**3. Results**

#### *3.1. Shelling of Growth Rings in Flat Western Larch Deckboards*

Shelling was clearly evident at the surface of flat western larch deckboards exposed pith-side-up to the weather. The confocal profilometry height map and adjacent matching photographic image in Figure 2 show that shelling of western larch deckboards was due to oblique checking at the interface between latewood and earlywood and the projection of latewood above the deckboard surface, as in Figure 2a,b.

**Figure 2.** Shelling of growth rings in a flat western larch deckboard oriented pith-side-up and exposed to the weather for 7 months in Vancouver, BC, Canada. Four shelled growth rings including a shelled latewood tip are labelled, 1, 2, 3, and 4: (**a**) confocal profilometry height map showing four shelled growth rings; (**b**) photograph of four shelled growth rings matching the height map in Figure 2a; and, (**c**) confocal profilometry line scan showing the heights of the shelled growth rings in Figure 2a,b.

The three-dimensional (3D) profilometry height map and line scan of a weathered western larch deckboard with growth rings oriented pith-side-up shows the extent to which latewood projected above the wood surface, Figure 2a,c. For example, the numbered (1, 2, and 4) red coloured regions of the

profilometry height map in Figure 2a correspond to the similarly numbered latewood bands in growth rings in Figure 2b. The heights of latewood bands in Figure 2a and the line scan Figure 2c indicate that one of the latewood bands (number 2) projected approximately 0.3 mm above adjacent earlywood. The latewood tip in Figure 2a,b (number 3) projected 0.15 mm above earlywood. The central area of Figure 2a,b is where a growth ring is parallel (0◦) to the underlying surface. The heights of shelled latewood tend to be greatest in growth rings adjacent to this central region (Figure 2c), and then diminish in height further away from the central part of the deckboard. Latewood separated from the earlywood in the adjacent growth ring (inter-ring failure) rather than from earlywood in the same ring (intra-ring failure).

Table 2 shows profilometry measurements of the heights that shelled growth rings projected above the surface of each of the six western larch deckboard samples oriented pith-side-up and exposed to the weather. This table also includes the density of the parent boards and the numbers of growth rings per cm. The extent to which shelled growth rings projected above the surface of deckboard samples varied between the six boards, but there appeared to be no relationship between wood density and growth ring width and severity of shelling, as in Table 2.

**Table 2.** Wood properties and shelling of six different flat western larch deckboards oriented pith-side-up and exposed to the weather in Vancouver, BC, Canada for 7 months.


1 Average height of shelled growth rings. Max and min in parentheses; 2 Total number of growth rings in parentheses.

There was an inverse relationship between the height of shelled growth rings and the angle growth rings made to the surface of flat boards oriented pith-side-up, as in Figure 3. In other words, shelled growth rings with a smaller angle to the surface tended to project further from the surface than growth rings that were inclined at a higher angle to the surface. The regression between shelled growth ring height and growth ring angle was very significant (*p* < 0.001), but the correlation coe fficient was low ( *R*<sup>2</sup> = 0.327). This may be caused by our small sample size.

Shelling did not occur at the surface of flat weathered deckboards oriented bark-side-up, although checking was more pronounced at the surface of boards oriented bark-side-up compared to those oriented pith-side-up, Figure 4, in accord with the results of previous studies [5,8]. Many of the checks in boards oriented bark-side-up occurred at the interface of latewood and earlywood (Figure 4), but did not cause growth rings to project from the surface of the wood, as was observed in boards oriented pith-side-up. Latewood was raised slightly above the surface of adjacent earlywood possibly due to springback or di fferential swelling of latewood and/or increased degradation and erosion of earlywood compared to latewood [6,26].

**Figure 3.** Linear regression (red line) of heights of latewood in shelled growth rings versus growth ring angle in flat western larch deckboards oriented pith-side-up and exposed to the weather for seven months in Vancouver, Canada; regression includes 95% confidence intervals (blue-lines).

**Figure 4.** Appearance of a western larch deckboard with growth rings oriented bark-side-up and exposed to the weather for 7 months in Vancouver, Canada: (**a**) confocal profilometry height map showing surface checking and roughening of the wood surface; (**b**) photograph matching the image in Figure 4a; and, (**c**) confocal profilometry line scan showing variation in height of images in Figure 4a,b.

#### *3.2. Shelling of Growth Rings in Flat ACQ-Treated Douglas Fir, Western Hemlock and White Spruce Deckboards*

Confocal profilometry imaging of flat ACQ-treated western hemlock, white spruce, and Douglas fir deckboards oriented pith-side-up and exposed to the weather for 18 months showed that shelling in these species was due to inter-ring delamination of growth rings and projection of latewood above adjacent earlywood, as in Figure 5. In western hemlock and white spruce, latewood in shelled growth rings projected over 1 mm above the surface, and splitting of delaminated latewood occurred in western hemlock creating sharp splinters. The delamination of growth rings and projection of latewood above weathered deckboard surfaces was more pronounced in western hemlock and white spruce than in Douglas fir boards.

