Implications of Climate Change for Carbon Stocks and Inventory

Climate change is one of the most important global environmental issues, which is likely to impact natural ecosystems as well as socio-economic systems (Ravindranath and Sathaye 2002). The global climate has warmed by 0.7 °C during the last century and is projected to rise by 1.8-4.0 °C during the current century (IPCC 2007a). This unprecedented projected rise in global mean temperature is likely to have adverse implications for forests, grassland and cropland vegetation. Climate is probably the most important determinant of vegetation patterns globally and has significant influence on the distribution, structure and ecology of forests (Kirschbaum et al. 1996). Climate vegetation studies have shown that certain climate regimes are associated with particular plant communities or forest types (Holdrige 1947). Thus, any change in the climate would alter the configuration of forest ecosystems (Kirschbaum 1996). Further, even a modest global warming of 1-2 °C will affect most ecosystems and landscapes through changes in species composition, productivity and biodiversity (Schröter et al. 2004). The coming decades will be characterized by increasing concentration of CO2 in the atmosphere along with warming and changes in precipitation patterns. These factors may lead to changes in plant productivity. Such an impact on plant productivity will have implications for carbon inventory, since carbon stocks in forests, plantations and grasslands may increase or decrease in different regions. Implications for carbon inventory include the following:

• Changes in the size of the carbon sink in forest and other land-use systems

• Changes in growth rates of biomass and soil carbon in forests, grasslands and agroforestry systems

• Projections of carbon sequestration or mitigation potential and rates per hectare

• Greenhouse gas inventory or CO2 emissions and removals in the long-term

• Estimation of timber or roundwood production

This chapter presents the projected impacts of climate change on carbon stocks and rates of change affecting the mitigation potential, particularly for forest and plantation systems, and implications for roundwood production, focusing on the implications for carbon inventory methods.

19.1 CO2 Concentrations and Climate Change Projections

Projected CO2 concentrations in the atmosphere The concentration of CO2 in the atmosphere at the beginning of Industrial Revolution was about 280 ppm, which had increased to 380 ppm by 2005. The CO2 concentration is projected to increase according to different scenarios in the range of 540-970 ppm by 2100 (Nakicenovic et al 2000). The main contributors to the increase are fossil-fuel combustion and deforestation, which increased from 1970 to 2004. The percent increase in emissions for different sectors included; 65% for industry, 120% for transport, 145% for energy supply while land use, land-use change and forestry increased with 40% (IPCC 2007c).

Recent evidence shows that CO2 concentration increased by 1.9 ppm annually during 1995-2005, compared to the long-term average rate of 1.5 ppm since the beginning of the Industrial Revolution (IPCC 2007a). The growing concentration of CO2 is likely to have positive implications for carbon sinks and net primary productivity so long as soil water or nutrients do not become the limiting factors. Projected climate change Global average surface temperature during the last 100 years has increased by 0.74 °C, at the rate of 0.13 °C per decade over the last 50 years. The average global warming under various scenarios is estimated to be in the range of 1.8-4 °C, the best estimate being 2.4 °C by the end of the current century, with land surface warmer than the ocean surface. In addition to the warming, precipitation is likely to increase at higher latitudes and decrease in most subtropical land regions (IPCC 2007a). The combination of warming and changes in precipitation will impact the vegetation, particularly in terms of carbon stocks and plant productivity. Nitrogen deposition Nitrogen deposition consists of the input of reactive nitrogen species from the atmosphere to the biosphere. A simulation study by Lamarque et al. (2005) showed that under the assumed IPCC SRES A2 scenario the global annual average nitrogen deposition over land is expected to increase by a factor of 2.5, mostly because of the increase in nitrogen emissions. On average, approximately 70% of the emitted nitrogen is deposited over the land masses. The results from this study suggest that the deposition over land ranges between 25 and 40 million tonnes (N) a year and, by 2100, will range between 60 and 100 million tonnes. Nitrogen deposition on forests is expected to double by 2100, which is likely to have implications for forest biomass stock and growth rates. The sources of nitrogen emissions include fossil fuel combustion, biomass burning, organic manure and application of nitrogenous fertilizer.

