High Nutrient Environments Research Articles

DEMOGRAPHIC PERFORMANCE PREDICTS INVASIVENESS OF SPECIES IN THE COMMELINACEAE UNDER HIGH-NUTRIENT CONDITIONS

Demographic models are powerful tools for making predictions about the relative importance of transitions from one life stage (e.g., seeds) to another (e.g., nonreproductives); however, they have never been used to compare the relative performance of invasive and noninvasive taxa. I use demographic models parameterized from common garden experiments to develop hypotheses about the role of different life stage transitions in determining differences in performance in invasive and noninvasive congeners in the Commelinaceae. I also extended nested life table response experiment (LTRE) analyses to accommodate interactions between nested and unnested factors. Invasive species outperformed their noninvasive congeners, especially under high-nutrient conditions. This difference in performance did not appear to be due to differences in elasticities of vital rates, but rather to differences in the magnitude of stage transitions. Self-compatible invasive species had greater fecundity in high-nutrient environments and a shorter time to first reproduction, and all invasive species had greater vegetative reproduction than their noninvasive congeners. Thus greater opportunism in sexual and asexual reproduction explained the greater performance of invasive species under high-nutrient conditions. Similar common garden experiments could become a useful tool to predict potential invaders from pools of potential introductions. I show that short-term and controlled experiments considering multiple nutrient environments may accurately predict invasiveness of nonnative plant species.

Abiotic constraints on the competitive ability of exotic and native grasses in a Pacific Northwest prairie

In prairie ecosystems, abiotic constraints on competition can structure plant communities; however, the extent to which competition between native and exotic plant species is constrained by environmental factors is still debated. The objective of our study was to use paired field and greenhouse experiments to evaluate the competitive dynamics between two native (Danthonia californica and Deschampsia cespitosa) and two exotic (Schedonorus arundinaceus and Lolium multiflorum) grass species under varying nutrient and moisture conditions in an upland prairie in the Willamette Valley, Oregon. We hypothesized the two invasive, exotic grasses would be more competitive under high-nutrient, moderate-moisture conditions, resulting in the displacement of native grasses from these environments. In the field, the experimental reduction of competition resulted in shorter, wider plants, but only the annual grass, Lolium multiflorum, produced more aboveground biomass when competition was reduced. In the greenhouse, the two exotic grasses produced more total biomass than the two native grasses. Competitive hierarchies were influenced by nutrient and/or moisture treatments for the two exotic grasses, but not for the two native grasses. L. multiflorum dominated competitive interactions with all other grasses across treatments. In general, S. arundinaceus dominated when in competition with native grasses, and D. cespitosa produced the most biomass in monoculture or under interspecific competition with the other native grass, D. californica. D. californica, D. cespitosa, and S. arundinaceus all produced more biomass in high-moisture, high-nutrient environments, and D. cespitosa, L. multiflorum, and S. arundinaceus allocated more biomass belowground in the low nutrient treatment. Taken together, these experiments suggest the competitive superiority of the exotic grasses, especially L. multiflorum, but, contrary to our hypothesis, the native grasses were not preferentially excluded from nutrient-rich, moderately wet environments.

Matrix Based Fertilizers Reduce Nitrogen and Phosphorus Leaching in Greenhouse Column Studies

