Ecosystem ecology

They Only Saw the Surface of Things

by J.W. Trull

Differential tree and shrub production in response to fertilization and disturbance by coastal river otters in Alaska

by Aaron Roe

Effects of urbanization on nutrient biogeochemistry of aridland streams

by David Lewis

Nancy B. Grimm, J. Ramón Arrowsmith, Chris Eisinger, James Heffernan, Amanda MacLeod, David B. Lewis, Lela Prashad, Tyler Rychener, W. John Roach, and Richard W. Sheibley
Pp. 129-146 in "Ecosystems and Land Use Change," Geophysical Monograph 153, editors R. DeFries, G. Asner, and R. Houghton. American Geophysical Union, Washington (2004)

Land-use and land-cover change affect the biogeochemistry of stream ecosystems in numerous ways, both direct and... more Land-use and land-cover change affect the biogeochemistry of stream ecosystems in numerous ways, both direct and indirect. Changes result from hydrologic modifications, including direct alterations of flow regimes and hydrologic flowpaths and indirect changes in hydrologic patterns via increased impervious cover in contributing areas of watersheds. Direct changes to channel morphology (i.e., reduced complexity) and to floodplains of streams and rivers also influence biogeochemistry, for example, by eliminating surface water–groundwater exchange. The nature of and strength of connections between the stream and its watershed may be altered by large-scale changes such as those brought about by urban and suburban development. Finally, in urban and agricultural areas, elevated nutrient loading is exacerbated by land-cover changes that increase the potential for erosion or overland flow, and decrease the opportunities for nutrient retention. Using comparative analysis based on published work, mined public data, and new research, we evaluate urbanization effects on stream ecosystems in the Sonoran Desert region of central Arizona. Five key characteristics of non-urban desert streams—nitrogen limitation, a flashy hydrologic regime that initiates succession, extensive groundwater–surface water interaction, episodic terrestrial–aquatic interactions, and high capacity for nutrient retention—are both dramatically altered and scarcely affected by urbanization. The similarities exhibited by aridland streams and their urban counterparts arise from large-scale constraints (e.g., episodic terrestrial–aquatic interaction is imposed by the climatic regime in both stream types), whereas the differences, like interrupted flowpath continuity in the urban landscape, likely result from the myriad direct modifications of streams and catchment land cover in cities.

Plasticity in resource allocation and nitrogen-use efficiency in riparian vegetation: implications for nitrogen retention

by David Lewis

John D. Schade and David B. Lewis
Ecosystems (2006) 9:740-755

In this work, we summarize our current understanding of the function of riparian zones and describe an investigation... more In this work, we summarize our current understanding of the function of riparian zones and describe an investigation of changes in the production per unit nitrogen (N) taken up, or nitrogen-use efficiency (NUE) and resource allocation of a riparian shrub in response to changes in N availability. Empirical work included measuring leaf %N and root-to-shoot ratios (R:S) of individual riparian shrubs (Baccharis salicifolia, or seepwillow) growing at a range of N availabilities in the field and growing in fertilized and unfertilized plots in a field fertilization experiment. In both observational and experimental work, N availability was related positively to %N of plant tissues and negatively to R:S. We used a simulation model to investigate feedbacks between seepwillow responses to and effects on N availability. In the model, plasticity in resource allocation and NUE in response to changes in N led to lower productivity at low N supply and higher productivity and lower retention at high N supply than was observed in plants constrained to a constant %N and R:S. Furthermore, uptake became relatively more important as a retention mechanism when plants responded to high N supply. These feedbacks could have significant effects on N retention by riparian zones in watersheds receiving large fertilizer inputs of N or on ecosystems exposed to high rates of atmospheric N deposition.

Making sense of the landscape: geomorphic legacies and the landscape position of lakes

by David Lewis

Timothy K. Kratz, Katherine E. Webster, Joan L. Riera, David B. Lewis, and Amina I. Pollard
Pp. 49-66 (Chap 3) in "Long-term Dynamics of Lakes in the Landscape: Long-term Ecological Research on North Temperate Lakes," editors J.J. Magnuson, T.K. Kratz, and B.J. Benson. Oxford University Press, Oxford, UK (2006)

One of the hallmarks of our North Temperate Lakes Long-Term Ecological Research (LTER) program is that we have... more One of the hallmarks of our North Temperate Lakes Long-Term Ecological Research (LTER) program is that we have designed our research to consider multiple spatial scales, ranging from within lake, to whole lake, to multiple lake, to lake districts, and to regions. Although we use multiple lakes, that is, subsets of lakes within a lake district, as the unit of study, the key distinguishing feature of our research is that we explicitly consider spatial positioning of individual lakes relative to each other. Here we further develop the concept of landscape position to describe patterns of spatially explicit variation of lake properties and dynamics as a function of the lakes' positions in the landscape. Thus, we provide a framework intermediate in scale between the individual lake and the lake district that recognizes the importance of the specific locations of lakes within the landscape. This emphasis on spatial gradients is similar conceptually to but quite different in detail from the River Continuum Concept.

