Juvenile growth and crown morphological plasticity of eastern white pines (Pinus strobus L.) planted along a natural light gradient: results after six years

摘要

Underplanting white pine (Pinus strobus) in stands of shade intolerant broadleaved trees has been suggested as a method to reduce competition and protect against white pine weevil (Pissodes strobi) damage. However, shading caused by overstorey treescan reduce growth, vigour and survival of white pine. The objective of this study was to determine the effects of a light gradient on the growth and overall crown morphology of white pine saplings planted in spring 1988 in 3-m strips within a hardwood forest in Quebec. In 1994, total height and diameter, leader length (in 1994) and numerous crown morphological variables were measured and the light environment was estimated above the crown of 63 young white pine saplings representing six families of close provenance in Quebec. White pine grew well (i.e.,>20 cm in height/year) for the first six years when planted at light levels between 10 and 66% of full sunlight. Total height and diameter after six years tended to decline more sharply below 30% full sunlight, confirming earlier experiments made in controlled conditions. No significant changes in crown morphology were evident along the light gradient. This lack of crown morphological plasticity presumably contributes to limiting the ability of white pine to grow and compete in a very low light environment. Various silvicultural options are discussed in relation to the results obtained in this study.