19.2 Impact of Climate Change on Forest Ecosystems

Forests account for nearly one third of the earth's land area with the tropics dominating by accounting for 42% of total global forest area. Forests store the largest portion of biospheric carbon stocks, estimated at 1,146 billion tonnes of carbon. A warming greater than 2-3 °C is projected to impact forest ecosystems (IPCC 2001, 2007b) in following ways.

• Significant forest dieback towards the end of the century in tropical, boreal and mountain areas, leading to loss in biodiversity, reduction in carbon sinks and loss of other forests services

• Substantial changes in structure and functioning of terrestrial ecosystems

• Nearly one third of the species assessed at increasing risk of extinction

Impact on forest biomass production and net primary productivity Scientific evidence of the impact of climate change on plant productivity is not yet conclusive. The projected climate change is likely to lead to gains and losses in plant productivity in different regions and over different periods.

• A review of impacts of climate change suggests varying levels of impacts (IPCC 2001, 2007b; Boisvenue and Running 2006). According to both field and satellite-based data, climate change over the last 55 years seems to have had a generally positive impact on forest productivity. However, this is only true of sites where water is not a limiting factor. Average productivity gains may result from CO2 fertilization, although this effect is likely to be much weaker than previously projected, and plant productivity can also be limited by availability of nutrients.

• The atmospheric system has experienced changes not only in temperature but also in precipitation and solar radiation, in addition to rise in CO2 concentration. Response of forest vegetation to rise in concentration of CO2 is still uncertain, although studies (Wittig et al. 2005) found that gross primary productivity increased dramatically in the initial years at the younger state of development of species.

• A study by Ravindranath et al. (2007), using the equilibrium model BIOME, estimated that the net primary productivity under A2 and B2 IPCC-SRES scenarios would increase by 70-100% over the control or baseline scenario for different forest types in India, provided water or soil nitrogen are not the limiting factors.

• Carbon stock in the present-day vegetation is estimated to be about 600-630 GtC; between 2060 and 2100, it is predicted to increase by 290 GtC under the Had CM2 climate scenario and 170 GtC under the Had CM3 climate scenario (White et al 1999).

• Individual species responses to doubling of CO2 can range from close to zero if nutrients, especially N, and water are the limiting factors (Oren et al. 2001; Reich et al. 2006) up to 70% when they are not (Morgan et al. 2004).

• Globally, forestry production is estimated to change only modestly with climate change in the short and medium term. The change in global forest products output could range from a modest increase to a slight decrease, although regional and local changes are likely to be large. Production increase is likely to shift from low-latitude regions in the short term to high-latitude regions in the long term.

There is little doubt that increasing atmospheric CO2 can increase carbon uptake, and possibly carbon sinks, but the magnitude and spatial distribution of these impacts are still debated (Morgan et al. 2004; Körner et al. 2005). Climate Change coupled with increasing CO2 concentration, nitrogen deposition and other factors, is likely to impact net primary productivity and biomass production in forest, plantation and grassland ecosystems. This will have implications for carbon inventory, in particular projection of future carbon stocks or biomass production in land-use systems.

19.3 Models for Projecting Carbon Stock Changes Under Climate Change Scenarios

Forests are highly complex biological ecosystems. Ecological modelling has been applied in forest science to make long-term projections of the impacts of climate change on forest vegetation dynamics. Predicting the effects of future climate change and human disturbances on the distribution of natural vegetation requires modelling. The models used to predict responses of vegetation to future climate change are categorized as follows and an illustrative list of models and their features is presented in Table 19.1.

