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 cottonwo od (Populus deltoides Bartr. ex Marsh.) as a nurse for slower growing, dist urbance-dependent species is under evaluation as a forest rehabilitation to ol on former agricultural land in the Lower Mississippi River Alluvial Vall ey, USA. The primary objectives of this study were to quantify understory l ight availability in the eastern cottonwood plantation, and describe the ph otosynthetic 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% of full sunlight , and was sufficient to meet leaf saturation requirements over 29% of the d iurnal cycle. Oak seedlings established in the cottonwood understory showed no change in blade area, and minimal shifts (< 19%) in dry mass per unit a rea relative to open grown seedlings. A 19% decrease in dark respiration ra te (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 influ enced by the presence of the cottonwood canopy, regardless of leaf area, ma ss or N content. Pn-sat was strongly determined by foliar N concentration o f seedlings in understory and open environments, increasing 1.2 mu mol m(-2 ) s(-1) for each 0.1% increase in N concentration. Our data indicate that N uttall oak seedlings established beneath an eastern cottonwood canopy devel oped leaves with a capacity for carbon assimilation similar to open-grown p lants. However, carbon assimilation by Nuttall oak may be sub-optimal on de graded sites where intensive row cropping has depleted soil N. (C) 2001 Els evier Science B.V. All rights reserved.