The Quiver Tree (Aloe dichotoma) is one of the best known desert plant species of southern Africa. Reaching heights of up to 10 metres, it is a conspicuous and striking component of the native landscape. Quiver Trees grow in arid regions of South Africa and Namibia. Within these two countries, the species is found predominantly in the west, and occurs throughout much of the Succulent Karoo Biodiversity Hotspot (IUCN 2009).
Weather station records from across the Quiver Tree’s range show that average temperatures in the region have increased in recent decades and regional climate predictions suggest that temperatures will increase and rainfall will decrease in the near future. Because Quiver Trees live so long (approximately 350 years) and continue to grow throughout their lives, they provide an invaluable living record of past climatic events. By examining the sizes of trees in a Quiver Tree population, it is possible to estimate when the infrequent wet periods suitable for seedling survival occurred. In addition, because the decay of dead trees happens slowly in the hot, dry desert conditions, the number of tree deaths can be recorded with reasonable accuracy. This makes Quiver Trees ideal for examining the ongoing impacts of climate change (IUCN 2009).
By 2001, large die-offs of Quiver Trees were occurring. Scientists found that most of these die-offs were occurring in the hotter equator-ward areas of the Quiver Tree’s range, and that these were most likely caused by drought stress. In contrast, populations farther from the equator and at the tops of high mountains were growing and reproducing successfully. Many species are known to respond to climatic warming by shifting their ranges to higher latitudes and higher altitudes, where conditions are typically cooler and moister. For mobile species, this is apparent as individuals migrate away, but for plants and other immobile organisms, individuals that are "left behind" by their shifting ranges are unlikely to reproduce successfully and eventually die (IUCN 2009).
Some higher latitude areas that were previously unsuitably cold or wet may become suitable habitat for a species being squeezed out of its range by changing climate. Mobile species can quickly take advantage of these new conditions, but stationary species such as plants must rely on rare long distance seed dispersal events to colonise new areas. Mapping patterns of population die-off across the Quiver Tree’s range has indeed revealed a pattern of a poleward range shift in response to warming. Die-offs are generally occurring close to the equator and at low altitudes, while populations on the southern "leading edge" and on mountain tops have been flourishing. It appears that limited seed dispersal may be slowing the Quiver Tree's colonization of new regions with suitable climate conditions (IUCN 2009).