Dynamics of vegetation after cable and ground-based logging

The relationship between forest resource management and the conservation of biological diversity. is a controversial one, with the steep country logging of oldgrowth forests in Tasmania being the source. of much of this attention, due to the perceived greater impacts of logging on steep slopes and the greater damage caused by the large modern cable systems. Retrospective (chronosequence) and permanent plot studies of cable and groundbased logging in forests aged 0-54 years on steep slopes in southern Tasmania and comparable unlogged sites indicated that following both logging treatments, there was a gradual recovery in floristic composition towards mature forest. Vascular plant species richness recovered more rapidly in the ground-based logging treatment than the cable logged treatment. Differences between treatments were confined to the initial recovery stages where treatment effects were evident in the extent of soil, woody debris and micro-habitat disturbance. Following canopy closure and self-thinning (i.e. after 20-30 years) sites logged by the two techniques resemble each other in vascular species composition and that of mature unlogged forest. However, differences still existed in forest structure and in the composition of the obligate epiphytic fern flora (particularly that present on Dicksonia caudexes). Generally cable logging did less physical damage to vegetation than ground-based logging. Ground-based logging led to a greater degree of floristic change than cable logging. The pattern of vegetation change was related to the degree and nature of soil and habitat disturbance. Conventional ground-based thinning of 30 year old regrowth forest led to a greater change in the composition of the ground stratum (an increase in the cover of weed and other herbaceous species) than cable thinning. Cable thinning caused little soil disturbance and ground fern species rapidly regenerated rhizomatously. Herbaceous and weed species were infrequent after cable thinning. Cable logging practices adjacent to or through retained stands of streamside vegetation can result in significant changes to the vegetation. This can occur during all phases of tree falling, establishment and movement of cable settings, yarding, fire break construction and the regeneration burn. Many of the detrimental effects of cable logging adjacent to retained stands of streamside vegetation are indirect, resulting from actions such as windthrow, tree health decline and crown scorch from regeneration burning. While there has been an improvement in forest practices since the 1980's when the attempted full suspension logging over streamsides was ended, the on-going health and viability of retained streamside vegetation is still in doubt. Ground-based logging inflicted greater levels of mortality than cable logging on populations of the tree fern .Dicksonia antarctica, a functionally important understorey species in· wet forests. Approximately 80% of the D. antarctica resprouted following cable logging prior to burning. Only 40% survived the regeneration burn. This was reduced to 27% after 10 years. While ground-based logging resulted in greater mortality, it led to a greater degree of new sporophytic recruitment to the D. antarctica population. The significant reduction in the abundance of this slow growing species following logging has implications for the conservation of obligate vascular epiphytes that use it's caudex as a substrate. The vascular epiphytic flora, the primary late successional specialist group, was almost completely lost following cable logging. Epiphytes were slow to recolonise following either cable or ground-based logging, although their recovery was more rapid following ground-based logging. While most of the vascular epiphytic species recovered 30-50 years following either logging method, their abundance was reduced and the diversity of substrates available for colonisation was limited. The response of vegetation to logging treatment was studied at the three levels; the community, the functional group and the species. No general statement can be made about the logging treatments across these levels as the treatment response was not uniform. However, the individual species level of response provided most of the understanqing of processes operating following either logging method in terms of how these treatments differ in their spa~ial variability, the biological legacies they create and t;heir intensity. The use of permanent plot monitoring associated with the chronosequence studies effectively validated the generalised chronosequence trends, at least for the short term predictions, and also questioned many of the trends described elsewhere in the literature from chronosequence studies which lack the rigour of an individual species level of experimental treatment and monitoring. The examination of cable and ground-based logging in a controlled manner has provided a depth of understanding to previous logging studies that was generally not available. It has added additional emphasis to the role of logging as both a process of vegetation disturbance and one of establishing biological legacies which mediate the regenerative response. It also questions much of the literature which describes vegetation responses in terms of generalised hypotheses, eg the intermediate disturbance hypothesis, as vegetation responses are not uniform across differing levels of examination.