The leaf surface is the contact point between plants and the environment and plays a crucial role in mediating biotic and abiotic interactions. Here, we took a phylogenetic approach to investigate ...the function, trade-offs, and evolution of leaf surface traits in the milkweeds (Asclepias). Across 47 species, we found trichome densities of up to 3000 trichomes cm⁻² and epicuticular wax crystals (glaucousness) on 10 species. Glaucous species had a characteristic wax composition dominated by very-long-chain aldehydes. The ancestor of the milkweeds was probably a glaucous species, from which there have been several independent origins of glabrous and pubescent types. Trichomes and wax crystals showed negatively correlated evolution, with both surface types showing an affinity for arid habitats. Pubescent and glaucous milkweeds had a higher maximum photosynthetic rate and lower stomatal density than glabrous species. Pubescent and glaucous leaf surfaces impeded settling behavior of monarch caterpillars and aphids compared with glabrous species, although surface types did not show consistent differentiation in secondary chemistry. We hypothesize that pubescence and glaucousness have evolved as alternative mechanisms with similar functions. The glaucous type, however, appears to be ancestral, lost repeatedly, and never regained; we propose that trichomes are a more evolutionarily titratable strategy.
Information on denitrification (particularly N₂) losses from dry ecosystems is limited despite their large area. Here, we present the first direct denitrification measurements for a northern ...hemisphere savanna, a Prosopis-dominated grassland/grove matrix in south Texas. We used the gas-flow intact soil core method to quantify N₂, N₂O and CO₂ losses and compared these with field measurements of N₂O, NOy, NH₃ and CO₂. Under field-realistic soil moisture and O₂ conditions (average 17.5–20 % O₂, minimum 15 %) incubated soils produced no measurable N₂ flux (detection limit 52.2 µg N m⁻² h⁻¹). Only in a subset of grove soils were fluxes of 70–75 µg N m⁻² h⁻¹ recorded after 102 h of incubation at 5–10 % O₂ following wetting of very dry soils. Making the assumption that potential N₂ production falls just below the detection limit (likely an overestimate given the conditions needed to generate measurable fluxes), N₂ flux rates would fall on the low end of that recorded for a tropical Australian savanna (45–110 µg N m⁻² h⁻¹) under comparable abiotic conditions. Assuming maximum possible production rates, N₂ could comprise <32–76 % of total soil N gas flux following soil wetting in summer. Lack of flux response to soil wetting in winter suggests that cold-season N₂ fluxes are negligible. N₂O fluxes for core incubations were significantly higher than for field chambers; thus it is likely that incubations may overestimate N₂O flux by reducing soil column consumption. Overall, results indicate that soil N₂ fluxes are less dominant in this savanna than in other ecosystems investigated.
Despite growing understanding of how rising temperatures affect carbon cycling, the impact of long-term and whole forest warming on the suite of essential and potentially limiting nutrients remains ...understudied, particularly for elements other than N and P. Whole ecosystem warming experiments are limited, environmental gradients are often confounded by variation in factors other than temperature, and few studies have been conducted in the tropics. We examined litterfall, live foliar nutrient content, foliar nutrient resorption efficiency (NRE), nutrient return, and foliar nutrient use efficiency (NUE) of total litterfall and live foliage of two dominant trees to test hypotheses about how increasing mean annual temperature (MAT) impacts the availability and ecological stoichiometry of C, N, P, K, Ca, Mg, Mn, Fe, Zn, and Cu in tropical montane wet forests located along a 5.2°C gradient in Hawaii. Live foliage responded to increasing MAT with increased N and K concentrations, decreased C and Mn concentrations, and no detectable change in P concentration or in foliar NRE. Increases in MAT increased nutrient return via litterfall for N, K, Mg, and Zn and foliar NUE for Mn and Cu, while decreasing nutrient return for Cu and foliar NUE for K. The N:P of litterfall and live foliage increased with MAT, while there was no detectable effect of MAT on C:P. The ratio of live foliar N or P to base cations and micronutrients was variable across elements and species. Increased MAT resulted in declining N:K and P:K for one species, while only P:K declined for the other. N:Ca and N:Mn increased with MAT for both species, while N:Mg increased for one and P:Mn increased for the other species. Overall, results from this study suggest that rising MAT in tropical montane wet forest: (i) increases plant productivity and the cycling and availability of N, K, Mg, and Zn; (ii) decreases the cycling and availability of Mn and Cu; (iii) has little direct effect on P, Ca or Fe; and (iv) affects ecological stoichiometry in ways that may exacerbate P-as well as other base cation and micronutrient - limitations to tropical montane forest productivity.
