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 ecosystemswith 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 forestsphotosynthesis, plant and heterotrophic respiration, and disturbance releasesare 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.
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).
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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