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Extinction (The Possibilities for Stationary Plants) -- Summary
In addition to migrating to more suitable locations, Earth's plants have other ways of successfully responding to various pressures that might otherwise lead to their extinction. The "acid test" for any extinction hypothesis is to examine what appears to be happening - or what appears to actually have happened - in the real world, which is what is done here with respect to studies of plants that are fighting to survive under experimental settings or real-world locations where they periodically face various threats to their survival.

One experiment that broached the subject of plants threatened by external biological and climatic factors was established in the spring of 1994 at the Cedar Creek Natural History Area in central Minnesota (USA), where a decade later Lambers et al. (2004) quantified the temporal evolution of the productivity and "staying power" of fourteen species of plants across an experimental grassland diversity gradient. Over the course of that long-term study, the five researchers learned that certain species were over-yielders, i.e., plants that grow better and produce more biomass when grown in competition with other species than when grown by themselves. In this study there were six such species, including a C3 grass, three C4 grasses and two legumes; and the five researchers noted that these "over-yielding species were either superior N competitors (C4 grasses) or N fixers (legumes)." On the other hand, they found there were also five under-yielding species, four of which were forbs that typically grew less robustly when in the presence of other species. Nevertheless, they found that the over-yielding species were not displacing the under-yielding species over time.

In discussing their findings, Lambers et al. concluded that diversity-promoting interactions also played a role in this experiment, and that "under-yielding species appear to be buffered from extinction." How common is this phenomenon? No one knows; but its operation in this study suggests that Earth's plants may be much better "buffered from extinction" than many have long supposed. More research should thus be directed to better elucidate the various "diversity-promoting interactions" that maintain the existence of under-yielding species in the face of what might logically be presumed to be significant competitive pressure from average and over-yielding species. In addition, consideration should be given to how the phenomena responsible for enabling under-yielding species to avoid extinction may be influenced by global warming and rising atmospheric CO2 concentrations, both of which phenomena could well prove helpful to them in this regard.

In another intriguing study, Stinson and Bazzaz (2006) grew well-watered stands of ragweed (Ambrosia artemisiifolia) out-of-doors in open-top-chambers maintained at either 360 or 720 ppm CO2 from seedling stage to the onset of senescence, after which the plants were harvested and the dry masses of their shoots, roots and reproductive structures determined, while prior to this time - at 14, 33 and 52 days after the start of the experiment - they also measured the heights and numbers of leaves of all the plants. This work revealed that doubling the atmosphere's CO2 concentration increased the mean stand-level biomass of the shoots of the ragweed plants by 44%, while it increased the biomass of their roots and reproductive structures by 46% and 94%, respectively, for a total CO2-induced biomass increase of 70%. Of perhaps even greater interest, however, was the two researchers' finding that the extra CO2 "reduced the coefficients of variation for all aspects of plant growth, especially reproductive biomass," such that the CO2-induced growth enhancements were "more pronounced in small, rather than large plants." That is to say, as they rephrased their findings, "growth enhancements to smaller plants diminished the relative biomass advantages of larger plants in increasingly crowded conditions," or as they stated in yet another place in their paper, "CO2-induced growth gains of subordinate A. artemisiifolia plants minimize differences in the reproductive output of small and large plants."

The Harvard University scientists thus concluded that "more homogeneous reproduction between subordinates and dominants also implies that a larger number of individuals will contribute propagules to future generations," which phenomenon, in their words, "could in turn affect evolutionary and population dynamics." And what they found to be true for within-species subordinates and dominants might also be true for among-species subordinates and dominants. And if it is, this preferential stimulation of growth responses to atmospheric CO2 enrichment in subordinate species, could well help those that are endangered to better withstand whatever forces might be pushing them towards extinction.

In another study that yielded some encouraging new insights, an international team of 33 researchers (Wills et al., 2006) analyzed seven tropical forest dynamics plots located throughout the New and Old World tropics that had a wide range of species richness and tree densities, and that had all been visited and "censused" more than once over the past few decades. These efforts of the team paid off handsomely, for they found that for all of the plots they studied, "rare species survive preferentially, which increases diversity as the ages of the individuals increase," or as they state for further clarity, "when species were rare in a local area, they had a higher survival rate than when they were common, resulting in enrichment for rare species and increasing diversity with age and size class in these complex ecosystems."