Agrarian legacy in soil nutrient pools of urbanizing arid lands

by David Lewis

David Bruce Lewis, Jason P. Kaye, Corinna Gries, Ann P. Kinzig, and Charles L. Redman
Global Change Biology (2006) 12:703–709

Today's worldwide expansion of dry land cities consumes cultivated and native ecosystems, providing laboratories for... more Today's worldwide expansion of dry land cities consumes cultivated and native ecosystems, providing laboratories for investigating imprints of former land use in places where people now live. Around Phoenix, USA, we compared soil nutrient pools in residential yards converted from farms with nutrient pools in yards developed on native desert. Organic matter, carbon (C), nitrogen (N), and soluble ions were >2-fold greater in yards that were previously agrarian than in yards that were not. These pools remain elevated 40 years after land conversion to residential use. Present N accumulation (1.5 g per m2 per yr) is not affected by prior land use, suggesting that rates of residential fertilizer application and retention are not affected by antecedent soil fertility. Bioavailable, inorganic phosphorus (Pav) is elevated in soil with a recent agrarian past, but this signal disappears after 10–30 years of residential use owing to an accumulation of Pav in never-farmed yards. Our results indicate a ‘direct agrarian legacy,’ wherein agrarian amendment of nutrient pools endures urbanization, more so than an ‘indirect legacy,’ wherein present land management is molded by former land use. Agriculture in dry lands thus sequesters material in soils, and—as we also found higher material contents in residential soils than in contemporary agrarian soils—residential land use simply adds to the agrarian legacy these soils already bear. Intense human use of arid lands may cause increases in material pools in soils, a condition with potential global consequence.

The spatial structure of variability in a semi-arid, fluvial ecosystem

by David Lewis

David Bruce Lewis, John D. Schade, Anne K. Huth and Nancy B. Grimm
Ecosystems (2006) 9:386-397

The arrangement and composition of flowpath types within a given network are thought to govern its functioning. This... more The arrangement and composition of flowpath types within a given network are thought to govern its functioning. This concept assumes that different flowpath types are functionally distinct. We investigated this assumption in a fluvial ecosystem by comparing the riparian zone, parafluvial zone (in-channel gravel bars), and surface stream. We hypothesized that differences in advection, uptake, and sorption would render material cycles more (a) open and (b) mutable in the surface stream, whereas the converse would occur in the riparian zone, and an intermediate state would be seen in the intervening parafluvial zone. To test our first hypothesis, we predicted that spatial heterogeneity in solute concentrations would be least in the surface stream, greater in the parafluvial zone, and greatest in the riparian zone. Using a null model, we ascertained that this pattern was shown by all solute species we examined (nitrate, ammonium, total dissolved inorganic nitrogen [DIN], dissolved organic N, total dissolved N, soluble reactive phosphorus, dissolved organic carbon, and chloride). To test our second hypothesis, we predicted that temporal change in spatial heterogeneity would be greatest in the surface stream, less in the parafluvial zone, and least in the riparian zone. Nitrate, DIN, and chloride showed this pattern. In particular, surface stream inorganic N was less spatially variable following months of high rainfall. According to an extant hypothesis, these results suggest that inorganic N processing may be a stable function in this ecosystem. Other solute species did not support our second prediction, perhaps because their retention and release dynamics are influenced principally by geochemistry. Generally, our findings indicate that a geomorphic template can generate spatial patterns in ecosystem function, warranting an expansion of the spiraling framework to a variety of flowpath types.

