Protocolo F. Destino Final: Eutanasia

The objectives of this chapter were to identify trends in the developmental profiles of several key anatomical properties in the Douglas-fir sample trees. In doing this two hypotheses were tested, the null hypotheses of which were:

H4-1: No change in the magnitude of the studied property takes place with increasing

cambial age.

H4-2: No difference exists in the magnitude of the studied property with increasing

longitudinal position in the stem.

Considering the radial profiles of each anatomical property in their entirety, all showed a change in the magnitude of the assessed values with increasing cambial age, and as such the null hypothesis given in H_4-1 can be rejected and the alternative hypothesis, that cambial age does have an effect on anatomical properties, accepted. A similar result was achieved when studying longitudinal variations within the stem, with all properties beside latewood width and earlywood density showing statistically significant differences between the two stem heights. As a result, the null hypothesis stated in H4-2 is rejected and the alternative hypothesis, that a

difference exists in the magnitude of studied anatomical properties with increasing longitudinal position in the stem, accepted.

Upon breaking the developmental profiles down into separate growth periods, a rather more mixed outcome regarding the acceptance of either the null or alternative hypothesis emerges. In general for all properties studied, large changes with cambial age were noted early in the growing cycle, with rates of change decreasing and eventually appearing to stabilise with increasing age for many of the properties assessed. Although the results obtained from older sample trees exhibited some unusual growth patterns in the early years of growth, they appeared to confirm the stabilisation of values with increasing cambial age seen in younger trees.

The variations in the results encountered can be better understood when the changes occurring during the life cycle of the living tree are considered. Variations in anatomical properties occur in response to the changing demands of nutrient transportation required by the live crown and the need to support the increasing weight of the tree above (Barnett, 2004). Within an individual growing season, environmental factors such as photoperiod, precipitation, temperature and the availability of nutrients have been shown to have a large influence on the nature of the wood material produced (e.g. Larson, 1960, Lebourgeois, 2000). Wood is produced within the cambial zone or cambium, with new growth in temperate regions triggered by increases in temperature and photoperiod. As observed in Figure 4-1, ring widths were typically greatest in the growth rings immediately adjacent to the pith. As all younger sample trees were grown within even aged stands this factor arises in part due to stand dynamics. At younger cambial ages little or no competition exists between adjacent trees and wide rings are therefore observed. As the crowns of neighbouring trees begin to meet, the competition for light and nutrients increases and as a result ring widths begin to decline. The fall in ring widths does not directly relate to a decrease in the rate of biomass accumulation, due to the fact that as the tree grows in circumference a growth ring of a certain width will contain a greater quantity of material than in trees with a smaller circumference. This factor also accounts for the decline in ring widths seen with increasing cambial age

The ring widths and changing proportions of early, transition and latewood within a growth ring reported in Sections 4.2 and 4.3 are highly dependent on the duration of tracheid maturation experienced following cambial division. Maturation takes place in two phases; radial expansion and secondary cell wall thickening (Dodd and Fox, 1990, Samuels et al., 2006). As seen in Sections 4.4 and 4.5, earlywood tracheids are observed to have larger diameters and thinner cell walls, with the reverse being true of latewood. The type of cell produced is thought to be under the control of hormonal signals and the availability of

materials produced through photosynthesis. The production of the phytohormone indole-3- acetic acid (IAA) during early terminal shoot growth and its basipetal flow through the stem, has been shown to be linked to increases in the production of xylem and phloem within the cambial zone and the promotion of cell expansion (Larson, 1962, Larson, 1969b), resulting in the production of large diameter tracheids. The production of new foliage within the crown at this time is a major metabolic sink of photosynthate, consequently the tracheids produced have thin cell walls which alongside their large diameter typify earlywood. As this newly produced foliage reaches maturity it becomes a net exporter of material to the stem, with the extra carbohydrate available used in the production of tracheid cell wall material. A similarly timed drop in the production of growth hormones sees tracheid expansion reducing and consequently the production of cells with latewood characteristics (Larson, 1964). Renninger (2006) showed that a decline in the production of IAA as a result of cessation of leader growth and an increase in the availability of photosynthate is an independent process in Douglas-fir, possibly correlated with the same environmental cues. The change in tracheid dimensions across the growing season from early to latewood that arises from the processes described above was shown in Sections 4.4 and 4.5.

Section 4.2 shows that whilst earlywood production remains relatively stable at all cambial ages, large changes in the proportions of transition and latewood tracheids take place. Typically, the wood produced within the live crown contains the greatest number of transitional tracheids due to the high concentrations of IAA and material produced through the process of photosynthesis. As cambial age and the distance to the live crown increases at a given sampling location, the concentration of growth hormones declines and as such latewood production usually commences first at the stem base (Larson, 1969a). This accounts for the increase in latewood proportion observed with age in Figure 4-8. Greater nutrient and hormonal concentrations due to the increased quantity of crown foliage at the time of production of wood at the 8 m sampling location, may account for the differences seen in the developmental profiles for early and latewood between the two sample heights at young cambial ages. While many studies of anatomical properties overlook the presence of transitional tracheids, instead apportioning them to either early or latewood, their importance in understanding the physiological changes taking place during the life of the tree have been clearly demonstrated here.

