Search

Nutrient addition experiments indicate that nitrogen and phosphorus limit plant processes in many tropical forests. However, the long‐term consequences for forest structure and species composition remain unexplored. We are positioned to evaluate potential long‐term consequences of nutrient addition in central Panama where we have maintained a factorial nitrogen–phosphorus–potassium fertilization experiment for 21 yr and an independent study quantified the species‐specific nutrient requirements of 550 local tree species. Here, we ask whether nutrients limit reproduction at the species and community levels. We also ask whether species‐specific reproductive responses to nutrient addition are stronger among species associated with naturally fertile soils, which could contribute to a shift in species composition. We quantified species‐level reproductive responses for 38 focal species in the 21st year of the experiment and community‐level reproductive litter production for the first 20 yr. Species‐level reproductive responses to nitrogen and potassium addition were weak, inconsistent across species, and insignificant across the 38 focal species. In contrast, species‐level responses to phosphorus addition were consistently and significantly positive across the 38 focal species but were unrelated to species‐specific phosphorus requirements documented independently for the same species. Community‐level reproductive litter production was unaffected by nutrient addition, possibly because spatial and temporal variation is large. We conclude that phosphorus limits reproduction by trees in our experiment but find no evidence that reproductive responses to phosphorus addition favor species associated with naturally phosphorus‐rich soils. [ABSTRACT FROM AUTHOR]

Questions remain as to which soil nutrients limit primary production in tropical forests. Phosphorus (P) has long been considered the primary limiting element in lowland forests, but recent evidence demonstrates substantial heterogeneity in response to nutrient addition, highlighting a need to understand and diagnose nutrient limitation across diverse forests. Fine-root characteristics including their abundance, functional traits, and mycorrhizal symbionts can be highly responsive to changes in soil nutrients and may help to diagnose nutrient limitation. Here, we document the response of fine roots to long-term nitrogen (N), P, and potassium (K) fertilization in a lowland forest in Panama. Because this experiment has demonstrated that N and K together limit tree growth and P limits fine litter production, we hypothesized that fine roots would also respond to nutrient addition. Specifically we hypothesized that N, P, and K addition would reduce the biomass, diameter, tissue density, and mycorrhizal colonization of fine roots, and increase nutrient concentration in root tissue. Most morphological root traits responded to the single addition of K and the paired addition of N and P, with the greatest response to all three nutrients combined. The addition of N, P, and K together reduced fine-root biomass, length, and tissue density, and increased specific root length, whereas root diameter remained unchanged. Nitrogen addition did not alter root N concentration, but P and K addition increased root P and K concentration, respectively. Mycorrhizal colonization of fine roots declined with N, increased with P, and was unresponsive to K addition. Although plant species composition remains unchanged after 14 years of fertilization, fine-root characteristics responded to N, P, and K addition, providing some of the strongest stand-level responses in this experiment. Multiple soil nutrients regulate fine-root abundance, morphological and chemical traits, and their association with mycorrhizal fungi in a species-rich lowland tropical forest. [ABSTRACT FROM AUTHOR]

Lianas are a prominent growth form in tropical forests, and there is compelling evidence that they are increasing in abundance throughout the Neotropics. While recent evidence shows that soil resources limit tree growth even in deep shade, the degree to which soil resources limit lianas in forest understories, where they coexist with trees for decades, remains unknown. Regardless, the physiological underpinnings of soil resource limitation in deeply shaded tropical habitats remain largely unexplored for either trees or lianas. Theory predicts that lianas should be more limited by soil resources than trees because they occupy the quick-return end of the ''leaf economic spectrum,'' characterized by high rates of photosynthesis, high specific leaf area, short leaf life span, affinity to high-nutrient sites, and greater foliar nutrient concentrations. To address these issues, we asked whether soil resources (nitrogen, phosphorus, and potassium), alone or in combination, applied experimentally for more than a decade would cause significant changes in the morphology or physiology of tree and liana seedlings in a lowland tropical forest. We found evidence for the first time that phosphorus limits the photosynthetic performance of both trees and lianas in deeply shaded understory habitats. More importantly, lianas always showed significantly greater photosynthetic capacity, quenching, and saturating light levels compared to trees across all treatments. We found little evidence for nutrient 3 growth form interactions, indicating that lianas were not disproportionately favored in nutrient-rich habitats. Tree and liana seedlings differed markedly for six key morphological traits, demonstrating that architectural differences occurred very early in ontogeny prior to lianas finding a trellis (all seedlings were self-supporting). Overall, our results do not support nutrient loading as a mechanism of increasing liana abundance in the Neotropics. Rather, our finding that lianas always outperform trees, in terms of photosynthetic processes and under contrasting rates of resource supply of macronutrients, will allow lianas to increase in abundance if disturbance and tree turnover rates are increasing in Neotropical forests as has been suggested. [ABSTRACT FROM AUTHOR]

