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Summary: Plants express diverse nutrient use and acquisition traits, but it is unclear how trait combinations at the species level are constrained by phylogeny, trait coordination, or trade‐offs in resource investment. One trait – nitrogen (N) fixation – is assumed to correlate with other traits and used to define plant functional groups, despite potential confounding effects of phylogeny.We quantified growth, carbon metabolism, fixation rate, root phosphatase activity (RPA), mycorrhizal colonization, and leaf and root morphology/chemistry across 22 species of fixing and nonfixing tropical Fabaceae trees under common conditions.Belowground trait variation was high even among closely related species, and most traits displayed a phylogenetic signal, including N‐fixation rate and nodule biomass. Across species, we observed strong positive correlations between physiological traits such as RPA and root respiration. RPA increased ~ fourfold per unit increase in fixation, supporting the debated hypothesis that N‐fixers 'trade' N for phosphatases to enhance phosphorus acquisition. Specific root length and root N differed between functional groups, though for other traits, apparent differences became nonsignificant after accounting for phylogenetic nonindependence.We conclude that evolutionary history, trait coordination, and fixation ability contribute to nutrient trait expression at the species level, and recommend explicitly considering phylogeny in analyses of functional groupings. [ABSTRACT FROM AUTHOR]

Tropical ecosystems face escalating global change. These shifts can disrupt tropical forests' carbon (C) balance and impact root dynamics. Since roots perform essential functions such as resource acquisition and tissue protection, root responses can inform about the strategies and vulnerabilities of ecosystems facing present and future global changes. However, root trait dynamics are poorly understood, especially in tropical ecosystems. We analyzed existing research on tropical root responses to key global change drivers: warming, drought, flooding, cyclones, nitrogen (N) deposition, elevated (e) CO2, and fires. Based on tree species‐ and community‐level literature, we obtained 266 root trait observations from 93 studies across 24 tropical countries. We found differences in the proportion of root responsiveness to global change among different global change drivers but not among root categories. In particular, we observed that tropical root systems responded to warming and eCO2 by increasing root biomass in species‐scale studies. Drought increased the root: shoot ratio with no change in root biomass, indicating a decline in aboveground biomass. Despite N deposition being the most studied global change driver, it had some of the most variable effects on root characteristics, with few predictable responses. Episodic disturbances such as cyclones, fires, and flooding consistently resulted in a change in root trait expressions, with cyclones and fires increasing root production, potentially due to shifts in plant community and nutrient inputs, while flooding changed plant regulatory metabolisms due to low oxygen conditions. The data available to date clearly show that tropical forest root characteristics and dynamics are responding to global change, although in ways that are not always predictable. This synthesis indicates the need for replicated studies across root characteristics at species and community scales under different global change factors. [ABSTRACT FROM AUTHOR]

This article explores the significance of studying tropical forests and their responses to global change. It emphasizes the unique characteristics of tropical forests and their crucial role in the global carbon cycle. The article calls for research and model development to better understand the changing nature of these forests. It also discusses the importance of large-scale global change experiments and cross-site comparisons, as well as the complexity of plant, soil, and microbial responses. The document further highlights the need for inclusivity, collaboration, standardization, and the incorporation of local knowledge and partnerships with local communities in tropical forest research. [Extracted from the article]

Summary: Tropical forests are often characterized by low soil phosphorus (P) availability, suggesting that P limits plant performance. However, how seedlings from different functional types respond to soil P availability is poorly known but important for understanding and modeling forest dynamics under changing environmental conditions.We grew four nitrogen (N)‐fixing Fabaceae and seven diverse non‐N‐fixing tropical dry forest tree species in a shade house under three P fertilization treatments and evaluated carbon (C) allocation responses, P demand, P‐use, investment in P acquisition traits, and correlations among P acquisition traits.Nitrogen fixers grew larger with increasing P addition in contrast to non‐N fixers, which showed fewer responses in C allocation and P use. Foliar P increased with P addition for both functional types, while P acquisition strategies did not vary among treatments but differed between functional types, with N fixers showing higher root phosphatase activity (RPA) than nonfixers.Growth responses suggest that N fixers are limited by P, but nonfixers may be limited by other resources. However, regardless of limitation, P acquisition traits such as mycorrhizal colonization and RPA were nonplastic across a steep P gradient. Differential limitation among plant functional types has implications for forest succession and earth system models. [ABSTRACT FROM AUTHOR]

Fine roots balance multiple essential plant functions that ultimately fuel plant productivity. The coordination of root functional traits is multidimensional, meaning that some traits are correlated while others can vary independently. This leads to a large variety of adaptive trait combinations and thus functional diversity in plant belowground morphology, anatomy, physiology and symbioses with soil biota. Within forest communities, co‐occurrence of such belowground trait combinations may be especially prevalent in tropical forests, not only due to their immense biodiversity but also because their soils are frequently weathered and limited in phosphorus. Phosphorus occurs in different forms (that differ in their mobility and accessibility) that may select for different trait combinations on relatively small spatial scales. While evidence accumulates of the myriad viable nutrient acquisition strategies on poor tropical soils, they are not well conceptualized. In this study, we outline criteria for the identification and investigation of fine‐root trait syndromes. We then apply these criteria to a case study of tropical tree species adapted to phosphorus‐poor soil and synthesize a range of traits. Five potentially coexisting syndromes are presented that are distinguished by root morphology, mycorrhizal association type, their interactions and implications for fine‐root functioning. We connect the functional variation of these species‐level syndromes in tropical communities to an established phosphorus partitioning hypothesis. The co‐occurrence of fine‐root trait syndromes could have implications for root sampling design, species coexistence, community structure and biogeochemical cycling. [ABSTRACT FROM AUTHOR]