Static biogeographical models Static biogeographical models assume equilibrium conditions in both climate and vegetation in predicting the distribution of potential vegetation by relating the geographic distribution of climatic parameters to vegetation. The equilibrium approach implicitly ignores the dynamic processes but generally requires far less information and estimates the potential magnitude of the response of vegetation from regional to global scales. These equilibrium models are restricted to the estimating steady-state conditions and include such models (Table 19.1) as BIOME and the mapped-atmosphere-plant-soil system model (MAPSS) (Woodward 1987).

Dynamic biogeographical models Dynamic biogeographical models capture the transient response of vegetation or biomes to a changing environment using explicit representation of key ecological processes such as establishment of a tree plantation, tree growth, competition, death and nutrient cycling (Shugart and West 1980; Shugart 1990; Botkin 1993). Dynamic models also require much more information on the characteristics of species than is easily available or even known for some areas of the globe (Solomon 1986). These models are used in predictions at the regional scale or for ecosystems, and have also been applied at the global scale. Examples of such models include HYBRID and IBIS (Table 19.1).

Process-based biogeochemistry models Biogeochemistry models project changes in basic ecosystem processes, such as the cycling of carbon, nutrients and water. These models are designed to predict changes in nutrient cycling and primary productivity. The inputs to these models are temperature, precipitation, solar radiation, soil texture and atmospheric CO2 concentration. The plant and soil processes simulated are photosynthesis, decomposition, soil nitrogen transformations mediated by microorganisms, evaporation and transpiration. Common outputs from biogeochemistry models are estimates of net primary productivity, net nitrogen mineralization, evapotranspiration fluxes and the storage of carbon and nitrogen in vegetation and soil. Examples of such models are BIOME-BGC (Hunt and Running 1992; Running and Hunt 1993), CENTURY (CENTURY

Table 19.1 Features of selected climate impact assessment models and their relevance to carbon inventory

Model

Features

Data needs

Outputs

Application

Hybrid Numerical process-based model;

considers the daily cycling of C, N and H2O within the biosphere and between the biosphere and atmosphere and projects C in soil and vegetation and productivity

- Number of wet days in each month

- Mean precipitation per wet day

- Monthly mean of maximum temperature recorded daily

- Monthly mean of minimum temperature recorded daily

- Solar irradiance, vapour pressure

- Past and projected CO2 concentrations

- Species-level parameters (33)

- Plot-level parameters (23)

- Individual-level parameters (17)

- Plant phenological parameters (2)

■ Annual gross primary productivity (kgC/m2/year)

■ Annual net primary productivity (kgC/m2/year)

Projecting C stocks and rates of change r

BIOME Coupled carbon and water flux scheme, which determines the seasonal maximum LAI that maximizes NPP for any given PFT

- Monthly mean temperature, precipitation, and sunshine hours

- Water-holding capacity of top 30 cm and 120 cm of soil.

- Water conductivity indices

- Area changes in forest types

- Changes in NPP

Projection cf NPP

RothC Terrestrial ecosystem biogeochemistry model

- Projected monthly air temperature, rainfall and evaporation.

- Clay content, sampling depth, bulk density and inert carbon in soil

- Total C in soil

Projection cf C in soil

C: carbon, N: nitrogen, LAI: leaf area index, NPP: net primary productivity, PFT: plant functional types RothC: http://www.rothamsted.bbsrc.ac.uk/aen/carbon/rothc.htm

Hybrid: http://eco.wiz.uni-kassel.de/model_db/mdb/hybrid.html, BIOME:http://mercury.ornl.gov/metadata/ornldaac/html/rgd/daac.ornl.gov_data_bluangel_ harvest_RGED_QC_metadata_models_biome4.html

C: carbon, N: nitrogen, LAI: leaf area index, NPP: net primary productivity, PFT: plant functional types RothC: http://www.rothamsted.bbsrc.ac.uk/aen/carbon/rothc.htm

Hybrid: http://eco.wiz.uni-kassel.de/model_db/mdb/hybrid.html, BIOME:http://mercury.ornl.gov/metadata/ornldaac/html/rgd/daac.ornl.gov_data_bluangel_ harvest_RGED_QC_metadata_models_biome4.html

1992), and RothC models (Coleman and Jenkinson 1995). BIOME-BGC predicts stocks and fluxes of carbon in both vegetation and soil whereas CENTURY and RothC predict stocks of carbon in soil.

