Photosynthesis of Nuttall oak (Quercus nuttallii Palm.) seedlings interplanted beneath an eastern cottonwood (Populus deltoides Bartr. Ex Marsh.) nurse crop
An afforestation system which utilizes the pioneer species eastern cottonwood (Populus deltoides Bartr. ex Marsh.) as a nurse for slower growing, disturbance-dependent species is under evaluation as a forest rehabilitation tool on former agricultural land in the Lower Mississippi River Alluvial Valley, USA. The primary objectives of this study were to quantify understory light availability in the eastern cottonwood plantation, and describe the photosynthetic light response of interplanted Nuttall oak (Quercus nuttallii Palm.) seedlings. Photosynthetic photon flux density (PPFD) measured in the understory of a 3-year-old cottonwood plantation was 43 percent of full sunlight, and was sufficient to meet leaf saturation requirements over 29 percent of the diurnal cycle. Oak seedlings established in the cottonwood understory showed no change in blade area, and minimal shifts (<19 percent) in dry mass per unit area relative to open grown seedlings. A 19 percent decrease in dark respiration rate (Rd) was measured on a leaf area basis; but gross photosynthesis (Pg-sat), net photosynthesis (Pn-sat), quantum yield (Q), light compensation point (LCP), and the saturation constant (K) of Nuttall oak leaves were not influenced by the presence of the cottonwood canopy, regardless of leaf area, mass, or N content. (Pn-sat) was strongly determined by foliar N concentration of seedlings in understory and open environments, increasing 1.2 µmol m-2 s-1 for each 0.1 percent increase in N concentration. Our data indicate that Nuttall oak seedlings established beneath an eastern cottonwood canopy developed leaves with a capacity for carbon assimilation similar to open-grown plants. However, carbon assimilation by Nuttall oak may be sub-optimal on degraded sites where intensive row cropping has depleted soil N.