We tested the efficacy of matrix based fertilizer formulations (MBF) that reduce NH4, total phosphorus (TP), total reactive phosphorus (TRP) and dissolved reactive phosphorus (DRP) in leachate. The MBF formulations cover a range of inorganic N and P in compounds that are relatively loosely bound (MBF1) to more moderately bound (MBF2) and more tightly bound compounds (MBF3) mixed with Al(SO4)3 H2O and/or Fe2(SO4)3 and with the high ionic exchange compounds starch, chitosan and lignin. Glomus interadicies, a species of arbuscular mycorrhizal fungal spores that will form mycorrhizae in high nutrient environments, was added to the MBF formulations to increase plant nutrient uptake. When N and P are released from the inorganic chemicals containing N and P the matrix based fertilizers likely bind these nutrients to the Al(SO4)3 H2O and/or Fe2(SO4)3 starch–chitosan–lignin matrix. We tested the efficacy of the MBFs to reduce N and P leaching compared to Osmocote® 14-14-14, a slow release fertilizer (SRF) in sand filled columns in a greenhouse study. SRF with and without Al and Fe leached 78–84% more NH4, 58–78% more TP, 20–30% more TRP and 61–77% more than MBF formulations 1, 2, and 3 in a total of 2.0 liters of leachate after 71 days. The concentration and amount of NO3 leached among SRF and MBF formulations 1 and 2 did not differ. The SRF treatment leached 34% less NO3, than MBF3. Total plant weight did not differ among fertilizer treatments. Arbuscular mycorrhizal infection did not differ among plants receiving SRF and MBF formulations 1, 2 and 3. Although further greenhouse and field testing are called for, results of this initial investigation warrant further investigation of MBFs.

Nitrogen fixation by the savanna tree Philenoptera violacea (Klotzsch) Schrire (Apple leaf) of different ages in a semi-arid riparian landscape

The acquisition of nitrogen for growth and maintenance is essential for plants, and having multiple strategies for that acquisition is especially important for those colonizing nutrient poor substrates. Philenoptera violacea (Apple leaf) is a prominent tree in nutrient poor savanna and alluvial soils near rivers in southern Africa, where nutrient availabilities are highly variable in space and time. We investigated nitrogen fixation in P. violacea within riparian corridors flanking the Sabie River in Kruger National Park (KNP) in the Lowveld in northeastern South Africa using the natural 15N abundance technique. Results indicated that P. violacea fixes atmospheric nitrogen and this varies with life history stage. We found that foliar δ 15N levels were significantly lower in all life stage classes of P. violacea compared with the reference plant D. mespiliformis growing in open riparian forest. In addition δ 15N values were significantly different within the different life stages of P. violacea with the leaves of saplings and juvenile plants having significantly lower δ 15N levels than mature plants. While δ 15N values increased with age, foliar nitrogen concentration values declined, with leaves from sapling P. violacea having significantly higher total nitrogen than adults and juveniles, which were in turn significantly higher than juvenile D. mespiliformis. However, foliar δ 15N levels in seedlings of P. violacea growing in a high nutrient environment in flood debris piles did not differ from levels recorded in seedlings of the reference tree. This study confirms that P. violacea is able to fix nitrogen, but it is dependant on soil conditions and the life stage of the trees.

Early Cretaceous (Valanginian ‐ Hauterivian) calcareous nannofossils and isotopes of the northern hemisphere: proxies for the understanding of Cretaceous climate

From three boreholes (DSDP Site 535; ODP Site 638; BGS borehole 81/43) of the Central Atlantic and the North Sea Basin 379 samples of early Cretaceous age (Valanginian-Hauterivian) were examined. The localities cover a S-N transect of approximately 3000 km stretching from 17°N to 40°N palaeolatitude. The distribution of calcareous nannofossils and fluctuations of the stable isotopes (δ13C, δ18O) have been recorded and were compared with results of recent studies. We differentiate between high nutrient indicators and oligotrophic taxa and propose a four step scheme to characterize the trophic level of the surface water. (1) High abundances of the fertility group (Biscutum constans/Zeugrhabdotus spp.) combined with a high dominance of B. constans and low abundances of Watznaueria barnesae/W. fossacincta represent a high nutrient environment (eutrophic setting). (2) High abundances of the fertility group combined with a high dominance of Zeugrhabdotus spp. and low abundances of W. barnesae/W. fossacincta reflect enhanced nutrient contents of the surface water (mesotrophic setting). (3) Enhanced abundances of the fertility group combined with high abundances of W. barnesae/W. fossacincta indicate slightly increased nutrient contents of the surface water (meso- to oligotrophic setting). (4) Low abundances of the fertility group and high abundances of W. barnesae/W. fossacincta are of low nutrient affinities (oligotrophic setting). Our estimations of seawater palaeotemperatures in combination with literature data show a distinctive trend for the Valanginian to Hauterivian interval. A general decrease of water temperature from the Valanginian to the early Hauterivian is obvious. This decrease of temperature coincides with the southward migration of the high latitudinal cold water species Crucibiscutum salebrosum to lower latitudes. Our findings shed new light on the evolution of the earliest Cretaceous climate, which may be characterized as a warm greenhouse world with interludes of short cooling.

