Canopy arthropods and herbivory on the Tasmanian southern beeches, Nothofagus cunninghamii and Nothofagus gunnii

The Gondwanan relict tree genus Nothofagus has generated much research due to its biogeographical significance in understanding the biota of the Southern Temperate Zone. However, the southern temperate forest arthropod fauna, including that of Nothofagus, has received less attention than northern temperate forest and tropical rainforest. The extant Tasmanian Nothofagus canopy arthropod fauna has been considered depauparate. Yet with a palaeohistory of long persistence, high diversity and widespread distribution of the genus within Australia, the endemic Tasmanian Nothofagaceae – the evergreen Nothofagus cunninghamii and winter deciduous N. gunnii - could be expected to have accreted a large and diverse arthropod fauna, as have its sister Fagalean families in the northern hemisphere. Conversely, range contraction of Tasmanian Nothofagus in the increasingly arid conditions of the late Tertiary, and its cyclical isolation from the Australian mainland during the glacial cycles of the Pleistocene, could have depleted the associated arthropod fauna. Thus the aims of this project were to document in detail the diversity and structure of the invertebrate communities associated with both species of Tasmanian Nothofagus; to build an inventory of canopy arthropod fauna according to feeding guild; and to measure the levels of herbivory, as an indication of herbivore activity, on the these host species. These data enabled comparison of the arthropod communities on the two hostplants, and comparison with similar studies on closely related tree species (Nothofagus elsewhere, northern temperate Fagales) and other Australian tree taxa, now widespread (Acacia and Eucalyptus). A cross section of survey sites was chosen, 17 for N. cunninghamii, and 12 for N. gunnii, including three where the host plants were sympatric. Canopy arthropods were sampled by branch beating, and leaves were collected from, or examined in situ on marked branches. The sampling schedule from March 1998 until March 2002 yielded data sets by Nothofagus species, site, sampling aspect, season, region, and between years. Priority was given to arthropod orders containing herbivorous taxa or scavenging taxa known to forage on trees. These were sorted to family, then genus and species where possible, otherwise to morphospecies; and allocated to feeding guilds. The leaves were designated ‘chewed’ or ‘intact’ and the leaf area lost from the chewed leaves was estimated. Mean percentage leaf area loss was calculated as a measure of herbivory. The main finding from these investigations was that there existed a strong contrast in the diversity and taxonomic profile of the arthropod fauna between the two Nothofagus species. For N. cunninghamii, the associated fauna was relatively rich in taxa and comparable with other temperate zone trees in both its diversity and guild structure. Similarly the N. cunninghamii herbivory levels were within the range of those found on temperate and subtropical tree taxa. N. gunnii in contrast was clearly depauparate, particularly lacking many important folivorous groups such as chrysomelid beetles, leaf miners and aphids. However there was, for the two Nothofagus species, considerable local variation in the arthropod communities and levels of herbivory - between regions; between sites within a region; and within a site, seasonal variation and also differences between consecutive sampling years - insights which have significance for effective biodiversity surveys and conservation strategies.