Why would that be? Some of the reasons the researchers gave were that (1) "diversity should increase as a group of individuals ages, because more common species are selectively removed by pathogens and predators," especially those that are commonly associated with them, (2) "individuals compete more intensively with conspecifics than with individuals of other species," and (3) "diversity may increase if an individual facilitates (benefits) nearby non-conspecifics," which facilitation "has the effect of making interspecific interactions more positive than intraspecific interactions and thus provides an advantage to locally rare species." Likewise, in a commentary on these important findings and the phenomena underpinning them, Pennisi (2006) wrote that "being closer together, common trees are more prone to deadly infections," and "they may also face stiffer competition for certain resources," while "rarer trees, by depending on slightly different sets of resources, may not have this problem."

Consequently, for whatever reason or reasons, and in the face of historical increases in air temperature and atmospheric CO2 concentration (which may or may not be as dramatic as climate alarmists claim them to be), the biodiversities of real-world tropical forests are increasing, and rare species are becoming more abundant, which is just the opposite of what climate alarmists continually claim is occurring. In addition, Pennisi quotes Scott Armbruster of the UK's University of Portsmouth as saying that the fact that "these patterns are found to be so consistent across so many distant tropical forests suggests to me that the conclusion may eventually be found to hold for other diverse ecosystems as well."

In another paper dealing with plants, Londre and Schnitzer (2006) wrote that all around the globe, woody vines or lianas are "competing intensely with trees and reducing tree growth, establishment, fecundity, and survivorship," possibly because "increasing levels of CO2 may enhance growth and proliferation of temperate lianas more than of competing growth forms (e.g., trees)," and possibly because "warmer winter temperatures may also increase the abundance and distribution of temperate lianas, which are limited in their distribution by their vulnerability to freezing-induced xylem embolism in cold climates." Consequently, the two researchers decided to see if these phenomena had impacted liana abundance and distribution over the prior 45 years in 14 temperate deciduous forests of southern Wisconsin (USA), during which time (1959-1960 to 2004-2005) the atmosphere's CO2 concentration rose by some 65 ppm, mean annual air temperature in the study region rose by 0.94°C, mean winter air temperature rose by 2.40°C, but mean annual precipitation (another important growth-altering factor) did not change. So what did the Wisconsin scientists find?

As they described it, and contrary to their initial hypothesis, "liana abundance and diameter did not increase in the interiors of Wisconsin (USA) forests over the last 45 years." In fact, they reported that Toxicodendron radicans - a liana popularly known as poison ivy, which they say "grew markedly better under experimentally elevated CO2 conditions than did competing trees (Mohan et al., 2006)" - actually decreased in abundance over this period, and did so significantly. But how did it happen that what had seemed to be so logical turned out to be so wrong?

In broaching this question, Londre and Schnitzer wrote that "the lack of change in overall liana abundance and diameter distribution in [the] study suggests that lianas are limited in the interiors of deciduous forests of Wisconsin by factors other than increased levels of CO2," and in this regard they suggested it was likely that the interior-forest lianas were limited by the historical increase in atmospheric CO2 via the enhanced tree growth provided by the CO2 increase, which likely resulted in the trees becoming more competitive with the vines because of CO2-induced increases in tree leaf numbers, area and thickness, all of which factors would have led to less light being transmitted to the lianas growing beneath the forest canopy, which phenomenon likely negated the enhanced propensity for growth that likely was provided to the vines by the historical increase in the atmosphere's CO2 concentration.

Support for this net-zero competing effects hypothesis was provided by Londre and Schnitzer's finding that "compared to the forest interior, lianas were >4 times more abundant within 15 m of the forest edge and >6 times more abundant within 5 m of the forest edge," which "strong gradient in liana abundance from forest edge to interior," in the words of the two researchers, "was probably due to light availability." In addition, they said their results "are similar to findings in tropical forests, where liana abundance is significantly higher along fragmented forest edges and within tree fall gaps," and, one might add, where the interior tropical trees have also not suffered what some have claimed would be the negative consequences of CO2-induced increases in liana growth, as described in the review of the study of Phillips et al. (2002).

In commenting on the significance of their findings, Londre and Schnitzer wrote that because "forest fragmentation (and thus edge creation) has increased significantly over the last half-century, particularly in the northeastern and midwestern United States (e.g., Ritters and Wickham, 2003; Radeloff et al., 2005), liana abundance has likely increased in temperate forests due to forest fragmentation." Consequently, they opined that "as forest fragmentation continues, liana abundance will also likely continue to increase, and the effects of lianas on temperate forests, such as intense competition with trees (Schnitzer et al., 2005), reduced tree growth rates and biomass sequestration (Laurance et al., 2001), and the incidence of arrested gap-phase regeneration (Schnitzer et al., 2000) may become even more pronounced."

