Volume 10, Number 35: 29 August 2007
In an important paper published in the Proceedings of the National Academy of Sciences, USA, Houlton et al. (2007) write that "whether a given plant species can adjust to different nitrogen sources will determine its ability to adapt to environmental change." Given the huge changes in climate that are predicted by folks such as Al Gore, James Hansen and various Intergovernmental Panel on Climate Change functionaries, as well as the heed that is paid to their pronouncements by much of the media and innumerable public officials at all levels of government worldwide, this topic is of much more than purely academic interest; and, therefore, the four researchers developed a comprehensive protocol for attempting to address the concerns it creates, the major one of which, in their words, is that "if species specialize on a particular form of nitrogen [N] in the soil, either nitrate, ammonium, or dissolved organic N, then any changes in the N cycle could trigger marked changes in community composition and species distributions," some of which changes could be disastrous for species unable to shift their geographical locations fast enough or far enough to find the form of nitrogen needed to sustain themselves.
So what, exactly, did the Princeton and University of Florida scientists do? As they describe it, they used "natural stable isotopes to constrain [i.e., identify] the sources of N that fuel the growth of a community of functionally diverse tropical plant species in response to differences in precipitation," making use of "six well characterized sites of montane tropical forest from the windward slopes of Mt. Haleakala on the island of Maui, Hawaii, across which mean annual precipitation (MAP) changes from 2,200 to 5,050 mm," but over which "other state factors such as mean annual temperature, geologic substrate age, and biotic composition are relatively constant." In addition, they focused on four dominant species that "encompass the growth strategies that characterize forest ecosystems more generally: Metrosideros polymorpha, a dominant canopy tree; Cheirodendron trigynum, a subdominant canopy tree; Cibotium glaucum, a tree fern; and Melicope clusiifolia, an understory woody plant."
The results of Houlton et al.'s ambitious data acquisition campaign revealed that "the dominant source of N for vegetation growth changes sharply with precipitation, from nitrate at the drier sites to ammonium at the wetter sites," and they further found that the "shift in proximal N source closely tracks the shift in N availability as measured by concentrations of extractable nitrate and ammonium." In fact, they report that "plants fed almost exclusively (>80%) on nitrate in forests with <=3,350 mm of MAP where nitrate is more abundant than ammonium," while "in contrast, >95% of plant growth requirements are met by ammonium in the wetter climates, where nitrate is scarce but ammonium is abundant." What is more, these results were obtained for all four of the diverse species studied.
In summarizing their findings, the researchers state that "a functionally diverse group of dominant species in Hawaiian tropical forest are inherently flexible in their capacity to grow on different N forms, consistently assessing the most abundant form of inorganic N in the soil," and that "these species seem to share a coherent and tightly regulated strategy for addressing changes in the abundance of N forms in their environment." These results, in their words, "are consistent with the idea that these tropical plant species have evolved a uniformly plastic ability to switch among different N sources." Consequently, they conclude that "the apparent community-wide flexibility in nitrogen uptake suggests that diverse species within tropical forests can physiologically track changes in nitrogen cycling caused by climate change," which is welcome news for the entire biosphere.
Sherwood, Keith and Craig Idso
Reference
Houlton, B.Z., Sigman, D.M., Schuur, E.A.G. and Hedin, L.O. 2007. A climate-driven switch in plant nitrogen acquisition within tropical forest communities. Proceedings of the National Academy of Sciences, USA 104: 8902-8906.