Herbivory and a Simple Field Test for Total Phenolics in Trees
Many deciduous and coniferous trees contain varying levels of phenolics, including tannins. “Blackwater rivers” in forested areas of the world, such as the Amazon basin or the southern United States, carry phenolics (tannins) leached out of trees. Such rivers differ in plant and animal life from “whitewater rivers.” The biological functions of such plant secondary metabolites (PSMs) have been debated for a long time. They often have antimicrobial functions, but also serve as repellents and feeding inhibitors against herbivorous insects and vertebrates, notably birds and mammals. Animals have evolved many mechanisms to cope with phenolics in their diet. These start with food processing. For instance, beavers consume experimental sticks of the phenolics-rich witch hazel only after leaving them in the water for 2–3 days, apparently to leach out unpalatable compounds (Müller-Schwarze et al. 2001). Many birds and mammals eat clay to adsorb phenolics so they never will be absorbed in the intestines. If they are taken up in the blood stream, such PSMs will eventually be rendered harmless by oxidation and other processes, followed by conjugation, in the liver. They then will be excreted in the urine. The levels of phenolics vary with plant parts and season. Most valuable parts such as buds, flowers, and catkins are more heavily defended. For the winter, more phenolics are translocated to the bark. This is thought to intensify defense at a time when the tree is dormant and cannot respond to herbivore damage by wound healing and regrowth. Among the phenolic glycosides, salicin (saligenin glycoside) and salicortin occur in the bark of willow (Salix spp.) and “poplar” (Populus spp.), tremulacin in the bark of P. tremula and P. tremuloides. The heartwood of P. tremuloides contains tremulone and related compounds.