Gradient Of Nutrient Availability Research Articles

Nutrient fluxes via litterfall and leaf litter decomposition vary across a gradient of soil nutrient supply in a lowland tropical rain forest

The extent to which plant communities are determined by resource availability is a central theme in ecosystem science, but patterns of small-scale variation in resource availability are poorly known. Studies of carbon (C) and nutrient cycling provide insights into factors limiting tree growth and forest productivity. To investigate rates of tropical forest litter production and decomposition in relation to nutrient availability and topography in the absence of confounding large-scale variation in climate and altitude we quantified nutrient fluxes via litterfall and leaf litter decomposition within three distinct floristic associations of tropical rain forest growing along a soil fertility gradient at the Sepilok Forest Reserve (SFR), Sabah, Malaysia. The quantity and nutrient content of small litter decreased along a gradient of soil nutrient availability from alluvial forest (most fertile) through sandstone forest to heath forest (least fertile). Temporal variation in litterfall was greatest in the sandstone forest, where the amount of litter was correlated negatively with rainfall in the previous month. Mass loss and N and P release were fastest from alluvial forest litter, and slowest from heath forest litter. All litter types decomposed most rapidly in the alluvial forest. Stand-level N and P use efficiencies (ratios of litter dry mass to nutrient content) were greatest for the heath forest followed by the sandstone ridge, sandstone valley and alluvial forests, respectively. We conclude that nutrient supply limits productivity most in the heath forest and least in the alluvial forest. Nutrient supply limited productivity in sandstone forest, especially on ridge and hill top sites where nutrient limitation may be exacerbated by reduced rates of litter decomposition during dry periods. The fluxes of N and P varied significantly between the different floristic communities at SFR and these differences may contribute to small-scale variation in species composition.

EVIDENCE FOR MICRONUTRIENT LIMITATION OF BIOLOGICAL SOIL CRUSTS: IMPORTANCE TO ARID-LANDS RESTORATION

Desertification is a global problem, costly to national economies and human societies. Restoration of biological soil crusts (BSCs) may have an important role to play in the reversal of desertification due to their ability to decrease erosion and enhance soil fertility. To determine if there is evidence that lower fertility may hinder BSC recolonization, we investigated the hypothesis that BSC abundance is driven by soil nutrient concentrations. At a regional scale (north and central Colorado Plateau, USA), moss and lichen cover and richness are correlated with a complex water–nutrient availability gradient and have approximately six-fold higher cover and approximately two-fold higher species richness on sandy soils than on shale-derived soils. At a microscale, mosses and lichens are overrepresented in microhabitats under the north sides of shrub canopies, where water and nutrients are more available. At two spatial scales, and at the individual species and community levels, our data are consistent with t...

DEVELOPMENTAL PATTERNS AND NUTRITION IMPACT RADIATION USE EFFICIENCY COMPONENTS IN SOUTHERN PINE STANDS

A number of contemporary forest productivity models use some variation of a growth efficiency (ε) approach. Typically, these models predict production (aboveground net primary production, ANPP, in units of dry mass biomass per unit of area per unit of time) as the product of two terms: radiation use efficiency (ε, in units of dry mass biomass per megajoule of photosynthetically active radiation [PAR] intercepted or absorbed by the plant canopy) and the sum of PAR intercepted or absorbed by the canopy (Φpar, in units of megajoules per unit of radiation area per unit of time). Predicting productivity in a biologically realistic manner requires an understanding of how model components are affected by natural and anthropogenic environmental factors, as well as other influences such as aging or stand development. We measured or calculated all components of the ε model (aboveground woody biomass increment, IWB; foliage biomass increment, IFB; aboveground net primary production, ANPP; leaf area index, LAI; Φpar; and aboveground radiation use efficiency, εA) from ages 4 to 18 yr in loblolly and slash pine stands in north-central Florida grown under replicated fertilizer and understory vegetation control treatments that induced a large gradient in soil nutrient availability. Treatments impacted all measured components, which in turn led to strong responses in aboveground radiation use efficiency. Age 6–9 yr εA, averaged across species, ranged from 0.78 g/MJ to 0.83 g/MJ in treatments receiving fertilizer or vegetation control vs. 0.53 g/MJ in untreated plots. Stand developmental processes modified these responses, however, with εA declining by over 40% in treated plots from age 6–9 yr to age 15–16 yr. Variation in εA in both species was linked to development of stand basal area (BA), with a positive, linear relationship between εA and BA for values of BA < 18 m2/ha and a declining, linear relationship for values of BA > 18 m2/ha. Loblolly pine εA was positively correlated with foliar nitrogen concentration, [N]. These data highlight the dynamic nature of εA and suggest that both stand developmental and nutritional processes drive changes in ANPP and εA in southern pines.