The mechanistic links between nitrogen (N) availability and investment in plant phosphorus (P) acquisition have important implications for plant growth, species distributions, and responses to CO2 fertilization under global change, especially in P‐poor tropical ecosystems. Currently, it is unclear whether investment in strategies that enhance plant P acquisition (arbuscular mycorrhizal, AM; colonization or root phosphatase activity, RPA) are determined primarily by phylogeny, or whether these strategies differ among N2‐fixing legumes and nonfixing plants as a result of differing N availability.We hypothesized that plant N status, which can vary widely independent of N fixation, correlates with investment in P acquisition, because: (a) N and P concentrations scale in plant tissue indicative of coupled demand and (b) plants with more N may have more resources available to allocate to acquisition strategies.We grew seedlings of eight tropical tree species from three families (including three N2‐fixing and one nonfixing legume) under greenhouse conditions in native forest soil for four months. Species represented almost the full range of foliar N observed in tropical trees.Neither foliar N nor P concentrations correlated with investment in P acquisition. Across all species, we found an inverse relationship between investment in AM colonization and RPA, but this trade‐off was unrelated to foliar N or P and did not differ between functional types (i.e., N2 fixers vs. nonfixers).Within legumes (family Fabaceae), two strategies were evident that were unrelated to fixation status. High‐fixing Inga and nonfixing Dialium displayed high foliar N and P concentrations and greater proportional investment in RPA versus AM, while lower fixing Ormosia species were characterized by lower foliar nutrient concentrations and proportionally more investment in AM.Synthesis.Investment in P acquisition strategies in tropical trees is not dependent on foliar N or functional group, but instead may be controlled in part by resource trade‐offs. High diversity in nutrient strategies between related species cautions again the use of simple functional groupings to draw conclusions about nutrient acquisition in tropical trees. Although nitrogen and phosphorus are stoichiometrically coupled in foliage, we found no evidence that investment in phosphorus‐acquisition mechanisms is related to either foliar nitrogen or to the presence of nitrogen fixation in tropical tree seedlings. Instead, species show trade‐offs between P acquisition mechanisms (arbuscular mycorrhizal colonization and root phosphatase enzymes), and display diverse nutrient strategies, even within plant families. [ABSTRACT FROM AUTHOR]

Biological nitrogen fixation ( BNF) in woody plants is often investigated using foliar measurements of δ15 N and is of particular interest in ecosystems experiencing increases in BNF due to woody plant encroachment. We sampled δ15 N along the entire N uptake pathway including soil solution, xylem sap and foliage to (1) test assumptions inherent to the use of foliar δ15 N as a proxy for BNF; (2) determine whether seasonal divergences occur between δ15 Nxylem sap and δ15 Nsoil inorganic N that could be used to infer variation in BNF; and (3) assess patterns of δ15 N with tree age as indicators of shifting BNF or N cycling. Measurements of woody N-fixing P rosopis glandulosa and paired reference non-fixing Z anthoxylum fagara at three seasonal time points showed that δ15 Nsoil inorganic N varied temporally and spatially between species. Fractionation between xylem and foliar δ15 N was consistently opposite in direction between species and varied on average by 2.4‰. Accounting for these sources of variation caused percent nitrogen derived from fixation values for P rosopis to vary by up to ∼70%. Soil-xylem δ15 N separation varied temporally and increased with P rosopis age, suggesting seasonal variation in N cycling and BNF and potential long-term increases in BNF not apparent through foliar sampling alone. [ABSTRACT FROM AUTHOR]

Biological nitrogen fixation (BNF) in woody plants is often investigated using foliar measurements of δ(15) N and is of particular interest in ecosystems experiencing increases in BNF due to woody plant encroachment. We sampled δ(15) N along the entire N uptake pathway including soil solution, xylem sap and foliage to (1) test assumptions inherent to the use of foliar δ(15) N as a proxy for BNF; (2) determine whether seasonal divergences occur between δ(15) Nxylem sap and δ(15) Nsoil inorganic N that could be used to infer variation in BNF; and (3) assess patterns of δ(15) N with tree age as indicators of shifting BNF or N cycling. Measurements of woody N-fixing Prosopis glandulosa and paired reference non-fixing Zanthoxylum fagara at three seasonal time points showed that δ(15) Nsoil inorganic N varied temporally and spatially between species. Fractionation between xylem and foliar δ(15) N was consistently opposite in direction between species and varied on average by 2.4‰. Accounting for these sources of variation caused percent nitrogen derived from fixation values for Prosopis to vary by up to ∼70%. Soil-xylem δ(15) N separation varied temporally and increased with Prosopis age, suggesting seasonal variation in N cycling and BNF and potential long-term increases in BNF not apparent through foliar sampling alone., (© 2014 John Wiley & Sons Ltd.)