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Biodiversity (Grasslands) -- Summary
What do we know about the possible impacts of atmospheric CO2 enrichment and/or global warming on the biodiversity of grasslands?  Experiments conducted in various places around the globe are beginning to give us a glimpse of what might be expected in a CO2-enriched (and possible warmer and wetter) world of the future.

Edwards et al. (2001) conducted a FACE experiment utilizing atmospheric CO2 concentrations of 360 and 475 ppm on a sheep-grazed dry-land pasture located in Manawatu, New Zealand.  In each of the two years of their study, elevated CO2 increased seed production and dispersal in seven of the eight most abundant pasture species, including the grasses Anthoxanthum odoratum, Lolium perenne and Poa pratensis, the legumes Trifolium repens and Trifolium subterranean, and the herbs Hypochaeris radicata and Leontodon saxatilis.  In some of these species, the elevated CO2 increased the number of seeds produced per reproductive structure, while in all of the species it increased the number of reproductive structures per unit ground area.

These CO2-induced increases in seed production contributed to the increase in the numbers of species observed within the CO2-enriched experimental plots.  In addition, atmospheric CO2 enrichment helped maintain biodiversity by increasing the number of H. radicata, L. saxatilis, T. repens, and T. subterranean seedlings that survived for at least seven months in both study years, while it additionally lengthened the survival time of A. odoratum and L. perenne in the initial year of experimentation.  As the atmospheric CO2 concentration increases further, therefore, it should help to maintain, and maybe even increase, the biodiversity of these dry-land pasture communities by increasing the numbers of both common and uncommon species they contain.

Johnson et al. (2003) grew communities of 14 common prairie plants in 12 greenhouse chambers at Flagstaff, Arizona, USA, six of which chambers were maintained at an atmospheric CO2 concentration of 450 ppm and six of which were maintained at 688 ppm (equivalent to 368 and 560 ppm at sea level, respectively) during daylight hours.  Each of the CO2 treatments was also subdivided into treatments possessing living or dead arbuscular mycorrhizal (AM) fungal inoculum and low or enriched soil nitrogen (N) content. After one growing season under these conditions, it was determined, in Johnson et al.'s words, that "plant species richness was highest in mesocosms with elevated CO2, +AM fungi, and low soil N."  They also concluded that "in some plant species elevated CO2 can increase the net benefits of mycorrhizae by reducing their relative carbon cost."  Hence, their study provides a second instance where not only did the extra CO2 directly help the various plant species involved in the study, but where it also helped them indirectly, i.e., by promoting the growth of AM fungi that provided additional benefits to the plants.  This study, too, thus suggests that as the air's CO2 content continues to climb, it should have a tendency to maintain, and possibly even increase, the species richness of prairie ecosystems, especially those where soil nitrogen content is less than optimal.

Dukes (2002) grew the invasive forb Centaurea solstitialis in (1) monoculture on nutrient-poor serpentine soils that occur along the Coast Range and Sierra Nevada Mountains of California, USA, and (2) model microcosms of native California grasslands.  In both situations, the air to which the plants were exposed was maintained at ambient and twice-ambient CO2 concentrations in an attempt to determine if elevated CO2 favors the expansion of the non-native species into species-rich grassland communities.

In the monoculture experiments, the doubled CO2 increased the invasive forb's photosynthetic rate by 130% and its aboveground biomass by 70%.  When grown in competition with native serpentine grassland species, however, C. solstitialis did not exhibit much of a response: at ambient CO2 it comprised 4.3% of the community biomass, while at doubled CO2 it comprised only slightly more, i.e., 5.5%.  Concurrently, the total community biomass rose by 28%.  As the air's CO2 content continues to rise, therefore, the invasive forb likely will not significantly affect the biodiversity of native grasslands growing on California's nutrient-poor serpentine soils.

