The nutrient status of epiphytes and their host trees along an elevational gradient in Costa Rica

Vascular epiphytes are a conspicuous and highly diverse group in tropical wet forests; yet, we understand little about their mineral nutrition across sites. In this study, we examined the mineral nutrition of three dominant vascular epiphyte groups: ferns, orchids, and bromeliads, and their host trees from samples collected along a 2600 m elevational gradient in the tropical wet forests of Costa Rica. We predicted that the mineral nutrition of ferns, orchids, and bromeliads would differ because of their putative differences in nutrient acquisition mechanisms and nutrient sources—atmospherically dependent, foliar feeding bromeliads would have lower nitrogen (N) and phosphorous (P) concentrations and more depleted δ¹⁵N values than those in canopy soil-rooted ferns because canopy soil is higher in available N, and more enriched in δ¹⁵N than the atmospheric sources of precipitation and throughfall. We also predicted that epiphyte foliar chemistry would mirror that of host trees because of the likely contribution of host trees to the nutrient cycle of epiphytes via foliar leaching and litter contributions to canopy soil. In the same vein, we predicted that epiphyte and host tree foliar chemistry would vary with elevation reflecting ecosystem-level nutrients—soil N availability increases and P availability decreases with increasing elevation. Our results confirmed that canopy soil-rooted epiphytes had higher N concentrations than atmospheric epiphytes; however, our predictions were not confirmed with respect to P which did not vary among groups indicating fixed P availability within sites. In addition, foliar δ¹⁵N values did not match our prediction in that canopy soil-rooted as well as atmospheric epiphytes had variable signatures. Discriminant function analysis (DFA) on foliar measurements determined that ferns, orchids, and bromeliads are statistically distinct in mineral nutrition. We also found that P concentrations of ferns and orchids, but not bromeliads, were significantly correlated with those of host trees indicating a possible link in their mineral nutrition’s via canopy soil. Interestingly, we did not find any patterns of epiphyte foliar chemistry with elevation. These data indicate that the mineral nutrition of the studied epiphyte groups are distinct and highly variable within sites and the diverse uptake mechanisms of these epiphyte groups enhance resource partitioning which may be a mechanism for species richness maintenance in tropical forest canopies.     

Natural 15N abundance of epiphytes depends on the position within the forest canopy: source signals and isotope fractionation

The natural ¹⁵N abundance (δ¹⁵N) of epiphytes and its N sources were studied in the canopy of a lowland rainforest in Costa Rica. Vascular and non‐vascular epiphytes and canopy soils were collected from four canopy zones and analysed for N contents and δ¹⁵N signals. In addition, the N concentrations and δ¹⁵N signatures of bulk precipitation, throughfall and stemflow were measured during the wet and the dry season. The δ¹⁵N values of epiphyte leaves decreased significantly from the lower zones (means of ?3·9 and ?4·3‰) to the upper zones (means of ?5·4 and ?6·1‰) of the canopy. In contrast, δ¹⁵N signatures of canopy soils (average ?0·3‰) differed little between the zones. Bulk deposition was enriched in ¹⁵N (+4·3‰) compared to all other potential N sources and was higher than throughfall and stemflow (+0·5 to ?1·3‰). δ¹⁵N values of atmospheric deposition were inversely related to those of the epiphyte leaves, whereas N isotopic composition of canopy soils did not vary significantly. Consequently, it is concluded that the variations in foliar N isotope composition of epiphytes were not simply caused by utilization of isotopically different N sources, but by different ¹⁵N discrimination during N acquisition.     

Size-Dependent Variation of Carbon and Nitrogen Isotope Abundances in Epiphytic Bromeliads

