Root distribution of Pinus banksiana (Lamb.) from natural and planted stands

In Quebec, the law requires forestry companies to look after the regeneration of the forests they exploit. The natural cycle of jack pine (Pinus banksiana Lamb.) is related to fire, which allows opening of the cones and dispersal of the seeds. But after harvesting, the exploited sectors are normally replanted. It has been shown that the current methods of seedling production and planting cause significant deformations of root systems of jack pine, such as winding of the roots, poor distribution, no taproot, shallow rooting, and these defects may affect more than 90% of planted jack pine (Sheedy 1997). In contrast, these types of deformation and the development of poor root systems are not known for naturally regenerated jack pines. To understand the extent of the problems, it is necessary to quantify the damage observed on planted jack pines and to compare with natural trees. The purpose of this project is to relate the spatial distribution of roots to stand origin of jack pine. Two sites located in the Quebec boreal forest were selected. The first site is a plantation of jack pine which was planted in 1987, using seedlings grown for one year in containers. The second site is a natural forest which burned in 1983. Since the release of seeds by jack pine cones takes place from one to five years after fire, the trees on the two sites are comparable ages. A total of 40 trees and root systems were collected (20 trees on each site). Each root system was excavated, down to the finest possible diameter (< 0.4mm). A precise map of the roots was drawn, the root drawings were digitized and MTG files were constructed, to enable classification and extraction of information about root system architecture, using the AMAPmod software (Godin 2000 ; Godin et al. 1999). Three observation scales were used in this project : the individual, the axis, and the segment (Danjon et al., 1999). The minimal length of one segment was fixed by large changes in orientation of the roots in space or by the presence of ramifications. The orientation of the structures in space and rooting depth were analysed. The results confirm the observation of Sheedy (1997) on 7-8 years-old planted jack pine. No taproot had developed in 15 years-old planted jack pine, and although these trees had produced a large number of lateral roots, they were poorly distributed in space, both around the stem and in depth. In comparison, the taproot of the trees resulting from natural regeneration were well developed. The lateral roots were distributed all around the stem and originated at different depth along the taproot. These differences in the distribution of roots may explain the difference in sinuosity of the stem between the two stands, due to a difference in the potential stability of trees. The methods used in Quebec province can therefore cause serious root defects. Trees with these deformations may: 1) be unable to resist to strong wind; 2) have reduced timber quality as the instability may lead to stem deformations. As exploited jack pine forests are routinely regenerated in plantations, it is important to find as soon as possible new silvicultural practices that will positively influence the quality of this species. Possible solutions would include improved nursery practice, site cultivation and planting method.