Biomes contain a collection of plants, animals, and microorganisms that have similar climate tolerances and therefore overlapping geographic ranges associated with specific temperature and precipitation conditions (see Figure 3.4 in the textbook). A major emphasis of Unit 1 is the importance of physiological tolerance of the physical environment as the ultimate determinant of where a species can exist. In addition, species’ geographic distributions are subject to biological interactions such as competition and predation, as well as dispersal ability, which will be considered in Units 2, 3, and 4.
Although discussion of climate change usually focuses on temperature, patterns of precipitation have changed as well. Simultaneous changes in temperature and precipitation of differing magnitudes may result in the occurrence of climate combinations that are not currently found on Earth. These “novel” or “non-analog” climates could cause a reshuffling of organisms due to their climate tolerances, resulting in the appearance of biomes that do not occur today. For example, two species occurring under current climate conditions may have different ranges of precipitation where they can occur, despite similar temperature tolerances. Under a warmer, drier climate, one species may drop out of the biome, one may remain, and a new species may occur due to its ability to tolerate the drier conditions (Figure 1).
Such change in climate conditions and associated novel biomes are known to have occurred in the past (Overpeck et al. 1992; Jackson and Williams 2004). During the period of deglaciation following the last ice age, climates with greater seasonal temperature changes than those of today occurred. Several novel biome types occurred in association with these climates (Figure 2; see also Figure 25.16), including an association of spruce and ash trees not found in any biomes today (Williams et al. 2001).
What can we expect in the future as climate continues to change? Will novel combinations of climate variables occur? John Williams and colleagues (Williams et al. 2007; Williams and Jackson 2007) examined the potential for the appearance of novel climates using models of future climate change from the report of the Intergovernmental Panel on Climate Change (IPCC 2007). They evaluated the uniqueness of predicted future climates relative to today’s climates using mean summer and winter temperatures and mean summer and winter precipitation, as these variables are particularly important to plant survival and establishment. Novel climates were determined by whether the climate differed from existing climates in a 500 km (300 mile) radius around a target region. Their analysis indicated that large areas of Earth would experience novel climates by the end of the twenty-first century (Figure 3). The appearance of these novel climates was particularly pronounced in the tropical and temperate zones. In addition, the predicted trajectory of greenhouse gas emissions influenced the occurrence of novel climates, with greater emissions resulting in more area experiencing a novel climate.
We can get a sense of the potential for the emergence of novel biomes by examining the initial stages of vegetation responses to climate change. In particular, multiple reports of vegetation change in the Arctic indicate that parts of the tundra are shifting toward a shrub-dominated biome (Sturm et al. 2001; Epstein et al. 2008; Elmendorf et al. 2012). Birch and willow shrubs are increasing in abundance throughout the lower-latitude portions of the Arctic. The increase in shrub cover corresponds with the substantial warming that has been recorded over the past two decades (Macias-Fauria et al. 2012) and is also consistent with warming experiments carried out in Alaskan Arctic tundra (Chapin et al. 1995). It is also reflective of vegetation patterns that occurred during warming in the early Holocene, about 10,000 years ago (Anderson and Brubaker 1993).
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