Abstract: While atmospheric species of bromeliads have narrow leaves, densely covered with water‐absorbing trichomes throughout their life cycles, many tank bromeliads with broad leaves, forming phytotelmata, go through an atmospheric juvenile phase. The effect of the different habits and the phase change in tank‐forming bromeliads on water and nutrient relations was investigated by analysing the relationship between plant size, C/N ratios and the natural abundance of ¹³C and ¹⁵N in five epiphytic bromeliad species or morphospecies of a humid montane forest in Xalapa, Mexico. The atmospheric species Tillandsia juncea and T. butzii exhibited full crassulacean acid metabolism, with δ¹³C values (mean ‐ 15.3 ‰ and ‐ 14.7 ‰, respectively) independent of size. In Tillandsia species with C₃ photosynthesis, δ¹³C decreased with increasing plant size, indicating stronger drought stress in juveniles. The increase of the C/N ratio with size suggests that, at least in heteroblastic bromeliads, the availability of water is more limiting during early growth, and that limitations of nitrogen supply become more important later on, when water stored in the tank helps to bridge dry periods, reducing water shortage. δ¹⁵N values of the two atmospheric species were very negative (‐ 12.6 ‰ and ‐ 12.2 ‰, respectively) and did not change with plant size. Tank‐forming bromeliads had less negative δ¹⁵N values (c ‐ 6 ‰), and, in species with atmospheric juveniles and tank‐forming adults, δ¹⁵N values increased significantly with plant size. These differences do not appear to be an effect of the isotopic composition of N sources, but rather reflect N availability and limitation and stress‐induced changes in ¹⁵N discrimination.     

15N natural abundance of vascular rainforest epiphytes: implications for nitrogen source and acquisition

The foliar natural abundance of ¹⁵N was analysed to compare the potential nitrogen sources of vascular rainforest epiphytes and associated soil-rooted trees. Leaves of epiphytes collected from six rainforest communities in Brazil, Australia and the Solomon Islands were depleted in ¹⁵N relative to the trees at each site. Epiphyte δ¹⁵N was as low as -6.4%_o, while trees were generally enriched in ¹⁵N (0.7 to 3.5%_o). These results indicate either that epiphytes use nitrogen sources depleted in ¹⁵N or that discrimination against ¹⁵N is an intrinsic function of epiphyte physiology. At three sites, epiphytes could be grouped into those having both low δ¹⁵N and low leaf-nitrogen content and those possessing both high δ¹⁵N and high leaf-nitrogen content. The second group had δ¹⁵N values in the range sometimes attributable to N₂ fixation (-2 to 0%o). There was no correlation between growth form and δ¹⁵N. It is concluded that epiphytes may utilize ¹⁵N-depleted nitrogen from atmospheric deposition and N₂ fixation.     

Variations in nitrogen-15 natural abundance of plant and soil systems in four remote tropical rainforests, southern China

The foliar stable N isotope ratio (δ¹⁵N) can provide integrated information on ecosystem N cycling. Here we present the δ¹⁵N of plant and soil in four remote typical tropical rainforests (one primary and three secondary) of southern China. We aimed to examine if (1) foliar δ¹⁵N in the study forests is negative, as observed in other tropical and subtropical sites in eastern Asia; (2) variation in δ¹⁵N among different species is smaller compared to that in many N-limited temperate and boreal ecosystems; and (3) the primary forest is more N rich than the younger secondary forests and therefore is more ¹⁵N enriched. Our results show that foliar δ¹⁵N ranged from ?5.1 to 1.3 ‰ for 39 collected plant species with different growth strategies and mycorrhizal types, and that for 35 species it was negative. Soil NO₃ ^? had low δ¹⁵N (?11.4 to ?3.2 ‰) and plant NO₃ ^? uptake could not explain the negative foliar δ¹⁵N values (NH₄ ⁺ was dominant in the soil inorganic-N fraction). We suggest that negative values might be caused by isotope fractionation during soil NH₄ ⁺ uptake and mycorrhizal N transfer, and by direct uptake of atmospheric NH₃/NH₄ ⁺. The variation in foliar δ¹⁵N among species (by about 6 ‰) was smaller than in many N-limited ecosystems, which is typically about or over 10 ‰. The primary forest had a larger N capital in plants than the secondary forests. Foliar δ¹⁵N and the enrichment factor (foliar δ¹⁵N minus soil δ¹⁵N) were higher in the primary forest than in the secondary forests, albeit differences were small, while there was no consistent pattern in soil δ¹⁵N between primary and secondary forests.     

