The level of occlusion of included bark affects the strength of bifurcations in hazel (Corylus avellana L.)

Bark-included junctions in trees are considered a defect as the bark weakens the union between the branches. To more accurately assess this weakening effect, 241 bifurcations from young specimens of hazel (Corylus avellana L.), of which 106 had bark inclusions, were harvested and subjected to rupture tests. Three-point bending of the smaller branches acted as a benchmark for the relative strength of the bifurcations. Bifurcations with included bark failed at higher displacements, and their modulus of rupture was 24% lower than normally formed bifurcations, while stepwise regression showed that the best predictors of strength in these bark-included bifurcations were the diameter ratio and width of the bark inclusion, which explained 16.6% and 8.1% of the variability, respectively. Cup-shaped, bark-included bifurcations where included bark was partially occluded by xylem were found, on average, to be 36% stronger than those, where included bark was situated at the bifurcation apex. These findings show that there are significant gradations in the strength of bark-included bifurcations in juvenile hazel trees that relate directly to the level of occlusion of the bark into the bifurcation. It therefore may be possible to assess the extent of the defect that a bark-included bifurcation represents in a tree by assessing the relative level of occlusion of the included bark

Grasses and the resource availability hypothesis: the importance of silica-based defences

The resource availability hypothesis (RAH) predicts that allocation of resources to anti-herbivore defences differs between species according to their growth rate. We tested this hypothesis by assessing the growth and defence investment strategies of 18 grass species and comparing them against vole feeding preferences. In addition, we assessed the effectiveness of silica, the primary defence in many grasses, in influencing vole feeding behaviour. Across species, we found that there was a strong negative relationship between the overall investment in defence and growth rate, thus supporting predictions of the RAH. However, no such relationship was found when assessing the various individual anti-herbivore defences, suggesting that different grass species show significant variation in their relative investment in strategies such as phenolic concentration, silica concentration and leaf toughness. Silica was the most influential defensive factor in determining vole feeding preference. Experimentally induced increases in leaf silica concentration deterred vole feeding in three of the five species tested, and altered feeding preference ranks between species. The strong positive relationship between silica concentration and leaf abrasiveness, when assessed both within and between species, suggests that increased abrasiveness is the mechanism by which silica deters feeding. Although grasses are often considered to be tolerant of herbivore damage rather then defended against it, they do follow predictions of defence allocation strategy based on their growth rates, and this affects the feeding behaviour of generalist grass-feeding herbivores

The mechanical properties of wood and the design of Neolithic stone axes

Despite the importance of wooden tools for early man, and the development of woodworking in the Mesolithic and Neolithic culture, there has been surprisingly little research on how wood can be worked by stone tools or how wooden handles for composite tools were designed. This paper outlines an approach based on an understanding of the structure and mechanical properties of wood. The cell arrangement in wood makes it far less stiff, strong and tough across the grain, especially tangentially. This makes it hard to harvest wood or break it into lengths because it splits down its centre rather than breaking right across. Fortunately, this also makes wood easy to split along the grain, especially radially through its centre into sections and planks. A model of the splitting process predicted that wood is best split using blunt, broad but smooth wedges, as these would use less energy and would be less likely to get stuck in the wood. The predictions were verified in tests in which hazel coppice poles were split using wedges of contrasting angle, width and surface texture. The results help explain the change from the flaked flint Mesolithic tranchet axes to the broader polished stone Neolithic axe and adze heads. However, further experiments are also needed cutting wood obliquely to test this hypothesis. The splitting model also helps to understand the design of socketed axe hafts. Failure usually occurs when the handles split at the distal and proximal ends of the socket. To prevent this, handles are best designed with the growth rings parallel to the socket, and with an expanded head, especially with flanges on the distal and proximal ends of the socket. These designs are seen in some of the Neolithic axe handles that have been found in Britain, including the Etton, Ehenside and Shulishader axes. More experimental research is needed to understand the optimal way of hafting axe heads