). Sambucus nigra ssp. canadensis (syn: S. canadensis L.) grows in damp, rich soils in much of eastern North America (Dirr, 1998), whereas Sambucus nigra ssp. cerulea (syns: S. cerulea Raf., S. mexicana Presl.) in the San Francisco Bay area grows in habitats ranging from upper riparian terraces to seasonally dry elderberry savannahs and chaparrals (Ackerly et al., 2002; Koch-Munz and Holyoak, 2008). Ptelea trifoliata inhabits moist understories in eastern North America (Dirr, 1998), whereas Ptelea crenulata grows in dry chaparrals and woodlands in California, where it is endemic (Corelli, 2005). For simplicity, we will refer to these taxa as eastern and western congeners.
The Mediterranean mesoclimate near the San Francisco Bay is characterized by an average annual precipitation of 25 to 75 cm, whereas the region from which we obtained germplasm of the eastern congeners (southern Wisconsin and southern Illinois for Ptelea and Sambucus, respectively) averages 80 to 120 cm of precipitation annually [Changnon, 2003; National Climatic Data Center (NCDC), 2012]. Temporal distribution of precipitation differs also. Habitats supporting the western congeners receive ≈1 cm of precipitation from June through September, whereas 30 to 40 cm of rain is typical during those 4 months where we collected germplasm farther east (NCDC, 2012). Such seasonality of precipitation may correspond to drought-adaptive characteristics of closely related woody taxa (Chunyang et al., 2000). The presence of congeners in these divergent habitats could be viewed as evidence for physiological divergence between the species, but the natural distributions of taxa are not necessarily predictive of their capacities to withstand stressors (Parolin et al., 2010; Schrader et al., 2005; Stewart et al., 2007). Therefore, we questioned whether the differences in precipitation in the habitats of the western and eastern congeners of the species of Sambucus and Ptelea we studied are predictive of their responses to dry and inundated root zones. Several traits suggest that eastern and western congeners of these genera have evolved to possess different degrees of drought resistance. Although the eastern congener of both genera has glabrous or nearly glabrous leaves, the western congener of Sambucus is pubescent on both the abaxial and adaxial foliar surfaces, and the western congener of Ptelea has adaxial pubescence (Corelli, 2005; Dirr, 1998). The western congener of Ptelea also has smaller leaves than its eastern counterpart. We tested the hypothesis that deleterious effects of inundation and drought are more pronounced among plants from western and eastern North America, respectively, compared with their congeners by measuring RGR, photosynthesis, and biomass accrual in response to different irrigation treatments in a common environment.
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