Willow oak ( <i>Quercus phellos</i> ) seedling roots continue respiration and growth during fall and winter in a soil temperature-dependent manner

Many greentree reservoirs (GTRs) and other bottomland hardwood forests have experienced a shift in tree species composition away from desired red oaks (Quercus section Lobatae), like willow oak (Quercus phellos L.), due to flood stress mortality. Trees experience flood stress primarily through their root system, so it is surmised that GTR flooding may be occurring before root systems have reduced their activity entering the winter. Because soils buffer seasonal temperature changes, we hypothesized that root activity would respond to the belowground environment rather than the aboveground environment. To investigate whether cold soil temperatures reduce root growth and respiration in willow oak during winter, soil temperatures for container seedlings were either held at 15 °C or transitioned to 10 or 5 °C in the late fall. Root elongation was measured in seedlings grown in rhizotron pots by analyzing repeated images of roots during the fall-winter transition period. Root respiration, measured at soil temperature levels, was used as an indicator of root energetic expenses. Also, root respiration was measured at 15 and 5 °C to determine Q10 values to test for acclimation to low soil temperature. Root elongation continued in winter, even after stem elongation stopped in soil temperatures ≥5 °C, a condition usually met throughout most of the native range of willow oak. Both root elongation and respiration rates decreased in cooler soil temperatures. However, Q10 values were unaffected by soil temperature treatment. These findings do not support root dormancy or cold acclimation of root respiratory activity but indicate that temperature directly and reversibly affected root respiration rate. Root elongation may have been dependent on photoassimilates produced by green leaves that were retained through much of winter. Overall, our results suggest that willow oak roots may continue a high rate of growth throughout winter, unlike most temperate species measured to date, and that soil temperature has a major influence over their growth and respiration rates.