19.4 Implications of Climate Change for Carbon Inventory

Climate change is important especially for long-term biomass production and carbon mitigation projects. Projections of future carbon stocks or biomass production are required during the project development phase. The implications of climate change for carbon inventory for different programmes and projects are given in Table 19.2. Karjalainen et al. (2003) have prepared a carbon inventory of European forest sector for the year 1990 and projected it for the year 2050 by incorporating the growth functions calibrated by process-based models into the European Forest Information Scenario Model (EFISCEN) framework. Total amount of carbon in

Table 19.2 Implications of climate change for carbon inventory

Carbon inventory programme or project

Estimates required

Implications for carbon inventory

Feasibility and reliability of projections of climate impacts

Carbon mitigation

■ Biomass and soil carbon stock projections

■ Growth rates of biomass and soil carbon

- Estimating carbon gains or losses due to CO2 fertilization and climate change

- Significant implications for long-term afforestation and avoided deforestation projects

- Need to exclude credits due to indirect effects of human action or global change

- Projection of impacts of climate change and CO2 fertilization is feasible using dynamic global vegetation models

- High uncertainty of projections at regional, species and project level

Biomass production Roundwood (biomass)

Significant for long

- Limitations exists with

stocks and growth

rotation timber

respect to assessment

rates

projects for eco

of combined effects of

nomic analysis

climate change, CO2

concentrations and

other factors

GHG inventory

Factoring out of CO2 fertilization effects on carbon stock changes

Need for assignment of C credits and debits for projections of future carbon sink

Limitations of model prevent factoring out the effects of indirect human impacts

GHG inventory

Factoring out of CO2 fertilization effects on carbon stock changes

Need for assignment of C credits and debits for projections of future carbon sink

Limitations of model prevent factoring out the effects of indirect human impacts

1990 was estimated to be 12.8Gt, with 94% of it in tree biomass and forest soil and 6% in wood products in use. Average total carbon stock is projected to be 35% higher after 60 years under the business-as-usual scenario. Average total carbon stocks are projected to be about 5% higher under a climate change scenario consisting of a mean temperature increase of 2.58 °C and annual precipitation increase of 5-15% between 1990 and 2050 than those under current climatic conditions.

19.5 Conclusions

Biomass production, carbon sequestration rates and carbon stocks in forest, plantation, grassland and agroforestry systems are influenced by climate parameters, carbon dioxide concentrations, nutrient supply, soil moisture status, management practices and other factors. This dependence makes projections of future biomass production or carbon sequestration rates too complex a process. Models help in making projections and are available for projecting the impacts of climate change and increase in CO2 concentration on carbon stocks and net primary plant productivity of forest and plantation systems. These models require a whole range of parameters related to climate, plant physiology, soil, moisture and so on to be defined for each location and plant functional type. Most vegetation models make projections only for the plant functional types incorporated in the model. Thus, climate impacts cannot be assessed for all the diverse forest or plantation types occurring in a project location or even in a country. The currently available models have limitations in addressing the combined effects of climate, soil nutrients, water status and management practices. The projected climate changes, particularly those in temperature and precipitation and increase in concentration of CO2 in the atmosphere will have impacts on biomass production and carbon sequestration. Such impacts are particularly relevant to long-term projects such as afforestation, reforestation and avoided deforestation. In future, impacts of climate change and elevated CO2 concentration on land-use systems will become important for programmes and projects aimed at addressing climate change, particularly mitigation, as well as estimation of national greenhouse gas emissions. Future developments in methods and models may enable a better understanding of the implications of impacts of climate change, such as elevated CO2 concentration and nitrogen deposition, on biomass production or carbon sequestration.

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