Australian mulga ecosystems –13C and 15N natural abundances of biota components and their ecophysiological significance

Samples of recently produced shoot material collected in winter/spring from common plant species of mulga vegetation in eastern and Western Australia were assayed for ¹³C and ¹⁵N natural abundance. ¹³C analyses showed only three of the 88 test species to exhibit C₄ metabolism and only one of seven succulent species to be in CAM mode. Non-succulent winter ephemeral C₃ species showed significantly lower mean δ¹³C values (– 28·0‰) than corresponding C₃-type herbaceous perennials, woody shrubs or trees (– 26·9, – 25·7 and – 26·2‰, respectively), suggesting lower water stress and poorer water use efficiency in carbon acquisition by the former than latter groups of taxa. Corresponding values for δ¹⁵N of the above growth and life forms lay within the range 7·5–15·5‰. δ¹⁵N of soil NH₄⁺ (mean 19·6‰) at a soft mulga site in Western Australia was considerably higher than that of NO₃^– (4·3‰). Shoot dry matter of Acacia spp. exhibited mean δ¹⁵N values (9·10 ± 0·6‰) identical to those of 37 companion non-N₂-fixing woody shrubs and trees (9·06 ± 0·5‰). These data, with no evidence of nodulation, suggested little or no input of fixed N₂ by the legumes in question. However, two acacias and two papilionoid legumes from a dune of wind-blown, heavily leached sand bordering a lake in mulga in Western Australia recorded δ¹⁵N values in the range 2·0–3·0‰ versus 6·4–10·7‰ for associated non-N₂-fixing taxa. These differences in δ¹⁵N, and prolific nodulation of the legumes, indicated symbiotic inputs of fixed N in this unusual situation. δ¹⁵N signals of lichens, termites, ants and grasshoppers from mulga of Western Australia provided evidence of N₂ fixation in certain termite colonies and by a cyanobacteria-containing species of lichen. Data are discussed in relation to earlier evidence of nitrophily and water availability constraints on nitrate utilization by mulga vegetation.     

Estimating tissue‐specific discrimination factors and turnover rates of stable isotopes of nitrogen and carbon in the smallnose fanskate Sympterygia bonapartii (Rajidae)

This study aimed to estimate trophic discrimination factors (TDFs) and metabolic turnover rates of nitrogen and carbon stable isotopes in blood and muscle of the smallnose fanskate Sympterygia bonapartii by feeding six adult individuals, maintained in captivity, with a constant diet for 365 days. TDFs were estimated as the difference between δ¹³C or δ¹⁵N values of the food and the tissues of S. bonapartii after they had reached equilibrium with their diet. The duration of the experiment was enough to reach the equilibrium condition in blood for both elements (estimated time to reach 95% of turnover: C t95%_blood = 150 days, N t95%_blood = 290 days), whilst turnover rates could not be estimated for muscle because of variation among samples. Estimates of Δ¹³C and Δ¹⁵N values in blood and muscle using all individuals were Δ¹³C_blood = 1·7‰, Δ¹³C_muscle = 1·3‰, Δ¹⁵N_blood = 2·5‰ and Δ¹⁵N_muscle = 1·5‰, but there was evidence of differences of c.0·4‰ in the Δ¹³C values between sexes. The present values for TDFs and turnover rates constitute the first evidence for dietary switching in batoids based on long‐term controlled feeding experiments. Overall, the results showed that S. bonapartii has relatively low turnover rates and isotopic measurements would not track seasonal movements adequately. The estimated Δ¹³C values in S. bonapartii blood and muscle were similar to previous estimations for elasmobranchs and to generally accepted values in bony fishes (Δ¹³C = 1·5‰). For Δ¹⁵N, the results were similar to published reports for blood but smaller than reports for muscle and notably smaller than the typical values used to estimate trophic position (Δ¹⁵N c. 3·4‰). Thus, trophic position estimations for elasmobranchs based on typical Δ¹⁵N values could lead to underestimates of actual trophic positions. Finally, the evidence of differences in TDFs between sexes reveals a need for more targeted research.     

Trophic level delineation and resource partitioning in a South African afromontane forest bird community using carbon and nitrogen stable isotopes

Southern African forests are naturally fragmented yet hold a disproportionately high number of bird species. Carbon and nitrogen stable isotopes were measured in feathers from birds captured at Woodbush (n = 27 species), a large afromontane forest in the eastern escarpment of Limpopo province, South Africa. The δ¹³C signatures of a range of forest plants were measured to categorise the food base. Most plants sampled, including two of five grass species, had δ¹³C signatures typical of a C₃ photosynthetic pathway (?29.5 ± 1.9‰). Three grass species had a C₄ signature (?12.0 ± 0.6‰). Most bird species had δ¹³C values representing a predominantly C₃‐based diet (?24.8‰ to ?20.7‰). δ¹⁵N values were as expected, with higher levels of enrichment associated with a greater proportion of dietary animal matter. The cohesive isotopic niche defining most species (n = 22), where the ranges for δ¹³C and δ¹⁵N were 2.4‰ and 3.4‰, respectively, highlight the difficulties in understanding diets of birds in a predominantly C₃‐based ecosystem using carbon and nitrogen stable isotopes. However, variation in isotopic values between and within species provides insight into possible niche width and the use of resources by different birds within a forest environment.     

