1. The concentration, stoichiometry and resorption of nitrogen (N) and phosphorus (P) in plant leaves are often used as proxies of the availability of these growth-limiting nutrients, but the ...responses of these metrics to changes in nutrient availability remain largely untested for tropical forest trees. 2. We evaluated changes in N and P concentrations, N/P ratios and resorption for four common tree species after 13 years of factorial N and P additions in a lowland tropical forest in Panama. 3. Chronic P addition increased foliar P concentrations, decreased P resorption proficiency and decreased N/P ratios in three locally common eudicot tree species (Alseis blackiana, Heisteria concinna, Tetragastris panamensis). The increase in foliar P involved similar proportional increases in organic and inorganic P in two species and a disproportionately large increase in inorganic P in A. blackiana. 4. Nitrogen addition did not alter foliar N concentrations in any species, but did decrease N resorption proficiency in H. concinna. 5. A fourth species, the palm Oenocarpus mapora, demonstrated remarkably static foliar nutrient concentrations, responding only with a marginal decrease in P resorption proficiency under N plus P co-addition. 6. Synthesis. Collectively, these results suggest that adjustment of N/P ratios can be expected in eudicots exposed to elevated P, but foliar N appears to already be at optimal levels in these lowland rain forest tree species. The complexity of species-specific responses to altered nutrient availability highlights the difficulty in predicting future responses of tropical forest trees to a changing world.
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.
Temperature is a primary driver of the distribution of biodiversity as well as of ecosystem boundaries. Declining temperature with increasing elevation in montane systems has long been recognized as ...a major factor shaping plant community biodiversity, metabolic processes, and ecosystem dynamics. Elevational gradients, as thermoclines, also enable prediction of long-term ecological responses to climate warming. One of the most striking manifestations of increasing elevation is the abrupt transitions from forest to treeless alpine tundra. However, whether there are globally consistent above- and belowground responses to these transitions remains an open question. To disentangle the direct and indirect effects of temperature on ecosystem properties, here we evaluate replicate treeline ecotones in seven temperate regions of the world. We find that declining temperatures with increasing elevation did not affect tree leaf nutrient concentrations, but did reduce ground-layer community-weighted plant nitrogen, leading to the strong stoichiometric convergence of ground-layer plant community nitrogen to phosphorus ratios across all regions. Further, elevation-driven changes in plant nutrients were associated with changes in soil organic matter content and quality (carbon to nitrogen ratios) and microbial properties. Combined, our identification of direct and indirect temperature controls over plant communities and soil properties in seven contrasting regions suggests that future warming may disrupt the functional properties of montane ecosystems, particularly where plant community reorganization outpaces treeline advance.
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 premontane forest in Panama. We traced the movement of three 15N 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 15N was recovered in microbes than in plants, while all pools (extractable organic and inorganic N, microbial biomass, and leaves) contained greater amounts of 15N from nitrate than from ammonium or glycine. Furthermore, 13C 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 15N recovery in plants with arbuscular mycorrhizal (AM) associations or proteoid roots compared to orchids. Some root trait groups differed in 15N 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.