A comparison of invasive and non‐invasive dayflowers (Commelinaceae) across experimental nutrient and water gradients

ABSTRACTLittle is known about the traits and mechanisms that determine whether or not a species will be invasive. Invasive species are those that establish and spread after being introduced to a novel habitat. A number of previous studies have attempted to correlate specific plant traits with invasiveness. However, many such studies may be flawed because they fail to account for shared evolutionary history or fail to measure performance directly. It is also clear that performance is context dependent. Thus, an approach that corrects for relatedness and incorporates multiple experimental conditions will provide additional information on performance traits of invasive species.I use this approach with two or three pairs of invasive and closely related non‐invasive species of Commelinaceae grown over experimental gradients of nutrient and water availability. Invasive species have been introduced, established, and spread outside their native range; non‐invasive species have been introduced, possibly (but not necessarily) established, but are not known to have spread outside their native range. The invasive species had higher relative growth rates (RGR) than non‐invasive congeners at high nutrient availabilities, but did not differ from non‐invasive species at low nutrient availabilities. This is consistent with a strategy where these particular invasive species are able to rapidly use available resources. Relative growth rates were also higher for two out of three invasive species across a water availability gradient, but RGR did not differ in plasticity between the invasive and non‐invasive species. This suggests that nutrient addition, but not changes in water availability, might favour invasion of dayflowers.This approach is novel in comparing multiple pairs of invasive and non‐invasive congeners across multiple experimental conditions and allows evaluation of the robustness of performance differences. It also controls for some of the effects of relatedness that might confound multispecies comparisons.

Assessing biodiversity at landscape level in northern Thailand and Sumatra (Indonesia): the importance of environmental context

Most biodiversity assessment methods tend to sample isolated areas of land cover such as closed forest or local land use mosaics. Contemporary methods of assessing biodiversity are briefly reviewed and focus on the relative roles of the Linnean species and plant functional types (PFTs). Recent case studies from central Sumatra and northern Thailand indicate how the range distributions of many plant and animal species and functional types frequently extend along regional gradients of light, water and nutrient availability and corresponding land use intensity. We show that extending the sampling context to include a broader array of environmental determinants of biodiversity results in a more interpretable pattern of biodiversity. Our results indicate sampling within a limited environmental context has the potential to generate highly truncated range distributions and thus misleading information for land managers and for conservation. In an intensive, multi-taxa survey in lowland Sumatra, vegetational data were collected along a land use intensity gradient using a proforma specifically designed for rapid survey. Each vegetation sample plot was a focal point for faunal survey. Whereas biodiversity pattern from samples within closed canopy rain forest was difficult to interpret, extending the sample base to include a wider variety of land cover and land use greatly improved interpretation of plant and animal distribution. Apart from providing an improved theoretical and practical basis for forecasting land use impact on biodiversity, results illustrate how specific combinations of plant-based variables might be used to predict impacts on specific animal taxa, functional types and above-ground carbon. Implications for regional assessment and monitoring of biodiversity and in improving understanding of the landscape dynamics are briefly discussed.

