Forest endophytes: pattern and process
Studies on taxonomy, distribution, and possible function have dominated the literature on forest endophytes since these fungi were first noted. Endophyte distributions have been described at a variety of scales ranging from those at a landscape level to those at the finest scales of resolution within a single leaf or patch of bark. At a landscape level, distribution patterns seem to be determined by host specificity, liquid precipitation, canopy cover, and geographic continuity or disjunctness. At the level of the individual stand or tree, height in the crown may become important. At the level of individual branch systems, age of substrate appears the most important factor. At a microscopic scale, tissue specificity, leaf topography, and phenology of the infection process with respect to leaf development all play a role. Endophytic fungi have seldom been found to cause widespread disease in trees; furthermore, their age-specific infection frequencies in conifer needles show continuous increases with needle age, in contrast to needle pathogens in which infections are largely confined to young needles. Endophytes are usually viewed as protective mutualists acting against herbivorous insects and pathogenic fungi. Proof of mutualism requires not only that endophytes be present in tissues under attack and their presence be correlated with, for example, increased insect mortality, but that inoculation experiments and comparisons with uninoculated plant tissues also be done. Such experiments often show no differences between treatment and control plants, results that may be related to unsuccessful inoculation of the endophyte. Such results may also be rooted in an inability to reproduce the biological complexity present in natural forest stands. Multiple infections of single leaves may lead to synergistic interactions of toxins produced by endophytes. Insect herbivores have evolved behavioral responses to the presence of endophytes, and protective mutualism by endophytes may occur only intermittently. Key words: endophyte, distribution, mutualism, gall midge, toxins, coevolution.