Galling in Beech Forests and the Effect of Forest Edge on Herbivore Resistance
Galling is a common problem encountered by almost any metalworker and can be one of the most frustrating parts of working with metal. When it occurs, the results can be a seized fastener that refuses to loosen or be removed. The cause of this is friction between two contacting metal surfaces, and the solutions to the issue can vary from simply using a different lubricant to changing the wrench speed or the direction of rotation.
The occurrence of galling can be seen on all types of metal, but it is most often found in threaded connections and is commonly a result of improper or insufficient lubrication. Stainless steel and aluminum, for example, are more prone to galling because of the formation of an oxide layer between them and the underlying metal. This film prevents the materials from corroding or wearing, but it can be broken under high compressive forces, which is what happens during screw tightening and installation. When this occurs, the metals bond together and the result is excessive heat, a locked fastener or “seize,” and often requires a costly and time-consuming procedure to remove.
The most common causes of galling are excessive heat and friction between the contacting surfaces. These conditions can be caused by several factors, including insufficient lubrication or the use of inappropriate lubricants, which can lead to shear of the protective coating between metals and increase the adhesion between them. In addition, the shearing can produce high local energy densities and elevated frictional forces, resulting in the formation of plastically deforming zones that generate surface protrusions. These asperities can then infiltrate into the brittle oxide layer of the complimentary mating surface and initiate material transfer, resulting in galling.
Although many bottom-up influenced herbivore resistance mechanisms have been proposed to explain patterns of gall density in beech forests, there is little research that explores the interaction between these factors and the effects of forest edge on herivores. We have shown that edge influence on both ecto- and endoparasitism rates of M. fagi significantly reduces galling density and that this is most pronounced for trees in isolated forest fragments. We conclude that the decrease in gall density at forest edges may be due to reduced parasitism and predatory pressure, as well as to climatic changes in temperature, which induce a delayed start of budburst in the host beech tree, F. sylvatica. These changes in vegetation are a consequence of historical habitat fragmentation and appear to be an indirect factor influencing herbivore abundance in beech forest fragments. These results suggest that negative edge-influence may play an important role in determining M. fagi population density. This is consistent with the view that edge-influenced trophic-level processes drive forest patterning at higher levels. A deeper understanding of this relationship could lead to new approaches for the management of herbivore-plant interactions and conservation of beech forest biodiversity.
