Root hemiparasites are autotrophic plants that extract water and nutrients from neighboring plants via root connections called haustoria. Hemiparasites are postulated to be particularly sensitive to shade early in life when they must invest in infrastructure aboveground for photosynthesis and belowground for acquiring hosts. To fix sufficient carbon, shaded hemiparasites are predicted to invest less into roots, reducing the opportunity to attach to hosts and increasing mortality. Surviving hemiparasites are predicted to grow more slowly under shade and consequently uptake less from host roots. In turn, these hosts experiencing smaller belowground losses to shaded hemiparasites are predicted to allocate relatively more to shoot growth compared to hosts supporting hemiparasites experiencing more sunlight. We tested these predictions with the perennial root hemiparasite Pedicularis canadensis L. using clusters of seedling and young plants transplanted from a restored prairie to pots containing hosts. Three shade treatments were applied: 50% shade cloth was applied to entire clusters of hemiparasites within pots early in development, 4 wk later, or never. No seedlings survived the experiment. Shade significantly increased mortality of young plants from the previous year, although no pairwise comparisons were significant. Regardless of timing, shade reduced mean biomass of surviving P. canadensis and reduced relative investment into roots. Shading the hemiparasites increased host shoot mass as expected if shade reduced hemiparasite growth and demand on host roots. Our results are consistent with predictions of the balanced-growth hypothesis when light limits hemiparasite growth and parasites limit host growth.
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