Nitrogen stable isotopes indicate differences in nitrogen cycling between two contrasting Jamaican montane forests

Background and aims

The aim of this study is to enhance our knowledge of nitrogen (N) cycling and N acquisition in tropical montane forests through analysis of stable N isotopes (δ¹⁵N).

Methods

Leaves from eight common tree species, leaf litter, soils from three depths and roots were sampled from two contrasting montane forest types in Jamaica (mull ridge and mor ridge) and were analysed for δ¹⁵N.

Results

All foliar δ¹⁵N values were negative and varied among the tree species but were significantly more negative in the mor ridge forest (by about 2 ‰). δ¹⁵N of soils and roots were also more negative in mor ridge forests by about 3 ‰. Foliar δ¹⁵N values were closer to that of soil ammonium than soil nitrate suggesting that trees in these forests may have a preference for ammonium; this may explain the high losses of nitrate from similar tropical montane forests. There was no correlation between the rankings of foliar δ¹⁵N in the two forest types suggesting a changing uptake ratio of different N forms between forest types.

Conclusions

These results indicate that N is found at low concentrations in this ecosystem and that there is a tighter N cycle in the mor ridge forest, confirmed by reduced nitrogen availability and lower rates of nitrification. Overall, soil or root δ¹⁵N values are more useful in assessing ecosystem N cycling patterns as different tree species showed differences in foliar δ¹⁵N between the two forest types.     

Nitrogen deposition and forest nitrogen cycling along an urban–rural transect in southern China

There is increasing concern over the impact of atmospheric nitrogen (N) deposition on forest ecosystems in the tropical and subtropical areas. In this study, we quantified atmospheric N deposition and revealed current plant and soil N status in 14 forests along a 150 km urban to rural transect in southern China, with an emphasis on examining whether foliar δ¹⁵N can be used as an indicator of N saturation. Bulk deposition ranged from 16.2 to 38.2 kg N ha^?1 yr^?1, while the throughfall covered a larger range of 11.7–65.1 kg N ha^?1 yr^?1. Foliar N concentration, NO₃^? leaching to stream, and soil NO₃^? concentration were low and NO₃^? production was negligible in some rural forests, indicating that primary production in these forests may be limited by N supply. But all these N variables were enhanced in suburban and urban forests. Across the study transect, throughfall N input was correlated positively with soil nitrification and NO₃^? leaching to stream, and negatively with pH values in soil and stream water. Foliar δ¹⁵N was between ?6.6‰ and 0.7‰, and was negatively correlated with soil NO₃^? concentration and NO₃^? leaching to stream across the entire transect, demonstrating that an increased N supply does not necessarily increase forest δ¹⁵N values. We proposed several potential mechanism that could contribute to the δ¹⁵N pattern, including (1) increased plant uptake of ¹⁵N‐depleted soil NO₃^?, (2) foliage uptake of ¹⁵N‐depleted NH₄⁺, (3) increased utilization of soil inorganic N relative to dissolved organic N, and (4) increased fractionation during plant N uptake under higher soil N availability.     

Gradation in Nutrient Composition and Photosynthetic Pathways Across the Restinga Vegetation of Brazil