Nutrient limitation of the macroalgaEnteromorpha intestinalis collected along a resource gradient in a highly eutrophic estuary

We conducted a laboratory experiment to quantify nutrient (nitrogen and phosphorus) limitation of macroalgae collected along a gradient in water column nutrient availability in Upper Newport Bay estuary, a relatively nutrient-rich system in southern California, United States. We collectedEnteromorpha intestinalis and water for use in the experiment from five sites ranging from the lower end of the estuary to the head. Initial algal tissue N and P concentrations and molar N∶P ratios—as well as water column NO3 and total Kjeldahl nitrogen (TKN)—increased along a spatial gradient from the lower end toward the head. Water column soluble reactive phosphorus (SRP) varied among sites as well but did not follow a pattem of increasing from the seaward end toward the head. Algae from each site were assigned to one of four experimental treatments: control (C), nitrogen enrichment (+N), phosphorus enrichment (+P), and nitrogen and phosphorus enrichment (+N+P). Each week for 3 wk we replaced the water in each unit with the appropriate treatment water to mimic a poorly flushed estuary. After 3 wk, the degree of nutrient limitation ofE. intestinalis varied spatially with distance from the head of the estuary. Growth ofE. intestinalis collected from several sites increased with N enrichment alone and increased further when P was added in combination with N This indicated that N was limiting and that when N was sufficient, P became limiting. Sites from whichE. intestinalis exhibited nutrient limitation spanned the range of background water column NO3 (12.9±0.4 to 55.2±2.1 μM) and SRP (0.8±0.0 to 2.9±0.2 μM) concentrations. Algae that were N limited had initial tissue N levels ranging from 1.18±0.03 to 2.81±0.08% dry weight and molar N∶P ratios ranging from 16.75±0.39 to 26.40±1.98.

Development of a nutrient pollution indicator using the seagrass, Zostera marina, along nutrient gradients in three New England estuaries

Worldwide, seagrasses provide important habitats in coastal ecosystems, but seagrass meadows are often degraded or destroyed by cultural eutrophication. Presently, there are no available tools for early assessment of nutrient over-enrichment; direct measurements of water column nutrients are ineffective since the nutrients typical of early enrichment are rapidly taken up by plants within the ecosystem. We investigated whether, in a gradient of nutrient availability but prior to actual habitat loss, eelgrass ( Zostera marina L.) plant morphology and tissue nutrients might reflect environmental nutrient availability. Eelgrass responses to nitrogen along estuarine gradients were assessed; two of these plant responses were combined to create an early indicator of nutrient over-enrichment. Eelgrass plant morphology and leaf tissue nitrogen (N) were measured along nutrient gradients in three New England estuaries: Great Bay Estuary (NH), Narragansett Bay (RI) and Waquoit Bay (MA). Eelgrass leaf N was significantly higher in up-estuary sampling stations than stations down-estuary, reflecting environmental nitrogen gradients. Leaf N content showed high variance, however, limiting its ability to discriminate the early stages of eutrophication. To find a stronger indicator, plant morphological characteristics such as number of leaves per shoot, blade width, and leaf and sheath length were examined, but they only weakly correlated with leaf tissue N. Area normalized leaf mass (mg dry weight cm −2), however, exhibited a strong and consistently negative relationship with leaf tissue N and a significant response to the estuarine nutrient gradients. We found the ratio of leaf N to leaf mass to be a more sensitive and consistent indicator of early eutrophication than either characteristic alone. We suggest the use of this ratio as a nutrient pollution indicator (NPI).