**Figure 5.** Confocal profilometry height maps showing shelling of growth rings in alkaline copper quaternary (ACQ) treated (**a**) western hemlock, (**b**) white spruce, and (**c**) Douglas fir deckboards with growth rings oriented pith-side-up and exposed to the weather for 18 months in Vancouver, BC, Canada.

Scanning electron microscopy was used to examine the interface between latewood and earlywood in shelled growth rings in Douglas fir, western hemlock, and white spruce deckboards, as in Figure 6. Images of shelled growth rings in all three species show a check between thicker-walled latewood cells and thinner-walled earlywood cells. In western hemlock, there was evidence that delamination of growth rings resulted from fracture of thin walled earlywood tracheids, as shown in Figure 6d,f. All three species showed evidence of photodegradation, for example, pit micro-checking (arrowed in Figure 6a,c) and colonization of the wood surface by mould fungi (arrowed in Figure 6d).

**Figure 6.** Scanning electron microscopy images of shelled growth rings in small samples from flat ACQ-treated Douglas fir, white spruce and western hemlock deckboards with growth rings oriented pith-side-up and exposed to the weather for 18 months in Vancouver, Canada: (**a**) Douglas fir sample showing separation of latewood (right) from earlywood. Note pit micro-checking in earlywood (arrowed bottom left); (**b**) white spruce sample showing separation of latewood (right) from earlywood; (**c**) western hemlock sample showing separation of latewood (right) from earlywood. Note pit micro-checking in earlywood (arrowed centre left); (**d**) western hemlock sample showing failure of earlywood cell wall (centre) where latewood separates from earlywood. Note mould in earlywood (arrowed bottom left); (**e**) a cross section through a shelled growth ring in western hemlock showing separation of latewood (left) from earlywood; and, (**f**) a cross section through a shelled growth ring in western hemlock showing failure of earlywood cell walls where latewood (left) separates from earlywood. Scale bars (**<sup>a</sup>**–**c**,**<sup>e</sup>**) = 100 μm; (**d**,**f**) = 20 μm.

#### *3.3. Shelling of Growth Rings in Profiled Western Larch Deckboards*

Weathering caused the separation of latewood and earlywood in some profile peaks in profiled deckboards with growth rings oriented pith-side-up. The separation of growth rings created lanceolate-shaped slivers (tips) of latewood that projected above the surface of deckboards, as in Figure 7. Figure 7a,c shows the extent to which the slivers projected above the rib of a profiled board. Shelling at the edges of growth rings was less obvious in profiled deckboards than in flat boards, even though latewood clearly separated from earlywood Figure 7b.

Growth rings rarely separated (shelled) at the interface of latewood and earlywood at the peaks of profiled deckboards with growth rings oriented bark-side-up. However, latewood tips were slightly raised above the surface of adjacent earlywood, as can be seen in the profilometry height map and photograph in Figure 8. The extent to which latewood was raised above the surface of the adjacent earlywood was much smaller than that of shelled latewood tips in profiled boards oriented pith-side-up (compare Figures 7c and 8c).

**Figure 8.** Tip of a growth ring raised above the peak of a profiled (short rib) western larch deckboard with growth rings oriented bark-side-up and exposed to the weather for seven months in Vancouver, Canada: (**a**) confocal profilometry height map of a latewood tip; (**b**) photograph of a latewood tip matching the image in Figure 8a; and, (**c**) confocal profilometry line trace showing variation in height of the arrowed rib in Figure 8b.

We counted the number of latewood tips that visibly projected above the surface of profile peaks in boards with different growth ring orientations (pith-side-up and bark-side-up) and profile geometries (rib, short rib, tall rib, ribble, and ripple). Analysis of variance indicated that the number of visible latewood tips at the surface of profiled and weathered deckboards oriented pith-side-up was significantly (*p* < 0.001) greater than that of similarly weathered boards oriented bark-side-up, as in Figure 9.

**Figure 9.** Effects of growth ring orientation on the numbers of latewood tips that projected above the surface of profiled western larch deckboards exposed to natural weathering for seven months in Vancouver, Canada. The results are averaged across boards with different profiles because there was no significant (*p* > 0.05) interaction of growth ring orientation and profile type on numbers of raised latewood tips. Error bars = Least significant difference, *p* < 0.05.

There was a small, but statistically significant (*p* = 0.036) effect of profile type (rib, ribble, and ripple, etc.) on the number of latewood tips that visibly projected above the surface of peaks in profiled boards, Figure 10, but the interaction of growth ring orientation *x* profile type was not significant (*p* > 0.05). Boards with a wavy ribble profile had significantly (*p* < 0.05) fewer visible latewood tips when compared to other profiled boards, except for boards with a wavy ripple profile (Figure 10).

**Figure 10.** Effects of profile type on the numbers of latewood tips that projected above the surface of profiled western larch deckboards exposed to natural weathering for seven months in Vancouver, Canada. Results are averaged across boards with different growth ring orientations because there was no significant (*p* > 0.05) interaction of growth ring orientation and profile type on numbers of raised latewood tips. Error bars = Least significant difference, *p* < 0.05.