In light of these latter observations, it is clear that it is not rising atmospheric CO2 concentrations that are to be feared in this regard, it is the encroachment of man upon the world of nature (Waggoner, 1995; Tilman et al.,2001, 2002; Raven, 2002); for it is this phenomenon that is destined to desecrate the globe's forests and drive innumerable species of both plants and animals to extinction, unless we can dramatically increase the water use efficiency of our crop plants, so we are not forced to encroach further upon the forests of the world to obtain the additional land and water resources (Wallace, 2000) we will otherwise need to grow the greater quantities of food that will be required to sustain our larger projected population at the midpoint of the current century.

Clearly, the most effective means of ensuring that the needed increase in plant water use efficiency actually comes to pass (in contrast to the grandiose schemes of men that promise much but produce little, especially where it is really needed) is to allow the atmosphere's CO2 concentration to continue its natural upward course, which will truly give crops throughout the world the productivity boost they will need to supply us with the food mankind will require but a few short decades from now without usurping further land and water resources from "wild nature," which should thereby preserve for future generations what yet remains of the world's forests and the great profusion of lifeforms they shelter and sustain (Idso and Idso, 2000).

Shifting gears just a bit to focus more directly on this latter challenge, Feeley and Silman (2009) have noted that "ongoing development of the Amazon, including natural gas and oil production, large-scale cattle ranching, soy farming, extended networks of improved roads, and the various synergistic activities that invariably accompany increased access, is causing the rapid loss and degradation of natural habitat," which can lead to the extinctions of species that live there. So just how serious is the situation?

To find out, the two researchers used various collections of pertinent data to map the potential ecoregion-based distributions of the more than 40,000 vascular plant species for which collections were available from the Amazon, after which they estimated rates of habitat loss due to future land-use changes, based on projections made by Soares-Filho et al. (2006) of areas that will be deforested by 2050 under a business as usual and a more optimistic governance scenario, which they finally translated into estimated extinction risks that will prevail in the year 2050. And these operations revealed that by AD 2050, human land-use practices will have reduced the habitat available to Amazonian plants by approximately 12-24%, resulting in 5-9% of species becoming "committed to extinction" at that future date.

Some regions, however, will suffer a whole lot more. In the case of the largest Amazonian ecoregion - the seasonal Cerrado savannahs of southwestern Brazil that cover about two million square kilometers - Feeley and Silman applied a habitat loss of 1.5%/year, characteristic of the past three decades, even though they indicated that "habitat loss in the Cerrado has actually accelerated to 3.1-4.3%/year." And they stated that if they include "historic habitat loss and use a contemporary habitat loss rate of 3.7%, extinction risk for Cerrado species rises to more than 2 times greater than for non-Cerrado species."

Is this something about which we should be particularly worried? You bet it is, because the Cerrado has been losing "natural habitat to agricultural and pastoral land over the past three decades," in the words of Feeley and Silman; and with the climate-alarmist push for greater biofuel production, those habitat losses will only accelerate with time. Indeed, this is the great threat to the Amazon's biodiversity, not the ongoing rise in the air's CO2 content. In fact, the region's only hope for salvation resides in the atmosphere's CO2 concentration continuing to rise; for it has been demonstrated that without the aerial fertilization and water-conserving effects of atmospheric CO2 enrichment, mankind will have to usurp most of the remaining untapped land and freshwater resources of the planet - leaving next to nothing for wild nature - just to provide the food our expanding population will require to sustain ourselves at the midpoint of the current century.

Focusing on what might be called an "out of sight, out of mind" ecosystem, Short et al. (2011) wrote that "seagrasses, a functional group of marine flowering plants rooted in the world's coastal oceans, support marine food webs and provide essential habitat for many coastal species, playing a critical role in the equilibrium of coastal ecosystems and human livelihoods," while noting that they are also "a component of more complex ecosystems within marine coastal zones, contributing to the health of coral reefs and mangroves, salt marshes and oyster reefs," while citing in this regard the studies of Dorenbosch et al. (2004), Duke et al. (2007), Heck et al. (2008) and Unsworth et al. (2008).