Athyrium distentifolium Tausch ex Opiz (A. alpestre (Hoppe) Rylands ex T. Moore‐non‐Clairv.) including A. distentifolium var. flexile (Newman) Jermy

Woodsiaceae. Athyrium distentifolium, Alpine lady-fern. Fronds bi-pinnate, ovate-lanceolate, yellow-green 20 cm to over 100 cm, arising in a 'shuttlecock' from a central crown. Stipe straight, one fifth to one quarter the length of the blade with pale brown scales, usually broad, but occasionally narrow. Pinnae widely spaced near base of the frond, and sometimes deflexed, crowded near the tip, meeting the rachis at an angle of 900 at the midpoint of the blade, or slightly ascending. Pinnules broadest at the base, tapering to a point. Sori circular, with irregular filaments making up a rudimentary indusium when young, soon obscured as the sori grow. Sori concentrated in the upper part of the frond, becoming sparse towards the base. Number of sori varies with size of frond. Plants frequently infertile (McHaffie et al. 2002). Athyrium distentifolium var.flexile Newman's lady-fern. Fronds bi-pinnate, narrow, blue-green, 10-40 cm, arising from a central crown, erect or sharply angled near the base of the rachis to lie nearly flat against the substrate. Stipe short, one sixth to one eighth the length of the blade, often densely covered with broad, pale scales that may continue beyond the midpoint of the blade. Blade broadest near the base, or nearer the mid-point. Pinnae close together near base of frond, often strongly deflexed to at least half way up the frond. Uppermost pinnae widely spaced. Pinnules taper towards the base. Sori circular, sometimes with only a few sporangia; rudimentary indusium visible while the sorus is still immature. Sori concentrated at base of the frond, becoming less frequent towards the tip. Plants usually fertile (McHaffie et al. 2002). Athyrium distentifolium is a chionophilous fern that grows at altitudes or latitudes which ensure long-lying winter snow cover. Deep snow provides insulation from severe winters, and prevents early growth in a mild spring. Only a limited number of species can tolerate these conditions and this reduces competition. Although previously known as a Scandinavian and continental European species, A. distentifolium was not recognized in Britain until 1844 (Newman 1851). Within Britain it is found only in Scotland where it is confined to the remote Highlands. Soon after it was identified, another similar taxon of uncertain taxonomic status was described (Backhouse 1852). Subsequent research has determined that this is a recessive variety which is now known as A. distentifolium var.flexile. Both taxa are covered in this account.

Long- and short-term induction of defences in seedlings of Shorea leprosula (Dipterocarpaceae): support for the carbon:nutrient balance hypothesis

The induction of carbon-based secondary metabolites in leaves following damage has been proposed to be a result of a shift in the carbon:nutrient balance, when growth is limited by nutrients in relation to carbon. Here we test this hypothesis using seedlings of a tropical tree, Shorea leprosula (Dipterocarpaceae). In the short term, we analysed the phenolic content of leaves 7 d after damage on seedlings grown under differing light and nutrient treatments. In the long term, we examined the effect of nutrients, over 12 mo, on leaf phenolic concentration and seedling growth. In both the long and short term, levels of phenolics increased in damaged leaves under low nutrient treatments. No changes in leaf phenolics were detected under high nutrient regimes, or in the short term under low light. In addition, it was found that defoliation of seedlings in high-nutrient environments led to greater rates of leaf production than in undamaged seedlings, suggesting compensation.

Stoichiometrically explicit competition between grazers: species replacement, coexistence, and priority effects along resource supply gradients.