Subsystems, flowpaths, and the spatial variability of nitrogen in a fluvial ecosystem

by David Lewis

David Bruce Lewis, Nancy B. Grimm, Tamara K. Harms, and John D. Schade
Landscape Ecology (2007) 22:911–924

Nutrient dynamics in rivers affect biogeochemical fluxes from land to oceans and the atmosphere. Fluvial ecosystems... more Nutrient dynamics in rivers affect biogeochemical fluxes from land to oceans and the atmosphere. Fluvial ecosystems are thus important environments for understanding spatial variability in nutrient concentrations. At the San Pedro River in semi-arid Arizona, USA, we investigated how variability in dissolved inorganic nitrogen (DIN) was regulated by subsystem type and hydrological flowpaths. The three subsystems we compared were the riparian zone, parafluvial (gravel bar) zone, and surface stream. DIN concentration was greater in the riparian zone than in the surface stream, suggesting that the riparian zone retains DIN and is a source of N for the surface stream. Parafluvial zones were too variable to generalize how they regulate DIN. Our hypothesis that subsystem type regulates DIN oxidation was too simple. The riparian and parafluvial zones host a mosaic of oxidizing and reducing conditions, as they exhibited highly variable ammonium to nitrate (NH4+:NO3-) ratios. Surface stream DIN was dominated by NO3-. Along a subsurface flowpath in the riparian zone, we did not observe spatial covariation among the N forms and transformations involved in mineralization. We also compared spatial variability in solute concentrations between flowpaths and nonflowpath reference areas. Our mixed results suggest that spatial variability is regulated in part by flowpaths, but also by solute-specific processing length along a flowpath. To understand the distribution of N in fluvial ecosystems, subsystem type and flowpaths are readily discernible guides, but they should be coupled with other mechanistic factors such as biota and soil type.

Hierarchical regulation of nitrogen export from urban catchments: interactions of storms and landscapes

by David Lewis

David Bruce Lewis and Nancy B. Grimm
Ecological Applications (2007) 17:2347–2364

In urban catchments of arid central Arizona, we investigate how the export of mineral and organic nitrogen (N) in... more In urban catchments of arid central Arizona, we investigate how the export of mineral and organic nitrogen (N) in storm runoff is regulated by interactions between local landscape characteristics and broader scale storm features. First, we test whether N export is more a function of (1) processes that affect N concentration in runoff or (2) the propensity of the catchment to convey rainfall as runoff. With data pooled across catchments, the mass of N in export (load) is determined by processes regulating runoff N concentration. There are exceptions when catchments are examined individually, where N load from some catchments is determined by the hydrologic responsiveness of the catchment. Second, we investigate the relationship between N export and catchment features. Loads per catchment area were greater from more impervious catchments, probably because impervious catchments held more N in a mobilizable phase and conveyed more rainfall as overland flow. Loads per area were lower from larger catchments, possibly owing to more N-retention hot spots in larger catchments. Catchments with the greatest N exports were those with commercial land use, and loads decreased as development became less prevalent or as residential replaced industrial land use. Third, we investigated how catchment features moderated direct responses of N export to storms. Export was less correlated with storm features in catchments that were larger, more pervious, and less industrial. Results support an ‘‘N build and flush’’ hypothesis, which purports that there is little biotic processing of N deposited to arid, urban surfaces with little organic matter. The rate and duration of deposition determine the size of the mobile N pool. Any amount of rainfall capable of generating overland flow would entrain nearly all mobilizable N and export it from the catchment. Nonetheless, these results suggest that, even with daunting seasonal and interannual variability in storm conditions, material export can be reduced by managing intrinsic catchment features.

Inorganic nitrogen immobilization in live and sterile soil of old-growth conifer and hardwood forests: implications for ecosystem nitrogen retention

by David Lewis

David Bruce Lewis and Jason P. Kaye
Biogeochemistry (2011) DOI: 10.1007/s10533-011-9627-6

Rapid immobilization of inorganic nitrogen (N) in soil contributes to ecosystem N accumulation, even in old-growth and... more Rapid immobilization of inorganic nitrogen (N) in soil contributes to ecosystem N accumulation, even in old-growth and chronically-fertilized forests once thought to have poor N retention capacity. In old-growth conifer and hardwood stands in Pennsylvania, we tested the hypotheses that biotic and abiotic N immobilization are regulated by N form and forest type. We added 15NH4+, 15NO2-, and 15NO3- to sterile (gamma-irradiated) and live organic-horizon soil and define N immobilization as the mass of added 15N remaining in soil following extractions conducted 15 min, 24 h, and 21 days later. Immobilization of NO2- (19–25% of added N) occurred in sterile soils within 15 min and was little changed thereafter. Tracer NO3- immobilization was not observed, although soils had been pretreated (refrigerated) so as to quantify the lower limit of immobilization potential. Immobilization of NH4+ (27%) occurred in live conifer soils by 21 days but not in other treatments. In 21-day incubations, tracer N immobilization was greater in NO3-poor and humic-rich soils. Immobilization was greater in sterile than in live soil, perhaps owing to artifacts of sterilization. Conifer stands exhibited more massive O-horizons, so NO2- immobilization per unit area was greater in conifer (1.46 mg N m-2) than hardwood (0.43 mg N m-2) stands, possibly accounting for lower N leaching from conifer forests. Areal immobilization rates appear to be fast enough to retain all N transformed to NO2-, so NO2- production may be a limiting step in soil N retention in old-growth ecosystems.

