Preface

Impacts of forest ecosystem management on greenhouse gas budgets

The rapid transformation of natural forest areas into fast-growing exotic species plantations, where the main objective is timber and pulp production, has led to a neglect of other services forests provide in many parts of the world. One example of such a problem is the county of Biscay, where the management of these plantations has negative impacts on the environment, creating the necessity to evaluate alternative tree species for use in forestry. The actual crisis in the forest sector of the region could be an opportunity to change to native species plantations that could help restore ecosystem structure and function. However, forest managers of the region are using the current interest on carbon sequestration by forest to persist with the “pine and eucalyptus culture”, arguing that these species provide a big C sequestration service. Moreover, they are promoting the expansion of eucalyptus plantations to obtain biomass for the pulp and paper industry and for bioenergy. The aim of this paper is to answer the following questions: Is this argument used by the foresters well-founded? or, could the use of native species in plantations improve the C sequestration service in Biscay while avoiding the environmental problems the actual plantations cause?

To answer these questions we created three alternative future scenarios: a) the Services scenario, where there is a substitution of fast-growing exotic plantations by native broadleaf species plantations; b) the Biomass scenario, where there is a bet on eucalyptus plantations; and c) the Business as usual scenario. The changes in the C stock in living biomass in these scenarios have been simulated by a hybrid approach utilising inventories and models, and the period considered was 150 years.

Our results show that the substitution of existing exotic plantations by plantations of native species has the greatest potential for increasing C sequestration. Although short- and mid-term outcomes may differ, when the long-term (more than 50 years) is considered, the C stock in the living biomass in the Services scenario is the greatest, accumulating 38% more C than the Business as usual scenario and 70% more C than the Biomass scenario at the end of the study period. Thus, changing pine and eucalyptus by native species in plantations, while solving some of the environmental problems of the actual plantations, sequesters more C in the long-term. As C sequestration initiatives only make sense if there is a good chance of long-term persistence of the C stocks created, there is no C sequestration argument for the foresters to continue with the actual policy of the use of fast-growing exotic species.

Climatic changes will affect the dynamics of a forest ecosystem. Consequently, carbon sequestration costs can only be estimated correctly if changes in climatic conditions are considered. This article determines the changes in mitigation costs of an optimal forest management regime in the presence of climatic changes and varying prices, and takes account of substitution processes between timber production and carbon sequestration at the stand level. The study demonstrates that in the presence of climate change the sequestration costs per ton of carbon increase with higher amounts of carbon sequestered per hectare. This finding can be used to identify a threshold for the amount of sequestered carbon per hectare below which the costs of carbon sequestration are hardly influenced by climate change.

Uncertainty associated with past land-use emissions restricts quantification of climate change effects. We identify the major affects of commercial forestry initiated over recent decades on Tasmanian primary-forest carbon (C), and search for means to mitigate its ongoing impacts. Spatio-temporal trends were derived from records of commercial operations combined with biomass data. Over the last two decades, the majority of forest C destined for short- or long-term emission (LTE, i.e. over several centuries and multiple harvests) was from clearfelling the higher-biomass wet-eucalypt forests on public land. Carbon dynamics at the unit-area-level for logging two disparate, wet-eucalypt forests were modelled. Parameters were varied to determine management options and model sensitivities under conversion by clearfell and intense burn to either eucalypt plantation or forest regeneration from local eucalypt seed. The first cycle of conversion of primary-forests contributed 43(±5)% to the LTE, and the LTE constituted ∼50% of the primary-forest C stock. Whether the first logging of even-aged primary-forests was prior to or after maturity, the LTEs were equivalent, although short-term emissions (STEs) were ∼2× higher from old-growth. Minor variations in soil organic carbon efflux during operations significantly altered LTEs. Conversion of wet-eucalypt by clearfell from 1999 to 2009 incurred an LTE of 2(±1.6) Tg from each year's logging. Lengthening the harvesting interval for sown forests from 80 to 200 years reduced LTEs by 42% and eucalypt wood-products by 26%; but yielded 40(±20) Mg ha⁻¹ of C in rainforest understorey—helping to sustain mixed-forest ecosystems and their products. Using 200-yr cycles for the wet-eucalypt already clearfelled could avoid LTEs of ∼15 Tg. Long-term C dynamics under harvest cycles were constrained by mathematical precepts that facilitate climate change modelling, e.g. the time to reach the harvesting-cycle's asymptote is correlated to the half-life of the longer-lived C pool. Emissions are not recovered by sequestration in wood-products unless their half-lives are ∼10× contemporary values—requiring 200–1000 years for recovery, during which time emissions would augment global climate change. Emissions can be reduced by product substitution, and by recycling wood-products, in a stable wood market. Primary-forest is part of a global commons. Comprehensive C accounting cannot occur if logging effects are omitted.