Soutenance mémoire
Comparison between sites
The natural boreal forest of Quebec constitutes a reservoir of trees with high ecological importance and economic potential that extends from the shoreline to Labrador and northwards to the 55th parallel. This ecosystem has 74% of its territory covered by conifers and the remaining area is either composed of regenerating stands (10%) or occupied by mixed (13%) or hardwood (3%) stands (Gauthier, 2009).
Black spruce (Picea mariana (Mill.) B.S.P.) largely dominates the other species of conifers and plays an important role in the boreal forest ecosystem (Parent and Fortin, 2008; Zhang and Koubba, 2008). The other main species are jack pine (Pinus banksiana Lamb), balsam fir (Abies balsamea (L.) Mill.), white spruce (Picea glauca (Moench) Voss) and American larch (Larix laricina (Du Roi) K. Koch), which can be found in most of the boreal forest distribution (Laird Farrar, 1997). Black spruce is the most abundant species of Quebec and can be found in many different soil types, wet or dry, which demonstrates the high adaptability of this species. Individual trees can reach a height of 20 meters and their life expectancy is around 200 years, although some specimens can reach an age of 280 years (Burns and Honkala, 1990). In the Saguenay-Lac-Saint-Jean region, black spruce represents nearly 80% of softwood and best characterizes the region (Fillion, 2004). Its abundance and properties make it a very popular species for the forest industry (Burns and Honkala, 1990; Vincent et al,, 2009). It was used mainly for pulp, but today it is more often used for structural purposes, in addition to being used in recent years in value added products (Alteyrac, 2005). The high economic values the forest industry attributes to black spruce are associated to the particular wood anatomy of the tree species, which changes indirectly due to environmental variations (Fritts, 1976). As wood quality is closely related to wood anatomy, it is very important to assess their related parameters (Makinen et al., 2002). As long as the growth and productivity of high-latitude altitude forests remain unknown, no strategy based on an economically advantageous exploitation of the remote areas can be evaluated or considered.
Conifer tree rings can be divided into two parts: early wood and latewood. The diameters in earlywood tracheids are two to three times larger than in latewood tracheids (Cuny et al., 2013). The environment (e.g. temperature, water availability, soil fertility, disturbances such as insects or fungi) directly and indirectly affect tree growth by influencing physiological processes such as photosynthesis, respiration, carbon assimilation, hormones, absorption of water and minerals, translocation of sugars and activity of the meristems (Fritts, 1976; Kozlowski and Pallardy, 1997). Xylem phenology is mainly controlled by the temperatures occurring at the boundaries of the growing period by activating or stopping growth. Moreover, the length of the thermally favorable period decreases with altitude and latitude, and increases with a warming scenario, thus influencing the length of the growing season (Rossi et al., 2011) However, the scientific observers are not unanimous about the response of trees along a geographical gradient, and there is a lack of information how the environment parameters affect the different anatomical properties.
The properties and products of wood are related to their anatomical structure (Panshin and De Zeeuw, 1970). The dimensions and adjustments of the tracheids only partly determine the properties of pulp, paper and sawn wood (Dinwoodie, 1965). The morphology of tracheids influences its flexibility, plasticity and resistance and also plays an important role in the physical properties of the wood and paper (Panshin and De Zeeuw, 1970). Density is also recognized as one of the most important properties of wood in regard to mechanical resistance and varies according to the structure of the tracheids (Lindstrom, 1997). The changes in the conifer tree-ring density integrate the variation in the anatomy of the rings; in particular in the thickness of the cell wall, cell diameter and lumen diameter, but also in the proportion of the ring occupied by latewood (Biermann, 1996; Lindstrom, 1997; Vaganov, 1996; Vaganov and Sviderskaya, 1990) .
