A climate-driven switch in plant nitrogen acquisitionwithin tropical forest communitiesBenjamin Z. Houlton†‡, Daniel M. Sigman§, Edward A. G. Schuur¶, and Lars O. Hedin†Departments of†Ecology and Evolutionary Biology and§Geosciences, Princeton University, Princeton, NJ 08544; and¶Department of Botany,University of Florida, Gainesville, FL 32611Edited by Robert Howarth, Cornell University, Ithaca, NY, and accepted by the Editorial Board March 29, 2007 (received for review November 9, 2006)The response of tropical forests to climate change will depend onindividual plant species’ nutritional strategies, which have notbeen defined in the case of the nitrogen nutrition that is critical tosustaining plant growth and photosynthesis. We used isotopenatural abundances to show that a group of tropical plant specieswith diverse growth strategies (trees and ferns, canopy, andsubcanopy) relied on a common pool of inorganic nitrogen, ratherthan specializing on different nitrogen pools. Moreover, the trop-ical species we examined changed their dominant nitrogen sourceabruptly, and in unison, in response to precipitation change. Thisthreshold response indicates a coherent strategy among species toexploit the most available form of nitrogen in soils. The apparentcommunity-wide flexibility in nitrogen uptake suggests that di-verse species within tropical forests can physiologically trackchanges in nitrogen cycling caused by climate change.global change 兩 isotope 兩 community ecologyStrategies of plant nitrogen (N) acquisition control manydif ferent aspects of ecological systems (1–3), with importantimplications for modeling and predicting ecosystem responses toclimate change, rising levels of atmospheric CO2, and N pollution(4, 5). Whether a given plant species can adjust to different Nsources will determine its abilit y to adapt to environment alchange. For instance, if species specialize on a particular for m ofN in the soil, either nitrate, ammonium, or dissolved organ ic N(DON) (6–11), then any changes in the N cycle c ould triggermarked changes in community composition and species distri-butions. Alternatively, if plants are less specialized (12, 13),environmental changes to the N cycle may not result in dramaticspecies turnover, but instead could induce increased competitionfor N together with more subtle changes in plant communities.Studies of extratropical land plant c ommunities (6–11) andtheories of plant competition have since Hutchinson’s “paradoxof the plankton” (14, 15) largely emphasized the first strategy,that species coexist by partition ing nutrient sources into rela-tively specialized ‘‘niches.’’ Little is known, however, about thesources of N that support plants in tropical forests, the sensitivityof N sources to climate change, and the resulting links betweenplant diversity and the N cycle.Here, we use natural stable isotopes to constrain the sourcesof N that fuel the growth of a communit y of functionally diversetropical plant species in response to differences in precipitationclimates. We make use of six well characterized sites of montanetropical forest from the windward slopes of Mt. Haleak ala on theisland of Maui, Hawaii (16), across which mean annual preci-pit ation (MAP) changes from 2,200 to 5,050 mm. Although thisrange in precipitation spans that observed for many tropicalrainforests globally (17), other state factors such as mean annualtemperature (16°C), geologic substrate age (⬇400,000 years),and biotic composition (dominated by native species) are rela-tively constant across this sequence (16, 18).At each of our sites, Schuur and Matson (16) measured the15N/14N of foliage f rom four dif ferent plant species that togetherc ontribute ⬎80% of tot al aboveg round biomass and productivityof the forests. When c ombined, the species also encompass thegrowth strategies that characterize forest ecosystems more gen-erally: Metrosideros polymorpha, a dominant canopy tree; Chei-rodendron trigynum, a subdominant canopy tree; Cibotium glau-cum, a tree fern; and Melicope clusiifolia , an understory woodyplant. These data show only slight (⬇1–3‰) differences in the␦15N of species’ leaves within a given site (Table 1) [␦15Ninunitsof per mil (‰) vs. air ⫽ (15N/14Nsample/15N/14Nair⫺ 1) ⫻ 1,000],and a broad decline in plant␦15N with increasing precipitation(see Fig. 2a ), similar to the pattern identified for plant commu-n ities worldwide (19, 20). Although bulk soil␦15N also decreasesacross the precipitation gradient, the␦15N decrease in the foliageis nearly twice as great (Table 1).Regardless of whether individual plants are at steady st atewith respect to the environment, the␦15N of their leaves shouldbe close to that of their N source(s) (Fig. 1). Plant N uptake doesnot express an appreciable isotope effect under natural soilc onditions (21–25). Thus, this process causes a min imal isotopicdif ference between a plant and its N source, and it similarly doesnot significantly modify the isotopic ratio of the acquired Nsource. In addition, isotopic discrimination is min imal during themajor loss processes, leaf fall and root turnover (26, 27). Internalplant f ractionation has been shown to cause ⬇ 2‰ differencesbet ween roots and shoots across a diversity of ecosystems (23, 25,33) that include tropical rainforests (34). Finally, whereas ecto-myc orrhizae may influence plant␦15N relative to a soil N source(28, 29), they are virtually absent from native Hawaiian flora(30–32). The dominant type of mycorrhizae in Hawaiian soils,arbuscular mycorrhizae (AM) (30–32), may impart a slightadditional fractionation of 2‰ (35) or less (28). We thereforeassume a combined isotopic effect of 4‰ owing to plant Nallocation and arbuscular mycorrhizae, causing leaves to be⬇2‰ lower than the preferred N source (Fig. 1 and Methods).Given the relatively minor f ractionation during plant upt ake,the impact of plants on isotopic dif ferences among available soilN pools is largely caused by competition between plant uptakeand the more fractionating microbial transformations (e.g.,n itrification and denitrification). For instance, a lower ratio ofplant nitrate uptake relative to denitrification will increase the␦15N of soil n itrate owing to discrimination against the heavierisotope of N by denitrif ying bacteria, all else held constant.Similarly, to the extent that plants alter the relative importanceof leaching vs.