Thursday, August 5, 2010
Exhibit Hall A, David L Lawrence Convention Center
Background/Question/Methods Precipitation patterns in the southern Sonoran Desert are highly variable from year to year. Variation in drought tolerance strategies of desert woody species is important for explaining their survival and distribution patterns. Four Parkinsonia small-tree species occurring sympatrically in the Sonoran Desert where used to test the survival and trajectories of drought tolerance of seedlings in a common garden. We evaluated the survival of seedlings to prolonged drought by conducting a pot experiment in the dry season (May-Jun) using 60 well-watered 10-month old seedlings of each of four coexisting Parkinsonia species (P. aculeata, P. florida, P. microphylla and P. praecox) in which wilting and mortality were monitored. We posed the following questions: (1) how seedling functional traits affect drought survival?, and (2) how seedling drought survival relates to the distribution of the sympatric species? Before the drought treatment, seedlings metrics, root:shoot ratio and specific leaf area of each species were determined. Survival was recorded and stages of wilting were monitored by registering stem dieback, apparent leaf wilting, leaf-fall, soil water potential, xylem water potential (Ψx), and stomatal conductance (gs) along the decreasing course of soil water potential.
Results/Conclusions Despite ample watering and controlled age, there were significant interspecific differences in all variables before seedlings were exposed to catastrophic drought: basal diameter (3.9–5.6 mm), total length (27–40 cm), branch number (2.0–3.7), leaf area (331–2112 mm2), SLA (200–399 cm2 g-1), gs at midday (18.4–129.6 mmol/m2s) and Ψx (-0.85 to -2.36 MPa). Wilting patterns differed among species. The time taken to 50% leaf wilting was shortest for P. aculeata and P. florida (12 days) which at this point reached less negative Ψx (ca. -4.0 MPa) than the other two species (ca. -5.0 MPa). Leaf-fall was complete after 30–37 days in all species when Ψx dropped to -6.2 MPa or lower. Stem dieback occurred earlier in P. aculeata (after 22 days of drought) and 1-2 weeks later in the other species. Also, P. aculeata had the highest seedling mortality (52%) and P. praecox the lowest (17 %). These functional differences among species in response to drought are reflected in the actual populations in the field, where the short-lived P. aculeata develops in nutrient rich, relatively well watered sites, and P. praecox and P. microphylla are long-lived species forming dense stands in drier, shallow, rocky soils in the undistubed desert.
Results/Conclusions Despite ample watering and controlled age, there were significant interspecific differences in all variables before seedlings were exposed to catastrophic drought: basal diameter (3.9–5.6 mm), total length (27–40 cm), branch number (2.0–3.7), leaf area (331–2112 mm2), SLA (200–399 cm2 g-1), gs at midday (18.4–129.6 mmol/m2s) and Ψx (-0.85 to -2.36 MPa). Wilting patterns differed among species. The time taken to 50% leaf wilting was shortest for P. aculeata and P. florida (12 days) which at this point reached less negative Ψx (ca. -4.0 MPa) than the other two species (ca. -5.0 MPa). Leaf-fall was complete after 30–37 days in all species when Ψx dropped to -6.2 MPa or lower. Stem dieback occurred earlier in P. aculeata (after 22 days of drought) and 1-2 weeks later in the other species. Also, P. aculeata had the highest seedling mortality (52%) and P. praecox the lowest (17 %). These functional differences among species in response to drought are reflected in the actual populations in the field, where the short-lived P. aculeata develops in nutrient rich, relatively well watered sites, and P. praecox and P. microphylla are long-lived species forming dense stands in drier, shallow, rocky soils in the undistubed desert.