Assuming key trade-offs among interactors, several models (resource ratio, keystone predation, intraguild predation) predict changes in species composition over resource supply gradients. Ecological stoichiometry could also predict compositional shifts of grazers over gradients of nutrient and light supply through a mechanism involving (mis)matches between elemental body composition of grazers and plants. This hypothesis is explored here using a suite of two-grazer, one-plant models that incorporate three key components: plant production depends on light and nutrients, nutrient content of plants can vary, and homeostatic grazers can be carbon or nutrient limited. The results from this suite closely resemble the classical resource ratio model describing plant competition for two resources. Here, the models predict shifts of grazer composition along resource supply gradients if species trade off competitive abilities for plant carbon and nutrients. Given this trade-off, superior nutrient competitors should dominate low nutrient environments, and superior carbon competitors should dominate high nutrient environments. At intermediate nutrient supply, species can coexist at a stable equilibrium, or alternative stable states emerge, depending on how grazers impact their resources. These results depend on food web architecture, however. For instance, predators can alter or reduce possibilities for stoichiometry-mediated coexistence of grazers.

Dynamics in bacterial cell surface properties assessed by fluorescent stains and confocal laser scanning microscopy

We investigated the dynamics of hydrophobic and hydrophilic properties of the bacterioplankton cell surface in different life stages, as they may play a crucial role in the uptake of nutrients and in regulating the grazing pressure of potential predators. Bacterial strains were grown in artificial seawater under controlled nutrient conditions and the dynamics of surface properties were assessed by staining living bacteria concurrently with a hydrophobic, a polar and a nucleic acid stain. Mean fluorescence intensity of the individual stain for each single cell was determined using a confocal laser scanning microscope and advanced image analysis. No proportional changes of the hydrophobic or hydrophilic properties with cell size were detectable, indicating their independence of cell size. While hydrophobic properties remained fairly constant and exhibited species-specificity, the polarity of the surface was more dependent on the life stage of free-living cells. Bacterial cells assembling in aggregates, however, exhibited a small but distinct elevated hydrophobicity compared to free-living cells. Based on their hydrophobicity, 2 distinctly different groups of bacteria could be distinguished. One group of bacterial strains exhibited a continuous increase in the ratio of hydrophilicity to hydrophobicity, whereas the less hydrophobic group exhibited 2 peaks in the ratio hydrophilicity to hydrophobicity. This ratio was significantly lower for the less hydrophobic group, indicating that less hydrophobic bacteria are also less hydrophilic. Increased growth rates were found with higher cell surface hydrophobicity. This might be interpreted as an expression of 2 different life strategies in bacterioplankton. Increased hydrophobicity and concomitant increase of polar moieties facilitate nutrient assimilation and seem to be advantageous for bacteria adapted to high nutrient environments maximizing growth at higher grazing losses. In contrast, bacteria with lower hydrophobicity exhibit a lower assimilation efficiency with reduced grazing pressure.

Responses to nitrogen, phosphorus, potassium and sodium chloride by three mangrove species in pot culture

An understanding of nutritional requirements is important for both management and restoration of mangrove areas. In this study, the effects of nutrients on three dominant Australian mangrove species (Avicennia marina, Ceriops tagal and Rhizophora stylosa) were examined in a sand culture experiment. A factorial design with three levels each of N, P and K, and two levels of NaCl, was used. Nutrient levels were selected to reflect low, medium and high concentrations found in mangrove habitats. Branching pattern, leaf expansion rate and stem and leaf growth were monitored. Medium (0.714 mM) and high (2.14 mM) N levels markedly increased leaf number in all species, and height and branching in A. marina. Interaction effects, particularly between N, P and K, were apparent. Leaf expansion was also affected by N, P and K and their interaction with salinity; however, the pattern of response differed between species. The response of these species to added nutrients reflects their ecological niches in the field, and the likely effects of high-nutrient environments. This information can assist in management decisions such as the selection of species to use in wastewater treatment, or the use of certain fertilizers to enhance growth during revegetation.