The diet-tissue discrimination factor is the amount by which a consumer's tissue varies isotopically from its diet, and is therefore a key element in models that use stable isotopes to estimate diet ...composition. In this study we measured discrimination factors in blood (whole blood, red blood cells and plasma), liver, muscle and feathers of Double-crested Cormorants (Phalacrocorax auritus) for stable isotope ratios of carbon, nitrogen and sulfur. Cormorants exhibited discrimination factors that differed significantly among tissue types (for carbon and nitrogen), and differed substantially (in the context of the isotopic variation among relevant prey species) from those observed in congeneric species. The Double-crested Cormorant has undergone rapid population expansion throughout much of its historic range over the past three decades, leading to both real and perceived conflicts with fisheries throughout North America, and this study provides an essential link for the use of stable isotope analysis in researching foraging ecology, diet, and resource use of this widespread and controversial species.
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.
Historically, anthropogenic fixed nitrogen has been purposely increased to benefit food production and global development. One consequence of this increase has been to raise concentrations of ...nitrogen in aquatic ecosystems. To evaluate whether nitrogen pollution promotes changes in the estimates of niche space of fish communities, we examined 16 sites along a Brazilian river basin highly impacted by anthropogenic activities, especially discharge of domestic and industrial sewage from a region with more than 5 million inhabitants. We analysed the carbon (δ
13
C) and nitrogen (δ
15
N) isotope ratios of fish species and both autochthonous (periphyton) and allochthonous (course and fine particulate organic matter) basal food resources. To estimate the magnitude of nitrogen pollution, we measured the nitrate and ammonium concentrations at each site. Sampling was conducted in the dry and wet seasons to evaluate the influence of seasonality. Nitrogen pollution generally increased estimates of niche space, and seasonality influenced only the niche estimates of fish communities from polluted sites. In addition, isotopic analyses of nitrogen polluted sites yielded unrealistic estimates of trophic positioning (detritivores at the top of the food web). We conclude that changes in niche space estimates reflect both alterations in baseline isotopic values and differential trophic behaviour among fishes. Our study suggests that under conditions of high pollution, other factors appear to influence isotopic estimates of niche, such as isotopically distinct sources that have not been sampled, and/or differences in δ
15
N turnover rates between fish tissue and basal resources, creating isotopic baselines that are challenging to interpret.
Mean annual temperature (MAT) is an influential climate factor affecting the bioavailability of growth‐limiting nutrients nitrogen (N) and phosphorus (P). In tropical montane wet forests, warmer MAT ...drives higher N bioavailability, while patterns of P availability are inconsistent across MAT. Two important nutrient acquisition strategies, fine root proliferation into bulk soil and root association with arbuscular mycorrhizal fungi, are dependent on C availability to the plant via primary production. The case study presented here tests whether variation in bulk soil N bioavailability across a tropical montane wet forest elevation gradient (5.2°C MAT range) influences (a) morphology fine root proliferation into soil patches with elevated N, P, and N+P relative to background soil and (b) arbuscular mycorrhizal fungal (AMF) colonization of fine roots in patches. We created a fully factorial fertilized root ingrowth core design (N, P, N+P, unfertilized control) representing soil patches with elevated N and P bioavailability relative to background bulk soil. Our results show that percent AMF colonization of roots increased with MAT (r2 = .19, p = .004), but did not respond to fertilization treatments. Fine root length (FRL), a proxy for root foraging, increased with MAT in N+P‐fertilized patches only (p = .02), while other fine root morphological parameters did not respond to the gradient or fertilized patches. We conclude that in N‐rich, fine root elongation into areas with elevated N and P declines while AMF abundance increases with MAT. These results indicate a tradeoff between P acquisition strategies occurring with changing N bioavailability, which may be influenced by higher C availability with warmer MAT.