Growth estimation
Discs were collected at the heights 0, 0.5, 1, 1.3 and 2 m from the root collar, above 2 m, discs were collected at intervals of 1 m for the remaining length of the stem. Discs were air-dried and sanded with progressively finer grade sandpaper. Tree-ring widths were measured with an accuracy of 0.01 mm using a WinDendro measuring system (Regent Instruments, 2005) along four paths (four cardinal directions) in the sections from zero to 2 meters and two paths in the sections above 2 meters, according to the uniformity of the tree rings on the disc. All ring width series were corrected by cross-dating performed both visually and using the COFECHA computer program (Holmes, 1983). Measurements were averaged for each disc and tree ring.
Mechanical strength
The modulus of rupture (MOR) is a widely used measure of fleXural strength (Bodig and Jayne, 1993) and is expressed as the applied stress at breaking divided by the unit area. This parameter is calculated as the maximum stress of the tracheids of the upper and lower side. MOR values are used to assess, for example, the potential of a material for use as beams and joists (Wangaard, 1950). The modulus of elasticity (MOE) according Wangaard (1950), expresses the stiffness of the timber, i.e., the ability to resist deformation induced by a load applied only to the proportional limit. This parameter is obtained from the ratio of stress/strain and by the derivation of the values obtained in a static bending test. MOE is used to calculate the deformation of beams and joists and also to calculate secure loads and allowable stresses for timber and columns (Passarini, 2011).
Anatomical features
Lumen area, cell diameter and hydraulic diameter in the five sites followed a quadratic function and cell wall thickness a linear function . Lumen area and hydraulic diameter were characterized by an initial increase and stabilized between the cambial ages of 30 to 60 years. These parameters stabilized very quickly at SIM, at cambial age 30, but much later at MIR (cambial age 60). High variation was found in sites BER and MIS.
Tracheid features
Tracheid length and tracheid diameter follow a spherical semi variance function, increasing quickly during the first years then continuing to increase at a lower rate until the end (Figure 11). Tracheid length at SIM and MIR presents lower values compared to the other three sites (Figure 11). Little variation between the five trees was obtained for site MIR and a lot of variation between trees in site SIM. Tracheid length at 80 years of age reached over 3 mm. Tracheid diameter at SIM had the lowest value at its maximum and stabilized at an earlier age than the other sites (Figure 11). MIR and MIS shpwed higher diameter values with age than the other sites. Little variation was measured between the five trees per site, except for MIS.
Comparison between ages
Principal components analysis was realized at ages of 20, 40, 60 and 80 years for a better understanding of the differences of all parameters shown at these ages (Figure 14). Component 1 accounts for the largest amount of the total variation in the data and varies from 63 to 71%. Component 2 accounts for the maximum amount of the remaining total variation and varies from 28 to 36%. The importance of components 1 and 2 for the five study sites changes little with cambial age. SIM and MIR are opposite in regard to the two components and BER, MIS and DAN are grouped together for cambial age 40,60 and 80 (Figure 14). At 20 years, SIM showed high values for mechanical strength, DBH and tracheid length (Figure 14). MIR presented low values for anatomical measurements, tracheid diameter, tree volume and stem height. MIS and DAN presented highest values of wood density. Component 1 and 2 accounted for 69% and 31%, respectively. At 40 years, BER, MIS and DAN showed higher values of tracheid diameter and hydraulic diameter. SIM showed highest values of mechanical strength, stem height, tree volume and DBH. MIR showed low values for anatomical measurements. Component 1 and 2 accounted for 63% and 37%, respectively. At 60 years, BER, MIS and DAN showed highest values of tracheid length and mechanical strength (Figure 14). SIM presented higher values of wood density, DBH, tree volume and stem height and low values for tracheid diameter and diameter hydraulic. MIR presented low values for mechanical strength, tracheid length and cell wall thickness. Component 1 and 2 accounted for 68% and 32%, respectively. At 80 years, once again, BER, MIS and DAN showed highest values of tracheid length and mechanical strength and low values of DBH. SIM presented higher values of wood density, tree volume and DBH and low values for tracheid diameter. MIR showed low values for mechanical resistance, tracheid length and cell wall thickness. Component 1 and 2 accounted for 72% and 28%, respectively.
