Ecology, 91(7), 2010, pp. 2121–2131 Ó 2010 by the Ecological Society of America Nitrogen and phosphorus additions negatively affect tree species diversity in tropical forest regrowth trajectories ILYAS SIDDIQUE, 1,5 IMA CE ´ LIA GUIMAR ˜ AES VIEIRA, 2 SUSANNE SCHMIDT, 1 DAVID LAMB, 1 CLA ´ UDIO JOSE ´ REIS CARVALHO, 3 RICARDO DE OLIVEIRA FIGUEIREDO, 3 SIMON BLOMBERG, 1 AND ERIC A. DAVIDSON 4 1 University of Queensland, School of Biological Sciences, Brisbane, Queensland 4072 Australia 2 Museu Paraense Emı´lio Goeldi, Coordenac¸ ˜ ao de Bot ˆ anica, CP 399, CEP 66040-170, Bele´m, Para´, Brazil 3 Empresa Brasileira de Pesquisa Agropecua´ria, EMBRAPA Amaz ˆ onia Oriental, CEP 66095-100, Bele´m, Para´, Brazil 4 Woods Hole Research Center, 149 Woods Hole Road, Falmouth, Massachusetts 02540-1644 USA Abstract. Nutrient enrichment is increasingly affecting many tropical ecosystems, but there is no information on how this affects tree biodiversity. To examine dynamics in vegetation structure and tree species biomass and diversity, we annually remeasured tree species before and for six years after repeated additions of nitrogen (N) and phosphorus (P) in permanent plots of abandoned pasture in Amazonia. Nitrogen and, to a lesser extent, phosphorus addition shifted growth among woody species. Nitrogen stimulated growth of two common pioneer tree species and one common tree species adaptable to both high- and low- light environments, while P stimulated growth only of the dominant pioneer tree Rollinia exsucca (Annonaceae). Overall, N or P addition reduced tree assemblage evenness and delayed tree species accrual over time, likely due to competitive monopolization of other resources by the few tree species responding to nutrient enrichment with enhanced establishment and/or growth rates. Absolute tree growth rates were elevated for two years after nutrient addition. However, nutrient-induced shifts in relative tree species growth and reduced assemblage evenness persisted for more than three years after nutrient addition, favoring two nutrient- responsive pioneers and one early-secondary tree species. Surprisingly, N þ P effects on tree biomass and species diversity were consistently weaker than N-only and P-only effects, because grass biomass increased dramatically in response to N þ P addition. The resulting intensified competition probably prevented an expected positive N þ P synergy in the tree assemblage. Thus, N or P enrichment may favor unknown tree functional response types, reduce the diversity of coexisting species, and delay species accrual during structurally and functionally complex tropical rainforest secondary succession. Key words: biodiversity loss; clay Oxisol; ecosystem fertilization; mixed-effects models; N and P co- limitation; N þ P synergism; niche dimensionality; spatial heterogeneity; tree–grass interactions; tree species richness; tropical moist forest recovery; vegetation development. INTRODUCTION Human food production and energy use have increased nitrogen and phosphorus fluxes in ecosystems by up to 10-fold in some regions, while other regions are experiencing nutrient impoverishment (Falkowski et al. 2000, Howarth et al. 2005). Both nutrient-enriched and impoverished regions occur in the tropics near or at hotspots of global biodiversity (Howarth et al. 2005). Enrichment with the nutrients limiting primary produc- tivity plays an important role in biodiversity loss in temperate ecosystems (Wassen et al. 2005, Clark et al. 2007). In biodiversity-rich, functionally complex, and highly dynamic ecosystems such as tropical secondary forests the relationship between biodiversity and nutri- ent limitation remains unknown. Low soil P availability limits primary productivity on the widespread, deeply weathered lowland tropical Oxisols (Vitousek and Sanford 1986, Elser et al. 2007). However, after land use change the resulting ecosystems may become limited by nitrogen (Davidson et al. 2007). Deforestation, subsequent land use, and repeated burning can cause large losses through volatilization of N and through ash transport, leaching, and erosion of both N and P (Kauffman et al. 1995, Williams and Melack 1997). Nutrient impoverishment may thus slow post-abandonment biomass accumulation during sec- ondary succession (Gehring et al. 1999, Davidson et al. 2004) and, thereby, accrual of mature forest species, which are most at risk among tropical forest species biodiversity (Martı´nez-Garza and Howe 2003, Chazdon 2008). By contrast, enrichment with limiting nutrients accelerates biomass accumulation and might thereby Manuscript received 14 April 2009; revised 17 September 2009; accepted 30 September 2009; final version received 26 October 2009. Corresponding Editor: J. J. Battles. 5 Present address: Centro de Investigaciones en Ecosiste- mas, Universidad Nacional Auto´noma de Me´xico, UNAM Campus Morelia, Antigua carretera a Pa´tzcuaro 8701, Col. Exhacienda San Jose´ La Huerta, C.P. 58190, Morelia, Michoaca´ n, Mexico. E-mail: ilysid@gmail.com 2121