Evaluating Carbon Stock Changes in Forest and Related Uncertainty

Pasquale Marziliano, Giuliano Menguzzato, Vittoria Coletta
2017 Sustainability  
For the evaluation of changes in the carbon stock of living biomass, two methods are reported in the Good Practice Guidance for Land Use, Land-Use Change, and Forestry: (1) the default method, which requires the biomass carbon loss to be subtracted from the biomass carbon increment for the reporting year; and (2) the stock change method, which requires two consecutive biomass carbon stock inventories for a given forest area at two points in time. We used three methods to estimate above-ground
more » ... mate above-ground biomass: (1) application of allometric equations, (2) constant BEF (biomass expansion factor), and (3) age-dependent BEF, following which we evaluated the changes in carbon stock and the related uncertainty. Our study was carried out in a Douglas fir plantation composed of plots with three different planting densities, monitored at three different ages (15, 25, and 40 years old). Results showed the highest uncertainty in the estimates based on the constant BEF, whereas the use of allometric equations led to the lowest uncertainty in the estimates. With a constant BEF, it is usually difficult to obtain a reliable value for the whole tree biomass because stem proportion increases with tree size at the expense of the other components. The age-dependent BEFs aim to reduce the bias representing the actual change in stock, thus we found a lower uncertainty in the estimates by using this method compared to the constant BEF. The default method had the highest uncertainty (35.5-48.1%) and gave an estimate higher by almost double compared to the stock change method, which had an uncertainty ranging from 2.9% (estimated by the allometric equation) to 3.4% (estimated by the constant BEF). a stand characteristic (i.e., dimensions of the median tree of a stand, merchantable stem volume, or stand age) [8, 9] . Investigation and quantification of tree biomass forms the basis of estimates of forest carbon pools and is therefore directly linked to some of the mechanisms for carbon offsetting and sequestration enshrined in the Kyoto protocol [10] . Accurate estimates of forest biomass are necessary since, as a Party to both the United Nations Framework Convention on Climate Change (UNFCCC) and its Kyoto Protocol, replaced in 2015 by the Paris Protocol, the European Community has to submit its annual GHG (greenhouse gases) inventory. In the second commitment period, which is ongoing, Parties must reduce GHG emissions by at least 18% below 1990 levels in the eight-year period from 2013 to 2020. Under these agreements [11, 12] it has become necessary to develop methods to estimate changes in carbon stocks and how these pools will change as a result of management [13] . However, substantial uncertainties remain in current model estimates of terrestrial carbon, and there is an increasing need to quantify and reduce these uncertainties [14, 15] . Uncertainty arises from the inability to perfectly measure key variables, the necessary use of models to make predictions, and the natural variability of ecosystem processes across the landscape [16] . According to the Intergovernmental Panel on Climate Change (IPCC) good practice guidance, the national reporting of changes of CO 2 equivalents in forest and other woody biomass stocks can be calculated by a default method as the difference between growth and drain estimated by considering all relevant processes (the growth of trees and carbon losses due to harvest, natural mortality, and natural disturbances), or by the stock change method as the change in stocks between two consecutive inventories. Information on the major uncertainties involved in the calculations of forest carbon stocks and stock changes is needed in the negotiations of the Climate Convention. The main aims of this study were: (i) to carry out a plot-based estimate of carbon stock and carbon stock change in a Douglas fir stand by using three biomass estimate models, according to three different planting densities (1667, 2000, and 2500 trees per hectare) and at three ages (15, 25, and 40 years old); and (ii) to quantify uncertainty in the carbon stock and carbon stock change estimates. Plot-based estimates of forest carbon stocks and carbon fluxes are derived metrics that contain multiple sources of uncertainty [17] [18] [19] . Carbon stock was calculated from biomass estimated with three different methods: (1) with constant biomass expansion factors (BEFs), (2) with age-dependent BEFs, and (3) with biomass equations applied directly to tree-wise data of the sample plots. The changes in carbon stock were therefore evaluated with the two methods suggested by the IPCC. The related uncertainty differs between the two methods since "the uncertainties in the default method are dominated by model errors due to the different components which only partly are usually derived from statistical forest inventory data, whereas the uncertainties in the stock change method, which are derived from net-carbon changes based on repeated statistical forest inventories, are dominated by the sampling error, especially in cases where the net-carbon changes are very small" [20] . Many studies have tended to focus on uncertainty in carbon stock estimates, rather than uncertainty in carbon change over time, but carbon change is arguably the most important of the two metrics, as it is the basis for UNFCCC reporting [21]. Materials and Methods Study Area The study area (39 • 25 N, 16 • 2 E) is located in the northern coastal chain of Calabria, in the Tyrrhenian side of south Italy. The climate of the area is typical Mediterranean. The annual rainfall is 1233 mm, with the minimum precipitation in summer (88 mm) and the maximum during the winter (501 mm). The average annual temperature is 11.5 • C at an altitude of 950 m a.s.l. This area is the property of the Regional Italian State Forestry Agency and, in 1967, Douglas fir (Pseudotsuga menziesii, (Mirb.) Franco) was extensively used to reforest the whole of the property. The plantation was part of a reforestation project carried out in the south of Italy with funding earmarked by the national government to restore forest ecosystems in abandoned agriculture areas [22] .
doi:10.3390/su9101702 fatcat:uzzxnnsrbncmxplq4urmvylfk4