Evergreen Conifer Forests

Evergreen conifer forests occupy a diverse range of climates. They exist at the cold extremes of the boreal zone, in the mild, humid maritime regions, on alpine mountain slopes, and in the semi-arid interior regions of continents. Consequently, the seasonal cycle of carbon exchange of evergreen conifer forests and its sensitivity to environmen tal forcings is not universal. Boreal, maritime, and subtropical conifer forests experience different seasonal patterns of CO2 exchange over a year's time (Figure 15.4).

Forests in maritime climates (Berbigier et al. 2001; Chen et al. 2002) and those growing close to the subtropics (Hui et al. 2003) have the potential to acquire carbon year-round. In contrast, conifer forests in cold boreal or alpine climates experience a restricted period for carbon uptake confined to the summer growing season (Goulden et al. 1996; Jarvis et al. 1997; Lindroth et al. 1998; Hollinger et al. 1999; Markkanen et al. 2001; Monson et al. 2002). During the short summer growing season in the extreme boreal and alpine climate zones, conifer forests frequently lose carbon on individual days. These situations either occur in low-productivity sites on hot, sunny days when high vapor pressure deficits promote stomatal closure (Baldocchi et al. 1997; Hollinger et al. 1999) or on cloudy days when canopy photosynthesis is reduced but the soil is warm and respiration is high (Monson et al. 2002).

Conifer forests in temperate, continental climates experience a longer growing season than forests in boreal and alpine climates. Because temperate conifer forests grow on more productive sites, they maintain higher rates of carbon uptake than do boreal forests (Aubi-net et al. 2001; Dolman et al. 2002). Furthermore, they do not experience summer episodes of carbon loss, like boreal forests. Conifers growing in semi-arid regions, such as the western United States, acquire carbon best during the spring and fall and can lose carbon during the prolonged summer drought (Goldstein et al. 2000; Anthoni et al. 2002).

Whether or not conifers gain carbon on fine spring days before the true growing season starts remains an important question. Needles are present and have the potential to assimilate carbon. Physiological studies on Picea abies show that uptake rates are low but positive, on fine days with frost events, but that carbon assimilation is suspended when air temperature remains below freezing (Larcher 1975). At the canopy scale, CO2 uptake for a boreal Scots pine forest is close to zero when the mean air temperature is above, but near, zero Celsius (Markkanen et al. 2001).

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