Changes in nutrient content and stable isotope ratios of C and N during decomposition of seagrasses and mangrove leaves along a nutrient availability gradient in Florida Bay, USA

The decomposition of the mangrove Rhizophora mangle and the seagrass Thalassia testudinum was examined using litterbags along a natural gradient in nutrient availability. Seagrass leaves had a higher fraction of their biomass in the labile pool (57%), compared to mangrove leaves (36%) and seagrass rhizomes (29%); the overall decomposition rates of the starting material reflected the fractionation into labile and refractory components. There was no relationship between the N or P content of the starting material and the decomposition rate. Nutrient availability had no influence on decomposition rate, and mass was lost at the same rate from litterbags that were buried in the sediment and litterbags that were left on the sediment surface. The dynamics of N and P content during decomposition varied as a function of starting material and burial state. N content of decomposing mangrove leaves increased, but seagrass rhizomes decreased in N content during decomposition while there was no change in seagrass leaf N content. These same general patterns held for P content, but buried seagrass leaves increased in P content while surficial leaves decreased. δ13C and δ15N changed by as much as 2‰ during decomposition.

Allometry and development in herbaceous plants: functional responses of meristem allocation to light and nutrient availability

We examined the relationship between meristem allocation and plant size for four annual plant species: Arabidopsis thaliana, Arenaria serphyllifolia, Brassica rapa, and Chaenorrhinum minus. Gradients of light and nutrient availability were used to obtain a range of plant sizes for each of these species. Relative allocation to reproductive, inactive, and growth meristems were used to measure reproductive effort, apical dominance, and branching intensity, respectively. We measured allocation to each of these three meristem fates at weekly intervals throughout development and at final developmental stage. At all developmental stages reproductive effort and branching intensity tended to increase with increasing plant size (i.e., due to increasing resource availability) and apical dominance tended to decrease with increasing plant size. We interpret these responses as a strategy for plants to maximize fitness across a range of environments. In addition, significant differences in meristem response among species may be important in defining the range of habitats in which a species can exist and may help explain patterns of species competition and coexistence in habitats with variable resource availability.

Adaptations of tropical marine microphytobenthic assemblages along a gradient of light and nutrient availability in Suva Lagoon, Fiji

How are microphytobenthic biofilms adapted to the high incident irradiances and temperatures, low inorganic nutrient concentrations and high desiccation stresses on intertidal flats present in tropical environments? This study investigated biofilms subject to different environmental conditions in a range of tropical sites in Suva lagoon, Fiji. PAM fluorescence was used to measure photophysiological responses to the light climate. Biofilm colloidal carbohydrate, extracellular polymeric substances (EPS) and low molecular weight (MW) carbohydrate concentrations and diel carbohydrate production patterns were measured. Average biomass (Chl a) ranged from 15 to 36 mg m−2, and was highest in seagrass bed sediments, but biomass was not correlated with water column or sediment porewater nutrient concentrations. Biofilm photophysiology differed significantly along a combined gradient of light and nutrient availability, with F v/F m, relative ETRmax and E k of biofilms highest in mangrove and intertidal main island sites and lowest in subtidal coral reef flats. Subtidal biofilms showed photoinhibition at irradiances > 1000 µmol m−2. Significant correlations between Chl a and colloidal carbohydrate concentrations were present (except on intertidal sandflats), and tropical biofilms had higher ratios of colloidal carbohydrate and EPS to Chl a than temperate estuarine biofilms, probably due to a combination of high irradiance and low nutrient availability leading to the production of excess photoassimilates. The percentage of EPS present in the colloidal fraction was highest in coral sand biofilms (42%), which had the lowest nutrient concentrations, compared with other sites (25–32%). Intertidal biofilms predominantly consisted of large motile taxa and showed strong rhythms of vertical migration. During tidal emersion, high sediment temperatures (41 °C), irradiance (>2300 µmol m−2 s−1) and salinity (49‰) stimulated downward migration. In silty sediments, migration resulted in a reduction in photosynthetic activity during the midday period but, in sands with high light penetration (to a depth of > 1700 µm), high production rates of EPS (18.2 µg carbo. µg Chl a−1 h−1) and low MW carbohydrate exudates (40.2 µg carbo. µg Chl a−1 h−1) occurred. Vertical migration, high E k and high rates of photoassimilate dumping are all adaptations to living in the tropical intertidal zone. Seagrass and reef flat biofilms consisted of a diverse non-migratory flora of motile and non-motile taxa that were not subject to such extreme temperature and irradiance conditions. Low values of photosynthetic parameters and high colloidal and EPS content indicated that these biofilms were nutrient-limited.