The restinga comprises coastal vegetation formations which dominate the Atlantic seaboard of Brazil. Exposed sand ridges and associated lagoon systems have poorly developed soils subject to pronounced water deficits. Distinct vegetation zones support a high diversity of life forms, and a comparative study has been undertaken to investigate interactions between degree of exposure, nutrient supply and photosynthetic pathway (C₃, or CAM) in selected species across the restinga. A number of species occurring throughout the restinga were chosen as representative species of different life forms, comprising C₃ pioneer shrubs (Eugenia rotundifolia and Erythroxylum ovalifolium), impounding (tank) terrestrial bromeliad (Neoregelia cruenta: CAM) and the atmospheric epiphyte (Tillandsia stricta: CAM). Comparisons of plant and soil nutrient composition, and airborne deposition were conducted for each zone. Soil nutrient content and organic matter were closely related, reaching a maximum in zone 4, the seaward face of the inner dune. Salt concentration in leaves was independent of atmospheric deposition for the terrestrial species, in contrast to the atmospheric epiphyte T. stricta. In the slack area, vegetation formed characteristic “islands” with the soil beneath enriched in nutrients, suggesting a complex interplay between plants and soil during the development of vegetation succession. Here, two additional trees were investigated, C₃ and CAM members of the Clusiaceae, respectively Clusia lanceolata and C. fluminensis. Stable isotope composition of nitrogen (δ¹⁵N) was generally more negative (depleted in ¹⁵N) in plants with low total nitrogen content. This was exemplified by the atmospheric bromeliad, T. stricta, with an N content of 2.91 g/kg and δ¹⁵N of ?12.3 per mil. Stable isotopes of carbon (δ¹³C) were used to identify the distribution of photosynthetic pathways, and while the majority of bromeliads and orchids were CAM, analysis of the soil organic matter suggested that C₃ plants made the major contribution in each zone of the restinga. Since δ¹³C of plant material also suggested that water supply was optimal in zone 4, we conclude that succession and high diversity in the restinga is dependent on exposure, edaphic factors, and perhaps a critical mass of vegetation required to stabilize nutrient relations of the system.     

Composition and ecology of vascular epiphyte communities along an altitudinal gradient in central Veracruz,Mexico

Abstract. Vascular epiphytes were studied in forests at altitudes from 720 to 2370 m on the Atlantic slope of central Veracruz, Mexico. The biomass of all trees of each species > 10 cm diameter at breast height within plots between 625 and 1500 m² was estimated. The number of species per plot ranged between 22 and 53, and biomass between 9 and 249 g dry weight/m². The highest values, both of species and biomass, were found at an intermediate altitude (1430 m). Habitat diversity may contribute to epiphyte diversity in humid forests, but the importance of this effect could not be distinguished from the influence of climate. A remarkably high number of bromeliads and orchids grew in relatively dry forests at low altitudes. In wet upper montane forests, bromeliads were replaced by ferns, while orchids were numerous at all sites, except for a pine forest. The number of epiphytic species and their biomass on a tree of a given site were closely related to tree size. According to Canonical Correspondence Analysis, the factor determining the composition of the epiphytic vegetation of a tree was altitude and to some extent tree size, whereas tree species had practically no influence. The only trees which had an evidently negative effect on epiphytes were pines, which were particularly hostile to orchids and to a lesser degree to ferns, and Bursera simaruba, which generally had few epiphytes due to its smooth and defoliating bark.     

Foliar δ15N is affected by foliar nitrogen uptake, soil nitrogen, and mycorrhizae along a nitrogen deposition gradient

Foliar nitrogen isotope (δ¹⁵N) composition patterns have been linked to soil N, mycorrhizal fractionation, and within-plant fractionations. However, few studies have examined the potential importance of the direct foliar uptake of gaseous reactive N on foliar δ¹⁵N. Using an experimental set-up in which the rate of mycorrhizal infection was reduced using a fungicide, we examined the influence of mycorrhizae on foliar δ¹⁵N in potted red maple (Acer rubrum) seedlings along a regional N deposition gradient in New York State. Mycorrhizal associations altered foliar δ¹⁵N values in red maple seedlings from 0.06 to 0.74 ‰ across sites. At the same sites, we explored the predictive roles of direct foliar N uptake, soil δ¹⁵N, and mycorrhizae on foliar δ¹⁵N in adult stands of A. rubrum, American beech (Fagus grandifolia), black birch (Betula lenta), and red oak (Quercus rubra). Multiple regression analysis indicated that ambient atmospheric nitrogen dioxide (NO₂) concentration explained 0, 69, 23, and 45 % of the variation in foliar δ¹⁵N in American beech, red maple, red oak, and black birch, respectively, after accounting for the influence of soil δ¹⁵N. There was no correlation between foliar δ¹³C and foliar %N with increasing atmospheric NO₂ concentration in most species. Our findings suggest that total canopy uptake, and likely direct foliar N uptake, of pollution-derived atmospheric N deposition may significantly impact foliar δ¹⁵N in several dominant species occurring in temperate forest ecosystems.     