They also noted, however, that for the first time, "the probability of extinction is determined for the world's seagrass species under the Categories and Criteria of the International Union for the Conservation of Nature (IUCN) Red List of Threatened Species," which effort they described as "a four-year process involving seagrass experts internationally, compilation of data on species' status, populations, and distribution, and review of the biology and ecology of each of the world's seagrass species." And in doing so, the twenty-six seagrass experts - hailing from eleven different countries - determined that ten seagrass species (comprising 14% of all seagrass species) are at elevated risk of extinction, with three other species qualifying as Endangered.

So what was the cause of the problem? Was it CO2-induced global warming and ocean acidification? Not much was said about these two villains; but the international team of experts named a number of others, including suspended sediments and siltation (Dennison et al., 1993; de Boer, 2007), coastal construction, land reclamation, shoreline hardening, and dredging (Erftemeijer and Lewis, 2006), damaging fisheries practices such as trawling and aquaculture (Pergent-Martini et al., 2006), mechanical damage from boats, boat moorings and docks (Burdick and Short, 1999; Kenworthy et al., 2002), introduced species (Williams, 2007) that compete for space and resources (Heck et al., 2000) and certain diseases (Rasmussen, 1997). And so it was that they concluded that "the most common threat to seagrasses is human activity," particularly that of the type which is responsible for most of the threats listed above.

Turning to another "out of the way" type of ecosystem, Gerdol and Vicentini (2011) wrote that "Sphagnum mosses are a fundamental component of bog vegetation in northern regions, where these plants play a major role in controlling important ecosystem processes." However, they added that "as heat waves are expected [by some folks] to become increasingly intense and frequent, especially in cold territories," they felt it important to see if they could determine the ability of the mosses to survive such extreme weather events. Therefore, scaling the south-eastern Alps of the Italian province of Bolzano, the two intrepid researchers collected cores of two Sphagnum species - S. fuscum and S. magellanicum - from three mountain heights above sea level: low (1090 m), intermediate (1780 m) and high (2100 m), which locations spanned, in their words, "almost the whole altitudinal range known for these species in mountainous regions of central-southern Europe." Then, back in the laboratory, they grew portions of the six cores for four days in a row at three 12-hour daytime temperature levels - ambient temperature (AT, 25°C), medium temperature (MT, 36°C) and hot temperature (HT, 43°C) - while they measured net CO2 exchange and chlorophyll a fluorescence, as well as plant tissue chemistry.

After analyzing their data, the two Italian scientists reported that normalized net CO2 exchange rates did not vary among species nor with altitude, and that net CO2 exchange rates in the plants experiencing the MT treatment declined during treatment but recovered noticeably six days after treatment stopped; and they said that despite receiving "severe damage," the plants experiencing the HT treatment also exhibited a capacity to recover six days after the conclusion of the temperature treatment. And, therefore, noting that their study suggested that "the two Sphagnum species possess moderate altitudinal plasticity to increased temperature," their ultimate conclusion, which they expressed in the final sentence of their paper, was that "heat waves, even stronger than ever recorded, will unlikely bring about die-off of Sphagnum mosses in bog ecosystems unless high temperatures are coupled with drought."

One year later, Dreesen et al. (2012) wrote that "in many regions of the world, climate change is projected to induce higher temperatures and drier conditions (IPCC, 2007)," as well as "more frequent and more intense climate extremes such as heat waves and droughts (Meehl et al., 2000; Schar et al., 2004)," and they said that the latter phenomena "are expected to be more detrimental to plant functioning and productivity than homogenously distributed changes in temperature and precipitation," citing Karl et al. (1997), Easterling et al. (2000) and Smith (2011).

Well, it sounded logical enough, but "to gain more insight into the effects of climate extremes on plants," in the words of Dreesen et al., they conducted two experiments (one in July and another in August) in an experimental field at the University of Antwerp (Belgium), in which plant mesocosms that consisted of three temperate annual or biannual herbaceous species were exposed to either a heat wave or serious drought, or both extreme stresses together, where ten-day heat waves were simulated with infrared lamps and drought (of either 20 or 17 days duration) was created by withholding water input and removing the water table, after which the plants experienced normal weather until the end of the growing season. And what did they thereby learn?