Arp et al. (1998) grew six perennial plants common to The Netherlands in giant steel pots containing soil of high or low nitrogen content within greenhouse compartments receiving 354 or 566 ppm CO2 for two years, the second of which years saw two levels of water stress imposed upon the plants to investigate three-way interactions.  The experimenters found that elevated CO2 increased biomass production in all six species under conditions of optimal water supply, but only in the presence of high soil nitrogen, and that under conditions of water stress the plants responded even better.  Water-use efficiency also increased with atmospheric CO2 enrichment in all species but one; and this response, on average, was nearly twice as great for plants growing at high nitrogen as opposed to low nitrogen.  Most important of all, however, from a biodiversity perspective, Arp et al. found that "elevated CO2 tends to favor species already best adapted to their environments."  Consequently, they concluded that "a rise in CO2 would not change the relationships between plant species in the natural environment, but would reinforce existing ones."

The study of De Deyn et al. (2003) injected even more complexity into the subject of CO2 and grassland biodiversity.  The group of eight scientists began by noting that aboveground vertebrate herbivores "can indirectly benefit subdominant plant species through selective feeding on dominants (Crawley, 1997; Olff and Ritchie, 1998)," and by reporting that root symbionts below the soil surface "can enhance plant species diversity by improving the nutrient uptake and growth of subdominants (van der Heijden et al., 1998)," while further noting that root pathogens "can do so by suppressing dominant host plant species (Bever, 1994)."  They then expanded the scope of these types of interactions by exploring the impact of invertebrate soil fauna on plant biodiversity.

De Deyn et al. established 32 microcosms of plant species mixtures characteristic of recently abandoned grassland (early succession), grassland under restoration for twenty years (mid-succession), and species-rich natural grassland (the ultimate target state).  These microcosms were all inoculated with soil fauna from one of the three grassland successional stages.  The density and composition of the soil fauna added to the microcosms were the same as those of the three grassland successional stages and included microfauna (nematodes), mesofauna (microarthropods) and macrofauna (beetle larvae).  After four and six months of these treatments, the microcosm plants were clipped at 4 cm above the soil surface and the harvested dry weights of all individual plant species were determined, while after twelve months the plants were clipped at the soil surface and their root dry weights additionally determined.

This protocol revealed, in the words of De Deyn et al., that "the soil fauna decreased the shoot biomass of the early succession plant species after 6 months, as well as plant species from the mid-succession stage, whereas the shoot biomass of the target plant species was increased."  Hence, as they note, "addition of the soil fauna also enhanced plant species diversity."  Results obtained at the end of the experiment further suggested that "the invertebrate root herbivores were selectively feeding on roots of dominant plants," which "provided an indirect advantage for the subdominant plant species, which were only marginally suppressed in the presence of soil fauna."  The researchers also report that the positive contributions of soil fauna and mycorrhizal fungi seemed to be additive.

So what do these several observations have to do with carbon dioxide?  Simply put, they suggest that the ongoing rise in the air's CO2 content may also enhance ecosystem species richness, as a consequence of the tendency for atmospheric CO2 enrichment to increase both mycorrhizal fungi and soil fauna populations.

With respect to soil fauna, for example, Rillig et al. (1999a) found that an approximate doubling of the air's CO2 content increased the numbers of microarthropods in sandstone and serpentine grasslands by 108% and 39%, respectively.  Likewise, in a study of well-fertilized poplar cuttings, Lussenhop et al. (1998) found that their approximately double-CO2 treatment supported twice as many microarthropods as their ambient-air treatment.  And in another microcosm study of terrestrial ecosystems, Jones et al. (1998) found that a 53% increase in atmospheric CO2 concentration led to a 52% increase in soil microarthropods.

With respect to mycorrhizal fungi, Rillig et al. (1998a) found that elevated CO2 increased percent root colonization by fungal hyphae in three grasses and two herbs that co-occur in Mediterranean annual grasslands; while Rillig et al. (1998b) determined that it increased percent root colonization by fungal arbuscles in the annual grass Bromus hordeaceus.  Lastly, Rillig et al. (1999b) found that elevated CO2 increased percent root colonization by arbuscules in serpentine and sandstone grasslands by three- and ten-fold, respectively.

