Temperate deciduous broad-leaved forests exist across large regions of Asia, Europe, and North America, typically north of 30°. Globally, they occupy only 2 percent of the land area of the terrestrial biosphere (Melillo et al. 1993), but they contribute significantly to the global carbon budget because they occur in relatively wet and highly productive regions.
During winter, the trees are leafless and dormant, and the ecosystem is respiring (Figure 15.2). The presence or absence of snow has a major impact on soil temperatures and the rates of soil respiration from these forests (Goulden et al. 1996). Snow insulates the soil surface, so soil temperatures are warmer when snow is present. Consequently, greater rates of soil respiration occur from snow-covered ground than from bare soil, which is exposed to the cold winter weather.
During spring, a pronounced peak in ecosystem respiration reflects increased growth respiration as leaves emerge (Greco and Baldocchi 1996; Valentini et al. 1996; Granier et al. 2000, 2002). Daily respiration rates typically range between 5 and 7 g C m-2 d-1 during this period, which is double to triple respiration rates earlier in the spring. As leaf-out occurs, the ecosystem experiences a pronounced switch from a net source of carbon to a net sink. This switch can represent a net change in the magnitude of carbon exchange that approaches 10 g C m-2 d-1. Consequently, the date of leaf-out has a major impact on the net annual carbon exchange of this biome.
The date of leaf-out can vary by 30 days at a given site (Goulden et al. 1996; Wilson and Baldocchi 2000) and by more than 100 days across the deciduous forest biome (Figure 15.3). Consequently, differences in length of growing season can alter NEE by up to 600 g C m-2 year-1.
Once a forest has attained full canopy closure, changes in available sunlight explain most of the variability in hourly rates of carbon exchange, with a nonlinear and saturating response (Goulden et al. 1996; Valentini et al. 1996; Baldocchi 1997; Granier et al. 2000; Schmid et al. 2000; Pilegaard et al. 2001). The transparency of the atmosphere, however, complicates the sensitivity of NEE to changes in sunlight. At the canopy scale, the initial slope of the light response curve (also known as light use efficiency, LUE) increases as the fraction of diffuse radiation increases (Baldocchi 1997; Gu et al. 2002).
The net effect of increasing diffuse radiation, as a consequence of either clouds or atmospheric aerosols, is complex. In general, short-term NPP is twice as sensitive to changes in diffuse light as it is to changes in direct light (Gu et al. 2002). On an annual basis, increasing diffuse radiation by 20 percent without affecting total incident radiation (as can occur with aerosol loading in the atmosphere) can theoretically increase annual NEE of a temperate deciduous forest by 10-15 percent (Baldocchi et al. 2002), about 70 g C m-2 year-1. Increasing cloud fraction increases diffuse radiation but lowers total solar radiation. In this situation, an expected reduction in GPP (from the decrease in total solar radiation) is minimized at the canopy scale by an increase in light use efficiency (from the increased fraction of diffuse radiation).
Droughts tend to be episodic across the temperate deciduous forest biome. Summer drought and high vapor pressure deficits reduce daytime photosynthesis (Goulden et al. 1996; Greco and Baldocchi 1996; Baldocchi 1997). Drying of the soil also decreases soil
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Figure 15.3. Impact of length of growing season on NEE of temperate broad-leaved forests. Length of growing season explains more than 80 percent of the variance of annual NEE across this biome (Baldocchi et al. 2001).
respiration (Hanson et al. 1993). Together, these features reduce NEE from values expected during normal, wet summer conditions (Figure 15.2).
In the autumn, photosynthesis ceases, and leaves senesce and drop. Soil respiration, on the other hand, experiences an enhancement due to the input of fresh litter that is readily decomposable, as soils are still warm and autumn rains stimulate litter decomposition (Goulden et al. 1996; Granier et al. 2000).
On a daily and monthly timescale, ecosystem respiration is mainly a function of soil temperature. The proportionality factor by which ecosystem respiration increases with a 10°C increase in temperature (Q10) ranges between 1.6 and 5.4 (Schmid et al. 2000). Based on such data, one would expect that annual ecosystem respiration would increase as one moves south in the Northern Hemisphere to warmer climates. But across forests in Europe, ecosystem respiration was greater in northern regions (Valentini et al. 2000; Falge et al. 2002). This observation may be an artifact of relatively recent disturbance
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