The focus of much of the research on plant responses to elevated CO2 over the past several decades has been on the role of vegetation in the global carbon cycle and the feedback that plant responses provide to atmospheric CO2 concentration. This has not always been the case. The initial thrust of vegetation effects research was toward crop plant physiology and agricultural productivity. Although carbon cycle researchers had recognized as early as 1970 that CO2 fertilization of the biosphere could slow the increase of atmospheric CO2, the potential effect was often dismissed as insignificant. In a seminal paper in 1981, Paul Kramer challenged whether the extensive literature on photosynthesis and dry matter production of crop plants was relevant to plants in nature, and he concluded that reliable predictions of the global effects of increasing CO2 concentration required long-term measurements of plant growth from experiments in which elevated CO2 is combined with water and nitrogen stress. With the research imperative refocused on global carbon cycle feedbacks, workshops and reports at that time identified many critical questions and research challenges for understanding CO2 fertilization in that context. We should ask now, are those questions still relevant today?
Results/Conclusions
Boyd Strain and Fakhri Bazzaz framed a fundamental issue about CO2 fertilization in 1983. Noting that the initial effect of elevated CO2 in most plant communities will be to increase NPP, they questioned whether increased NPP would lead to greater plant biomass or faster turnover of leaves and roots. Resolving this dichotomy has required a strong focus on ecosystem-scale responses: NPP, carbon allocation, turnover, and nutrient interactions. Hypotheses in the 1980’s considered soil microbial interactions (mycorrhizae, nitrogen fixation, and exudation-stimulated nutrient dynamics), and these remain prominent research priorities today. While many of these early-identified questions have been tackled, one important insight from 25 years ago has been only partially addressed. Hal Mooney put forth a hypothetical scheme relating the relative CO2 response of the world’s ecosystems to the degree of drought stress and nutrient availability. He called for larger-scale, longer duration experiments in each of the world’s six major biomes (tundra, boreal forest, temperate forest, tropical forest, grassland, and desert). FACE experiments were instituted in grasslands, desert, and temperate forests, but absence of CO2 enrichment experiments in the tropics remains a glaring hole in our body of knowledge of feedbacks between the terrestrial biome and the global carbon cycle.