In light of these several observations, there is a good likelihood that the ongoing rise in the air's CO2 content will hasten the conversion of abandoned agricultural fields and early and mid-successional grasslands into species-rich mature grasslands, while at the same time protecting the biodiversity of long-established natural grasslands.  Nevertheless, and in spite of the many positive findings noted above, the world's climate alarmists are always quick to latch onto seemingly negative research findings and blow them all out of proportion.

A case in point is the paper of Zavaleta et al. (2003), who conducted a study of a California (USA) grassland at the Jasper Ridge Biological Preserve in the San Francisco Bay area that dealt with essentially three groups of plants: annual grasses, perennial grasses, and forbs.  There, for a period of three years, they exposed a number of experimental plots to various combinations of increased atmospheric CO2 concentration (an extra 300 ppm), increased temperature (an extra 80 Wm-2 of ground-directed thermal radiation), increased precipitation (50% above normal, which extended the growing season by about 20 days), and enhanced nitrogen deposition (an extra 7 g N per square meter per year).

What did the scientists find?  The very first day after their report's publication, the Independent Digital (UK) Ltd published a story entitled "Global Warming May Wipe Out a Fifth of Wild Flower Species," stating in its opening paragraph that "one in every five species of wild flower could die out over the next century if levels of carbon dioxide in the atmosphere double."  Other news agencies ran similar stories; and the world was once again reminded of the devastating effects of CO2-induced global warming or, more accurately, of the predictions of such effects.

So what did Zavaleta et al. really find?  With respect to the core concern of climate alarmists, i.e., rising temperatures, they determined that their imposed warming actually tended to increase total plant diversity (albeit non-significantly) by about 6%.  With respect to the "wild flowers" of the media headlines (the forbs), however, there was a decline; but it too was non-significant and amounted to only about 2%, which is a far, far cry from the claimed 20% reduction.

So where did the 20% figure originate?  It came from what was observed in the plots where both temperature and CO2 concentration were increased together -- which for a presumed CO2-induced warming would appear to be a reasonable thing to do -- and under these circumstances, forb diversity did indeed decrease as reported.  However, if one is going to take this approach, one should actually look at the combination of all of the predicted climatic consequences of elevated CO2, which includes increased precipitation; and when the experimental increase in precipitation was added to the mix, the decrease in forb diversity was reduced to only about 10%.  Furthermore, with respect to total plant diversity, there was actually no change under this scenario.

Another problem with the news stories was their expansive generalizing of the results of but a single experiment conducted in one location.  The Independent/UK, for example, declared that "one in every five species of wild flower could die out over the next century if levels of carbon dioxide in the atmosphere double," while the Stanford Report (18 June 2003) claimed that "doubling the amount of carbon dioxide in the air significantly reduces the number of plant species that grow in the wild."  In actuality, however, one cannot deduce anything about "the number of plant species that grow in the wild" from this particular experiment, for the ecosystem studied had some unique characteristics that clearly preclude such a vast extrapolation.

Zavaleta et al. make this point perfectly clear themselves by stating "there is no rule of thumb for understanding combined global change responses in natural ecosystems," and by noting their study is but one of "many tests that will be required for a general picture of ecological response to multiple global changes."  In fact, their study does not even imply that the species that disappeared from their experimental plots are ultimately headed for extinction; for those particular plants could well begin appearing in other places that become more conducive to their well-being, as climatic zones gradually shift and the ranges of different plant species expand, contract and overlap in various ways in response to global environmental change.

When all is said and done, therefore, the study of Zavaleta et al. actually tells us nothing about the future well-being of wild flowers, or any other types of plants, in a potentially CO2-enriched world of the future that might, in the mean, become both warmer and wetter.  For that assessment we must look to real-world studies of how plants alter their ranges and actually change some of their physiological and physical characteristics as various climatic parameters change; and when this is done, there are no significant signs that any of earth's plants are teetering on the edge of a CO2-induced destruction.  For more on this topic, see our Major Report: The Specter of Species Extinction: Will Global Warming Decimate Earth's Biosphere?.

