In climates that are too dry for forest but wetter than desert, one can either have scrub or grassland. Whether scrub or grassland actually occurs in a particular place depends on a range of factors including soil type, the time of year when rain occurs, lire frequency and the abundance of grazing mammals. It also depends partly on what species of plants happen to have evolved locally; whether they are mostly grasses or mostly bushes. Also, if the soils are thin, infertile and rocky, scrub is more likely than grassland.
Many places around the world that would naturally be forested have been reduced to scrub by human influence, through frequent burning and goat-grazing (see below). Around the Mediterranean, a natural scrub vegetation known as garri-guc has expanded greatly in area over the past several thousand years due to these influences. If burning is prevented and goats arc kept out, this vegetation often reverts to forest over several dccadcs.
Some scrub areas of the world arc strikingly rich in species of plants. For example, there is the very species-rich fynbos vegetation of the Cape region of South Africa, which has some 600 species of heathers (the genus Erica) plus many other types of plants packed into an area only a couple of hundred kilometers across.
Grasslands arc as the name suggests dominated by grasses, but usually there arc low-shrubs and broad-leaved flowers mixed in amongst them too. Grasslands arc called "steppe" in temperate latitudes (from a Russian word), while in the tropics they arc called "tropical grassland". The term "savanna" tends to be applied to warm climate grasslands that have an open scattering of trees or shrubs. The steppe grasslands fade in to "tundra" in high latitudes and at high altitudes on mountains. Tundra can have a lot of grasses, and mosses, but is often dominated by a rather prickly mass of low shrubs: dwarf willows, dwarf alders and dwarf birches, mixed with lichens. Thus, sometimes tundra can be a grassland, sometimes it is essentially a low scrubland.
Grasses grow from buds right down next to or underneath the ground, and can recover from fire or grazing very easily. They also have long leaves that grow by-pushing out from their base, like toothpaste out of lube. This also means that they can regrow very easily if the tops of the leaves are burnt or eaten. In fact, most grasses seem to "need" frequent fires or grazing to keep other plants out. In the absence of either type of disturbance, the grasses are usually out-competed by other plants such as trees or shrubs. Thus, grasses can have a strange indirect arrangement with grazing animals; the grazers kill parts of the grass, but the grass needs the help of the grazers in seeing oil* the competition.
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