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 13 years 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. [ABSTRACT FROM AUTHOR]

Summary 1. Nutrients are a critical resource for plant growth, but the elements limiting growth in tropical forests have rarely been determined. 2. We investigated the influence of nitrogen (N), phosphorus (P), potassium (K) and micronutrients on seedling biomass and nutrient allocation in a factorial nutrient fertilization experiment in lowland tropical forest at the Barro Colorado Nature Monument, Panama. We also measured 8 years of herbivory and growth for 1800 seedlings. We sought to determine the identity of limiting elements and possible nutrient interactions. 3. The five study species were Alseis blackiana, Desmopsis panamensis, Heisteria concinna, Sorocea affinis and Tetragastris panamensis. Plants grew in deeply shaded understorey with a mean canopy openness of 4.9% (±0.7%; 1 SE). 4. Tissue N concentration increased by 11% with N addition. Tissue P concentration increased by 16% with P addition. Tissue K increased by 4% with K addition. K addition reduced root-to-shoot biomass ratio. There was no significant effect of fertilization on specific leaf area or leaf area ratio. 5. The proportion of leaves damaged and the mean level of damage by herbivory increased with P and K addition and showed a significant P × K interaction. 6.7ensp;Across all species and years, relative growth rate of height increased with K addition and with N and P in combination. Relative growth rate of leaf count trended 8.5% higher with K addition ( P = 0.076). 7. We also added micronutrients in a parallel experiment. There was no effect of micronutrient addition on any seedling parameter. 8. Synthesis. K addition affected seedlings by enhancing tissue nutrient concentration, increasing herbivory, reducing root-to-shoot biomass ratio and increasing height growth. Additional effects of N or P on tissue chemistry, herbivory and growth offer support for the multiple limiting resources hypothesis. Our results suggest that seedling growth is limited by nutrients, especially K, even under highly shaded conditions in this lowland tropical forest. [ABSTRACT FROM AUTHOR]

Climatic conditions should not hinder nutrient release from decomposing leaf-litter (mineralization) in the humid tropics, even though many tropical forests experience drought lasting from several weeks to months. We used a dry-season irrigation experiment to examine the effect of seasonal drought on nutrient concentrations in leaf-fall and in decomposing leaf-litter. In the experiment, soil in two 2.25-ha plots of old-growth lowland moist forest on Barro Colorado Island, Republic of Panama, was watered to maintain soil water potential at or above field capacity throughout the 4-month dry season. Wet-season leaf-fall had greater concentrations of nitrogen (N, 13.5 mg g−1) and calcium (Ca, 15.6 mg g−1) and lower concentrations of sulfur (S, 2.51 mg g−1) and potassium (K, 3.03 mg g−1) than dry-season leaf-fall (N = 11.6 mg g−1, Ca = 13.6 mg g−1, S = 2.98 mg g−1, K = 5.70 mg g−1). Irrigation did not affect nutrient concentrations or nutrient return from forest trees to the forest floor annually (N = 18 g m−2, phosphorus (P) = 1.06 g m−2, S = 3.5 g m−2, Ca = 18.9 g m−2, magnesium = 6.5 g m−2, K = 5.7 g m−2). Nutrient mineralization rates were much greater during the wet season than the dry season, except for K, which did not vary seasonally. Nutrient residence times in forest-floor material were longer in control plots than in irrigated plots, with values approximately equal to that for organic matter (210 in control plots vs 160 in irrigated plots). Calcium had the longest residence time. Forest-floor material collected at the transition between seasons and incubated with or without leaching in the laboratory did not display large pulses in nutrient availability. Rather, microorganisms immobilized nutrients primarily during the wet season, unlike observations in tropical forests with longer dry seasons. Large amounts of P moved among different pools in forest-floor material, apparently mediated by microorganisms. Arylsulfatase and phosphatase enzymes, which mineralize organically bound nutrients, had high activity throughout the dry season. Low soil moisture levels do not hinder nutrient cycling in this moist lowland forest. [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]

