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Introduction
Recently, several economic, ecological and environmental concerns have become a focal point of forestry research in the sustainable management of forest ecosystems (Başkent, Keleş, & Yolasığmaz, 2008). Environmental concerns about climate change associated with increasing concentrations of greenhouse gases in the atmosphere has led to addition of carbon sequestration value of forest ecosystems to the multiple-use forest planning process (Asante & Armstrong, 2012). Forest ecosystems absorb huge amounts of CO2 from the atmosphere through photosynthesis and contribute to greenhouse gas emission reductions (Cacho, Hean, & Wise, 2003). In this context, fast growing forest plantations have had a great importance at global scale.
A series of decisions have been taken to multiple-use benefit from forest ecosystems within the principle of sustainability. Determining optimum cutting age is one of the most important decisions in forest management. This age affects the quality and quantity of all values provided by forest ecosystems such as timber products, soil conservation, water production, aesthetics, biological diversity and carbon sequestration. To this day, a few scientific studies associated with the determination of optimum cutting age in forest management planning including wood production and carbon sequestration values were performed (Asante & Armstrong, 2012; Diaz-Balterio & Rodriguez, 2006; Diaz-Balteiro, Martell, Romero, & Weintraub, 2014; Kula & Gunalay, 2012; Torres, Perez, Fernandez, & Belda, 2010). All these studies were to determine the optimum harvest time of forest stands to benefit from multiple forest ecosystem values at the same time. However, there is a need to consider new studies related to optimum cutting ages of various tree species in forest or plantation areas of different regions to contribute the literature.
More recently, a number of studies integrating carbon sequestration value to forest management problems in Turkey have been published (Başkent, Keleş, & Kadıoğulları, 2014; Başkent, Keleş, Kadıoğulları, & Bingöl, 2011; Keleş & Başkent, 2007; Keleş, 2010). There are also some studies to analyse the effects of land use and land cover changes on carbon sequestration amounts of forest ecosystems (Kadıoğulları & Karahalil 2013; Keleş, Kadıoğulları, & Başkent, 2012; Sivrikaya, Keleş, & Çakır, 2007). However, determining optimum cutting ages in forest management including multiple forest ecosystem values has not yet been sufficiently appraised in the scientific literature in Turkey.
In the present paper, we address the optimum cutting age problem by considering wood production and carbon sequestration values in a hybrid poplar plantation (Populus x euramericana [Dode] Guinier cv. I-214). This study also analyses the effects of different discount rates and carbon prices on optimum cutting ages using net present value approach.
Materials and Methods
Plantation data
For the case study, growth and yield curves of hybrid poplar plantations (P. x euramericana cv. I-214) per hectare was considered. Figure 1 shows the growth cycle of this species planted using a 4 m x 4 m planting grid in the good sites (higher productive areas where the average dominant height is above 30 m at age fifteen) and its carbon sequestration amounts (Birler, 2009). Table 1 also presents estimated costs of an afforestation project with hybrid poplar plantations in Turkey. All these values were obtained from the related units of General Directorate of Forestry in Turkey.
Figure 1: Wood volume and carbon sequestration of hybrid poplar plantations (Populus x euramericana cv. I-214) cultivated in good sites (higher productive areas where the average dominant height is above 30 m at age fifteen).
Table 1: Standard operations and their costs by the years in hybrid poplar plantations (Populus x euramericana [Dode] Guinier cv. I-214).
| Operations | Cost details | Operation years | Expenditure (US$·ha-1) |
|---|---|---|---|
| Ploughing | 0 | 164 | |
| Establishment | Planting | 0 | 1,177 |
| Fertilization | 0 | 333 | |
| 1 | 333 | ||
| Beating up | 1 | 317 | |
| 2 | 317 | ||
| 3 | 317 | ||
| 4 | 317 | ||
| 5 | 214 | ||
| 6 | 112 | ||
| 7 | 56 | ||
| 8 | 56 | ||
| Hoeing | 1 | 61 | |
| 2 | 61 | ||
| 3 | 61 | ||
| 4 | 41 | ||
| Maintenance | 5 | 20 | |
| Irrigation | 1 | 85 | |
| 2 | 85 | ||
| 3 | 85 | ||
| 4 | 85 | ||
| 5 | 57 | ||
| 6 | 57 | ||
| 7 | 28 | ||
| 8 | 28 | ||
| Tree-top improvement | 2 | 29 | |
| Pruning | 4 | 42 | |
| 6 | 42 | ||
| 8 | 42 | ||
| General administration | Management, protection and control | Each year | 251 |
Wood production data
Hybrid poplar plantations produce peeler log, sawlog, chip-wood and fire-wood in Turkey. Volumes of these wood assortments as a result of clear-cutting at any age were determined by product rates table of the relevant species (Birler, 2009). The financial data associated with wood production includes harvesting costs and wood assortments revenues. The net values of wood assortments used in this study are 20, 17, 5 and 3 US$•m-3 for peeler log, sawlog, chip-wood and fire-wood, respectively.