Rising mean annual temperature (MAT) is predicted to increase the availability of inorganic nitrogen (IN) for biological use via impacts on rates of biogeochemical cycling, and with implications for plant N and phosphorus (P) acquisition. We found that fine root proliferation into N+P‐fertilized microsites declined across a natural temperature and soil‐IN gradient, while fine root mycorrhizal colonization increased across the gradient, with no effect of fertilization treatment. We conclude that in this tropical montane forest, plants may prioritize a mycorrhizal nutrient acquisition strategy over fine root proliferation in high MAT/N bioavailability conditions as a compensatory P acquisition strategy.
Non‐native invasive plants often outcompete native species under high resource availability. Restoration techniques that lower resources may, therefore, create favorable conditions for resource ...conservative native species over resource exploitative invasive species. Research on this topic has focused on temperate grass and forb‐dominated ecosystems and has rarely been tested for woody species or tropical vegetation. We evaluated growth, resource‐use efficiency (RUE), ecophysiology, and competition (i.e. a relative interaction index based on biomass) of four woody native and two dominant invasive species from Hawaiian wet and dry tropical ecosystems in a greenhouse experiment. Density of plants was constant and species were grown with either a conspecific or the invasive species from that ecosystem type across a gradient of soil nutrient availability. Instantaneous photosynthetic rates varied minimally across nutrient availability. However, both invasive and one native species increased leaf area as nutrients increased, providing more photosynthetic area and increasing total biomass. Nitrogen RUE decreased with increasing nutrient availability for all but one native species, while phosphorus RUE remained constant for all but one native species. Competitive interactions were weak, variable, and not significantly impacted by soil nutrients. Overall, plants categorized as invasive or resource exploitative had larger changes in response variables with increasing soil nutrients compared to those categorized as native or resource conservative. These results suggest that manipulating soil nutrient availability is a potentially viable restoration tool for at least some woody species in tropical ecosystems. However, predicting restoration success requires understanding species‐specific ecophysiological traits determining response to altered environmental conditions.
Body surface area and volume are both physiologically important traits in amphibians, as their ratio constrains transport rates for water and respiratory gases across the skin. This is especially ...true in the lungless salamanders (Urodela: Plethodontidae), whose lungless morphology restricts nearly all gas and water transport to the body surface. Due to methodological difficulties of measuring surface area and volume, estimation techniques are the most convenient way to produce usable phenotype scores. To this end, we used high-resolution computerized tomography (CT) scans of three plethodontid species varying in body size and shape to produce allometric regression models to estimate body surface area (SA), volume, and surface area-to-volume ratio (SA:V). We compared our model estimates to empirical measurements and established estimation methods employed in the Plethodontidae using linear models and Deming regressions. We found our model estimates are both accurate and generalizable across temperate plethodontid species. This method significantly improves SA estimation accuracy when compared to published allometric models. Our models are also significantly more accurate than SA or volume estimates following geometric measurements, although SA:V estimates are comparable between these techniques for most body sizes. We discuss the relative utility of different estimation methods for future research questions.
Cattle are a major methane (CH₄) source from pasture ecosystems; however, the underlying landscape can be a significant and unaccounted source of CH₄. In general, landscape CH₄ emissions are poorly ...quantified, vary widely across time and space, and are easily underestimated if emission hotspots or episodic fluxes are overlooked. In this study, CH₄ emissions from subtropical lowland pastures were quantified using static chambers, eddy covariance, and mobile spectrometer surveys. Landscape emissions were the dominant CH₄ source, and cattle were responsible for 19-30% of annual emissions. The entire ecosystem emitted 33.84 ± 2.25 g CH₄ m⁻² y⁻¹ as estimated by eddy covariance-measured fluxes. Landscape emissions were highly variable, and seasonal flooding drove high magnitude emissions from the underlying landscape. Large CH₄ emissions were observed from wetlands and, to a lesser extent, from the entire landscape during the wet season. In contrast, during the dry season, there were no appreciable landscape CH₄ emissions, although canals, which cover only 1.7% of the total land area, were responsible for 97.7% of dry-season emissions. Ecosystem CH₄ fluxes, measured by eddy covariance, varied seasonally and positively correlated to water table depth, soil and air temperatures, and topsoil water content. The results presented here are the first to use mobile spectrometers to map biogenic CH ₄ emissions at the landscape scale, and strongly suggest that the underlying landscape is a strong CH₄ source that must be considered in addition to cattle emissions.
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