Dynamics of carbon exchange in a <I>Eucalyptus</I> forest in response to interacting disturbance factors

Abstract

Carbon (C) sinks created by forests depend on the balance between C uptake through photosynthesis and loss through respiration. This balance varies depending on the relative effect of environmental drivers on these processes. Components and dynamics of the C cycle were measured in a native Eucalyptus delegatensis forest at Tumbarumba in south-eastern Australia during conditions of average rainfall (1998-2001) and droughts (2002-2003 and 2006-2007). In 2002-03 there were interacting disturbance factors of dry conditions and insect damage that reduced the photosynthetically active leaf area in the canopy. Conditions during the droughts included reduced soil moisture content, higher temperatures and increased vapour pressure deficit. Similarly low soil moisture contents occurred during both drought periods, but lasted for longer in 2002-03. The combined impact of drought stress and insect damage resulted in markedly reduced growth (45-80%) and higher mortality of trees (5-60%). Impacts were variable across the 50,000ha of forest, with mortality greatest in stands with normally highest growth rates, and in locations with greatest reductions in soil moisture. Growth rates were reduced during 2002-03, most severely in 2003-04, and recovering in 2004-05. Mortality remained high in 2004-05 indicating the prolonged effect of the stress conditions. The total C pool in the forest is 483tCha -1 with net C uptake of -6.7tCha -1yr -1 in the growing season prior to the insect attack (2001-02). Under conditions of drought and insect disturbance in 2003 the forest released a total of 1.7tCha -1 over 8 months, while under conditions of drought alone carbon uptake was -6.5tCha -1yr -1 in the growing season year of 2006-07 and -5.6tCha -1yr -1 in the calendar year of 2007. Interacting stress factors of drought and insect damage resulted in a large imbalance in the C budget with a 26% reduction in gross primary productivity and a 9% reduction in ecosystem respiration, whereas drought alone had a much lesser effect. Drought conditions result in (1) weather conditions that break the synchronisation of insects with parasites and predators resulting in insect outbreaks, (2) moisture stress that predisposes trees to attack by insects, and (3) moisture stress that restricts leaf regeneration after damage. Climate change and climatically induced changes in disturbance regimes may affect the processes of photosynthesis and respiration differently and hence change the balance of net ecosystem C exchange. Reduced sink strength of forests will lead to positive C cycle-climate feedbacks, which enhance the greenhouse effect and climate change.