Photosynthetic nutrient-use efficiency in three fast-growing tropical trees with differing leaf longevities.

Differences in nutrient-use efficiency have been attributed to differences in leaf habit. It has been suggested that evergreens, with their longer-lived leaves, and therefore longer nutrient retention, are more efficient than deciduous species in their use of nutrients. In tropical trees, however, leaf life span is not always a function of whole-tree deciduousness, leading to the proposal that nutrient-use efficiency is better related to leaf life span than to leaf habit. It was predicted that potential photosynthetic nutrient-use efficiency (maximum potential photosynthesis/leaf nutrient content) would decrease with increasing leaf life span, whereas cumulative photosynthetic nutrient-use efficiency (carbon assimilated over a leaf's life span/total nutrients invested in a leaf) would increase with increasing leaf life span. Potential and cumulative photosynthetic nutrient-use efficiencies (with respect to nitrogen and phosphorus) were measured for three fast-growing tropical trees: Cedrela odorata L. (Meliaceae), Cordia alliodora (R. & P.) Cham. (Boraginaceae), and Hyeronima alchorneoides Allemão (Euphorbiaceae). Mean leaf life spans of the three species varied about threefold and ranged from 50 to 176 days. The predictions were partially supported: Cedrela odorata had the shortest-lived leaves and the highest potential nitrogen-use efficiency, whereas Hyeronima alchorneoides had the longest-lived leaves and the highest cumulative nitrogen- and phosphorus-use efficiencies. Potential phosphorus-use efficiency, however, was invariant among species. It is suggested that there are potential tradeoffs between leaf characteristics that lead to high potential and cumulative nutrient-use efficiencies. High potential nutrient-use efficiency may be beneficial in high-nutrient environments, whereas high cumulative nutrient-use efficiency may be of greater benefit to species in low-nutrient environments.

Inbreeding depression in smooth cordgrass (Spartina alterniflora, Poaceae) invading San Francisco Bay

The magnitude of inbreeding depression in invading plant populations is often presumed to be small and of little consequence. The purpose of this study was to assess the magnitude of inbreeding depression in a pollen-limited, partially self-incompatible, invading plant population. The magnitude and timing of inbreeding depression were compared among ten maternal plants sampled from a population of smooth cordgrass (Spartina alterniflora) invading San Francisco Bay. Selfed and outcrossed progeny were compared for embryo abortion, survival of seedlings, and growth/survival at the end of the first growing season in three greenhouse environments. Estimates of inbreeding depression varied among environments, with competitive environment > high-nutrient environment > low-nutrient environment. Population-level estimates of inbreeding depression ranged from 0.61 to 0.81; however, maternal plants varied significantly in their magnitude of inbreeding depression, ranging from 0.1 to 0.97. The 95% confidence interval for inbreeding depression for some maternal plants included zero. There was a significant negative correlation between the overall magnitude of inbreeding depression and self-fertility rate among maternal plants. The few maternal plants with high self-fertility carried relatively little genetic load, and their selfed progeny are likely to survive on open mudflats. The noncompetitive, pollen-limited growing conditions associated with invasion may allow self-fertility to spread in this population.

Effects of Maternal Nutrient Environment and Maturation Position on Seed Heteromorphism, Germination, and Seedling Growth in Triplasis purpurea (Poaceae)

