1. Introduction
Forests can sequester carbon from the atmosphere and store it in their living biomass, which has been attracting increasing attention for mitigating carbon emissions [
1,
2,
3]. Harvested wood product (HWP) can still store parts of the sequestered carbon for a certain period of time, and besides HWP can also be a substitute for other materials (like steel and concrete) and fossil fuels (like petroleum and coal) that have higher carbon emission intensities. In recent years, the use of HWP has been encouraged due to its carbon emission reduction effects [
2], and thus more and more HWPs are being consumed. HWP presents a huge carbon pool, with an increasing trend from 59 MtC/year in 1990 to 74 MtC/year in 2040 [
4,
5,
6]. Therefore, much attention has been focused on estimating the HWP carbon storage [
3]. Based on national greenhouse gas inventories, the Intergovernmental Panel on Climate Change (IPCC) proposed three approaches to estimate the HWP carbon storage; after which scientists began to estimate the HWP carbon storage in their countries, including Russia, Iceland, the USA and China [
7,
8,
9,
10,
11].
However, the HWP manufacturing process can result in some environmental problems and correlated carbon emissions due to material consumption. In addition, carbon stored in HWP could be released gradually (by landfill) or immediately (by combustion) after their life cycle. Currently, increasing efforts are being made to understand the environmental impacts [
12], energy consumption [
13,
14], and carbon emissions [
3,
15] during the HWP manufacturing process.
In terms of the carbon storage and the carbon emission, HWP has already been an important part of global carbon cycle. Thus, it is of high importance to track the entire carbon flow during the complete life cycle of HWP. Life cycle assessment (LCA) is an important tool to evaluate environmental loads and material flows related to a process or an activity [
16]. In the LCA system, the entire life cycle or life span should be considered from the raw material extraction to the final disposal of products. Present studies have already tracked carbon flows along the life cycle of HWP, but only a few studies focused on the entire carbon flows from forest plantation to the HWP final disposal [
17,
18,
19]. Moreover, most of these studies were based on inventory data, and they lack survey data [
3].
The carbon footprint can be used to estimate the total amount of carbon emissions caused directly or indirectly by a process, product or service [
16]. The carbon footprint of a product is unique to its product system, and its carbon emissions occur during its complete life cycle [
16]. Therefore, many researchers have estimated carbon footprints of different products or services [
3,
16,
20], and they have defined their own system boundaries according to their study aims. The HWP carbon footprint, in this study, is defined as carbon emissions for producing this HWP due to material consumption (like fossil fuel, electricity, fertilizer and so on), and besides these carbon emissions occur in the forestry process as well as in the manufacturing process. Additionally, it is also important to clearly understand differences in the carbon footprint among different HWPs, especially for those HWPs that originate from the same forest. Moreover, since the HWP can store the sequestered carbon by forests, it is of interest to estimate the net carbon balance for HWP before it is disposed.
To better understand carbon flows and carbon footprints, this study chose five typical HWPs from the Larix principis-rupprechtii plantation forest, a widely distributed forest species in North China. Larix principis-ruppechtii adapts to low temperature well, and can even survive at the altitude of 2800 m. Larix principis-ruppechtii is a plantation species, and its timber can be used for construction, furniture, panels, poles and fuel. Therefore, it is of high necessity to understand and compare the carbon flow and the carbon footprint among different HWPs from Larix principis-ruppechtii, based on the life cycle analysis. This is the overall aim of this study. The specific objectives of this study are to: (1) establish the boundary system of HWPs for the life cycle analysis, from a cradle (the seedling cultivation) to gate (the HWP final disposal) perspective; (2) monitor, evaluate and compare the carbon flow, the carbon footprint and the net carbon balance of different HWPs along their entire life cycles, including the forestry process, the manufacturing process and the use and disposal process, by applying the approaches in the PAS 2050 and ISO (International Organization for Standardization); and (3) discuss the substitution effect on carbon mitigation by the use of HWP, as well as how to better achieve carbon mitigation using the HWP.
2. Materials and Methods
2.1. Study Area
The great distribution of Larix principis-rupprechtii exists in the Mulan Weichang State-Owned Forest, where the environment is very suitable for this species to grow. With a long time of cultivation, a suite of forest management practices have been established, and many wood product factories are also located nearby. Therefore, we chose the Mulan Weichang State-Owned Forest as our study area. We conducted our interviews and surveys with local forest managers and owners of HWP factories in 2014, to learn about forest management practices and manufacturing processes. In the Mulan Weichang State-Owned Forest, Larix principis-rupprechtii is assumed to be planted in bare land, and its harvest rotation period is 41 years, with an initial intensity of 3300 stands/ha. After the final harvest, new seedlings are planted again for the next rotation period. According to our surveys, parts of the harvested timbers are directly consumed as fuel wood and poles. The remaining harvested timbers are hauled to local HWP factories to produce construction, furniture and panel products.
