Root dynamics in bottomland hardwood forests of the Southeastern United States Coastal Plain

Effects of flooding on root dynamics appear nonlinear and therefore difficult to predict, leading to disparate and often contradictory reports of flooding impacts on production in bottomland hardwood forests. We explored root dynamics in two adjacent wetland habitats by comparing results obtained from several methods of estimating root processes. Also, we tested the influence of flooding on root dynamics of cherrybark, overcup, water and swamp chestnut oaks. Fine root biomass in the laurel oak habitat was greater (< 0.05) than in the swamp tupelo habitat (5.7 vs. 2.4 Mg ha^–1), as was fine root necromass (2.4 vs. 1.3 Mg ha^–1), productivity (2.3 vs. 0.3 Mg ha^–1 yr^–1 when the sum of significant increments method was used, 5.6 vs. 2.5 Mg ha ^–1 yr^–1 when the maximum minus minimum method was used, and 1.2 vs. 1.0 Mg ha^–1 yr ^–1, when the root screen method was used), and turnover (40% and 12% per year). Mortality estimates were lower in the laurel oak habitat (1.3 and 1.2 Mg ha^–1 yr^–1) than in the swamp tupelo community (2.8 and 2.1 Mg ha^–1 yr^–1) when significant increment and maximum minus minimum methods were used, respectively. This apparent contradiction between estimates of production and mortality may be due to more rapid decomposition rates in the more aerated soil of the laurel oak than in the swamp tupelo forest type. Roots in the swamp tupelo habitat appeared to be longer-lived than in the laurel oak habitat. We concluded that there was greater investment in roots in the laurel oak habitat, where a shallow rooting zone and episodes of flooding and drought required drastic changes in root structure and physiology. In contrast, the swamp tupelo habitat had a deeper rooting zone and more consistently moist to flooded hydroperiod, allowing flood adapted roots to persist. The four oak species varied in their phenology of root production and response to flooding, from no difference among treatments for overcup oak to dramatic reductions in root growth during and after flooding for cherrybark oak. Flooding enhanced or at least did not negatively influence root growth in overcup oak, but seriously impacted root growth and survival of cherrybark oak and swamp chestnut oak. Different responses were attributed to the timing of root production: root growth began early for cherrybark oak so spring flooding severely affected this species. Growth in overcup oak began later and ended earlier than the other species tested, allowing the species a means of avoiding flood stress.