RESUMEN
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.
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Bosques , Clima Tropical , Nitrógeno , Panamá , Fósforo , Suelo , ÁrbolesRESUMEN
Nitrogen (N) availability influences the productivity and distribution of plants in tropical montane forests. Strategies to acquire soil N, such as direct uptake of organic compounds or associations with root symbionts to enhance N acquisition in exchange for carbon (C), may facilitate plant species coexistence and ecosystem N retention. Alternatively, rapid microbial turnover of soil N forms in tropical soils might promote flexible plant N-uptake strategies and mediate species coexistence. We tested whether sympatric plant species with different root symbiont associations, and therefore potentially different nutrient acquisition strategies, partition chemical forms of N or show plasticity in N uptake in a tropical pre-montane forest in Panama. We traced the movement of three 15 N forms into soil pools, microbes, and seedlings of eleven species differing in root traits. Seedlings were grown in a split-plot field transplant experiment, with plots receiving equimolar mixtures of ammonium, nitrate, and glycine, with one form isotopically labeled in each block. After 48 h, more 15 N was recovered in microbes than in plants, while all pools (extractable organic and inorganic N, microbial biomass, and leaves) contained greater amounts of 15 N from nitrate than from ammonium or glycine. Furthermore, 13 C from dual-labeled glycine was not recovered in the leaves of any seedling, suggesting the studied species do not directly take up organic N or transform organic N prior to translocation to leaves. Nitrogen uptake differed by root symbiont group only for nitrate, with greater 15 N recovery in plants with arbuscular mycorrhizal (AM) associations or proteoid roots compared to orchids. Some root trait groups differed in 15 N recovery among N forms, with greater nitrate uptake than ammonium or glycine by AM-associated and N2 -fixing plants. However, only five of eleven species showed differences in uptake among N forms. These results indicate flexibility in uptake of N forms in tropical plants across root trait groups, with only a few species displaying weak preferences for a specific N form.
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Bosques , Nitrógeno/metabolismo , Plantas/metabolismo , Panamá , Raíces de Plantas , SueloRESUMEN
Ectomycorrhizal (EcM)-mediated nitrogen (N) acquisition is one main strategy used by terrestrial plants to facilitate growth. Measurements of natural abundance nitrogen isotope ratios (denoted as δ(15)N relative to a standard) increasingly serve as integrative proxies for mycorrhiza-mediated N acquisition due to biological fractionation processes that alter (15)N:(14)N ratios. Current understanding of these processes is based on studies from high-latitude ecosystems where plant productivity is largely limited by N availability. Much less is known about the cause and utility of ecosystem δ(15)N patterns in the tropics. Using structural equation models, model selection and isotope mass balance we assessed relationships among co-occurring soil, mycorrhizal plants and fungal N pools measured from 40 high- and 9 low-latitude ecosystems. At low latitudes (15)N-enrichment caused ecosystem components to significantly deviate from those in higher latitudes. Collectively, δ(15)N patterns suggested reduced N-dependency and unique sources of EcM (15)N-enrichment under conditions of high N availability typical of the tropics. Understanding the role of mycorrhizae in global N cycles will require reevaluation of high-latitude perspectives on fractionation sources that structure ecosystem δ(15)N patterns, as well as better integration of EcM function with biogeochemical theories pertaining to climate-nutrient cycling relationships.
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Micorrizas/fisiología , Ciclo del Nitrógeno , Nitrógeno/metabolismo , Plantas/metabolismo , Microbiología del Suelo , Clima , Ecosistema , Modelos Estadísticos , Isótopos de Nitrógeno/análisis , Suelo/químicaRESUMEN
Boletellus exiguus sp. nov. and Boletellus dicymbophilus sp. nov. (Boletaceae, Boletales, Basidiomycota) are described as new to science. These boletes were collected from tropical forests dominated by ectomycorrhizal Dicymbe corymbosa (Caesalpiniaceae) in the Pakaraima Mountains of western Guyana.
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Basidiomycota/clasificación , Basidiomycota/aislamiento & purificación , Fabaceae/microbiología , Basidiomycota/citología , Guyana , Microscopía , Esporas Fúngicas/citología , Árboles , Clima TropicalRESUMEN
This work tested the hypothesis that ectomycorrhizas (EM) of Dicymbe corymbosa alter leaf-litter decomposition and residual litter quality in tropical forests of Guyana. Mass loss of leaf litter in litter bags was determined on three occasions, in two experiments, during a 12-month period. Paired root-exclusion plots were located randomly within a D. corymbosa forest. Both D. corymbosa and mixed-species leaf litters were reciprocally transplanted into their respective forest types. Elemental analysis was performed on the residual D. corymbosa leaf litter after 1 yr. Leaf litter mass loss in the D. corymbosa forest was not influenced by EM, despite high EM colonization. Elemental analysis of D. corymbosa leaf litter residues demonstrated reduced calcium levels in the presence of EM, which were negatively correlated with EM rootlet-colonizing mass. The lack of EM effect on the litter decomposition rate, coupled with high EM colonization, suggests an important but indirect role in mineral nutrient acquisition. Lowered Ca concentration in leaf litter exposed to EM may suggest a high Ca demand by the ectotroph system.
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Fabaceae/microbiología , Micorrizas/fisiología , Hojas de la Planta/metabolismo , Clima Tropical , Calcio/análisis , Fabaceae/anatomía & histología , Fabaceae/fisiología , Guyana , Micorrizas/crecimiento & desarrollo , Hojas de la Planta/química , Raíces de Plantas/anatomía & histología , Raíces de Plantas/microbiología , ÁrbolesRESUMEN
In Guyana, we investigated seed output, and resulting seedling establishment and survival, during a 'mast' year, by the ectomycorrhizal, monodominant rainforest canopy tree Dicymbe corymbosa (Caesalpiniaceae), a species with high, synchronous seed production at intermittent years. By utilizing seed traps, the mast seed output, predation, carbon and mineral investment, and masting synchrony were quantified in 2003 in primary D. corymbosa forests. Establishment of seedling cohorts was monitored, and climatic conditions associated with masting were assessed. During 2003, D. corymbosa in the Pakaraima Mountains exhibited high, synchronous seed production with low dispersal and predation. Investment in reproductive biomass was large relative to that in other tropical forests. Recent D. corymbosa reproductive events followed El Nino-induced droughts, with little intervening seed production. Over 12 months, 40% of the 2003 seedling cohort survived. Our results suggest that D. corymbosa has a strongly bimodal fruiting pattern that allows the establishment of a large seedling bank, facilitating persistent monodominance. Resource investment in large seed crops may depend on mineral recycling via ectomycorrhizas, coupled with the reallocation of carbon from vegetative maintenance.