DISCUSSION
Age
All measured wood properties change with tree age and follow distinct trends. At the initial stage the tree produces a wood called, « juvenile wood » characterized by lower density, shorter tracheids, thinner cell walls, smaller tangential cell dimensions, larger fibril angles, larger cell lumen and lower strength properties (Walker, 2013; Yang and Hazenberg, 1994). The formation of « mature wood » follows that of juvenile, with little variation in the anatomical properties (Cown, 1992; Panshin and De Zeeuw, 1970). These two phases were clearly visible in most of the analyzed data set. In between juvenile and mature wood is the transition that occurs between the ages of 11 and 21 for black spruce (Alteyrac et al., 2005; Yang and Hazenberg, 1994).
Growth and latitudinal gradient
Xylem production decreased from south to north, SIM and MIR being the sites having the highest and lowest production of xylem respectively and BER, MIS and DAN having a similar development, this trend is due to the reduction in temperature. The length of the growing season is a determinant of xylem production; temperature and precipitation normally influence the annual growth of boreal conifers (Brooks et al., 1998; Kozlowski et al., 1991) because trees are active only from late spring to summer and become dormant in autumn in order to harden for winter (Rossi et al., 2008). However it has been observed that precipitation has no effect on the wood features in black spruce in the Saguenay-Lac Saint-Jean area (Krause et al., 2010), the main factor affecting xylem production in the northern area was the temperature, as demonstrated by several authors (Deslauriers and Morin, 2005; Gricar et al., 2006a; Gricar et al., 2006b; Oribe and Kubo, 1997; Rossi et al., 2008). Inferring the results of Lupi, warmer spring temperatures lead to earlier cambial reactivation; increasing cell production and delaying cell maturation in autumn (Lupi et al., 2010). It was also verified in Europe that elevation influenced the developmental phases, which were earlier at lower elevations and growth tended to decrease with increased elevation (Moser et al., 2009).
Wood quality
Wood quality for structural purposes had the highest values in the southern site and decreased northwards. Mechanical strength was closely linked to the relationship between cell wall thickness and wood density. Density is one of the most important physical properties of wood (Bowyer et al., 2007; Desch and Dinwoodie, 1996), defined by the relation between its mass (cell wall) and volume (cell wall plus air) at a given moisture content. Thus the cell wall thickness is an excellent indicator of wood density which is influenced by the environment (proportional to temperature in Norway spruce (Franceschini et al., 2013)) determining the rate of growth (Treacy et al., 2000), as well as the type and size of cells and the amount of late wood versus early wood (Barnett and Jeronimidis, 2003). The linear and strong relationship between wood density and mechanical strength has been also demonstrated by many studies (British Standard EN 384R, 2000; Forest Products Laboratory, 2010; O’Sullivan, 1976; Panshin and De Zeeuw, 1970).
CONCLUSION
This study has provided information on the effect of latitudinal and altitudinal gradients on wood productivity and quality; our work hypothesis was partially confirmed. In fact, the latitudinal and altitudinal gradient influenced wood productivity, density and mechanical resistance, achieving lower values at higher gradients. Age also influences the growth and wood quality. All the parameters associated to tree growth increase with the cambial age. In the case of measured parameters associated to wood quality, lower values were obtained during the first stages of the tree life, followed by an increase with age before stabilisation of the wood quality values.
|
Table des matières
1. INTRODUCTION
1.1. Hypothesis and Objectives
2. MATERIALS AND METHODS
2.1. Study area
2.2.Tree selection
2.3. Growth estimation
2.4. Anatomical features
2.5. Tracheid features
2.6. Wood density
2.7. Mechanical strength
2.8. Curves fittings
3. RESULTS
3.1. Growth
3.2. Anatomical features
3.3. Wood density
3.4. Tracheid features
3.5. Mechanical strength
3.6. Comparison between sites
3.7. Comparison between ages
4. DISCUSSION
4.1. Age
4.2. Growth and latitudinal gradient
4;3. Wood quality
5. CONCLUSIONS
Télécharger le rapport complet