N, P, and K Budgets along Nutrient Availability and Productivity Gradients in Wetlands

N, P, and K Budgets along Nutrient Availability and Productivity Gradients in Wetlands

Are nutrient availability and acidity-alkalinity gradients related in Sphagnum-dominated peatlands?

Abstract Gradients in acidity-alkalinity and nutrient availability were studied in 2 Sphagnum-dominated peatlands on the southeastern Italian Alps. Decreasing concentrations of most mineral elements (Ca2+, Mg2+, Mn2+, Al3+ and Si4+) in pore water indicated a progressively lower influx of mineral-soil water from the slightly minerotrophic conditions in the peatland margins to ombrogenous conditions in the central part of the peatlands. This was paralleled by decreasing concentrations of ash, bulk density, Ca, Fe and, partly, Mn in the peat. The nutrient gradient, as defined by pore water concentrations of N and P, was largely independent of the acidity-alkalinity gradient: NO3− and PO43− had similar concentrations throughout the gradient, whereas NH4+ concentrations increased with increasing pore-water pH. In contrast, the peat nutrient gradient coincided with the acidity-alkalinity gradient, with total concentrations of N and P decreasing from the margin to the centre. Bryophytes and vascular plants had d...

Nitrogen, phosphorus, and sulfur dynamics in a low salinity marsh system dominated by Spartina alterniflora

In the upper reaches of estuaries, the pulsing of nutrient inputs and salinity occurs on fine temporal and spatial scales. We investigated the supply and exchanges of N, P, and S along a salinity gradient in a Chesapeake Bay subestuary. Interactions among nutrients in surface water, sediment porewater, and shoots of the dominant marsh macrophyte Spartina alterniflora were also examined. The system was characterized by a spatial gradient in nutrient availability, with the low salinity region representing abundant allochthonous nitrogen and phosphorus inputs and minimal sulfur availability. Autochthonous production and consumption of nutrients were most important at the midpoint of the salinity gradient. These patterns were reflected in the N, P, and S content of S. alterniflora over one growing season. This brackish tidal creek system also had a gradient of temporal variability in levels of inorganic nitrogen, phosphorus, and sulfur. The higher salinity portion experienced the damping effect of waters of more constant nutrient composition, whereas the upper portion of the system was characterized by highly pulsed nutrient availability. Interchanges between nutrient pools were apparent throughout the system. Sediment, and even surface-water, concentrations of nutrients seemed to respond to plant root zone oxidation and uptake and release of nutrients. Porewater biogeochemical processes were linked to surface-water nutrient dynamics as well.

Coastal upwelling in the Canary Island region: spatial variability reflected by the surface sediment diatom record

Modern diatom and C org distribution patterns were investigated in 39 surface sediment samples of the Canary region in order to quantitatively determine the influence of coastal upwelling and Saharan dust on the flux of these sedimentary components. High diatom abundance in surface sediments (>2×10 6 valves g −1 dry sediment) as well as the dominance of the assemblages by spores of Chaetoceros, which is a genus related to upwelling conditions, reflect the stronger coastal upwelling conditions occurring at Cape Ghir off the NW African coast. Further offshore, total diatom abundance decreases one order of magnitude (±1×10 5 valves g −1 dry sediment) and the assemblages are dominated by oceanic species of the genus Nitzschia and fresh water diatoms, mainly the genera Aulacoseira, which is known to be a component of Saharan dust. This association suggest that offshore diatom concentrations may be generated by dust input from desert regions. Even though the sediments are expected to preserve only a mixed and averaged oceanographic signal, characteristic biogeographic patterns of a phytoplankton community occurring across a gradient of turbulence and nutrient availability, are recorded in surface sediments off Cape Ghir. This is seen in the subsequent maximum abundances of the genera Thalassiosira, Chaetoceros and Rhizosolenia.