Understanding ecosystem retrogression

by Duane Peltzer

Estimating uncertainty in ecosystem budget calculations

by Ruth Yanai

Open Access

Ecosystem nutrient budgets often report values for pools and fluxes without any indication of uncertainty, which makes... more Ecosystem nutrient budgets often report values for pools and fluxes without any indication of uncertainty, which makes it difficult to evaluate the significance of findings or make comparisons across systems.  We present an example, implemented in Excel, of a Monte Carlo approach to estimating error in calculating the N content of vegetation at the Hubbard Brook Experimental Forest in New Hampshire.  The total N content of trees was estimated at 847 kg ha-1 with an uncertainty of 8%, expressed as the standard deviation divided by the mean (the coefficient of variation).  The individual sources of uncertainty were as follows: uncertainty in allometric equations (5%), uncertainty in tissue N concentrations (3%), uncertainty due to plot variability (6%, based on a sample of 15 plots of 0.05 ha), uncertainty due to tree diameter measurement error (0.02%). In addition to allowing estimation of uncertainty in budget estimates, this approach can be used to assess which measurements should be improved to reduce uncertainty in the calculated values.  This exercise was possible because the uncertainty in the parameters and equations we used was made available by previous researchers.  It is important to provide the error statistics with regression results if they are to be used in later calculations; archiving the data makes resampling analyses possible for future researchers. When conducted using a Monte Carlo framework, the analysis of uncertainty in complex calculations does not have to be difficult and should be standard practice when constructing ecosystem budgets.

Regulation of animal size by eNPP, Bergmann’s rule,

by Steve Wolverton

Michael A. Huston and Steve Wolverton

Bergmann's rule, which proposes a heat-balance explanation for the observed latitudinal gradient of increasing animal... more Bergmann's rule, which proposes a heat-balance explanation for the observed latitudinal gradient of increasing animal body size with increasing latitude, has dominated the study of geographic patterns in animal size since it was first proposed in 1847. Several critical reviews have determined that as many as half of the species examined do not fit the predictions of Bergmann's rule. We have proposed an alternative hypothesis for geographic variation in body size based on food availability, as regulated by the net primary production (NPP) of plants, specifically NPP during the growing season, or eNPP (ecologically and evolutionarily relevant NPP). Our hypothesis, “the eNPP rule,” is independent of latitude and predicts both spatial and temporal variation in body size, as well as in total population biomass, population growth rates, individual health, and life history traits of animals, including humans, wherever eNPP varies across appropriate scales of space or time. In the context of a revised interpretation of the global patterns of NPP and eNPP, we predict contrasting latitudinal correlations with body size in three distinct latitudinal zones. The eNPP rule explains body-size patterns that are consistent with Bergmann's rule, as well as two distinct types of contradictions of Bergmann's rule: the lack of latitudinal patterns within the tropics, and the decline in body size above approximately 60° latitude. Both types of contradictions of Bergmann's rule are consistent with the eNPP rule, as are a wide range of other phenomena.

A catchment-based approach to mapping hydrological ecosystem services using riparian habitat: A case study from the Wet Tropics, Australia

by Petina Pert

Ecological Complexity
Volume 7, Issue 3, September 2010, Pages 378-388
doi:10.1016/j.ecocom.2010.05.002

