Flood-Ring Formation and Root Development in Response to Experimental Flooding of Young Quercus robur Trees.

Paul Copini,Jan Den Ouden,Jacques C Tardif,Elisabeth M R Robert,Leo Goudzwaard,Ute Sass-Klaassen,Walter A Loesberg

doi:10.3389/fpls.2016.00775

Abstract

Spring flooding in riparian forests can cause significant reductions in earlywood-vessel size in submerged stem parts of ring-porous tree species, leading to the presence of ‘flood rings’ that can be used as a proxy to reconstruct past flooding events, potentially over millennia. The mechanism of flood-ring formation and the relation with timing and duration of flooding are still to be elucidated. In this study, we experimentally flooded 4-year-old Quercus robur trees at three spring phenophases (late bud dormancy, budswell, and internode expansion) and over different flooding durations (2, 4, and 6 weeks) to a stem height of 50 cm. The effect of flooding on root and vessel development was assessed immediately after the flooding treatment and at the end of the growing season. Ring width and earlywood-vessel size and density were measured at 25- and 75-cm stem height and collapsed vessels were recorded. Stem flooding inhibited earlywood-vessel development in flooded stem parts. In addition, flooding upon budswell and internode expansion led to collapsed earlywood vessels below the water level. At the end of the growing season, mean earlywood-vessel size in the flooded stem parts (upon budswell and internode expansion) was always reduced by approximately 50% compared to non-flooded stem parts and 55% compared to control trees. This reduction was already present 2 weeks after flooding and occurred independent of flooding duration. Stem and root flooding were associated with significant root dieback after 4 and 6 weeks and mean radial growth was always reduced with increasing flooding duration. By comparing stem and root flooding, we conclude that flood rings only occur after stem flooding. As earlywood-vessel development was hampered during flooding, a considerable number of narrow earlywood vessels present later in the season, must have been formed after the actual flooding events. Our study indicates that root dieback, together with strongly reduced hydraulic conductivity due to anomalously narrow earlywood vessels in flooded stem parts, contribute to reduced radial growth after flooding events. Our findings support the value of flood rings to reconstruct spring flooding events that occurred prior to instrumental flood records.

Highlights

Trees growing in riparian forests must cope with regular flooding events and may survive the anoxic conditions associated with flooding (Kozlowski, 1984; Siebel et al, 1998; Glenz et al, 2006)
In 4-year-old pedunculate oak trees harvested immediately after the flooding treatments earlywood-vessel development was suppressed in submerged stem parts if flooding occurred at budswell or internode expansion
In the two trees that were flooded during leaf dormancy but had started leaf development while flooded, vessel development was totally absent in flooded stem parts

Summary

Introduction

Trees growing in riparian forests must cope with regular flooding events and may survive the anoxic conditions associated with flooding (Kozlowski, 1984; Siebel et al, 1998; Glenz et al, 2006). Species of oak (Quercus) and ash (Fraxinus) trees frequently occur along river systems in Europe (Q. robur L., F. excelsior L.), the United States of America, and Canada (e.g., Q. macrocarpa Michx., Q. lyrata Walter., F. nigra March., F. pennsylvanica March.). These species are ring porous and form large earlywood vessels in spring, followed by small latewood vessels later on in the growing season and have shown to be able to cope with 50 days of flooding as juveniles or even 100 days as adult trees (Siebel et al, 1998; Kreuzwieser et al, 2004; Glenz et al, 2006).

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