Carbon sequestration data
There are various approaches for estimating tree biomass. These approaches are generally based on biomass expansion factors and tree-level allometric equations developed as a function of tree species and some stand parameters (Keleş 2015; Labrecque, Fournier, Luther, & Piercey, 2006; Sanquetta, Corte, & da Silva, 2011). To predict above-ground biomass in this study, first the standing timber volumes of hybrid poplar plantations are multiplied by species-specific biomass conversion factor. This biomass factor is set at 1.31. The average wood basic density of 0.35 is used. In order to calculate the carbon content of hybrid poplar species, a 0.48 conversion factor is used. The below-ground/root biomass is predicted as a proportion of the above-ground biomass using given root-to-shoot ratio of 0.46. All these biomass and carbon conversion factors for the relevant tree species were obtained from the literature (Tolunay, 2011).
The model
To determine the optimal cutting age for hybrid poplar plantations including carbon sequestration, the methodology (ideal carbon-accounting system) proposed by Cacho et al. (2003) was used. According to the methodology, the value of a stand of forest in the presence of carbon sequestration payments and with redemption upon harvest is as follow:
where:
NPV1(T) = Net present value of a forest harvested in year T after planting (US$)
cE = Forest establishment cost (US$)
Pv = Net value of wood assortments that depend on the stand age of trees at harvest (US$)
Pc = Price of carbon sequestration in tree biomass in tonnes per hectare
v(T) = Wood volume (m3•ha-1)
b(T) = Carbon stock in tree biomass (t•ha-1).
The first term on the right-hand side corresponds to the value of the wood harvest. The second term corresponds to the sum of the annual net benefits from carbon captured in the interval (0-T). The last term in the equation corresponds to the assumption that credits obtained during forest growth have to be fully redeemed upon harvest. The annual rate of carbon sequestration is estimated as
Some assumptions were also taken into consideration in this study. It is supposed that no thinning regime is employed. Clear-cutting is only one silvicultural regime. Rental value of the land is not included to estimations of net present values for timber and carbon sequestration functions. The model uses the net present value approach. A discount rate of 3 % was used, and a sensitivity analysis was also carried out. A reference price of 20 US$•t-1 for carbon sequestration was used in the analysis, and followed by a sensitivity analysis.
Results and discussion
Table 2 shows the results of optimum cutting ages of a hybrid poplar plantation for wood benefits and carbon benefits plus wood benefits depending on various discount rates and carbon prices. When carbon sequestration is not included, the optimum cutting age is 17 years based upon wood benefits at a discount rate of 3 %. When carbon sequestration was considered as a significant forest value, the optimum cutting age arises to be 19 years. Carbon sequestration benefits of 20 US$•t-1 are assumed here. For monitoring the effects of various discount rates on optimum cutting ages, a sensitivity analysis was also implemented in this study. Figures 2 and 3 show the effects of different discount rates on net present value of wood and net present values of timber plus carbon sequestration per hectare in hybrid poplar plantations, respectively. According to the figures and Table 2, the optimum cutting ages decrease for both wood benefit and wood plus carbon sequestration benefits depending on the increase in discount rates. When the discount rate decrease to 2 %, optimum cutting age for wood benefits increased to 19 years, but it was not change for wood and carbon sequestration benefits considered together. When carbon sequestration benefits are not included, net present values obtained from wood production are 7,677, 6,242, 5,221 and 4,375 US$•ha-1 for the four different discount rates considered (2, 3, 4, 5 %), respectively. The integration of carbon sequestration benefits into the model increased the net present values (NPVs) obtained from the plantation by 9, 12, 14 and 16 % for four discount rates, respectively.
Table 2: Optimum cutting ages of planting hybrid poplar (Populus x euramericana cv. I-214) for various discount rates and carbon prices.
| Discount rate (%) | tW | tW+C | Definition |
|---|---|---|---|
| 2 | 19 | 19 | Carbon price (20 US$·t-1) |
| 3 | 17 | 19 | |
| 4 | 14 | 17 | |
| 5 | 13 | 14 | |
| Carbon price (US$) | t W | t W+C | |
| 10 | 17 | 17 | Discount rate (3 %) |
| 20 | 17 | 19 | |
| 30 | 17 | 19 | |
| 40 | 17 | 20 |
tw: the optimum cutting age for only wood benefits
tw+c: the optimum cutting age for wood and carbon sequestration benefits together
Figure 2 Net present values (NPV) of wood benefits of hybrid poplar plantations (Populus x euramericana cv. I-214) according to the various discount rates.
