Seedling Submergence Tolerances Accurately Predict Riparian Tree Species Distributions: Insights to Help Design Environmental Flows

Abstract

Quantification of flooding thresholds that govern species distributions on microtopographic gradients in floodplains can help design environmental flows, but the multiple correlated dimensions of flooding such as frequency, depth, duration, and timing are a challenge. We postulated that species distributions are limited by the most stressful combination of flooding dimensions when the plants are in their most susceptible developmental stage. To test this idea, we measured survival of young seedlings in pots subjected to flood treatments that completely submerged the seedlings in stagnant water with and without suspended sediments for durations of up to 6 weeks during the growing season. This measure of flood tolerance predicted floodplain distributional limits of 16 tree species with high accuracy (adj. r2 = 0.91). The strength of the result suggests that seedling ability to survive complete submergence in stagnant water for prolonged periods is an important mechanism limiting species distributions in riparian forests. We propose that environmental flows that completely submerge the seedling layer in floodplain forest with stagnant water for at least 3 weeks are likely to be more effective at eliminating invading upland species than flows that only flood the soil up to the root collars of seedlings.