The annual dunegrass Triplasis purpurea exhibits a position-dependent seed heteromorphism. From the lower to the upper nodes along a tiller of an individual there is an increase in the number of seeds set on leaf sheath-enclosed axillary panicles and a decrease in seed mass. Between individuals, seed number and mass are most affected by microenvironmental conditions. The interactive effects of maturation position and maternal nutrient environment on seed heteromorphism, germination, and seedling growth were investigated. Field-collected seedlings from two populations on Staten Island, N.Y., were grown in the greenhouse and subjected to three maternal environments: (1) high nutrients (3 g/L fertilizer every 2 wk), (2) medium nutrients (1.5 g/L every 2 wk), and (3) low nutrients (tap water). The seed number at each tiller node was recorded and mature tillers with sheath-enclosed, intact seeds were separated into fragments by node and subjected to a germination trial. For one population, seeds were also removed from leaf sheaths. The effects of node position and maternal environment on seedling growth were also studied. High soil nutrients resulted in significant enhancement of germination. For one population, germination increased with increasing node position. Seeds from emergent, terminal panicles showed the greatest germination. Removal of the leaf sheath increased the speed of germination. Root and shoot dry mass was greatest for seedlings from seeds of maternal parents in the high-nutrient environment. Shoot dry mass was also greater for seedlings from heavier, lower-node seeds (relative to lighter, upper-node seeds). In the heterogeneous dune habitat, fitness for this annual species, expressed as seed mass and seed production by an individual, may be influenced by soil that is relatively higher in nutrients in a way that increases the likelihood of successful seedling establishment the following year.

Patterns of Leaf Tannin Variation in Chestnut Oak (Quercus prinus) and Black Oak (Quercus velutina) with Respect to Topography in a Southeastern Ohio Oak-Hickory Forest

in low light and high nutrient environments. To test this hypothesis, we evaluated the patterns of leaf tannin chemistry in two species of oak (Quercus velutina L. and Q. prinus L.) utilizing protein precipitation methods. Leaves were sampled from mature, forest-grown, canopy trees found in a southeastern Ohio oak-hickory forest. To evaluate the effects of environment, we sampled along a natural gradient using trees from contrasting northand south-facing slopes. To assess the patterns of variation in tannin abundance among populations, we sampled multiple trees from three different sites. Thus, data were analyzed for patterns of variation with respect to slope aspect and population. Quercus prinus trees growing on south-facing slopes had significantly greater amounts of leaf tannins than those on north-facing slopes but did not exhibit significant population differences in foliar tannin content. In contrast, Q. velutina trees on north- and south-facing slopes did not differ significantly in foliar tannin content but did exhibit significant population differences in foliar tannin content. Different species may have varying responses to the environmental stress associated with topography. Environment may have a considerable effect on leaf tannin content at the landscape scale. However, in some cases, populations may also contribute to variation and need to be considered when evaluating patterns of secondary plant metabolite distribution and/or plant-animal interactions.

Genetic relationships between growth characters in Salix viminalis grown in Sweden

From an eight by eight factorial crossing with Salix viminalis, 40 of the 64 families obtained were selected for further analysis. Fourteen seedplants from each of these 40 families were planted in two pairs of contrasting environments: sand and clay soil, and low and high nutrient supply. The material in the soil contrast was harvested after 1, 4 and 6 years of growth. The material in the nutrient contrast was harvested each year for 3 years and analysed after the first and the third harvests. The correlation between number of shoots and weight in the clay environment changed from being negative in the first harvests to positive at the last harvest, compared with the sand environment where this correlation was positive in all years. In the nutrient contrast this correlation was positive at the last harvest in the high nutrient environment, but no correlation could be detected in the low nutrient environment. The differences in correlations between environments may be due to a different allocation of nutrients in the plants, depending on whether the plant is under stress or not. The data suggests that the genetic relationship between growth components is the same over age and environments when the plants are grown without stress.

Plasticity in Root/Shoot Partitioning: Optimal, Ontogenetic, or Both?

