Vegetation Data, Standing Above Ground Biomass, Far North Queensland Rainforest SuperSite, Robson Creek, Core 1 ha and 25 ha plots, 2013

For the first time we report a measurement of basal area and a calculation of above ground living biomass for an Australian tropical rainforest derived from a comprehensive ground survey of a large area. Previous published values (Liddell et al. 2007; Preece et al. 2012) have been based on small plot sizes which may lead to inaccurate estimates particularly if large trees are present. However, caution should be taken in assuming that our figure of 418 Mg ha-1 is representative of a natural forest, as this tract of forest and extensive areas of similar forests have been modified by logging during the 20th century. Although logging was seen as best practice at the time as it selected for species and stem size, it was a major driver of floristic and stand structure change of both this plot and Australian tropical rainforest in general. Values for extracted timber in the Wet Tropics (Crome et al. 1992) indicate that stem (6.6 trees ha-1) and volume (37 m3 ha-1) loss was minimal. However, the incidental damage imposed on the forest, including 22% loss of canopy cover, was presumably enough to alter short term growth and recruitment dynamics. On the Robson creek plot the large proportion of stems 10-20 cm DBH and the prominence of early successional species such as Litsea leefeana, Cardwellia sublimis and Flindersia bourjotiana more than 40 years after logging ceased are evidence of this. Whether the subsequent recruitment and growth of such species has recovered or even exceeded the biomass lost by extraction of timber and associated incidental loss is debateable and cannot be tested as there are no similar large areas of unlogged forest. However, a 0.5 ha unlogged plot adjacent to the Robson Creek plot has an equivalent AGB of 700 Mg ha-1 (Murphy et al. 2013) which is considerably larger than any value recorded on the 25 ha plot.

Recent severe disturbance events in Australian tropical rainforests are not restricted to anthropogenic drivers. Cyclones are particularly important structuring elements of rainforest in the wet tropics of Australia with historical data suggesting that a severe cyclone (categories 4–5) will cross the coast of Australia about every 75 years (Turton & Stork 2009). Severe cyclone Larry in 2006 caused major structural damage to the forest around the plot including the mortality of 74 trees ha-1 ≥10 cm DBH (Metcalfe et al. 2008; Murphy et al. 2013). Moreover, current aggregations on the Robson 25 ha plot of large Blepharocarya involucrigera (Anacardaceae), a successional species, suggest at least one large disturbance event on a centurial time scale.