Foliar nitrogen to phosphorus (N:P) ratios are widely used to indicate soil nutrient availability and limitation, but the foliar ratios of woody plants have proven more complicated to interpret than ratios from whole biomass of herbaceous species. This may be related to tissues in woody species acting as nutrient reservoirs during active growth, allowing maintenance of optimal N:P ratios in recently produced, fully expanded leaves (i.e., "new" leaves, the most commonly sampled tissue). Here we address the hypothesis that N:P ratios of newly expanded leaves are less sensitive indicators of soil nutrient availability than are other tissue types in woody plants. Seedlings of five naturally established tree species were harvested from plots receiving two years of fertilizer treatments in a lowland tropical forest in the Republic of Panama. Nutrient concentrations were determined in new leaves, old leaves, stems, and roots. For stems and roots, N:P ratios increased after N addition and decreased after P addition, and trends were consistent across all five species. Older leaves also showed strong responses to N and P addition, and trends were consistent for four of five species. In comparison, overall N:P ratio responses in new leaves were more variable across species. These results indicate that the N:P ratios of stems, roots, and older leaves are more responsive indicators of soil nutrient availability than are those of new leaves. Testing the generality of this result could improve the use of tissue nutrient ratios as indices of soil nutrient availability in woody plants [ABSTRACT FROM AUTHOR]

Tree death drives population dynamics, nutrient cycling, and evolution within plant communities. Mortality variation across species is thought to be influenced by different factors relative to variation within species. The unified model provided here separates mortality rates into growth-dependent and growth-independent hazards. This model creates the opportunity to simultaneously estimate these hazards both across and within species. Moreover, it provides the ability to examine how species traits affect growth-dependent and growth-independent hazards. We derive this unified mortality model using cross-validated Bayesian methods coupled with mortality data collected over three census intervals for 203 tropical rainforest tree species at Barro Colorado Island (BCI), Panama. We found that growth-independent mortality tended to be higher in species with lower wood density, higher light requirements, and smaller maximum diameter at breast height (dbh). Mortality due to marginal carbon budget as measured by near-zero growth rate tended to be higher in species with lower wood density and higher light demand. The total mortality variation attributable to differences among species was large relative to variation explained by these traits, emphasizing that much remains to be understood. This additive hazards model strengthens our capacity to parse and understand individual-level mortality in highly diverse tropical forests and hence to predict its consequences. [ABSTRACT FROM AUTHOR]

Aims: Tropical tree and lianas in the understory are limited by soil nutrients despite growing in extremely low light. It is not known if nomadic vines are also limited by nutrients in low light conditions.Methods: We measured differences in root architecture and mycorrhizal colonization, and leaf nutrients of a nomadic vine, Philodendron fragrantissimum (Araceae), in nitrogen (N) and phosphorus (P) fertilization plots in a lowland tropical moist forest in central Panama to measure potential nutrient limitation.Results: Relative to plants in control plots, leaf P concentration was 54% higher and leaf N concentration was 10% higher for plants in the P- and N-addition treatments, respectively. The N:P of leaves suggested P-limitation in the N-addition treatment and the control but not in the P-addition treatment. Root branching was highest in the P-addition treatment, and P-addition reduced mycorrhizal colonization.Conclusions: The large effect of P fertilization suggests that, like many tropical plants, P. fragrantissimum has the potential to be P-limited. Although further study is needed, we suggest that nomadic vines be added to the growth forms that respond to nutrient addition in the forest understory and conclude that nutrient-limitation seems like the rule rather than the exception in the light-limited understory. [ABSTRACT FROM AUTHOR]

The majority of terrestrial plants associate with arbuscular mycorrhizal (AM) fungi, which typically facilitate the uptake of limiting mineral nutrients by plants in exchange for plant carbon. However, hundreds of non-photosynthetic plant species—mycoheterotrophs—depend entirely on AM fungi for carbon as well as mineral nutrition. Mycoheterotrophs can provide insight into the operation and regulation of AM fungal relationships, but little is known about the factors, fungal or otherwise, that affect mycoheterotroph abundance and distribution. In a lowland tropical forest in Panama, we conducted the first systematic investigation into the influence of abiotic factors on the abundance and distribution of mycoheterotrophs, to ask whether the availability of nitrogen and phosphorus altered the occurrence of mycoheterotrophs and their AM fungal partners. Across a natural fertility gradient spanning the isthmus of Panama, and also in a long-term nutrient-addition experiment, mycoheterotrophs were entirely absent when soil exchangeable phosphate concentrations exceeded 2 mg P kg−1. Experimental phosphorus addition reduced the abundance of AM fungi, and also reduced the abundance of the specific AM fungal taxa required by the mycoheterotrophs, suggesting that the phosphorus sensitivity of mycoheterotrophs is underpinned by the phosphorus sensitivity of their AM fungal hosts. The soil phosphorus concentration of 2 mg P kg−1 also corresponds to a marked shift in tree community composition and soil phosphatase activity across the fertility gradient, suggesting that our findings have broad ecological significance. [ABSTRACT FROM AUTHOR]