Arp, W.J., Van Mierlo, J.E.M., Berendse, F. and Snijders, W.  1998.  Interactions between elevated CO2 concentration, nitrogen and water: effects on growth and water use of six perennial plant species.  Plant, Cell and Environment 21: 1-11.

Bever, J.D.  1994.  Feedback between plants and their soil communities in an old field community.  Ecology 75: 1965-1977.

Crawley, M.J.  1997.  Plant Ecology.  Blackwell Science, Oxford, UK.

De Deyn, G.B., Raaljmakers, C.E., Zoomer, H.R., Berg, M.P., de Rulter, P.C., Verhoef, H.A., Bezemer, T.M. and van der Putten, W.H.  2003.  Soil invertebrate fauna enhances grassland succession and diversity.  Nature 422: 711-713.

Dukes, J.S.  2002.  Comparison of the effect of elevated CO2 on an invasive species (Centaurea solstitialis) in monoculture and community settings.  Plant Ecology 160: 225-234.

Edwards, G.R., Clark, H. and Newton, P.C.D.  2001.  The effects of elevated CO2 on seed production and seedling recruitment in a sheep-grazed pasture.  Oecologia 127: 383-394.

Johnson, N.C., Wolf, J. and Koch, G.W.  2003.  Interactions among mycorrhizae, atmospheric CO2 and soil N impact plant community composition.  Ecology Letters 6: 532-540.

Jones, T.H., Thompson, L.J., Lawton, J.H., Bezemer, T.M., Bardgett, R.D., Blackburn, T.M., Bruce, K.D., Cannon, P.F., Hall, G.S., Hartley, S.E., Howson, G., Jones, C.G., Kampichler, C., Kandeler, E. and Ritchie, D.A.  1998.  Impacts of rising atmospheric carbon dioxide on model terrestrial ecosystems.  Science 280: 441-443.

Lussenhop, J., Treonis, A., Curtis, P.S., Teeri, J.A. and Vogel, C.S.  1998.  Response of soil biota to elevated atmospheric CO2 in poplar model systems.  Oecologia 113: 247-251.

Olff, H. and Ritchie, M.E.  1998.  Effects of herbivores on grassland plant diversity.  Trends in Ecology and Evolution 13: 261-265.

Rillig, M.C., Allen, M.F., Klironomous, J.N., Chiariello, N.R. and Field, C.B.  1998a.  Plant species-specific changes in root-inhabiting fungi in a California annual grassland: responses to elevated CO2 and nutrients.  Oecologia 113: 252-259.

Rillig, M.C., Allen, M.F., Klironomos, J.N. and Field, C.B.  1998b.  Arbuscular mycorrhizal percent root infection and infection intensity of Bromus hordeaceus grown in elevated atmospheric CO2Mycologia 90: 199-205.

Rillig, M.C., Field, C.B. and Allen, M.F.  1999a.  Soil biota responses to long-term atmospheric CO2 enrichment in two California annual grasslands.  Oecologia 119: 572-577.

Rillig, M.C., Field, C.B. and Allen, M.F.  1999b.  Fungal root colonization responses in natural grasslands after long-term exposure to elevated atmospheric CO2Global Change Biology 5: 577-585.

van der Heijden, M.G.A., Klironomos, J.N., Ursic, M., Moutoglis, P., Streitwolf-Engel, R., Boller, T., Wiemken, A. and Sanders, I.R.  1998.  Mycorrhizal fungal diversity determines plant biodiversity, ecosystem variability and productivity.  Nature 396: 69-72.

Zavaleta, E.S., Shaw, M.R., Chiariello, N.R., Mooney, H.A. and Field, C.B.  2003.  Additive effects of simulated climate changes, elevated CO2, and nitrogen deposition on grassland diversity.  Proceedings of the National Academy of Sciences, USA 100: 7650-7654.