Figure 3: Net present values (NPV) of wood and carbon sequestration benefits of hybrid poplar plantations (Populus x euramericana cv. I-214) according to the various discount rates.
With the purpose of evaluating the response to changes in carbon prices, a sensitivity analysis was also implemented in the case study. Figure 4 shows the effects of various carbon prices on total net present value per hectare of wood and carbon sequestration benefits in hybrid poplar plantations. Table 2 shows the optimum cutting ages of hybrid poplar forest stands planted with a 4 m x 4 m planting grid in the good sites depending on different carbon prices at a fixed discount rate of 3 %. When carbon prices increased, the optimum cutting ages of hybrid poplar plantations including wood and carbon sequestration benefits also increased from 17 years at 10 US$ to 20 years at 40 US$. NPVs obtained from wood production and carbon sequestration benefits per hectare also increased between 6 and 26 % (from 6,242 US$ for 0 US$•t-1 carbon to 7,896 US$ for 40 US$•t-1 carbon).
As expected, when the discount rate increased, optimal rotation ages were shortened. When carbon prices were introduced, results varied. The integration of carbon sequestration benefits into wood benefits increased the optimum cutting ages of hybrid poplar plantations. Further, higher carbon prices (10 and 40 US$•t-1) resulted in longer cutting ages. On the other hand, other studies have also presented the similar results for various tree species (Asante & Armstrong, 2012; Diaz-Balteiro & Rodriguez, 2006; Diaz-Balteiro et al., 2014; Kula & Gunalay, 2012; Sohngen & Brown 2008; Romero, Rios, & Diaz-Balteiro, 1998; Torres et al., 2010).
Forestry projects such as afforestation and reforestation lead to the generation of carbon credits and these credits are sold to regulated entities or to those who wish to reduce their carbon emissions (Demirci & Öztürk, 2015). Carbon projects are neglected in carbon markets now, however these markets will provide huge opportunities for the generation of forest carbon credits in the future. Turkey is one of the most active players in voluntary carbon markets in the world, and currently cannot benefit from flexible mechanisms to reduce emissions by developing forest projects because it was not a party to the United Nations Framework Convention on Climate Change (UNFCCC) at the time the Kyoto Protocol was adopted. In this context, voluntary carbon market is extremely important for Turkey now, but Turkey will be able to benefit from the flexibility mechanisms in the future (Demirci & Öztürk, 2015). The results obtained from the current study will serve to develop reforestation or afforestation projects to reduce greenhouse gas emissions in Turkey. If sufficient monetary incentives are provided to forest landowners, plantation management in the future will encourage short rotations producing large production of wood and carbon sequestration (Nepal, Grala, & Grebner, 2012).
Figure 4: Net present values (NPV) of wood and carbon sequestration benefits of hybrid poplar plantations (Populus x euramericana cv. I-214) according to the various carbon prices.
However, there are many ways in which the model reported in this study might be extended and improved. This study considers only clear-cutting management practice and not take into consideration the effects of different management practices on forest stand structure and values provided. Although a sensitivity analysis is carried out using different discount rates and carbon prices, there is a need to integrate some economic uncertainties such as forest product prices, market supply and demand situation, carbon prices and plantation establishment costs. It means that the model used in this study is deterministic, not stochastic. Risks caused by natural events (e. g. climate change, forest fire) and uncertainties of timber and carbon markets are not taken into consideration in this study. The high volatility in the market for carbon credits leads to significant uncertainty in future carbon revenues and it will remain an important factor to consider in economic analysis of afforestation for carbon sequestration (Nijnik, Pajot, Moffat, & Slee, 2013; Olschewski & Benitez, 2010). Although costs and prices are likely to change in future, the results derived from this study will provide valuable information on financial viability of increasing wood production and carbon sequestration benefits.
Conclusions
Fast growing tree plantations are the source of many wood products, and provide a series of environmental services like carbon sequestration by promoting climate change mitigation. It is also expected that the importance of fast growing tree plantations will increase in the future. However, they must be managed according to the sustainability principles in order to maximum benefit from forest ecosystem values such as carbon sequestration, soil protection, biodiversity conservation and water production. The determination of optimum cutting ages when considering timber production and the other forest ecosystem values together is very important for sustainable and multiple-use forest management. The results of this study showed that the integration of carbon capture benefits with wood increased optimum cutting ages of hybrid poplar forests (Populus x euramericana cv. I-214). The optimum cutting age decreased from 19 to 14 years depending on the increase in discount rates. When carbon prices increased, optimum cutting ages also increased from 17 to 20 years. Determining optimum cutting ages including wood and carbon sequestration benefits for each fast growing tree species is extremely important for sustainable forest management in Turkey.