Comparative ecophysiology of CAM and C3 bromeliads. IV. Plant water relations

Abstract An investigation was carried out into the water relations of CAM and C₃ bromeliads in their natural habitat during the dry season in Trinidad. Measurements were made of xylem tension with the pressure chamber and of cell-sap osmotic pressure and titratable acidity on crushed leaf samples. A steady-state CO₂ and H₂O-vapour porometer was also used so that changes in leaf water relations during individual day-night cycles could be directly related to gas-exchange patterns in situ. Xylem tension changed in parallel with transpiration rate and in general reached its maximum value in CAM bromeliads at night and in C₃ bromeliads during the day. In addition, large nocturnal increases in cell-sap osmotic pressure and titratable acidity (ΔH⁺) typically occurred in the CAM bromeliads. The C₃-CAM intermediate Guzmania monostachia showed slight nocturnal acidification, but had higher values of xylem tension during the day. Very high values of AH+ were observed in the CAM species when the tanks of the epiphytic bromeliads contained water: Aechmea nudicaulis showed a mean maximum ΔH⁺ of 474 mol m^?3, the highest value so far observed for CAM plants. On some nights dew formed on the leaf surfaces of the epiphytes, partially curtailing gas exchange and leading to a marked decrease in xylem tension in both C₃ and CAM species. Between-site comparisons were also made for a wide range of habitats from arid coastal scrub to montane rain forest. Compared with values characteristic of other life-forms, xylem tension and cell-sap osmotic pressure were low for all bromeliads, and did not differ significantly in co-occurring CAM and C₃ bromeliads. Mean maximum xylem tension (10 species in total) ranged from 0.29 M Pa at the montane sites to 0.67 MPa at the most arid site, and mean minimum osmotic pressure (17 species) from 0.51 to 0.97 MPa. At the arid sites the bromeliads were exclusively CAM species, two of which (Aechmea aquilega and Bromelia plumieri) grew terrestrially in the undergrowth of the coastal scrub. Xylem tension in these species was low enough to indicate that they must be functionally independent of the substratum during the dry season. In the wetter part of Trinidad, no between-site differences in leaf water relations were found along an altitudinal gradient in the Northern Mountain Range; seasonal differences in this area were also small. Overall, leaf water relations and gas exchange in the bromeliads were strongly affected both by short-term changes in water availability and by longer-term climatic differences in the various regions of the island.     

Altitudinal changes in the incidence of crassulacean acid metabolism in vascular epiphytes and related life forms in Papua New Guinea

Summary The occurrence of Crassulacean acid metabolism (CAM), as judged from ¹³C values, was investigated in epiphytes and some related plant species at a series of sites covering the approximate altitudinal range of epiphytes in Papua New Guinea. Comprehensive collections were made at each site and the occurrence of water storage tissue and blade thickness was also determined. Some 26% of epiphytic orchids from a lowland rainforest (2–300 m.a.s.l) showed ¹³C values typical of obligate CAM and possessed leaves thicker than 1 mm. A second group of orchids, mostly with succulent leaves, possessed intermediate ¹³C values between -23 and -26% and accounted for 25% of the total species number. Some species of this group may exhibit weak CAM or be facultative CAM plants. The remainder of the lowland rainforest species appeared to be C₃ plants with ¹³C values between -28 and -35%. and generally possessed thin leaves. Obligate CAM species of orchids from a lower montane rainforest (1175 m.a.s.l) comprised 26% of the species total and mostly possessed thick leaves. The remainder of the species were generally thin-leaved with ¹³C values between -26 and -35%. largely indicative of C₃ photosynthesis. Orchids with intermediate ¹³C values were not found in the lower montane rainforest. Obligate CAM appeared to be lacking in highland epiphytes from an upper montane rainforest and subalpine rainforest (2600–3600 m.a.s.l). However the fern, Microsorium cromwellii had a ¹³C value of -21.28%. suggesting some measure of CAM activity. Other highland ferns and orchids showed more negative °¹³C values, up to-33%., typical of C₃ photosynthesis. The highland epiphytic orchids possessed a greater mean leaf thickness than their lowland C₃ counterparts due to the frequent occurrence of water storage tissue located on the adaxial side of the leaf. It is suggested that low daytime temperatures in the highland microhabitats is a major factor in explaining the absence of CAM. The increased frequency of water storage tissue in highland epiphytes may be an adaptation to periodic water stress events in the dry season and/or an adaptation to increased levels of UV light in the tropicalpine environment.     

