The Insects Infesting Pinus Andnothofagus

The distribution ofliving and fossil Nothofagus in the southern hemisphere (Skottsberg 1949) suggests the possibility that it once played a part in colonising primitive and unstable soils comparable with that played by Pinus in the northern hemisphere. Each genus includes species which colonise rocky soils of mountain ranges and glacial, alluvial and volcanic deposits; each genus is, or has been, represented from circumpolar regions to the highlands of the tropics. Those species best adapted as pioneers are most tolerant of variations in soil moisture, nutrients and temperature, but either require insolation of the soil for germination or are intolerant of shade as seedlings so that HItle regeneration survives under closed canopy. Since adjacent trees are commonly interconnected through root grafts, deaths of individual stems do not normally provide root space for the development of seedlings. Larger gaps in the canopy resulting from storm damage or other natura] causes of group mortality tend to be rapidly filled with dense regeneration. For these reasons, forests of pioneer species are typically even-aged except near advancing margins or where group mortality has occurred (Wardle 1970, Wardle 1974, p. 23 for Nothofagus solandri). Mortality as a result of competition in even-aged stands ensures a continuing supply of host material in addition to that provided intermittently by climatic damage and senescence for those organisms which thrive on weakened or moribund tissues. Such a coupling of extensive geographic distribution and mortality resulting from intraspecific competition has probably provided environments conducive to the evolution of aggressive insects and pathogens of forest trees. To the extent that this is so, comparison of the insect complexes of Pinus and Nothofagus may be meaningful although the genera belong in different subdivisions of the Spermatophyta.