5.6.4.2. Vulnerability Associated with NPP, NEP, and NBP
Given their inertia, forested systems may exhibit low vulnerability and low
climate sensitivity, unless drought and disturbance are driving factors (Peterken
and Mountford, 1996). Extant forests may persist and appear to exhibit low vulnerability
and low climate sensitivity (see Section 5.2), but they
may be climate sensitive in ways that are not immediately apparent. Thus, their
vulnerability may occur as a reduction in quality (degradation) even where the
forest persists as an entity. Increases in disturbances, however, may lead to
rapid structural changes of forests, with replacement by weedy species (Overpeck
et al., 1990). For example, an increase in area burned by fire or destroyed
by invading insects and disease could rapidly undergo changes in species composition,
successional dynamics, rates of nutrient cycling, and many other aspects of
forest ecosystems—with impacts on goods and services provided by these
forests. From this perspective, in the context of all of the direct and indirect
impacts of climatic change and their interactions, the potential vulnerability
of forests is high.
The main processes that determine the carbon balance of forests—photosynthesis,
plant and heterotrophic respiration, and disturbance releases—are regulated
by different environmental factors. Carbon assimilation is a function mainly
of available light, temperature, nutrients, and CO2, and this function shows
a saturation characteristic. Respiration is a function of temperature; it increases
exponentially with temperature. Thus, close correlation between GPP and NPP
does not exist (allocation plays an important role). Similarly, there is no
strict correlation between NPP and NEP (high NEP is possible at high and low
rates of assimilation and respiration). Disturbances such as harvest or fire
export carbon from forests, bypassing respiration; thus, NBP is not expected
to correlate with NEP and NPP. Photosynthesis, plant and heterotrophic respiration,
and disturbance are vulnerable in different ways and are quite sensitive to
climate change and other global change forces.
5.6.4.3. Vulnerability of Unmanaged Systems
Increases in disturbances such as insect infestations and fires can lead to
rapid structural and functional changes in forests (species composition, successional
dynamics, rates of nutrient cycles, etc.), with replacement by weedy species.
The effects of these vulnerabilities in unmanaged systems on goods and services
vary. They are not likely to have large effects on market products given that
unmanaged forests constitute an increasingly small portion of timber harvests.
They could have large impacts on local provision of timber products, NWFP, and
fuelwood. Local services from forests could be highly vulnerable, particularly
if the services are tied to specific forest functions that change (e.g., hiking
in old-growth forests).
5.6.4.4. Vulnerability in Managed Systems
Managed systems are vulnerable to direct impacts on NPP (related to volume
production) and species composition (related to timber quality), as well as
market forces. The global supply of market products exhibits low vulnerability
because timber markets have high capacity to adapt to change. Temperate and
boreal producers are vulnerable to dieback effects and lower prices caused by
potential global increases in timber growth. Managed forests in subtropical
regions have low vulnerability given their high growth rates and short rotation
periods, which provide multiple opportunities to adapt to changes.
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