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Patterns in forests across environmental gradients can also be examined in terms of structure and species composition. Trees tend to be taller under favorable environmental conditions of moderate‐to‐warm temperatures and high rainfall (Figure 2.3). Part of the geographic pattern in tree heights relates to the physics of supplying leaves high above the ground with sufficient water to meet the evaporative demand of the atmosphere; dry air requires more water than the tallest trees could transport (see Chapter 4). The number of species found in forests is low at higher elevations, and high at lower elevations and in warmer areas.

FIGURE 2.3 The tallest trees in the world occur in cool, wet locations, and most tall trees are limited to relatively wet areas. More severe growing conditions limit average tree heights

(Source: upper map, from Pan et al. 2013/Annual reviews,Inc.).

The diversity of tree species in a tropical forest in Costa Rica dropped by about half with a 1000 m increase in elevation (lower left,

Source: Data from Veintimilla et al. 2019, photo by Cristian Montes).

Across eastern China, the number of tree species that co‐occur in 600 m² (=0.06 ha) plots increases rapidly with temperature, but levels off (and might even decline) on the hottest sites.

Source: Based on Wang et al. 2009.