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Root foraging for patchy nutrients: Morphological, physiological and demographic plasticity in four species.
Bliss, Kristin*,1, Jones, Robert 2, Wagner, Rachel2, Mou, Paul3, 1 Randolph-Macon Woman's College, Lynchburg, VA, USA2 Virginia Tech, Blacksburg, VA, USA3 Department of Biology, Greensboro, NC, USA
ABSTRACT- In response to soil nutrient heterogeneity, plant root systems exhibit morphological, physiological, or demographic plasticity. We predicted that species adapted to different positions within a successional sequence would vary in their foraging ability. We tested this prediction using two pot experiments, each with two herb and two tree species that co-occur in the southeastern USA but differ in successional status. In the first experiment, we created heterogeneity by concentrating slow-release fertilizer in one half of the pot; homogeneity was created in additional pots by distributing the same amount of fertilizer evenly. Nutrient heterogeneity led to an increase in root mass in all four species (P < 0.001), however all species responded similarly with respect to morphological plasticity. Plants were dosed with a low concentration of 15N labeled ammonium nitrate 48 hours prior to harvest. Uptake rates per unit of root mass were greater for roots conditioned to low nutrient levels (P < 0.009), but species tended to differ in the degree of this response (P < 0.080), indicating variable levels of physiological plasticity. In a second experiment, seedlings were grown in rhizotron boxes with high and low fertility halves (19:1 nutrient ratio). Over 21 weeks, biweekly root tracings were used to estimate production, mortality and turnover. In all species, the high fertility half had significantly greater mortality (P = 0.033) and turnover (P = 0.006). Significant differences among species were detected for each demographic variable; however, species did not differ in the degree of demographic plasticity as measured by the relative difference between high and low nutrient patches. Plants exposed to fertile soil patches show within-root system plasticity in morphology, physiology, and demography. Contrary to our prediction, degree of plasticity was not strongly variable across the four tested species.
Key words: root production, root mortality, root foraging, root turnover