Forest ecosystems provide a number of economic (such as timber and non-timber products), ecological (such as carbon sequestration, water resources, and biodiversity) and social values (such as recreation and employment opportunities for local residents). However, in many countries, forest management policies have generally focused on timber production over the past decades.
The carbon sequestration of forest ecosystems is now receiving substantially more attention as a result of global climate change. For example, China is an extensively forested country with productive forests that in 2013 covered approximately 22 percent of the land area, provided a volume of 15,137 million cubic metres, and 8427 million stored tonnes of carbon.
The annual volume of forest growth was estimated to be 283 million cubic metres, the annual harvest was 84 million cubic metres, and the annual carbon sequestration in forests was estimated to be 115 million tonnes in the 8th Forest Resources Inventory (2009–2013).
Therefore, China’s forests play important roles as carbon sinks in mitigating global climatic changes. However, the carbon emissions from fossil fuels in China reached 2260 million tonnes in 2010. It is estimated that approximately five percent of these emissions can be sequestered in forest biomass.
Obviously, China faces serious pressure to reduce carbon emissions from fossil fuels and increase carbon stocks in forest biomass. Thus, understanding the carbon dynamics of the forest ecosystem and its response to forest management efforts and climate change has become one of the most important research topics in forestry.
Adaptive Forest Management
Previous studies have demonstrated that planting trees in suitable areas and increasing forest biomass carbon stocks through adaptive forest management are among the most effective alternative strategies for reducing atmospheric CO2 concentrations.
The first strategy has been widely implemented in many countries over past decades. For example, it was estimated that planted forests (such as forestation and reforestation) in China have sequestered 0.45 Pg of carbon since the mid-1970s.
The second strategy has been long overlooked. However, in the last few decades, it has drawn more attention worldwide. The literature indicates that adaptive forest management strategies could significantly affect carbon dynamics in forest ecosystems and thus are effective for mitigating global climate change.
The carbon benefits of forest management prescriptions (such as selective cutting) typically vary according to the methods used to utilize trees (such as part or whole tree), the methods used to harvest trees (such as thinning from below vs. from above), the rotation length, the management intensity, and the length of time after a thinning treatment.
In addition, species composition, development stage, and characteristic variables (such as stand density and site quality) within a stand influence the carbon benefits of different management prescriptions.
In general terms of short-term monitoring, live biomass carbon pools have significantly decreased relative to the no-harvest forest because trees have been directly removed from stands. However, because the soil carbon pool is more resistant to forest management changes, it may not distinctly change.
For long-term monitoring, the differences between the total and all sub-carbon pools of forest ecosystems may be not significant. In addition, forest management prescriptions produce certain end-use products that can store significant amounts of carbon for much longer periods than dead wood and residue materials (such as branches and barks) in forest ecosystems. From this perspective, most forest management prescriptions should be effective in improving the amount of carbon in forest ecosystems.
However, an experiment to evaluate the effects of these forest management prescriptions on improving the level of carbon stocks across a broad forest landscape would be costly and time-consuming. Therefore, the forest management planning process, which can provide details of where, when and how management prescriptions should be scheduled across a landscape, has been widely used to support the assessment of economic and ecological (such as wildlife habitat, water quality and carbon sequestration) goals during the last few decades.
Several studies have incorporated the carbon objective into the forest management planning model. These studies evaluate trade-offs between harvested timber and carbon sequestration and examine the effects of different constraints or policies on forest carbon management objectives using traditional and exact mathematical techniques, such as goal programming and linear programming.
However, harvest adjacency and green-up constraints have become some of the most important components of laws, regulations and forest certification programs worldwide. These constraints may have significant effects on the fragmentation of the forest landscape and other ecological processes.
However, few studies have incorporated the necessary spatial requirements (the adjacency and green-up constraints) into forest carbon objective management planning models for forest economic and commodity production goals.
Based on a broad survey of the forestry literature, one can state that numerous variables affect forest management decision making (such as forest characteristics, management policies and economic factors). Of these variables, economic factors might be one of the clearest and most credible bases for forest management practices.
