Contrasts between whole-plant and local nutrient levels determine root growth and death in Ailanthus altissima (Simaroubaceae).

UNLABELLED: • PREMISE OF THE STUDY: There is an ongoing debate about the importance of whole-plant control vs. local modular mechanisms for root growth. We conducted a split-root experiment with different patch/background levels of nitrogen to examine whether local root growth and death are controlled by local resource levels or at the whole-plant level.• METHODS: Three microrhizotrons with 0, 10, and 100 µg N/g growth medium levels (74 g growth medium each) were attached to pots of high or low soil N in which one Ailanthus altissima individual was growing. One fine root was guided into each of the microrhizotrons and photographed every 4 d. Plants were harvested after 28 d; root growth and mortality in the microrhizotrons were recorded. Changes in root length, number of laterals, and interlateral length were determined from the photos and analyzed.• KEY RESULTS: While overall plant growth was influenced by background N level, both patch and background N levels influenced root growth and mortality in patches. Local roots proliferated most when the patch N level was high and background level low, and they proliferated least and showed highest mortality when patch N was low and the background level high.• CONCLUSIONS: The fate of roots growing in a patch is influenced by the resource environment of the plant's other roots as well as the resource levels in the patch itself. Thus, the growth and death of roots in patches is determined by both modular and whole-plant mechanisms.

Are competitive interactions influenced by spatial nutrient heterogeneity and root foraging behavior?

• Nutrient heterogeneity, root foraging and competitive interactions were investigated for six species native to south-eastern USA. • Monocultures, two- and six-species garden plots were fertilized to create spatially homogeneous or heterogeneous nutrient conditions. After 3.5 months, root proliferation in rich patches (precision), mean above-ground biomass per plant (scale) and influence of nutrient treatment on total plot biomass (sensitivity) in monocultures were measured. Competition (above-ground biomass) was assessed in two- and six-species plots. • In monoculture plots, two species were relatively precise foragers, but no species showed significant sensitivity to nutrient treatment. Correlations between precision, scale and sensitivity were weak (-0.40 < r < 0.17), which contrasts with previous work showing a scale-precision trade-off. In two-species plots, competition was influenced by soil heterogeneity in two of six cases tested (anova, P < 0.05), and precise foragers grew larger in heterogeneous than in homogeneous conditions. In six-species plots, nutrient treatment had no influence on growth or competition. • In our study system, heterogeneity effects on competition are context specific, generally weak and potentially mediated by the degree of root foraging precision.