There is much interest in understanding ecosystem responses to local‐scale soil fertility variation, which has often been studied using retrogressive chronosequences that span thousands of years and ...show declining fertility and plant productivity over time. There have been few attempts to experimentally test how plant nutrient limitation changes during retrogression. We studied a well‐characterized system of 30 forested lake islands in northern Sweden that collectively represent a 5350‐year post‐fire retrogressive chronosequence, with fertility and productivity decreasing as time since fire increases. For each island, we set up four plots on understorey vegetation, each subjected to a different fertilizer treatment over 6 years: no additions, nitrogen (N) only, phosphorus (P) only and N + P. We found that both N and P additions reduced feather moss and thus total plant biomass. Meanwhile, the three dominant vascular plant species showed contrasting biomass responses, but similar responses of foliar nutrient concentrations to nutrient additions. Fertilization reduced most microbial groups and altered CO₂ fluxes, most likely through feather moss reduction. Against expectations, the majority of interactive effects of N and P were antagonistic. Changes in effects of nutrient additions during retrogression were usually modest. Empetrum hermaphroditum biomass was increasingly promoted by P and N + P addition, while vascular plant N‐to‐P ratios were increasingly reduced by P addition, indicating increasing plant limitation by nutrients (notably P) during retrogression. Below‐ground, positive effects of N addition on soil mineral N increased, while negative effects of N addition on soil fungi decreased during retrogression; no other below‐ground effects of fertilization changed along the gradient. Synthesis. Our results show that forest understorey communities on islands of different fire history and thus stages of retrogression show relatively modest differences in how they respond to nutrient addition despite large changes in ecosystem productivity and soil fertility, probably because of high species turnover and adaptation of communities to infertile conditions. While increased nutrient availability (as expected through global change) may have important ecological consequences, these effects are likely, especially below‐ground, to be rather similar across ecosystems that differ greatly in nutrient availability and productivity.
Mycorrhizal and saprotrophic (SAP) fungi are essential to terrestrial element cycling due to their uptake of mineral nutrients and decomposition of detritus. Linking these ecological roles to ...specific fungi is necessary to improve our understanding of global nutrient cycling, fungal ecophysiology, and forest ecology. Using discriminant analyses of nitrogen (δ¹⁵N) and carbon (δ¹³C) isotope values from 813 fungi across 23 sites, we verified collector-based categorizations as either ectomycorrhizal (ECM) or SAP in > 91% of the fungi, and provided probabilistic assignments for an additional 27 fungi of unknown ecological role. As sites ranged from boreal tundra to tropical rainforest, we were able to show that fungal δ¹³C (26 sites) and δ¹⁵N (32 sites) values could be predicted by climate or latitude as previously shown in plant and soil analyses. Fungal δ¹³C values are likely reflecting differences in C-source between ECM and SAP fungi, whereas ¹⁵N enrichment of ECM fungi relative to SAP fungi suggests that ECM fungi are consistently delivering ¹⁵N depleted N to host trees across a range of ecosystem types.
The Clavariaceae is a diverse family of mushroom-forming fungi composed of species that produce simple clubs, coralloid, lamellate-stipitate, hydnoid and resupinate sporocarps. Here we present a ...systematic and ecological overview of the Clavariaceae based on phylogenetic analysis of sequences of the nuclear large subunit ribosomal RNA (nLSU), including nine from type collections. Forty-seven sequences from sporocarps of diverse taxa across the Clavariaceae were merged with 243 environmental sequences from GenBank and analyzed phylogenetically to determine major clades within the family. Four major clades or lineages were recovered: (i) Mucronella, (ii) Ramariopsis-Clavulinopsis, (iii) Hyphodontiella and (iv) Clavaria-Camarophyllopsis-Clavicorona. Clavaria is paraphyletic, within which the lamellate and pileate-stipitate genus Camarophyllopsis is derived and composed of two independent lineages. The monotypic genus Clavicorona also appears nested within Clavaria. The monophyly of Clavaria and Camarophyllopsis, however, cannot be statistically rejected. We compared differing classification schemes for the genera Ramariopsis and Clavulinopsis, most of which are inconsistent with the molecular phylogeny and are statistically rejected. Scytinopogon, a genus classified in the Clavariaceae by several authors, shares phylogenetic affinities with the Trechisporales. Overall 126 molecular operational taxonomic units can be recognized in the Clavariaceae, roughly half of which are known only from environmental sequences, an estimate that exceeds the known number of species in the family. Stable isotope ratios of carbon and nitrogen were measured from specimens representing most major phylogenetic lineages to predict trophic strategies. These results suggest that most non-lignicolous species feature a biotrophic mode of nutrition. Ancestral state reconstruction analysis highlights the taxonomic significance of at least nine morphological traits at various depths in the family tree.
