Extinction: Terrestrial Plants and Soils
From ClimateWiki
Scherrer and Korner (2010) write, “climate warming scenarios predict higher than average warming in most alpine areas,” and therefore, they state, “alpine regions are often considered as particularly threatened.” In testimony presented to the Select Committee on Energy Independence and Global Warming of the United States House of Representatives on 26 April 2007, NASA’s James Hansen declared life in alpine regions is in danger of being “pushed off the planet” as the Earth warms, since it has “no place else to go.”
In a study designed to test this contention, Scherrer and Korner employed thermal imagery and microloggers to assess the fine-scale detail of both surface and root zone temperatures in three temperate-alpine and subarctic-alpine regions: one in the Swiss Alps, one in Northern Sweden, and one in Northern Norway. All of these sites were located on steep mountain slopes above the climatic tree line that exhibited a rich microtopography but no significant change in macroexposure. The two Swiss scientists report observing, “microclimatic variation on clear sky days was strong within all slopes, with 8.4 ± 2.5°C (mean ± SD) surface temperature differences persisting over several hours per day along horizontal (i.e., equal elevation) transects.” These differences, as they describe them, “are larger than the temperature change predicted by the IPCC.”
These findings, in the words of Scherrer and Korner, are “important in the context of climate change” because they show “species do not necessarily need to climb several hundred meters in elevation to escape the warmth.” Quite often, in fact, a “few meters of horizontal shift will do,” so that for plants “unable or too slow to adapt to a warmer climate, thermal microhabitat mosaics offer both refuge habitats as well as stepping stones as atmospheric temperatures rise.” Discussing the greater implications of their results, the Swiss scientists state their data “challenge the stereotype of particularly sensitive and vulnerable alpine biota with respect to climatic warming,” noting “high elevation terrain may in fact be more suitable to protect biodiversity under changing climatic conditions than most other, lower elevation types of landscapes.” Thus, in what would appear to be a bit of good advice to all—and James Hansen in particular—the two researchers state they “advocate a more cautious treatment of this matter.”
In another report on the status of alpine communities, this one in the Swedish Scandes, Kullman (2010) writes, “alpine plant life is proliferating, biodiversity is on the rise, and the mountain world appears more productive and inviting than ever,” which is about as far from being “pushed off the planet” as one could imagine. The professor of physical geography at Sweden’s Umea University states this particular course of biotic landscape evolution “has reached historical dimensions and broken a multi-millennial trend of plant cover retrogression, alpine tundra expansion, floristic and faunal impoverishment, all imposed by progressive and deterministic neoglacial climate cooling.” And he concludes “continued modest warming over the present century will likely be beneficial to alpine biodiversity, geoecological stability, resilience, sustainable reindeer husbandry and aesthetic landscape qualities.” He came to these conclusions, he writes, via “an integrative review of results from long-term monitoring of subalpine/alpine vegetation.”
Enlarging on some of these positive warming-induced impacts, Kullman writes, “plant species diversity will further increase, both in remaining treeless alpine areas and emerging forest outliers on the former alpine tundra,” and this “new alpine landscape may come to support a previously unseen mosaic of richly flowering and luxuriant plant communities of early Holocene character,” citing the works of Smith (1920), Iversen (1973), and Birks (2008). In describing what already has been documented, he states, “in contrast to model predictions, no single alpine plant species has become extinct, neither in Scandinavia nor in any other part of the world, in response to climate warming over the past century,” citing the studies of Pauli et al. (2001, 2007), Theurillat and Guisan (2001), and Birks (2008).
Tracing the evidence to the source of all these warming-induced ecological benefits, Kullman writes, “many alpine species are extremely tolerant of high temperatures per se,” citing Dahl (1998) and Birks (2008), as indicated “by their prospering and spread along roadsides far below the treeline, where emerging trees and shrubs are regularly mechanically exterminated (Kullman, 2006; Westerstrom, 2008).” He notes “another argument against the much-discussed option of pending mass-extinction of alpine species in a warmer future is that some alpine and arctic plant species contain a variety of ecotypes, pre-adapted to quite variable microclimatic and edaphic conditions, which could buffer against extinction in a possibly warmer future (Crawford, 2008).” In addition, he writes, this view is supported “by the fact that in the early Holocene, alpine plants survived, reproduced and spread in accordance with higher and more rapidly rising temperatures than those projected for the future by climate models (Oldfield, 2005; Birks, 2008).”
Kullman observes the “extended ranges of many flowering species and increasing plant species richness and habitat diversity imply a highly variable and aesthetically appealing mountain landscape, which should be positive from a nature conservation point of view (Jurasinski and Kreyling, 2007).” In fact, he states “such a course of landscape evolution adds to physical and ecological stability, functional efficiency, resilience and assures against ‘system failure’,” citing McCann (2000), Korner (2002), and McLaren (2006). Therefore, as Kullman concludes, “continued warming throughout the present century would be potentially and predominantly advantageous for alpine flora and vegetation.”
