The presence of slopes modifies climate by its aspect and its effect on air drainage. Aspect is the combination of the slope and the direction that it faces. North-facing slopes offer plants less sunlight than a south-facing one. This is dramatically illustrated when observing the snow on opposite sides of an east- west valley (or roofs in a street), when the north facing sides are left white long after the snow has melted on the other side (see Figure 2.9); much
more radiation is intercepted by the surface on the south facing slope. Closer examination reveals considerable differences in the growth of the plants in these situations and it is quite likely that different species grow better in one situation compared with the other. Plants on such slopes experience not only different levels of light and heat, but also different water regimes; south-facing slopes can be less favourable for some plants because they are too dry.
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