Microbiological activity clearly affects the mechanical strength of leaves. Although it cannot be denied that with most species the loss of mechanical strength is the result of both invertebrate feeding and microbiological breakdown, the example of Fagus sylvatican illustrates loss without any sign of invertebrate attack being evident. Fagus shows little sign of invertebrate attack even after being exposed for eight months in either lake or stream environment, but results of the rolling fragmentation experiment show that loss of mechanical strength,even in this apparently resistant species, is considerable.
Most species appear to exhibit a higher rate of degradation in the stream environment than in the lake. This is perhaps most clearly shown in the case of Alnus. Examination of the type of destruction suggests that the cause for the greater loss of material in the streamprocessed leaves is a combination of both biological and mechanical degradation. The leaves exhibit an angular fragmentation,which is characteristic of mechanical damage, rather than the rounded holes typical of the attack by large particle feeders. As the leaves become less strong,the fluid forces acting on the stream nylon cages caused successively greater fragmentation.
Mechanical fragmenation, like biological breakdown, is to some extent influenced by leaf structure and form. In some leaves with a strong midrib, the lamina break up, but the pieces remain attached by means of the midrib. One type of leaf may break clean while another tears off and is easily destroyed once the tissues are weakened by microbial attack.
In most species, the mechanical breakdown will take the form of gradual attrition at the margins. If the energy of the environment is sufficiently high, brittle species may be broken across the midrib, something that rarely happens with more pliable leaves. The result of attrition is that, where the areas of the whole leaves follow a normal distribution, a bimodal distribution is produced, one peak composed mainly of the fragmented pieces, the other of the larger remains.
To test the theory that a thin leaf has only half the chance of a thick on for entering the fossil record, all other things being equal, Ferguson (1971) cut discs of fresh leaves from 11 species of different leaf thickness and rotated them with sand and water in a revolving drum. Each run lasted 100 hours and was repeated three times, but even after this treatment, all species showed little sign of wear. It therefore seems unlikely that leaf thickness alone, without substantial microbial preconditioning, contributes much to the probability that a leaf will enter a depositional environment in a recognizable form. The result of experiments with whole fresh leaves show that they are more resistant to fragmentation than leaves exposed to microbiological attack. Unless the leaf is exceptionally large of small, leaf size and thickness are not likely to be as critical in determining the preservation potential of a leaf type as the rate of microbiological degradation.
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