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CAS Workshop on Ecosystem Succession Theory and Practice of Ecological Restoration Effects of reforestation, afforestation and harvest on forest ecosystem
carbon cycling: Managing forests to mitigate climate change Pan Yude Richard Birdsey (USDA Forest
Service, Global Change Program, Newtown Square, PA 19073, USA) Abstract:
The UNFCCC was negotiated with the aim of reducing fossil fuel
emissions because of the likely climate effects of increasing CO2
concentrations. Aside from limiting fossil fuel consumption, the
protocol includes provisions for managing natural terrestrial carbon
sinks, primarily afforestation and reforestation, to increase sink
strength and reduce atmospheric CO2. Thus, understanding
the nature of terrestrial carbon sinks, their distribution, control,
longevity, and reliability is an increasing demand for both the
scientific community and government agencies.
Forest ecosystems play dominant
roles in the carbon cycle because they store a large amount of C in
vegetation and soil, and interact with atmospheric processes through
the absorption and respiration of CO2. Recent research has
highlighted the role of the Northern Hemisphere as a carbon sink and
suggested that the mid-latitude forests were likely the primary places
where carbon sequestration has been enhanced. There is no doubt that
forest ecosystems can be managed to reduce carbon emission and
increase carbon sink size significantly; yet there are different
opinions about how to manage forests to increase sinks. The Kyoto
Protocol manages young forest stands because of their greater capacity
for assimilating carbon. However, some scientists suggest that
preservation of natural old growth forests may have larger effects on
the carbon cycle than promotion of regrowth because harvesting may
cause short-term losses of ecosystem carbon. According to the Kyoto
Protocol, carbon sources and sinks from the Kyoto forests that consist
of post-1990 reforestation, afforestation and deforestation can be
counted as a country’s efforts to reduce emission. Replacement of
old-growth forests by young Kyoto stands can gain the credit as part
of forest management. Many studies have suggested that
reforestation and afforestation could be an effective way to sequester
carbon. Field data and the simulation results of the succession model
TEM-LPJ illustrated that annual NPP in forest stands was likely to
reach the maximal at ages between 20 and 30 and then decline before
level off. The carbon
storage in forest vegetation could saturate as forests aged over 50
years old. One of our recent studies on China forest carbon estimation
indicated that the carbon pool in China forest trees had increased
approximately 13% since early 1970s, reflecting the impact of
reforestation and afforestation programs in China since the 1960s. Our
results showed that the carbon sequestration rate from the late 1980s
to the early 1990s was significantly higher, which was likely related
to the change of age structure in China forests that reached more
productive stages. In this study, we compared forest carbon estimates
of China, the conterminous U.S., and Russia for the period of
1988-1993. The results further showed the effect of ages on forest
carbon sequestration. Although China had much less forested lands and
a smaller forest carbon pool than the U.S. and Russia, the
area-weighted C sequestration rate was highest because of younger
forests and greater C sequestration capacity.
The criticism of the forest
management strategy under the Kyoto Protocol is that the protocol is
not based on a full ecosystem carbon budget including all ecosystem
components and applies only to specific “commitment” periods
instead of a long time period. It is argued that the carbon budget of
forests is determined more by respiration than assimilation.
In forest ecosystems, most carbon is stored in pools such as
wood, litter and soil organic matter that differ in their turnover
time ranging from daily to millennium timescales. About two thirds of
terrestrial carbon is stored in soils and generally has slower
turnover rates than aboveground carbon. Without disturbances, soil
organic carbon in passive pools can be maintained over longer period
of time. The studies of forest ecosystems
revealed that large carbon losses could take place after a stand
harvest because of tremendous increase of respiratory and leaching
losses. A full harvest resets the vegetation to an early stage of
succession. TEM-LPJ simulated soil carbon dynamic in a secondary
succession of a stand in Harvard Forest after an assumed clear
cutting. The soil carbon pools appeared to decline for several years
because the litter input of the regrowth was lower than the decay rate
of organic matter already present in the soil. The modeled results of
secondary succession indicated that it took 60-80 years for soil
carbon pools to recover to the precutting level in forest sites, which
agreed with experimental results at Harvard Forest. The raised concern
is that the forest management credited by the Kyoto protocol, such as
reforesting old-growth forests, may lead to massive carbon losses by
reducing passive soil carbon pools and create an incentive for actions
that can actually increase cumulative emission. Thus, net biome
production (NBP), a new concept that represents a full carbon budget
over sufficient time scales to reflect changes in long-term carbon
storage, is suggested as an appropriate basis for any accounting
system for terrestrial carbon regarding the development of C emission
restriction in the Kyoto protocol.
Because of large uncertainties
in calculating carbon budget associated with land-use change and
forestry, it requires our better understanding about regulation
processes of terrestrial carbon balance across scales from plots to
continents and from daily to millennium. For forest ecosystems,
statistically accurate estimates for aboveground change of carbon
storage are readily available, but the estimates for belowground
components are poor and uncertain. We lack knowledge for verifying
soil carbon changes in the past decades attributed to land-use changes
and disturbances. Nowadays, the impacts of human-induced disturbances
have become such a prominent property of all ecosystems that affects
carbon fluxes at all spatial and temporal scales. Our recent research
indicated that changed atmospheric chemistry such as CO2
concentration and N deposition not only affected forest sequestration
capacity, also altered allocation pattern of carbon to different pools
of forest ecosystems. We should understand that
managing forest ecosystems to reduce carbon emission only serves as a
temporary solution because a substantial fraction of the fossil fuel
carbon sequestered in terrestrial sinks is vulnerable to return to the
atmosphere on time scale from decades to century. Terrestrial sinks
are thus best viewed as buying valuable time to reduce industrial
emissions. Although many processes that influence the net terrestrial
sink are beyond direct human management, careful evaluation and
adoption of appropriate strategy for managing forest ecosystems,
especially enhancing sizes of stable carbon pools, are certainly
important for achieving our goals to mitigate climate change.
Key words:
Carbon sequestration; Kyoto Protocol; forest management;
deforestation/afforestation; full harvest; net biome production (NBP);
climate change
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