The four researchers reported finding, "surprisingly," that total above- and below-ground end-of-season community biomass was stimulated "in response to drought extremes in both periods," and that "effects of heat extremes varied but never reduced biomass," as was also the case with respect to combined heat and drought. "This increase in total community biomass," as they continued, "originated exclusively from stimulated root growth," and they indicated, in this regard, that "the exact mechanism for this unexpected result could not be ascertained," but they went on to say that "greater whole-plant nitrogen stocks clearly indicated enhanced nutrient availability," which they opined "may have arisen from increased net mineralization or from greater root exploration under the influence of 'mid-season drought'."

Most recently, and also studying the effects of heat waves, were Ameye et al. (2012), who introduced the report of their study by noting that in studies where the air's CO2 content has been doubled, "increases in net photosynthesis were reported ranging from 43% to 192% in Pinus taeda (Teskey, 1997; Tissue et al., 1997; Ellsworth, 1999; Wertin et al., 2010; Frenck et al., 2011) and from 30% to 256% in Quercus rubra (Kubiske and Pregitzer, 1996; Anderson and Tomlinson, 1998; Cavender-Bares et al., 2000)." Likewise, they indicated that "generally, an increase in air temperature also has a positive effect on net photosynthesis and growth," citing Sage and Kubien (2007) and Way and Oren (2010). But how might loblolly pine and northern red oak trees respond to the extreme heat waves that are often predicted to occur in a future CO2-enriched world?

Working with the most recent fully-developed leaves of well watered and fertilized seedlings grown in 7.6-L pots out-of-doors at Athens, Georgia (USA) within polyethylene chambers maintained at ambient and elevated air temperatures (Tamb and Tamb + 3°C), as well as seven-day heat waves consisting of a biweekly +6°C heat wave or a monthly +12°C heat wave - which treatments were maintained throughout the growing season - Ameye et al. measured rates of net photosynthesis before, during and after the mid-summer heat waves they created. And this work revealed that "an immediate and significant decline in net photosynthesis was observed in seedlings that were subjected to a +12°C heat wave, but not in seedlings subjected to a +6°C heat wave." Also, they found that "after the third day of the +12°C heat wave, net photosynthesis values stabilized at positive values and did not show signs of further reduction, indicating that the photosynthetic apparatus did not accrue additional stress or damage as the heat wave continued."

As a result of these observations, Ameye et al. concluded that "if soil moisture is adequate, trees will experience negative effects in photosynthetic performance only with the occurrence of extreme heat waves." And in light of the fact that "elevated CO2 diminished these negative effects," they opined that "the future climate may not be as detrimental to plant communities as previously assumed," which is essentially the same message one gets from the results of all of the other studies reviewed in this document.

In one final study, Frei et al. (2014) introduce their work by noting that "mountain ecosystems are particularly susceptible to climate change," and, therefore, they say "characterizing intraspecific variation of alpine plants along elevational gradients is crucial for estimating their vulnerability to predicted changes." In an effort to do just that, Frei et al. collected seeds of three nutrient-poor montane grassland species - Ranunculus bulbosus L., Trifolium montanum L. and Briza media L. - from across the Swiss Alps in the summer of 2008 at elevations of 1200 and 1800 meters above sea level (m a.s.l.), after which they established a reciprocal transplant experiment with experimental gardens located at 600, 1200 and 1800 m a.s.l. in order to test for plant adaptation to its elevation of origin.

According to the five Swiss scientists, their study revealed "no evidence for local adaptation to elevation of origin and hardly any differences in the responses of low and high elevation populations." And they state that "the consistent advanced reproductive phenology observed in all three species shows that they have the potential to plastically respond to environmental variation." Such findings, in the words of Frei et al., "support the conclusion of a recent meta-analysis including 32 other plant species that local adaptation is less common than generally assumed (Leimu and Fischer, 2008)." And they go on to state that "since plant populations were not adapted to their respective elevations of origin, we conclude that, for our study species, the expected upward shift of optimal climatic conditions will not necessarily lead to a shift of population and species ranges to higher elevations in the context of climate change," since "plasticity will buffer the detrimental effects of climate change."

Based on the results of the several studies reviewed above, it would thus appear that the plant extinction hypothesis resides on very shaky ground, for there is much evidence to refute it. Indeed, as demonstrated here and in reviews of numerous other experiments posted on our CO2 Science website (see the various subheadings under the topic of Extinction in our Subject Index), the future of Earth's many plant species is one full of optimism.

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Last updated 13 February 2015