Foliar nitrogen (N) isotope ratios (δ¹⁵N) are used as a proxy for N-cycling processes, including the “openness” of the N cycle and the use of distinct N sources, but there is little experimental ...support for such proxies in lowland tropical forest. To address this, we examined the δ¹⁵N values of soluble soil N and canopy foliage of four tree species after 13 years of factorial N and P addition to a mature lowland rainforest. We hypothesized that N addition would lead to ¹⁵N-enriched soil N forms due to fractionating losses, whereas P addition would reduce N losses as the plants and microbes adjusted their stoichiometric demands. Chronic N addition increased the concentration and δ¹⁵N value of soil nitrate and δ¹⁵N in live and senesced leaves in two of four tree species, but did not affect ammonium or dissolved organic N. Phosphorus addition significantly increased foliar δ¹⁵N in one tree species and elicited significant N × P interactions in two others due to a reduction in foliar δ¹⁵N enrichment under N and P co-addition. Isotope mixing models indicated that three of four tree species increased their use of nitrate relative to ammonium following N addition, supporting the expectation that tropical trees use the most available form of mineral N. Previous observations that anthropogenic N deposition in this tropical region have led to increasing foliar δ¹⁵N values over decadal time-scales is now mechanistically linked to greater usage of ¹⁵N-enriched nitrate.
The concentration, stoichiometry and resorption of nitrogen (N) and phosphorus (P) in plant leaves are often used as proxies of the availability of these growth-limiting nutrients, but the responses ...of these metrics to changes in nutrient availability remain largely untested for tropical forest trees. We evaluated changes in N and P concentrations, N/P ratios and resorption for four common tree species after 13years of factorial N and P additions in a lowland tropical forest in Panama. Chronic P addition increased foliar P concentrations, decreased P resorption proficiency and decreased N/P ratios in three locally common eudicot tree species (Alseis blackiana, Heisteria concinna, Tetragastris panamensis). The increase in foliar P involved similar proportional increases in organic and inorganic P in two species and a disproportionately large increase in inorganic P in A.blackiana. Nitrogen addition did not alter foliar N concentrations in any species, but did decrease N resorption proficiency in H.concinna. A fourth species, the palm Oenocarpus mapora, demonstrated remarkably static foliar nutrient concentrations, responding only with a marginal decrease in P resorption proficiency under N plus P co-addition.Synthesis. Collectively, these results suggest that adjustment of N/P ratios can be expected in eudicots exposed to elevated P, but foliar N appears to already be at optimal levels in these lowland rain forest tree species. The complexity of species-specific responses to altered nutrient availability highlights the difficulty in predicting future responses of tropical forest trees to a changing world.
Global patterns in soil, plant, and fungal stable isotopes of N (δ 15 N) show promise as integrated metrics of N cycling, particularly the activity of ectomycorrhizal (ECM) fungi. At small spatial ...scales, however, it remains difficult to differentiate the underlying causes of plant δ 15 N variability and this limits the application of such measurements to better understand N cycling. We conducted a landscape-scale analysis of δ 15 N values from 31 putatively N-limited monospecific black spruce (Picea mariana) stands in central Alaska to assess the two main hypothesized sources of plant δ 15 N variation: differing sources and ECM fractionation. We found roughly 20% of the variability in black spruce foliar N and δ 15 N values to be correlated with the concentration and δ 15 N values of soil NH 4 + and dissolved organic N (DON) pools, respectively. However, 15 N-based mixing models from 24 of the stands suggested that fractionation by ECM fungi obscures the 15 N signature of soil N pools. Models, regressions, and N abundance data all suggested that increasing dependence on soil DON to meet black spruce growth demands predicates increasing reliance on ECM-derived N and that black spruce, on average, received 53% of its N from ECM fungi. Future research should partition the δ 15 N values within the soil DON pool to determine how choice of soil δ 15 N values influence modeled ECM activity. The C balance of boreal forests is tightly linked to N cycling and δ 15 N values may be useful metrics of changes to these connections.
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