Comparative ecophysiology of CAM and C3 bromeliads. III. Environmental influences on CO2 assimilation and transpiration

Abstract Field measurements of the gas exchange of epiphytic bromeliads were made during the dry season in Trinidad in order to compare carbon assimilation with water use in CAM and C₃ photosynthesis. The expression of CAM was found to be directly influenced by habitat and microclimate. The timing of nocturnal CO₂ uptake was restricted to the end of the dark period in plants found at drier habitats, and stomatal conductance in two CAM species was found to respond directly to humidity or temperature. Total night-time CO₂ uptake, when compared with malic-acid formation (measured as the dawn-dusk difference in acidity, ΔH⁺), could only account for 10–40% of the total ΔH⁺ accumulated. The remaining malic acid must have been derived from the refixation of respired CO₂ (recycling). Within the genus Aechmea (12 samples from four species), recycling was significantly correlated with night temperature at the six sample sites. Recycling was lowest in A. fendleri (54% of ΔH⁺ derived from respired CO₂), a CAM bromeliad with little water-storage parenchyma that is restricted to wetter, cooler regions of Trinidad. Gas-exchange rates of C₃ bromeliads were found to be similar to those of the CAM bromeliads, with CO₂ uptake from 1 to 3 μmol m^?2 s^?1 and stomatal conductances generally up to 100 mmol m^?2 s^?1. The midday depression of photosynthesis occurred in exposed habitats, although photosynthetically active radiation (PAR) limited photosynthesis in shaded habitats. CO₂ uptake of the C₃ bromeliad Guzmania lingulata was saturated at around 500 μmol m^?2 s^?1 PAR, suggesting that epiphytic plants found in the shaded forest understorey are shade-tolerant rather than shade-demanding. Transpiration ratios (TR) during CO₂ fixation in CAM (Phase I and IV) and C₃ bromeliads were compared at different sites in order to assess the efficiency of water utilization. For the epiphytes displaying marked uptake of CO₂, TR were found to be lower than many previously published values. In addition, the average TR values were very similar for dark CO₂ uptake in CAM (42 ± 41, n= 12), Phase IV of CAM (69 ± 36, n= 3) and for C₃ photosynthesis (99 ± 73, n= 4) in these plants. It appears that recycling of respired CO₂ by CAM bromeliads and efficient use of water in all phases of CO₂ uptake are physiological adaptations of bromeliads to arid microclimates in the humid tropics.     

Are vascular epiphytes nitrogen or phosphorus limited? A study of plant (15) N fractionation and foliar N : P stoichiometry with the tank bromeliad Vriesea sanguinolenta

Although there is unambiguous evidence for vascular epiphytic plants to be limited by insufficient water and nutrient supply under natural conditions, it is an open debate whether they are primarily phosphorus (P) or nitrogen (N) limited. Plant (15) N fractionation and foliar N : P stoichiometry of a tank epiphyte (Vriesea sanguinolenta), and its response to combined N-P fertilization, were studied under semi-natural conditions over 334 d to clarify the type of nutrient limitation. Plants collected in the field and experimental plants with limited nutrient supply showed significant plant (15) N fractionation (mean 5‰) and plant N : P ratios of c. 13.5. Higher relative growth rates and declines in plant (15) N fractionation (0.5‰) and in foliar N : P ratios to 8.5 in the high N-P treatment indicated that these epiphytes were P limited in situ. The critical foliar N : P ratio was 10.4, as derived from the breakpoint in the relationship between plant (15) N fractionation and foliar N : P. We interpret the widespread (15) N depletion of vascular epiphytes relative to their host trees as deriving from (15) N fractionation of epiphytes as a result of P limitation. High foliar N : P ratios (> 12) corroborate widespread P limitation (or co-limitation by N and P) of epiphytic bromeliads and, possibly, other epiphyte species.     