1. We tested whether plants increase root :shoot ratios to compensate for limitations of below-ground resources in a manner consistent with optimal partitioning theory or whether the relative production of roots and shoots is controlled by species-specific developmental patterns. Individuals of two annual plant species, Abutilon theophrasti and Chenopodium album, were grown from seed in controlled greenhouse conditions under high- or low-nutrient regimes. Mid-way through the experiment, a sub-set of low-nutrient-grown plants were given high nutrient availability and a sub-set of high-nutrient-grown plants were transferred to a low nutrient environment. 2. Under continuous nutrient regimes : (1) high-nutrient-grown plants of both species grew faster and had a lower root :shoot ratio than low-nutrient-grown plants, consistent with optimal partitioning theory ; (2) both species exhibited a substantial amount of ontogenetic drift as root :shoot ratios decreased through ontogeny (subsequent to an initial increase in R/S shortly after germination) ; (3) allometric analyses revealed that increased allocation to roots occurred very early in ontogeny for both species, after which the relative growth of shoots exceeded that of roots in low-nutrient-grown plants compared to their high nutrient-grown counterparts - a result inconsistent with optimal partitioning theory. 3. Under temporally varying nutrient regimes : (1) growth substantially increased in low-nutrient-grown plants that were switched to a high-nutrient environment without a change in root :shoot partitioning ; (2) there was no change in growth or partitioning when plants were switched from a high- to a low-nutrient regime. 4. We conclude that, for these annual species, root/shoot partitioning is partially consistent with optimal partitioning theory but that is also highly ontogenetically constrained. This constraint is evident both in substantive ontogenetic drift in partitioning and in the period during development that plasticity in partitioning can be expressed.

Growth, decomposition, and nutrient retention of Cladium jamaicense Crantz and Typha domingensis Pers. in the Florida Everglades

Estimates of phosphorus (P) and nitrogen (N) gains and losses during annual macrophyte growth, death and 2 years decomposition were made along a gradient of surface water nutrient concentrations in the Florida Everglades. Annual rates of P and N allocation to growing leaves, translocation or leaching from dying leaves, and retention in dead leaves of Cladium jamaicense Crantz and Typha domingensis Pers. were correlated to soluble reactive P and nitrate concentrations in surface water. Rates of each of these processes were higher in T. domingensis than in C. jamaicense, Cladium jamaicense rates increased linearly along the nutrient gradients, but did not fluctuate with yearly variations in soluble reactive P or nitrate concentrations. For T. domingensis, annual rates were strongly correlated with mean annual soluble reactive P and nitrate concentrations during specific sampling years. Responses of C. jamaicense to the nutrient gradient were characteristic of species competitive in an infertile habitat, while responses of T. domingensis were characteristic of species competitive in a fertile habitat. The main effect of P and N enrichment on leaf nutrient flux was to accelerate translocation or leaching from dying tissue, rather than to increase retention in standing dead leaves. Freshly dead leaves retained only slightly greater quantities of P and N under enriched conditions in comparison to background conditions. After 2 years of decomposition, approximately half of the leaf litter mass remained intact. Increasing P and N concentrations in decomposing leaf litter resulted in net uptake or retention of these elements after 2 years despite decreasing litter mass. The total amounts of P and N that were sequestered annually by T. domingensis after processes of leaf production, mortality and 2 years decomposition were lowest under non-enriched conditions and reached a maximum under a moderate level of enrichment. Wetland ecosystems such as the Everglades, which developed under conditions of low nutrient supply, may offer a finite potential for accelerated nutrient retention when the exogenous nutrient supply increases as a result of human activities. However, a plant species such as C. jamaicense, that is adapted to a low-nutrient environment, may have a low nutrient threshold before it loses its competitive capability and its habitat is invaded by a species such as T. domingensis that is better adapted to a high-nutrient environment.

SEASONAL PRIMARY PRODUCTION OF UNDERSTORY RHODOPHYTA IN AN UPWELLING SYSTEM1

ABSTRACTThe in situ primary production of three common under‐story members of the Rhodophyta in South African west coast kelp beds was determined monthly for a year using dissolved oxygen techniques. Strong seasonal patterns of photosynthesis and respiration were evident in all three species. Net photosynthesis of all three species was greatest in spring (October) and lowest in winter (June). Increasing photosynthesis in late winter coincided with increasing ambient irradiance and photoperiod, whereas decreasing photosynthesis in summer was not explained by changes in the environmental parameters measured. We suggest that this may he due to an innate pattern related to some other seasonal plant activity such as reproduction. Seasonal Pmax and Ik values reveal that the obligate understory species, B. prolifera and E. obtusa, are shade‐adapted whereas G. radula, a low intertidal and shallow subtidal dominant, is sun‐adapted. Low C: X ratios consistent with a high nutrient environment and high rates of productivity were found in all three species. Net photosynthesis to respiration (Pn:R) ratios were fairly constant for B. prolifera and E. obtusa, implying that then photosynthetic processes were governed more by seasonal variations in irradiance than by instantaneous light availability. The Pn: R ratio of G. radula was variable, suggesting that this species is more responsive to rapid fluctuations in irradiance and may therefore be adapted for rapid growth during periods of high irradiance.

