Autumn flooding disrupts seasonal nitrogen storage and impacts spring growth in Quercus texana seedlings

Abstract

Autumn flooding reduced nitrogen uptake and fine root mass. Leaf nitrogen resorption increased, but overall autumn nitrogen accumulation was negligible in flooded seedlings. Subsequently, spring survival and growth were decreased. Since nitrogen (N) is often limiting in terrestrial ecosystems, N is conserved in trees by resorption before leaf senescence. Bottomland forests are prone to flooding, which could reduce N uptake, and may decrease phloem transport, which is essential for N resorption. Therefore, we hypothesized that autumn flooding may diminish both N uptake and N resorption in trees, and this would reduce spring growth. Two-year-old Quercus texana seedlings either had no flood or had their complete root system flooded during dormancy induction, the period in autumn when trees prepare metabolically and physiologically for winter dormancy. We measured seedling growth and nutrient contents before flooding and after leaf fall and determined impacts of autumn flooding on growth during the subsequent spring. Autumn flooding resulted in a small increase in N resorption from leaves. In non-flooded seedlings, much more N accumulated in stems and roots than the amount resorbed from leaves, suggesting that there was substantial N uptake during the autumn dormancy induction period. However, flooding severely reduced accumulation of N in roots and stems during autumn, probably by directly reducing uptake and by increasing fine root mortality. Winter survival was reduced 50% by autumn flooding. Autumn-flooded seedlings that survived winter had greater new root growth in spring than non-flooded seedlings, but substantially decreased stem diameter growth. Our results indicate that Q. texana seedlings which are flooded during dormancy induction may be less competitive the following spring due to fine root mortality and reduced nutrient storage, which negatively impact spring growth.