Читать книгу Forest Ecology - Dan Binkley - Страница 31

The distributions of forests can be mapped across the world, showing regions where major forest types occupy most of the landscapes (Figure 2.2). Forests essentially occur everywhere there is enough water for trees to cope with evaporative demands driven by temperature and atmospheric humidity. The two exceptions to this generalization would be areas that are too cold, and areas that have enough water but high frequency of fires leads to greater success of grasses and fire‐tolerant shrubs.

The productivity of the world's vegetation has been mapped by satellites, where the wavelengths of light reflected from vegetation across time periods are combined with simulation models to estimate growth rates. Such global estimates may come close to pegging the actual global averages and totals, but the accuracy for any single region or landscape would need to be verified for confidence (as with any map).

FIGURE 2.2 The distributions of major types of forests can be mapped across the world (upper), along with typical rates of aboveground net primary production of forests and other vegetation types (middle; for more information on production, see Chapter 7;

Source: maps based on Pan et al. 2013).

The spatial patterns can also be examined in a functional way, where aboveground net primary production is plotted relative to annual temperatures, and the balance between precipitation and the energy available to evaporate water (wetter conditions occur above the 0 point on the Y‐axis;

Source: Based on Running et al. 2004.

The same information can be examined in relation to environmental factors rather than spatial locations (Figure 2.2). It's not surprising that the most productive forests (and other vegetation types) tend to occur in warmer locations where precipitation is close to (or higher than) the water that could be evaporated by available energy. Is it possible to be too warm for high growth rates, even where plenty of water is available? This is not clear from Figure 2.2, but the hottest tropical forests do appear to have lower growth rates than somewhat cooler forests (see Figure 2.18).