As well as the changes observed to take place within the growing season described above, more gradual changes in the nature of the cell wall material produced were observed at each

sampling location with increasing cambial age. These changes have been shown to be linked to maturation processes occurring within the cambium (Olesen, 1978, Olesen, 1982). The large differences in properties for a given growth ring between the two sampling heights have also been shown to be linked to cambial maturation, with wood produced higher in the stem being ontogenetically older than that produced at the stem base (Schweingruber et al., 2006).

Wood density is a measure of the total amount of cell wall material per unit volume. As such, the profiles of wood density development displayed in Section 4.6 are closely related to the relative proportions of the different cell types described in Section 4.3, as well as the dimensions of the cells and the thicknesses of the cell walls presented in Sections 4.4 and 4.5. The overall trend of increasing density exhibited with age is therefore a result of both increasing latewood proportions and changes to cell wall dimensions due to cambial maturation.

As with density, the decrease in whole ring, early, transition and latewood microfibril angle with cambial age and height observed in Section 4.7 can be attributed to increases in the proportion of latewood within a growth ring, which was observed to have a low microfibril angle, and due to the maturation of the cambium. The exact mechanism by which microfibril orientation is controlled is still uncertain, however it is thought to be associated with the orientation of microtubules within the living cell protoplast during wall formation (Donaldson, 2008). The functional significance of the changes observed in fibril angle within and between growth rings is better understood. The three primary loading conditions imposed upon a stem are lateral forces from wind, bending to a critical angle by animals and gravitational forces from material above (Lichtenegger et al., 1999). The decrease in whole ring mean microfibril angle seen in Figure 4-29 with increasing cambial age is therefore a direct response to these loading conditions, providing the optimal resistance to stem damage as a result of fracture stresses, crack propagation, strains and compressive stress at different times during the life of the tree (Booker and Sell, 1998). The decrease in S2 microfibril angle

observed upon transitioning from early to latewood was shown by Booker and Sell (1998) to be related to the increasing tracheid wall thickness observed over the same period. Thinner cell walls allow for larger lumens in earlywood for the conduction of nutrients through the stem, while vertically aligned microfibrils provide the most efficient vertical load carrying capacity. As such the most efficient orientation of cellulose microfibrils in all parts of a growth ring in older trees, where supporting the weight of the tree above is the primary imposed load, would appear to be vertically aligned. However, thin cell walls and low microfibril angles

would present a large risk of tracheid transwall fracture, hence the relatively high microfibril angles seen in earlywood tracheids at all ages. As the need for sap conduction decreases, the cell wall thickens and cell diameters narrow resulting in a smaller lumen, thicker cell walls are able to sustain lower microfibril angles without increasing the risk of transwall fracture, hence the intra-ring microfibril angle profile seen in Figure 4-33.

Differences between the mean values of the assessed properties due to site were found to be low in the variance components analysis of random effects, generally accounting for less than 10 % and remaining relatively constant in progressive growth periods for each property assessed. A low difference between the mean values for each site was to be expected, given the close geographic spacing, growth taking place over similar timescales and a similar distribution of tree average growth rates within each site. Differences that exist between sites could be accounted for by a number of factors including differences in silvicultural treatments, micro climatic effects at the site, topography, soil conditions and genetic differences. The large variances associated with trees nested within sites were to be expected, given the sampling methodology used to select trees of varying growth rates. It was generally found that in anatomical properties demonstrating a stabilisation of values in later years of growth, that the variance associated with trees increased. This was most likely due to the fall in variance associated with cambial age in successive growth periods, which would have been a component of the variation due to error.

4.9 Concluding remarks

Within this chapter, radial and longitudinal variations of wood anatomical properties known to be of importance in the determination of wood quality attributes have been assessed. All properties showed a change in magnitude upon moving away from the pith, particularly so at younger cambial ages in most cases. Results showing variations in the properties of transition- wood, which have not been widely reported for any softwood species, were of particular interest as they allowed a greater understanding of the developmental processes occurring to be gained. The variations, along with those observed between the two sampling heights, were attributed to the changing roles of the cellular material both within a growing season and as the cambial zone matured. The decline in the rate of change of many anatomical characteristics at older cambial ages was verified in the assessment of a sub-sample of older Douglas-fir trees. As well as providing insightful information on their own, the findings presented here form the basis of the interpretation of many of the results presented in the subsequent chapters.