Physiological and growth responses of three sizes of containerized Picea mariana seedlings outplanted with and without vegetation control M.S. Lamhamedi 1,a , P.Y. Bernier a,* , C. He Âbert a , R. Jobidon b a Natural Resources Canada, Canadian Forest Service, Laurentian Forestry Centre, P.O. Box 3800, Sainte-Foy, Quebec G1V 4C7, Canada b Ministe Áre des Ressources naturelles du Que Âbec, Direction de la Recherche, 2700 rue Einstein, Sainte-Foy, Que Âbec G1P 3W8, Canada Received 11 December 1997; accepted 5 February 1998 Abstract Three different stock sizes of containerized black spruce (Picea mariana [Mill.] B.S.P.) seedlings were planted in an abandoned agricultural ®eld. The small planting stock was of a conventional type produced in 110 cm 3 containers. The experimental medium and large stock types were produced in 340 and 700 cm 3 containers, respectively. Gas exchange, xylem water potential and dry masses were measured six times during each of the ®rst two growing seasons in ®eld plots with and without vegetation control. During the ®rst growing season, the effect of planting shock masked most physiological and growth differences among seedling types. During the second growing season, in plots with vegetation control, small and medium seedlings had similar values of physiological variables and of growth as measured by relative growth rates (RGR), but the large seedlings showed lower values of both net photosynthesis and of RGR, a difference attributed to low initial quality of the root system in the larger seedlings. In plots without vegetation control, the trend was identical, but differences were not signi®cant; the greater height of the larger seedlings, and the resulting greater access to light, compensated for their lower initial quality. The similarity in response between the medium and the small seedlings shows that a fourfold increase in shoot size (1.68±6.82 g) in the initial size and a doubling of the shoot : root ratio (2.17±4.54) of the planting stock did not result in increased planting shock or reduced growth in these containerized conifer stock types. The results also show the importance of the interaction between stock height and the vertical light pro®le created by the competing vegetation in the ®nal assessment of stock performance. # 1998 Elsevier Science B.V. Keywords: Picea mariana; Competing vegetation; Gas exchange; Water relations; Relative growth rate 1. Introduction In boreal and sub-boreal forest ecosystems, com- peting vegetation is an important constraint on the survival and growth of conifer seedlings (Brand, 1990, 1991 and Ruel, 1992). However, forestry practices in parts of Canada are quickly evolving towards very Forest Ecology and Management 110 (1998) 13±23 *Corresponding author. Tel.: (418) 648-4524; fax: (418) 648- 5849; e-mail: pbernier@cfl.forestry.ca 1 Present address: Centre de recherche en biologie forestie Áre, Pavillon Abitibi-Price, Universite  Laval, Sainte-Foy, Que Âbec, Canada G1K 7P4. 0378-1127/98/$19.00 # 1998 Elsevier Science B.V. All rights reserved. PII S0378-1127(98)00267-9