Changes in Net Primary Production of Calcareous Grasslands through Nutrient Input from Adjacent Farmland

Calcareous grasslands are semi-natural, nutrient-limited plant communities. Because of their high species richness they are of great interest for nature conservation. During the second half of this century the utilization of these areas became uneconomic and so they decreased considerably in area due to the abandonment of great parts of these formerly widespread plant communities. Nowadays the areas which have been set aside as nature reserves are also undergoing undesirable changes. Today the most severe threat to the existence of calcareous grassland communities is eutrophication. The main sources of nutrient input are the atmosphere and agroecosystems. Eutrophication causes changes in species composition and phytomass production. In this article it is shown that a gradient in nutrient availability along transects from farmland into calcareous grasslands corresponds to a gradient in aerial net primary production of the plant stands and a decrease in plant number.

Ecological significance of leaf life span among Central European grass species

Interspecific variation in leaf life span reflects the variation in nutrient conservation ability among different plant species and is considered to be associated with nutrient availability in the characteristic habitat. As defoliation interferes with nutrient conservation by the long‐lived leaves, we hypothesized that disturbance rate is another important environmental factor working as a selective force on interspecific variation in leaf life span. In order to investigate this, we measured leaf life span of 32 grass species in mature garden‐grown individuals. Variation in leaf life span was compared to measured leaf traits, to available data on species occurrence along gradients of nutrient availability and disturbance, and to published relative growth rates of the species. Leaf life span was associated positively with leaf tissue mass density and negatively with specific leaf area. Leaf life span correlated negatively with the disturbance rate in the characteristic habitat of a species, but not with nutrient availability. The latter relationship did not come about due to the long leaf life spans of species from nutrient‐rich habitats with a relatively low disturbance rate, and to some extent also due to the short leaf life spans of annual species from relatively nutrient‐poor sites. We conclude that although leaf longevity is an important means of reducing nutrient losses, this is a selective advantage only if the plant is not subjected to frequent defoliation. The frequently postulated association between leaf life span of a species and nutrient availability in its characteristic habitat may occur among species of habitats with positively correlated nutrient availability and disturbance rate. Leaf life span is negatively associated with seedling RGR, but there may be deviations in this relationship due to species with contrasting characteristics at seedling stage and at maturity.

Understorey vegetation development in North Finnish Picea forests after disturbance: re‐analysis of Sirén's data

Abstract. Sirén (1955) studied understorey species composition, tree stand properties and humus‐layer thickness in 64 unlogged forest stands on topographically and pedologically comparable sites. The stands were of even age (6 – 300 yr), stocked with the first or second tree generation after wildfire. The view of Sirén and several authors after him, that the vegetation of old‐growth boreal Picea forests is homogeneous on a broad scale, was examined by applying, in parallel, the partial variants of two ordination methods (DCA and PCA) to Sirén's vegetation data. Two main vegetation gradients were found: a major gradient running from recently burnt plots with prominence of pioneer species to plots with stand age &gt; 100 yr, a well stocked tree layer and a thick humus layer, dominance of feather‐mosses and ample occurrence of shade‐tolerant as well as light‐preferring vascular plant species, and a second gradient along which first‐ and second‐generation plots segregate. The more prominent element of Betula trees in first‐ than in second‐generation stands &lt; 100 yr contributed to the latter. A minor third gradient related to humus‐layer thickness was recovered by partial DCA only. The main vegetation gradient reappeared in separate ordinations of data from 47 mature forest stands (&gt; 100 yr), but without being correlated with forest age. Variation among mature‐forest stands in the importance of pioneer species is considered mainly to be brought about by fine‐scale disturbance processes such as tree uprooting. Increasing importance of factors operating on within‐stand scales [development of a varied gap structure and stronger gradients in tree influence (radiation at ground level), soil moisture, soil depth and nutrient availability] with time is also reflected in the second and third mature‐forest ordination axes. Possible implications of the results for conservation of biological diversity and monitoring of changes in boreal forests are discussed.