Today, long-term carbon trading or carbon taxes have been implemented in many countries. However, carbon prices vary from region to region and country to country. For example, the carbon prices in countries that have adopted carbon trading are typically less than RMB100/tonne.
In contrast, the carbon prices in countries that employ carbon taxes are approximately RMB1,000 /tonne. Therefore, the quantitative assessment of the effects of carbon prices on decision making is an important prerequisite when a forest planning model is applied in practice.
To evaluate the effects of carbon prices on trade-offs between forest carbon and timber management objectives in spatial harvest scheduling problems, a persuasive multi-objective forest management planning model was formulated.
We integrated carbon management objectives in a wood supply model. However, in addition to using the traditional forest management model, which only includes the typical even flow of harvested timber and minimum harvest age constraints, we considered the adjacency constraints,area restriction model (ARM) of assigned management prescriptions in temporal and spatial scales.
We also employed a well-known heuristic technique (simulated annealing) capable of greatly accelerating the search process for complex combinatorial optimization problems.
The results indicated that the assigned harvest timber and its corresponding differences of carbon stock (DoC) both presented typical nonlinear responses as the carbon price increased.
Given the harvest quota management constraints of China’s State Forestry Bureau, the realistic annual harvest level was set at approximately 15 thousand cubic metres per year for the study region for the long term, which resembles the simulated forest management scenario in which a carbon price of RMB3,500/tonne was used.
The results of this scenario corresponded to an average annual DoC of approximately 0.49 tonne/ha. However, the planning model ignored several important factors, which require additional discussion.
The first and perhaps most important factor is that not all the components of carbon sequestration related to forest and forestry were included, such as carbon in the soil, litter, shrub and herbal layers in forest ecosystems.
Forest soil carbon stocks typically account for 29–56 percent of the total carbon stocks for different development stages of the forests in this region. However, the amount of soil carbon stocks may change dramatically over a short period, particularly in forests that grow in high latitudes, such as our study area.
Additionally, because of the significant spatial heterogeneity of other vegetation layers (shrub and herbal), the carbon stocks in these components were not considered.
Moreover, the harvested timber within the entire time horizon can also store a significant amount of carbon. Based on an average biomass expansion factor of 0.56 tonnes/cubic metre for all forest types within this region and a carbon content of 0.45, the carbon stocks of the harvested timber within the entire time horizon for different management planning scenarios were estimated to be 0.36–0.92 million tonnes. Therefore, the carbon emissions process of various forest products (sawlogs) should also be integrated into the planning process.
A second factor that requires additional discussion is the effect of climate change on the forest management planning process.
Under boreal conditions, climate changes are expected to increase the annual temperature and precipitation and thus may increase forest growth and productivity. However, the frequency and severity of unproductive events (fire, wind and insect damage) may also be increased significantly.
To evaluate the effects of climate change, a process-based growth model or a modified empirical model can be integrated into the forest planning process. Among the various counterproductive events, forest fires represent a key natural disturbance factor in this region.
As many as 1614 forest fires with a burned area of approximately 35.23 million hectares have been reported from 1965 to 2010. The total carbon emissions of these fires were estimated to be 29.32 million tonnes, and the mean annual carbon emissions were approximately 0.64 tonnes per year, which accounted for six percent of the total carbon emissions from forest fires in China.
Therefore, these unproductive events should also be integrated into traditional forest planning models.
The final factor that requires discussion is that forest characteristics, economic factors, and management policies have significant effects on the forest planning process. For example, early studies have examined the effects of site productivity on forest harvest scheduling problems that involved green-up and maximum area restrictions.
Prices not only significantly affect the search process of heuristics but also reflect the priorities of multi-objective forest management. Therefore, in this study, we only focused on the effects of carbon prices.
However, the prices of various forest products and management activities represent a complex system from the economic perspective. A change to one economic parameter may affect other parameters. Thus, the interactions among such parameters might be important in forest management practices.
Finally, the management policies of adjacency and green-up constraints also have significant effects on the forest planning process. Earlier studies have confirmed that the economic profitability of an optimal management plan typically increased as the period of green-up constraints and the size of the maximum clear-cut area increased. However, the fragmentation of forest landscapes or habitat patches might be aggravated significantly.