2.2. Functional Unit
In this study, the functional unit for estimating the carbon flow is defined as the carbon storage in the HWP from 1 ha of Larix principis-ruppechtii plantation forest. The functional unit for estimating the HWP carbon footprint is defined as the carbon emission from producing 1 m3 of HWP. The HWP carbon storage intensity is defined as the carbon storage in the 1 m3 of HWP. Therefore, the net carbon balance of HWP can be estimated by balancing the carbon storage and the carbon footprint in 1 m3 of HWP.
2.3. System Boundary
For better tracking the carbon flow and estimating the carbon footprint, the HWP carbon cycle can be divided into three processes: (1) the forestry process; (2) the manufacturing process; and (3) the use and disposal process (
Figure 1). These three processes can completely illustrate the entire life cycle of HWP, from seedling cultivation to its final disposal.
2.3.1. The Forestry Process
The forestry process includes seedling cultivation, tree plantation, forest management and wood harvest. Therefore, in the forestry process, carbon is firstly sequestered by the forest, and then is transferred into timbers after harvest. The carbon footprint in the forestry process is defined as carbon emissions caused by material consumption in the forestry process, including fossil fuel, fertilizer, pesticide and so on.
Based on inventory data and Logistic Growth Model, Lun [
21] established the allometric growth models for tree height and diameter at breast height (DBH) (Equations (1) and (2)). Additionally, Liu [
22] built the stem biomass model with the parameters of tree height and tree DBH (Equation (3)) for each tree. With the stem carbon concentration of 0.5107 [
23], we can estimate the stem carbon storage by Equation (4). All of the harvested timbers originate from forest stems, and thus carbon storage in the harvested timbers can be estimated by Equations (1)–(4),
where
H and
D are tree height (m) and tree DBH (cm), respectively;
t refers to forest age;
Vstem is stem biomass (m
3); and
Cstem is carbon storage in stem (tC/ha).
Based on data from our interviews,
Table 1 summarizes forestry management practices and material consumption at different forest ages, while
Table 2 illustrates the utilization ratio of harvested timber for different uses at different forest ages. Therefore, the carbon flows of harvested timbers can be tracked with all this information.
According to approaches in the PAS 2050 and ISO, two types of data are needed for the HWP carbon footprint calculation in relation to carbon emissions: material consumption and carbon emission factors [
3]. Therefore, the carbon footprint in the forestry process (
CFforestry) can be calculated by material consumption (
Mi) and carbon emission parameters of different materials (
δC-Mi) using Equation (5) (
Table 3).
2.3.2. The Manufacturing Process
According to our surveys, harvested timbers were used for construction, furniture, panels, poles and fuel wood. Fuel wood is directly combusted as bio-energy in the harvest year, while poles are buried underground for use (
Figure 1). Therefore, there is no manufacturing process for harvested timbers for poles and fuel wood. Other harvested timbers would be processed and manufactured for producing further HWPs, including construction products, furniture products and panel products (
Figure 1). Apart from produced HWPs, some byproducts (wood waste) also originate from these harvested timbers, and all of these byproducts are used as wood fuel for energy. In the HWP manufacturing process, coal and petroleum are consumed, resulting in carbon emissions, which contribute to the HWP carbon footprint in the manufacturing process. Previous studies have already estimated timber consumption, material consumption, and byproducts production during production of 1 m
3 of different HWPs (See
Table 4). Therefore, carbon footprints of HWPs in the manufacturing process can be calculated by material consumption and their carbon emission parameters (Equation (6)). Carbon flows in manufacturing process are reallocated to new produced HWPs and byproducts (wood waste as fuel). The carbon stored in the byproducts as fuel can be estimated by their production and their carbon concentrations, while the remaining carbon in harvested timbers is transferred into produced HWPs.
