The large ranges in productivity in Table 17.2 and the wide confidence intervals in Figure 17.1 emphasize the productivity shows considerable temporal variation
considerable variation that exists within a given class of ecosystems. It is important to note also that productivity varies from year to year in a single location (Knapp & Smith, 2001). This is illustrated for a temperate cropland, a tropical grassland and a tropical savanna in Figure 17.2. Such annual fluctuations no doubt reflect year-to-year variation in cloudless days, temperature and rainfall. At a smaller temporal scale, productivity reflects seasonal variations in conditions, particularly in relation to the consequences of temperature for the length of the growing season. For example, the period when daily GPP is high persists for longer in temperate than in boreal situations (Figure 17.3). Moreover, the growing season is more extended but the amplitude of seasonal change is smaller in evergreen coniferous forests than in their deciduous counterparts (where the growing season is curtailed by the shedding of leaves in the fall).
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