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Abstract: We present a meta‐analysis of plant responses to fertilization experiments conducted in lowland, species‐rich, tropical forests. We also update a key result and present the first species‐level analyses of tree growth rates for a 15‐yr factorial nitrogen (N), phosphorus (P), and potassium (K) experiment conducted in central Panama. The update concerns community‐level tree growth rates, which responded significantly to the addition of N and K together after 10 yr of fertilization but not after 15 yr. Our experimental soils are infertile for the region, and species whose regional distributions are strongly associated with low soil P availability dominate the local tree flora. Under these circumstances, we expect muted responses to fertilization, and we predicted species associated with low‐P soils would respond most slowly. The data did not support this prediction, species‐level tree growth responses to P addition were unrelated to species‐level soil P associations. The meta‐analysis demonstrated that nutrient limitation is widespread in lowland tropical forests and evaluated two directional hypotheses concerning plant responses to N addition and to P addition. The meta‐analysis supported the hypothesis that tree (or biomass) growth rate responses to fertilization are weaker in old growth forests and stronger in secondary forests, where rapid biomass accumulation provides a nutrient sink. The meta‐analysis found no support for the long‐standing hypothesis that plant responses are stronger for P addition and weaker for N addition. We do not advocate discarding the latter hypothesis. There are only 14 fertilization experiments from lowland, species‐rich, tropical forests, 13 of the 14 experiments added nutrients for five or fewer years, and responses vary widely among experiments. Potential fertilization responses should be muted when the species present are well adapted to nutrient‐poor soils, as is the case in our experiment, and when pest pressure increases with fertilization, as it does in our experiment. The statistical power and especially the duration of fertilization experiments conducted in old growth, tropical forests might be insufficient to detect the slow, modest growth responses that are to be expected. [ABSTRACT FROM AUTHOR]

I present a meta‐analysis of plant responses to 48 nutrient addition experiments conducted with native species in naturally growing tropical forests, exclusive of mangrove forests. The added nutrients include nitrogen (N) in 36 experiments, phosphorus (P) in 33 experiments, calcium and potassium in one experiment each, and various mixtures of essential nutrients in the remaining experiments. I evaluate the hypotheses that nutrients limit tropical forest plants, nutrient limitation is stronger in successional than in old‐growth forests, P but not N is limiting in lowland forests, and N is limiting in montane forests. Responses to the most complete nutrient mix used in each experiment were strong for plant functions that contribute to aboveground production (Hedges' g averages 0.87) and nonsignificant for fine root biomass. Responses to N addition and to P addition were strong for tissue concentrations of the added element (Hedges' g averages 0.75 and 1.4, respectively), moderate for fine litter production (0.64 and 0.65, respectively), moderate to weak for plant growth (0.46 and 0.37, respectively) and nonsignificant for fine root biomass. Growth responses were stronger in successional than in old‐growth forests. All responses were unrelated to elevation. The 48 experiments included 30 factorial nitrogen‐phosphorus experiments that enable additional direct tests of the widely cited hypotheses that P limitation is stronger than N limitation in lowland forests and vice versa in montane forests. Both hypotheses were rejected. The N × P interaction effect was nonsignificant across the factorial experiments. In conclusion, nutrients clearly limit tropical forest plants. Limitation by N is widespread in both lowland and montane forests, and the same is true for P. Single experiments identify limitation by calcium and potassium, and correlative studies suggest limitation by calcium, potassium, and magnesium. The available evidence is consistent with the possibility that most macronutrients limit tropical forest plants; however, experiments focus almost exclusively on N and P. The way forward will include taking fuller advantage of existing nutrient addition experiments, siting new experiments strategically, and developing cost‐effective methods to assay responses to all of the essential nutrients soils supply to plants. [ABSTRACT FROM AUTHOR]