2.3.3. The Use and Disposal Process
Based on our interviews and surveys, it was found that fuel wood and poles have very short life spans, while other HWPs have long life spans. Moreover, different HWPs have different disposal treatment pathways. Therefore, we surveyed 128 families in the local area to estimate life spans and disposal treatment pathways for these five typical HWPs, and their results are illustrated in
Table 5. In the use and disposal process, HWP is disposed after their life spans and thus their stored carbon is released after their final disposal. When the disposed HWPs are directly combusted, all of their stored carbon would be immediately released back into the atmosphere. For the landfill treatment, carbon in these disposed HWPs would be gradually released into the atmosphere. The Yasso Model in CO2FIX model can be used to estimate carbon emissions from these landfilled HWPs [
29], which was also used in this study. There was no external material consumption in this process, and thus there were no carbon emissions and carbon footprint in the HWP use and disposal process.
2.4. Carbon Flows and Carbon Footprints in Their Entire Life Span
For the entire life cycle of HWP, the carbon flow referred to carbon embedded in the HWP. After the final disposal, HWP is treated by combustion or landfill, and thus their stored carbon would be released back into the atmosphere. Thus, the entire carbon flow of HWP included: (1) sequestered carbon in harvested timber; (2) carbon in produced HWP; and (3) carbon emissions from the disposed HWP. The carbon footprint of HWP referred to carbon emissions from material consumption, occurring in the forestry process and in the manufacturing process. Therefore, in this study, the total carbon footprint of HWP for its entire life cycle was the sum of carbon footprints in the forestry process and also in the manufacturing process.
2.5. The Substitution Carbon Storage of HWP
The harvested wood products can substitute some materials (like steel and concrete) and fossil fuels (like petroleum and coal) with higher carbon emission intensities. This reduction of carbon emissions can be considered as the substitution carbon storage of HWP [
29]. Here, we only focused on the carbon reduction from the substitution of wood fuel for fossil fuel.
In our study area, all of the wood fuel is used for warming, and thus it can reduce the consumption of coal. Based on previous studies [
30,
31,
32], we assumed that the utilization efficiencies of wood fuel and coal were 10% and 30%, respectively. Therefore, the substitution carbon storage of HWP can be estimated using Equation (7),
where
Sc is the substitution carbon storage by wood fuel (tC/ha);
Fc is the carbon emission parameter of standard coal, approximately 0.755 tC/ha;
Cw is carbon storage in wood fuel (tC/ha);
Rc is the carbon concentration of wood fuel, approximately 0.4971;
Hw and
Hc are the heat values for wood fuel and standard coal, approximately 12 MJ/kg and 29.27 MJ/kg, respectively; and η
w and η
c are the utilization efficiencies of wood fuel and standard coal, 10% and 30%, respectively.
5. Conclusions
Based on our interviews and surveys, this study evaluated the cradle-to-gate carbon flows and carbon footprints of different HWPs from a forest plantation of Larix principis-rupprechtii in China. For one forest rotation period, a total of 26.81 tC/ha sequestered carbon was transferred into harvested timbers for producing HWPs, 38.0% of which was used for producing construction products. In addition, the forestry process resulted in a total carbon emission of 1.59 tC/ha, leading to a carbon footprint of 10.40 kgC/m3 for the forestry process. After the manufacturing process, this carbon was relocated into different HWPs as follows: construction products (9.99 tC/ha), furniture products (2.43 tC/ha), panel products (5.03 tC/ha), poles (2.50 tC/ha) and wood fuel (6.87 tC/ha, including waste wood as fuel). For the entire 41-year rotation period, the manufacturing process resulted in a total carbon emission of 10.56 tC/ha. When the forest rotation period had ended, there was still 17.75 tC/ha of carbon stored in HWPs. However, the HWP carbon storage decreased to 0.25 tC/ha in the 100th year after forest plantation, mostly stored in panel products and poles. Including the carbon emissions in the forestry process and in the manufacturing process, the total carbon footprints of construction products, furniture products and panel products were 154.81 kgC/m3, 408.34 kgC/m3 and 228.73 kgC/m3, respectively. The manufacturing process was the most important contributor to carbon emissions for the entire life cycle of HWPs, accounting for more than 90% of the total carbon footprint. Considering the carbon emission and the carbon storage, the net carbon balance of construction products were much larger, compared to furniture products and panel products. HWP can store parts of carbon sequestered from the atmosphere for a certain time, which was beneficial for carbon mitigation. Furthermore, all of the wood fuel and parts of disposed HWP can be combusted for energy, resulting in the reduction of fossil fuel consumption, and thus it was also considered to have a positive impact on carbon mitigation. The substitution of fossil fuel by HWP can result in the substitution carbon storage, which should attract more attention in the future. In addition, with regard to further carbon mitigation, there are many alternatives for HWP, such as replacing fossil fuel with cleaner energy and increasing the utilization efficiency of wood fuel.