Le système assimilation-régénération des sels nutritifs dans les eaux superficielles de l'océan Austral

Low primary-productivity levels in high-nutrient environments of the Southern Ocean are not yet well understood. An accurate knowledge of nutrient assimilation by phytoplankton, at the base of the pelagic food-web, therefore appears to be essential. A study of the mesoscale hydrological structure and chemical observations in the upper layers of the Indian sector of the Antarctic Ocean (Cruise MD-25 FIBEX of M.S. “Marion Dufresne”, January–February 1981) permitted us to develop a method for estimating nutrient consumption by phytoplankton. After strong vertical mixing in winter, the homogeneous Antarctic Surface Water (ASW) is, in summer, divided into two parts by a well-formed pycnocline. In the upper layer, corresponding approximately to the photic zone, we observed nutrient depletion resulting from photosynthetic activity, while the underlaying thermal minimum layer displayed unaltered winter characteristics, including nutrient concentrations typical of winter surface-water conditions. Taking into account the nutrient depletion in summer, we calculated the assimilation ratios for Antarctic phytoplankton as follows: ([NO3-lower]-[NO3-upper]):([PO4≡upper])=ΔN:ΔP=11.2±2.4 and ΔN:ΔSi=0.27±0.05. These ratios are lower than the classical Redfield ratios (ΔN:ΔP=16 and ΔN:ΔSi=1), but are in good agreement with the elementary composition of phytoplankton samples collected during the same cruise. Moreover, they agree with previously published data on stoechiometric determinations of particulate matter in surface-water samples. Besides, in the “transition layer” between the lower layer and “Warm Deep Water” (WDW), the calculated mineralization ratios were slightly higher than the assimilation ratios: ΔN:ΔP=14.6 and ΔN:ΔSi=0.37. From these ratios, it would appear that, in these surface layers of the Southern Ocean (down to 250 m), assimilation-regeneration mechanisms operate in such a way that phosphate and silicate contents decrease much more than nitrate content during the northward drift of the surface waters. The determination of assimilation ratios taking into account the nutrient depletion of the upper layer allowed us to evaluate the carbon net-production which integrates time-space variations in the photic layer over the whole early summer period. We estimated a production rate (0.4 g C m-2 d-1) of the same order of magnitude as the mean value obtained by the 14C method (0.2 g C m-2 d-1). The theoretic silicium consumption by phytoplankton along a meridian transect, calculated using an estimated assimilation ratio of ΔC:ΔSi=1,85, indicates that the marked south-north decrease in silicate concentration in the Antarctic Surface Water would mainly result from biological activity. The silicate concentrations thus calculated were in good agreement with concentrations measured along a previous transect across the Southern Ocean.

The Influence of Mineral Nutrition on the Root Development of Trees

Lodgepole pine seedlings were grown in solution culture with their root systems divided between two contrasting nutrient regimes, viz. a high regime, which provided near optimum conditions for growth, and a low regime in which nutrients were limiting. The high nutrient regime stimulated root growth whereas the roots receiving a low level of nutrients grew comparatively slowly and in certain instances stopped elongating and became brown. After differential root growth had been induced certain plants were transferred to a uniform high nutrient environment and the previously deprived parts of the root system responded by renewed growth and their growth rate increased in comparison with roots which remained in low nutrient conditions. The results show that initial nutritional differences in the environment do not lead to dominance in certain roots; the root system remains remarkably plastic.