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.

本研究首次报道了基于大面积全面地面调查的澳大利亚热带雨林基面积测量结果，以及地上活体生物量的计算结果。此前已发表的相关数值（Liddell等，2007；Preece等，2012）均基于小型样地，此类样地可能导致估算结果存在偏差，尤其当样地内存在大型乔木时偏差更为显著。不过，需谨慎看待本研究得到的418 Mg ha⁻¹这一数值能否代表天然林现状——因为该林区及其他大片同类林区在20世纪均经历过采伐改造。尽管当时的采伐作业遵循彼时的最佳实践，即针对特定物种和径级开展择伐，但它仍是导致该样地乃至澳大利亚热带雨林整体区系组成与林分结构发生改变的主要驱动因素。湿热带地区提取木材的相关统计值（Crome等，1992）显示，样地的单株损失量（6.6株 ha⁻¹）和材积损失量（37 m³ ha⁻¹）均较低。但采伐过程中对森林造成的附带损伤——包括22%的冠层覆盖率损失——想必已足以改变森林的短期生长和更新动态。在罗伯森溪样地，采伐停止40余年后，径级10-20 cm的茎干占比偏高，且Litsea leefeana、Cardwellia sublimis和Flindersia bourjotiana等早期演替物种占比突出，这正是上述采伐影响的佐证。目前尚无定论的是，此类物种后续的更新与生长是否已恢复甚至超过了因木材采伐及附带损失所流失的生物量，且由于未存在同类大面积未采伐林区，该问题也无法得到验证。不过，毗邻罗伯森溪样地的一块0.5 ha未采伐样地，其地上生物量（above ground biomass, AGB）达到700 Mg ha⁻¹（Murphy等，2013），远高于该25 ha样地的所有观测值。澳大利亚热带雨林近期遭受的严重干扰事件并非仅由人为活动驱动。热带气旋是澳大利亚湿热带雨林林分结构形成的关键自然因子，历史数据显示，每75年左右就会有一场强热带气旋（4~5级）登陆澳大利亚沿海地区（Turton与Stork，2009）。2006年的强热带气旋“拉里”对该样地周边森林造成了严重结构损伤，导致胸径（Diameter at Breast Height, DBH）≥10 cm的74株 ha⁻¹乔木死亡（Metcalfe等，2008；Murphy等，2013）。此外，罗伯森溪25 ha样地内现存的大片大型演替物种Blepharocarya involucrigera（漆树科，Anacardiaceae），暗示该区域在百年尺度上至少发生过一次大型干扰事件。