Northern Wetlands Tundra

Snow Cover

Ts.„<°

GPP=0

Reco ~> 0

snow melt

GPP > 0

Tmln < 0; Senescence

GPP-0

\ T.,r<+) J~

\ R.co (++); Gpp(+) r

\ Reco > GPP

V ^J water table(-)

\J R.=o(+)

NEE > 0

Figure 15.9. Seasonal variation in net CO2 exchange of a northern wetland and tundra. Also shown are the environmental and biological factors that affect the seasonal dynamics of CO2 exchange. Data are gap filled and smoothed. Data sources: Shurpali et al. (1995); Griffis et al. (2000); Soegaard et al. (2000); Vourlitis et al. (2000); Lloyd (2001).

variations in a given region cannot be simply represented and modeled as a single "green slab." A reasonably accurate land use/cover (forests, grasslands, crops, etc.) map is a prerequisite for correctly representing the aggregation of different functional responses and biospheric fluxes.

The examples in this chapter also highlight the vulnerability of the terrestrial carbon sink. Climate changes and disturbance regimes affect the rates and patterns of biospheric carbon exchanges. In boreal regions changes in seasonal dynamics and length of the growing season can have a profound impact on carbon oxidation, even when the annual mean temperature is not perturbed. And if warm and dry spells promote forest fires, the landscape can quickly change from a weak carbon sink to a large source of carbon to the atmosphere. In temperate regions water limitations on soil organic matter decomposition plays a significant role and can limit carbon oxidation, despite changes in temperature. In the tropics light-use efficiency and changes in the ratio of direct versus diffuse radiation can also play an important role in the future carbon exchanges.

These are only few examples of the complexity of the terrestrial ecosystem responses and the parameters that drive their carbon balance. More work is needed to understand these emergent properties, which can have important impacts on the future of the land carbon sink, and to improve our global models. For this reason the concept of "ecosystem physiology," the integrated response of a vegetation community to environmental factors including carbon, energy, and water, is becoming a useful new tool for understanding the dynamics of the terrestrial biota and its relation with Earth system processes.

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