Caribbean octocorals record changing carbon and nitrogen sources from 1862 to 2005

During the last century, the global biogeochemical cycles of carbon (C) and nitrogen (N) have been drastically altered by human activities. A century of land‐clearing and biomass burning, followed by fossil fuel combustion have increased the concentration of atmospheric CO₂ by approximately 20%, and since the mid‐1900s, the use of agricultural fertilizers has been the primary driver of an approximate 90% increase in bioavailable N. Geochemical records obtained through stable isotope analysis of terrestrial and marine biota effectively illustrate rising anthropogenic C inputs. However, there are fewer records of anthropogenic N, despite the enormous magnitude of change and the known negative effects of N on ecosystem health. We used stable isotope values from independent octocorals (gorgonians) sampled across the Western Atlantic over the last 143 years to document human perturbations of the marine C and N pools. Here, we demonstrate that in sea plumes δ¹³C values and in both sea plumes and sea fans δ¹⁵N values declined significantly from 1862 to 2005. Sea plume δ ¹³C values were negatively correlated with increasing atmospheric CO₂ concentrations and corroborate known rates of change resulting from global fossil fuel combustion, known as the Suess effect. We suggest that widespread input of agricultural fertilizers to near‐shore coastal waters is the dominant driver for the decreasing δ ¹⁵N trend, though multiple anthropogenic sources are likely affecting this trend. Given the interest in using δ ¹⁵N as an indicator for N pollution in aquatic systems, we highlight the risk of underestimating contributions of pollutants as a result of source mixing as demonstrated by a simple isotope‐mixing model. We conclude that signals of major human‐induced perturbations of the C and N pools are detectable in specimens collected over wide geographic scales, and that archived materials are invaluable for establishing baselines against which we can assess environmental change.     

Nitrogen-15 signals of leaf-litter-soil continuum as a possible indicator of ecosystem nitrogen saturation by forest succession and N loads

Understanding forest carbon cycling responses to atmospheric N deposition is critical to evaluating ecosystem N dynamics. The natural abundance of ¹⁵N (??¹⁵N) has been suggested as an efficient and non-invasive tool to monitor N pools and fluxes. In this study, three successional forests in southern China were treated with four levels of N addition. In each treatment, we measured rates of soil N mineralization, nitrification, N₂O emission and inorganic N leaching as well as N concentration and ?? ¹⁵N of leaves, litters and soils. We found that foliar N concentration and ??¹⁵N were higher in the mature broadleaf forest than in the successional pine or mixed forests. Three-year continuous N addition did not change foliar N concentration, but significantly increased foliar ?? ¹⁵N (p < 0.05). Also, N addition decreased the ?? ¹⁵N of top soil in the N-poor pine and mixed forests and significantly increased that of organic and mineral soils in N-rich broadleaf forests (p < 0.05). In addition, the soil N₂O emission flux and inorganic N leaching rate increased with increasing N addition and were positively correlated with the ¹⁵N enrichment factor (?? _p/s) of forest ecosystems. Our study indicates that ?? ¹⁵N of leaf, litter and soil integrates various information on plant species, forest stand age, exogenous N input and soil N transformation and loss, which can be used to monitor N availability and N dynamics in forest ecosystems caused by increasing N deposition in the future.     

Discrimination of nitrogen isotopes during absorption of ammonium and nitrate at different nitrogen concentrations by rice (Oryza sativa L.) plants

Studies of uptake of ionic sources of N by two hydroponically grown rice (Oryza sativa L.) cultivars (paddy‐field‐adapted Koshihikari and dryland‐adapted Kanto 168) showed that the magnitude of the nitrogen isotope fractionation (?) for uptake of NH₄⁺ depended on the concentrations of NH₄⁺ and cultivar (averaging –6·1‰ for Koshihikari and –12·0‰ for Kanto 168 at concentrations from 40 to 200 mmol m^?3 and, respectively, –13·4 and –28·9‰ for the two cultivars at concentrations from 0·5 to 4 mol m^?3). In contrast, the ? for uptake of NO₃^? in similar experiments was almost insensitive to the N concentration, falling within a much narrower range (+3·2‰ to –0·9‰ for Koshihikari and –0·9‰ to –5·1‰ for Kanto 168 over NO₃^? concentrations from 0·04 to 2 mol m^?3). From longer term experiments in which Norin 8 and its nitrate‐reductase deficient mutant M819 were grown with 2 or 8 mol m^?3 NO₃^? for 30 d, it was concluded that the small concentration‐independent isotopic fractionation during absorption of this ion was not related to nitrate reductase activity.