Plant responses to fertilization experiments in lowland, species‐rich, tropical forests

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.

Plant host identity and soil macronutrients explain little variation in sapling endophyte community composition: Is disturbance an alternative explanation?

“…We applied 125 kg N ha −1 year −1 as urea, 50 kg P ha −1 year −1 as triple super‐phosphate, and 50 kg K ha −1 year −1 as KCl. Our rates of application were similar to those used in previous forest nutrient addition studies (reviewed by Wright et al, ).…”

“…Our results suggest that the addition of macronutrients either do not limit or are not available to bacterial endophytes. This is in spite of numerous studies demonstrating that the addition of these same macronutrients in situ enhanced the performance of our focal species and increased leaf nutrient concentrations of each nutrient by 4%–16% (Wright et al, ; Pasquini & Santiago, ; Santiago et al, ; Pasquini et al, ; Griffin et al, ; reviewed by Wright et al, ). Because bacterial endophytes reside in the apoplast (McCully, ), it remains unclear whether these bacteria can access these nutrients (Griffin & Carson, ; Vorholt, ).…”

“…Recent studies in tropical understories demonstrate that soil nutrient availability regulates many aspects of plant performance, even in deep shade (Griffin et al, ; Pasquini & Santiago, ; Pasquini, Wright, & Santiago, ; Santiago et al, ; Wright et al, ). For example, long‐term additions of macronutrients (N, P, K) cause increases in sapling growth, photosynthetic rates, leaf nutrient concentrations and even the amount of pest damage found on leaves (reviewed by Wright et al, ). Whether macronutrients regulate foliar bacterial communities in natural systems is unknown, though macronutrients often do so in crop species (Ikeda et al, ; Manching, Balint‐Kurti, & Stapleton, ).…”

The Effects of Phosphorus Cycle Dynamics on Carbon Sources and Sinks in the Amazon Region: A Modeling Study Using ELM v1

“…It has also been found that P limitation is pervasive at the species level but not at the community level across a steep natural P gradient in Panama tropical forests (Alvarez‐Clare et al, ; Condit et al, ; Turner et al, ). Wright et al () found that plant responses to P addition are not stronger than to N addition, but they also pointed out their findings are not conclusive because of small numbers of experiments, insufficient statistical power, and especially the short duration of the experiments. We advocate more long‐term nutrient manipulation experiments in the Amazon tropical forests to help us better understand the importance of P limitation in lowland tropical forests.…”

Drivers of subtropical forest dynamics: The role of functional traits, forest structure and soil variables

“…On the other hand, the effect of soil K levels on recruitment rates was different from what was initially expected. Soil K level is considered to be a key element of forest dynamics and structure (Lloyd et al., ) as it is positively related to tree growth and gross forest primary productivity (Tripler et al., ; Wright et al., , ). However, in the studied forest the level of soil K was negatively correlated with tree recruitment.…”