Norway spruce heartwood : properties related to outdoor use

Abstract: Degradation of wood is a complex problem dependent on many parameters, but especially the presence of excessive amounts of water causes many problems.The objective of this work was to study the influence of various parameters on water absorption and desorption in Norway spruce (Picea abies (L.) Karst). Focus has been on parameters that are measurable in the industrial chain and that affect liquid water absorption and desorption in end grain of Norway spruce. Most damage occurs in the end grain on products exposed outdoors, and this is often due to liquid water absorption.Computed tomography (CT) scanning and image processing were used to measure liquid water in end grain for 1, 3, 7 and 14 days absorption and desorption in room climate. To ascertain whether the differences observed correspond to differences in microbiological activity and cracking, test objects were exposed outdoors aboveground for 5.5 years. For these studies, spruces were selected with a strategy to obtain as large differences as possible between parameters that could be expected to influence water distribution in the tree, such as crown size, density, age and access to water. All trees were selected from Vindelns Försöksparker (Vindeln's Research Site) in the north of Sweden.The following are the most important findings from this study:Heartwood absorbs less water than sapwood and when dried in room climate reaches a dry level more rapidly than sapwood. Heartwood moisture content (MC) gradients were generally steeper, with lower moisture content than sapwood. After aboveground exposure for 5.5 years, heartwood panels had fewer cracks, shorter crack lengths and less growth of discoloration fungi on the undersides of the panels than sapwood.The MC gradients increase with time in a uniform way. The amount of water absorbed was expressible as a linear function of the square root of time.In the young spruces (37 years old), the border between sapwood and heartwood was less pronounced than in the older trees. There was an intermediate zone with MC gradients more similar to heartwood than to sapwood.In sapwood, there was no difference in MC gradient during absorption and desorption within the height of the tree. In heartwood, there was a tendency to differences in MC gradients from specimens 0.8 m from the butt cut compared to 5.8 and 9.5 m. When absorption and desorption cycles were repeated 3 times, moisture-content gradients in sapwood decreased from cycle 1 through cycles 2 and 3. In heartwood, the moisture content gradients followed almost the same curve for cycles 1, 2 and 3.Separation of sapwood and heartwood in dry state with near-infrared spectroscopy (NIRS) and multivariate analysis is possible. The visible-wavelength spectrum has a significant influence on the predictive power of separation models.Saw simulation has shown that it is possible to produce boards for outdoor cladding with 100% heartwood. However, the products have to be sawn from bigger logs than are normally used today. This has a negative effect on the yield, but this can be compensated for by extracting more side boards or by higher sales prices.