Length of the growing season

The productivity of a community can be sustained only for that period of the year when the plants have photosynthetically active foliage. Deciduous trees have a self-imposed limit on the period when they bear foliage. In general, the leaves of deciduous species photosynthesize fast and die young, whereas evergreen species have leaves that photosynthesize slowly but for longer (Eamus, 1999). Evergreen trees hold a canopy throughout the year, but during some seasons they may barely photosynthesize at all or may even respire faster than they photosynthesize. Evergreen conifers tend to dominate in nutrient-poor and cold conditions, perhaps because in other situations their seedlings are outcom-peted by their faster growing deciduous counterparts (Becker, 2000).

The latitudinal patterns in forest productivity seen earlier (see Table 17.2) are largely the result of differences in the number of days when there is active photosynthesis. In this context, Black et al. (2000) measured net ecosystem productivity (NEP) in a boreal deciduous forest in Canada for 4 years. First leaf emergence occurred considerably earlier in 1998 when length of the growing season: a pervasive influence on productivity

Species Richness Index

Figure 17.12 Seasonal patterns in leaf area index (area of leaves divided by ground area beneath the foliage) of (a) overstory aspen (Populus tremuloides) and (b) understory hazelnut (Corylus cornuta) in a boreal deciduous forest during four study years with contrasting spring temperatures. (c) Cumulative net ecosystem productivity (NEP). (After Black et al., 2000.)

Figure 17.12 Seasonal patterns in leaf area index (area of leaves divided by ground area beneath the foliage) of (a) overstory aspen (Populus tremuloides) and (b) understory hazelnut (Corylus cornuta) in a boreal deciduous forest during four study years with contrasting spring temperatures. (c) Cumulative net ecosystem productivity (NEP). (After Black et al., 2000.)

the April/May temperature was warmest (9.89°C) and a month later in 1996 when the April/May temperature was coldest (4.24°C) (Figure 17.12a, b). Equivalent spring temperatures in 1994 and 1997 were 6.67 and 5.93°C. The difference in the length of the growing season in the four study years can be gauged from the pattern of cumulative NEP (Figure 17.12c). During winter and early spring, NEP was negative because ecosystem respiration exceeded gross ecosystem productivity. NEP became positive earlier in warmer years (particularly 1998) so that overall total carbon sequestered by the ecosystem in the four years was 144, 80, 116 and 290 g C m-2 year-1 for 1994, 1996, 1997 and 1998, respectively.

In our earlier discussion of the study of Argentinian pampas communities (see Figure 17.10) we noted that higher NPP was not only directly affected by precipitation and temperature but was partly determined by length of the growing season. Figure 17.13 shows that the start of the growing season was positively related to mean annual temperature (paralleling the boreal forest study above), whereas the end of the growing season was determined partly by temperature but also by precipitation (it ended earlier where temperatures were high and precipitation was low). Again we see a complex interaction between water availability and temperature.

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