Combined effects of arbuscular mycorrhizas and light on water uptake of the neotropical understory shrubs, Piper and Psychotria.

•  Root hydraulic conductance (K ) was measured for five understory shrub species of the neotropical moist forest to determine the effects of arbuscular mycorrhizas (AM) for both carbon-rich and carbon-limited host plants. •  K was measured using a high pressure flow meter (HPFM) for potted plants grown in a factorial combination of AM fungi (presence/absence) and light (3.5 and 30% of full sun, low/high). •  AM colonization improved K for the more shade-tolerant species plants when growing in low light. By contrast, water uptake efficiency of the light-demanding species was significantly decreased by AM fungi in high light. Regardless of AM colonization, light-demanding species had a lower capacity than shade-tolerant species to meet transpirational demands, and they allocated substantially more to fine root production relative to leaf area when colonized. •  The differential effects of AM colonization and light on a species' root hydraulic conductance in relation to phylogeny and light adaptation demonstrate that AM fungi may be critical in determining early plant succession and community composition not only due to effects on nutrient uptake, but on water uptake as well.