Humans are changing the global environment to such an extent that they are changing global climate (IPCC 2007; http://www.ipcc.ch/). The pace, if not the eventual extent, of this climate change may be rare in Earth's history, and these rapid changes in climate will have profound negative impacts on biodiversity of the planet and on the quality of human life (Millennium Ecosystem Assessment; http://www.maweb.org//en/Index.aspx). It is already widely believed that the Earth has entered a new geological period termed the Anthropocene, in recognition of the impact that humans are having on the Earth, and it is predicted that human-caused changes to Earth's environment and climate will likely be the foremost problems facing humans in the coming decades.
Plants are directly or indirectly the primary source of food for most terrestrial organisms, and humans are often dependent on plants for fuel, fibers, medicine, clean water, etc., as well as for food. Hence, our ability to understand and predict the effects of global environmental change on terrestrial ecosystems and humankind requires an understanding of the effects of global environmental change on plants. Not surprisingly then, much scientific research effort has been devoted to plants and global environmental change, and this research effort is not likely to diminish in the coming decades. To date, much of the global-change research on plants has focused on single-factor effects, particularly effects of elevated CO2 and increased mean growth temperature, with less effort, understandably, on simultaneous multi-factor effects. However, since the major global-change factors can have opposite (e.g., elevated CO2 and temperature) or additive/synergistic effects (e.g., elevated CO2 and N) on plant performance, additional multi-factor studies will be necessary to understand plant responses to global environmental change. For practical reasons, much of the past research in this area has been carried out in laboratory studies, especially for certain global-change factors for which field experiments are logistically difficult (e.g., drought or warming effects on large plants or plant communities). In many instances, alternative approaches, such as modeling and long-term monitoring, will be necessary to elucidate effects of global environmental change on plants, communities, and ecosystems. Also, relatively less previous work in this arena has been devoted to investigating global-change effects on interactions of plants with non-plant organisms, and such work will be necessary to understand global-change effects on the biosphere.
In recognition of the tremendous ecological and societal importance of human-caused changes in the Earth's environment and climate, this issue of the Journal of Integrative Plant Biology features a diverse collection of papers concerning plants (or algae) and global environmental change. An initial goal of this special issue was to high-light younger scientists, and in this regard, we have succeeded. Of the 13 papers in this issue concerning global environmental change, 12 have lead authors in early stages of their scientific careers (three are graduate students, three are post-doctoral researchers, and six are in early stages of academic careers). These papers span the range from describing results of laboratory experiments to field studies to modeling to reviews or analyses of pre-existing literature results. Included here are papers examining the effects of several abiotic global-environmental-change factors (CO2, temperature, ozone, water, nitrogen), as well as biotic factors (invasive species, herbivory, species composition). These papers make important contributions to our understanding of plants and global environmental change, and several of them put forth intriguing novel hypotheses that should generate additional research.
Among the exciting findings in these global-change papers are those derived from review or analysis of literature results or from modeling approaches to understanding global-change effects on plants or algae. For example, in a review and analysis of pre-existing results, Taub and Wang examine existing hypotheses as to why growth of plants under elevated CO2 typically decreases tissue N concentration. They conclude that such decreases in N concentration occur due to growth dilution, as is often assumed, but also because of decreases in specific uptake rates of N by roots, and they discuss the likely reasons for this decrease in uptake rate. Bidart-Bouzat and Imeh-Nathaniel conduct an in-depth review of existing literature on how global-change factors (CO2, O3, UV, and temperature) impact plant chemical defenses against herbivores, and they find that despite much variability in past results, there are unique patterns that emerge for each global-change factor. Burton and colleagues combine analysis of literature results and their own experimental data to re-examine the influence of temperature on root respiration, and they provide compelling evidence that long-term adaptive responses to higher temperature are lower than those often observed in short-term responses, and this will have important relevance to the wide-spread use of Q10= 2 in most modeling studies. Phillips and co-authors also combine analysis of pre-existing results with their own data to provide evidence that, contrary to conventional wisdom, mature trees are capable of strong growth responses to global environmental change (elevated CO2 in this case). Using a modeling approach, Chiang and colleagues investigate the relative importance of changes in forest tree composition compared with changes in climate (here temperature, precipitation, and light) on forest net primary production, and they conclude that although climate effects will be larger, the effects of changes in forest composition will be significant. Also using a modeling approach in combination with experimental results, Cecala and co-authors estimate the impact that the highly-invasive mussel species, Dreissena polymorpha, zebra mussels, has on benthic primary production in temperate lakes in the northeast USA, showing that these ecosystem engineers increase benthic production and decrease its year-to-year variability.
Three of the papers in this special feature use an experimental approach on field-grown plants to investigate plant responses to global-change factors, which is often logistically more difficult than conducting laboratory experiments. Chen and co-authors used mesocosms installed in the field to examine the effects of warming and water-table manipulation on soil temperatures of peatland communities, finding that both factors alter the length of the non-frozen season, though in sometimes unexpected ways. Mishra and colleagues conduct the first field study of effects of elevated CO2 and O3 on heat-stress tolerance in plants, and find that elevated CO2 increases thermotolerance in soybean, thereby confirming previous laboratory studies. And Wang and co-authors examine the effects of N on heat-wave tolerance in prairie plants, finding that heat stress and N have differential effects on the C and N relations of the two co-dominant plants, and these effects have implications for community composition, herbivory, and decomposition.
The remaining four papers in this feature describe results from laboratory studies of global-change effects on plants. Ward and co-authors examine the influence of reduced growth temperatures at low CO2 on the growth and physiology of a C3 and C4 plant, providing evidence that during low CO2 periods in the geological past, C4 species likely had a competitive advantage over C3 species, even at lower growth temperatures. Hamilton and colleagues examine the interactive effects of growth temperature and CO2 on photosynthetic tolerance to heat stress, finding that elevated CO2 often decreases heat tolerance in C4 species, especially at supra-optimal growth temperatures, and in C3 species, high CO2 generally benefits heat tolerance, except at supra-optimal growth temperatures. Barua and co-authors examine differences in heat-shock protein (HSP) production and leaf thermotolerance among ecotypes of a species and how this is influenced by growth temperature, and their results indicate that increased heat tolerance, either among ecotypes or with acclimation to higher growth temperatures, is associated with decreased HSP levels and increased basal (vs. inducible) thermotolerance – a finding that has implications for population ecology of individual species in a warmer world. Finally, Kassem and colleagues investigate the effects of elevated CO2 and drought on soil microbial communities associated with roots, finding that CO2 and drought have unique and interactive effects on microbial activity and that elevated CO2 ameliorates drought effects, but that neither factor affects the composition of the microbial community.
In addition to increasing our knowledge of the effects of global environmental change on plants, and in stimulating new research in this area, we are hopeful that this special feature will serve to stimulate consideration of JIPB as an outlet for global-change research, in part by introducing JIPB to researchers unfamiliar with this journal. Finally, we express our appreciation to JIPB for supporting this special issue and its focus on younger scientists, and we thank our colleagues who contributed to this feature for their patience and cooperation, as well as their high-quality papers.