In a very different type of study, Willis et al. (2010) identified past historical periods in which climate was either similar to that projected by global climate models for the next century or so, or in which the rate of temperature change was unusually rapid. They examined these real-world periods to see if any real-world climate-related extinctions occurred.
The first period they examined was the Eocene Climatic Optimum (53–51 million years ago), when the atmosphere’s CO2 concentration exceeded 1,200 ppm and tropical temperatures were 5–10°C warmer than modern values. Yet far from causing extinctions of the tropical flora (where the data are best), the four researchers report “all the evidence from low-latitude records indicates that, at least in the plant fossil record, this was one of the most biodiverse intervals of time in the Neotropics.” They also note “ancestors of many of our modern tropical and temperate plants evolved ...when global temperatures and CO2 were much higher than present ... indicating that they have much wider ecological tolerances than are predicted based on present-day climates alone.”
The second period they examined included two rapid-change climatic events in the Holocene—one at 14,700 years ago and one at 11,600 years ago—when temperatures increased in the mid- to high-latitudes of the Northern Hemisphere by up to 10°C over periods of less than 60 years. There is evidence from many sites for rapid plant responses to rapid warming during these events. The researchers note “at no site yet studied, anywhere in the world, is there evidence in the fossil record for large-scale climate-driven extinction during these intervals of rapid warming.” On the other hand, they report extinctions did occur due to the cold temperatures of the glacial epoch, when subtropical species in southern Europe were driven out of their comfort zone.
The Willis et al. study also makes use of recent historical data, as in the case of the 3°C rise in temperature at Yosemite Park over the past 100 years. In comparing surveys of mammal fauna conducted near the beginning and end of this period, they detected some changes but no local extinctions. Thus they determined that for all of the periods they studied, with either very warm temperatures or very rapid warming, there were no detectable species extinctions.
In a study that may help explain how some researchers could have gotten things so wrong in predicting massive extinctions of both plants and animals in response to projected future warming, Nogues-Bravo (2009) explains the climate envelope models (CEMs)—often employed to predict species responses to global warming (and whether or not a species will be able to survive projected temperature increases)—“are sensitive to theoretical assumptions, to model classes and to projections in non-analogous climates, among other issues.” To determine how appropriate these models are for determining whether a particular species will be driven to extinction by hypothesized planetary warming, Nogues-Bravo reviewed the scientific literature pertaining to the subject and found several flaws. Nogues-Bravo writes, “the studies reviewed: (1) rarely test the theoretical assumptions behind niche modeling such as the stability of species climatic niches through time and the equilibrium of species with climate; (2) they only use one model class (72% of the studies) and one palaeoclimatic reconstruction (62.5%) to calibrate their models; (3) they do not check for the occurrence of non-analogous climates (97%); and (4) they do not use independent data to validate the models (72%).” Nogues-Bravo writes, “ignoring the theoretical assumptions behind niche modeling and using inadequate methods for hindcasting” can produce “a cascade of errors and naïve ecological and evolutionary inferences.” Hence, he concludes, “there are a wide variety of challenges that CEMs must overcome in order to improve the reliability of their predictions through time.” Until these challenges are met, contentions of impending species extinctions must be considered little more than guesswork (see also Chapman, 2010).
Employing yet another way of assessing the potential for plants to avoid extinction in a warming world, De Frenne et al. (2010) collected seeds of Anemone nemorosa L.—a model species for slow-colonizing herbaceous forest plants—found in populations growing along a 2,400-km latitudinal gradient stretching from northern France to northern Sweden during three separate growing seasons (2005, 2006, and 2008). They then conducted sowing trials in incubators, in a greenhouse, and under field conditions in a forest, where they measured the effects of different temperature treatments (growing degree hours or GDHs) on various seed and seedling traits.
The 19 researchers—from Belgium, Estonia, France, Germany, and Sweden—determined “seed mass, germination percenta ge, germinable seed output and seedling mass all showed a positive response to increased GDHs experienced by the parent plant,” noting seed and seedling mass increased by 9.7 percent and 10.4 percent, respectively, for every 1,000 °C-hours increase in GDHs, which they say is equivalent to a 1°C increase in temperature over a 42-day period. Therefore, they conclude, “if climate warms, this will have a pronounced positive impact on the reproduction of A. nemorosa, especially in terms of seed mass, germination percentage and seedling mass,” because “if more seeds germinate and resulting seedlings show higher fitness, more individuals may be recruited to the adult stage.” In addition, since “rhizome growth also is likely to benefit from higher winter temperatures (Philipp and Petersen, 2007), it can be hypothesized that the migration potential of A. nemorosa may increase as the climate in NW-Europe becomes warmer in the coming decades.” And, we would add, increasing migration potential implies a decreasing chance of extinction.
References
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