Soil organic matter is an important pool of carbon and nutrients in tropical forests. The majority of this pool is assumed to be relatively stable and to turn over slowly over decades to centuries, although changes in nutrient status can influence soil organic matter on shorter timescales. We measured carbon, nitrogen, and phosphorus concentrations in soil organic matter and leaf litter over an annual cycle in a long-term nutrient addition experiment in lowland tropical rain forest in the Republic of Panama. Total soil carbon was not affected by a decade of factorial combinations of nitrogen, phosphorus, or potassium. Nitrogen addition increased leaf litter nitrogen concentration by 7 % but did not affect total soil nitrogen. Phosphorus addition doubled the leaf litter phosphorus concentration and increased soil organic phosphorus by 50 %. Surprisingly, concentrations of carbon, nitrogen, and phosphorus in soil organic matter declined markedly during the four-month dry season, and then recovered rapidly during the following wet season. Between the end of the wet season and the late dry season, total soil carbon declined by 16 %, total nitrogen by 9 %, and organic phosphorus by between 19 % in control plots and 25 % in phosphorus addition plots. The decline in carbon and nitrogen was too great to be explained by changes in litter fall, bulk density, or the soil microbial biomass. However, a major proportion of the dry-season decline in soil organic phosphorus was explained by a corresponding decline in the soil microbial biomass. These results have important implications for our understanding of the stability and turnover of organic matter in tropical forest soils, because they demonstrate that a considerable fraction of the soil organic matter is seasonally transient, despite the overall pool being relatively insensitive to long-term changes in nutrient status. [ABSTRACT FROM AUTHOR]

Elevated atmospheric CO2 concentrations (ca) will undoubtedly affect the metabolism of tropical forests worldwide; however, critical aspects of how tropical forests will respond remain largely unknown. Here, we review the current state of knowledge about physiological and ecological responses, with the aim of providing a framework that can help to guide future experimental research. Modelling studies have indicated that elevated ca can potentially stimulate photosynthesis more in the tropics than at higher latitudes, because suppression of photorespiration by elevated ca increases with temperature. However, canopy leaves in tropical forests could also potentially reach a high temperature threshold under elevated ca that will moderate the rise in photosynthesis. Belowground responses, including fine root production, nutrient foraging and soil organic matter processing, will be especially important to the integrated ecosystem response to elevated ca. Water use efficiency will increase as ca rises, potentially impacting upon soil moisture status and nutrient availability. Recruitment may be differentially altered for some functional groups, potentially decreasing ecosystem carbon storage. Whole-forest CO2 enrichment experiments are urgently needed to test predictions of tropical forest functioning under elevated ca. Smaller scale experiments in the understorey and in gaps would also be informative, and could provide stepping stones towards stand-scale manipulations. [ABSTRACT FROM AUTHOR]

The question of how tropical trees cope with infertile soils has been challenging to address, in part, because fine root dynamics must be studied in situ. We used annual fertilization with nitrogen (N as urea, 12.5 g N m year), phosphorus (P as superphosphate, 5 g P m year) and potassium (K as KCl, 5 g K m year) within 38 ha of old-growth lowland tropical moist forest in Panama and examined fine root dynamics with minirhizotron images. We expected that added P, above all, would (i) decrease fine root biomass but, (ii) have no impact on fine root turnover. Soil in the study area was moderately acidic (pH = 5.28), had moderate concentrations of exchangeable base cations (13.4 cmol kg), low concentrations of Bray-extractable phosphate (PO = 2.2 mg kg), and modest concentrations of KCl-extractable nitrate (NO = 5.0 mg kg) and KCl-extractable ammonium (NH = 15.5 mg kg). Added N increased concentrations of KCl-extractable NO and acidified the soil by one pH unit. Added P increased concentrations of Bray-extractable PO and P in the labile fraction. Concentrations of exchangeable K were elevated in K addition plots but reduced by N additions. Fine root dynamics responded to added K rather than added P. After 2 years, added K decreased fine root biomass from 330 to 275 g m. The turnover coefficient of fine roots <1 mm diameter ranged from 2.6 to 4.4 per year, and the largest values occurred in plots with added K. This study supported the view that biomass and dynamics of fine roots respond to soil nutrient availability in species-rich, lowland tropical moist forest. However, K rather than P elicited root responses. Fine roots smaller than 1 mm have a short lifetime (<140 days), and control of fine root production by nutrient availability in tropical forests deserves more study. [ABSTRACT FROM AUTHOR]