Nitrogen resorption from needles of Pinus thunbergii Parl. growing along a topographic gradient of soil nutrient availability

To examine the relative importance of nutrient resorption in increasing the nutrient‐use efficiency of Pinus thunbergii Parl., we investigated the nitrogen contents of green and senescent needles of P. thunbergii trees growing at five positions along a slope (LS, lower slope; TR, transitional site; MS, middle slope; US, upper slope; RG, ridge) and found that soil nitrogen availability tended to decrease upslope. Nitrogen concentration in green and senescent needles decreased upslope. Nitrogen resorption efficiencies (percentage change in nitrogen content between green and senescent needles) increased upslope from 43 to 77% with decreasing soil nitrogen availability. Nitrogen resorption efficiency was related to green needle dry mass per unit length, but there was no clear correlation between nitrogen resorption efficiency and nitrogen content in green needles. We concluded that the increase in nitrogen resorption efficiency of P. thunbergii enhanced the nitrogen‐use efficiency as a response to the low nitrogen availability.

Scale-dependent patterns of soil enzyme activity in a forested landscape

We evaluated the patterns of variation in the activity of four soil enzymes in oak forests soils at spatial scales from 10s of km to &lt;3 m. The four enzymes (β-glucosidase, chitinase, phenol oxidase, and acid phosphatase) are specific for substrates that vary widely in lability or recalcitrance. Significant variations in enzyme activity were observed at the regional (among forested areas), topographic (along elevation gradients within a watershed), and single-tree (1 m upslope and downslope of an individual tree) scales, but not at the local scale (contiguous watersheds within forested areas). However, the specific patterns of variation in relation to spatial scale were unique to each enzyme system. Detrended correspondence analysis (DCA) ordination of the activity of the four enzymes in each soil sample suggested a strong nutrient availability gradient underlying these spatial scale differences. Exploratory path analysis produced relatively strong predictive models based on soil nutrients, organic matter, and moisture for all the individual enzymes except phenol oxidase. However, path analysis produced an even stronger model for the activity of all four enzymes together, using the DCA axis scores as the dependent variable. The results indicate that the four enzyme systems could help resolve spatial dependencies at a range of scales and could also be used to develop a scale-independent metric to be used for regional analyses in a geographic information system environment.

Nutrient efficiency along nutrient availability gradients

The validity of nutrient use efficiency as a central concept in ecosystem ecology has recently been subject to challenge based upon arguments over autocorrelation of data, interpretation of graphical approaches, and appropriate statistical analyses. Much of the confusion on the measurement and interpretation of nutrient use efficiency results from the lack of a sound theoretical basis with which to examine experimental results. In this paper, we develop a theory of nutrient use efficiency based upon fundamental mass balance, present a graphical approach to appropriate testing of alternative hypotheses to avoid problems of autocorrelation in data, and suggest critical areas where experiments must be performed to distinguish among hypotheses. We show that nutrient use efficiency (production per unit nutrient uptake) must be distinguished from nutrient response efficiency (production per unit nutrient available). In contrast to the monotonic increase of nutrient use efficiency with decreasing nutrient availability originally proposed in the 1982 model of P.M. Vitousek, nutrient response efficiency is unimodal with maximum efficiency at intermediate levels of nutrient availability. However, nutrient use efficiency dynamics at low nutrient availability cannot yet be theoretically defined. We also show theoretically which plant traits control responses of ecosystem nutrient use or nutrient response efficiency along gradients of nutrient availability. Finally, we show how our model naturally leads to species replacement along nutrient availability gradients.