Hydrological processes have been identified as delivering ecosystem services that are fundamental to both human... more Hydrological processes have been identified as delivering ecosystem services that are fundamental to both human well-being and the maintenance of biodiversity. If we can map the hydrological processes and the threats to them, the integrity of the provision of ecosystem services from the catchment can be identified, highlighting areas in need of further protection. Using the Tully-Murray catchment in the Wet Tropics as a case study, we constructed a simplified description of the ecosystem functions and related processes, and identified the roles of the ecosystem service providers and beneficiaries. Mapping the riparian zone and adjacent land uses enabled us to visualise areas where non-remnant vegetation existed and also vegetation that was classified as ‘Endangered’ or ‘Of Concern’. An assessment of the riparian vegetation and adjacent land use revealed that 51% of the catchment contained non-remnant riparian vegetation (i.e. was cleared or under agricultural production) and 36% of remnant vegetation was described as “Endangered” with a further 35% being classified as “Of Concern”. This is consistent with the reported current threats from water quality degradation, habitat fragmentation and other anthropogenic influences driven by rapid economic growth. The findings and data derived from this project can be used for a range of purposes in managing the riparian zone. Mapping the riparian zone and adjacent land use has shown the different types of land uses that impact on waterways and this will assist natural resource managers in prioritizing areas where remnant vegetation should be retained or rehabilitated. With this information, adjacent landholders can improve their management of the riparian zone and remnant vegetation through gaining the ecosystem service benefits, and further are able to profit from grant applications and incentives. Maintaining a healthy waterway and retaining remnant riparian vegetation will offer the possibility to obtain many economic, biodiversity and aesthetic benefits both within the catchment and downstream as far as the Great Barrier Reef.

Impacts of increased sediment loads on the ecology of lakes

by Ian Donohue

Donohue, I. & Garcia-Molinos, J. (2009) Impacts of increased sediment loads on the ecology of lakes. Biological... more Donohue, I. & Garcia-Molinos, J. (2009) Impacts of increased sediment loads on the ecology of lakes. Biological Reviews 84: 517-531

Increased sediment loading comprises one of the most important and pervasive anthropogenic impacts on aquatic ecosystems globally.  In spite of this, little is known of the overall effects of increased sediment loads on lakes.  By modifying both bottom-up and top-down ecological processes and restructuring energy flow pathways, increased sediment loads not only alter biotic assemblage structure and ecological functioning significantly, but frequently result in reduced biological diversity and productivity.  Although lake food-webs can be subsidised to some extent by the adsorption of organic carbon to fine sediments, trophic structure and the composition of biotic assemblages remain likely to be modified considerably.  The mineralogy and particle size of sediments and the availability of nutrients, by influencing both the scale and nature of impacts, are key determinants of the overall effects of increased sediment loads on lake ecosystems.  Although interactions with other global anthropogenic pressures, such as invasion by exotic species and climate change, are likely to be significant, little remains known about the nature or likely strength of those interactions.  Widespread increases in sediment loading to lakes have, therefore, profound implications for the conservation and management of global aquatic biological diversity.

Temporal variability within disturbance events regulates their effects on natural communities

by Ian Donohue

García Molinos, J. & Donohue, I. (2011) Temporal variability within disturbance events regulates their effects on... more García Molinos, J. & Donohue, I. (2011) Temporal variability within disturbance events regulates their effects on natural communities. Oecologia 166: 795-806.

Disturbances are inherently variable in time and space. Such heterogeneity comprises a key determinant of ecosystem responses to disturbance. Temporal patterns can, however, vary significantly both among and within individual disturbance events. While recent research has demonstrated an importance of the former, studies on the effects of variability within perturbations have consistently confounded temporal variability with other disturbance attributes (e.g. overall intensity or duration). We established a field experiment to test explicitly the hypothesis that the temporal pattern within perturbations can drive ecosystem responses independently of other disturbance traits. We examined the effects of two disturbance regimes comprising sediment pulses of contrasting temporal pattern (constant and temporally variable intensities) on the benthic invertebrate assemblage of a headwater stream. The overall intensity, duration, timing and frequency of the perturbations were, however, identical. Invertebrates drifting during the temporally variable pulses were more abundant and differed in taxonomic and trophic structure than those exposed to constant perturbations. Moreover, whereas temporal patterns of disturbance events had no immediate effect on benthic invertebrate assemblages in situ, assemblages exposed to the constant perturbations took longer to recover from sediment disturbances than those exposed to temporally variable perturbations. Our results demonstrate that variability in the temporal pattern of intensity within individual perturbations can regulate, independently of other disturbance attributes, the extent and type of ecosystem responses to, and recovery from, disturbances. Effective environmental management and policy therefore necessitate the explicit quantification of temporal patterns of intensity both within and among perturbations.

Above- and below-ground competition effects of two heathland species: Implications for growth and response to herbivory in birch saplings.

by Jonathan Millett

Basic and Applied Ecology, 9(2008), 55-66.

We examined experimentally the effect of competition from Calluna vulgaris or Molinia caerulea on Betula pubescens... more We examined experimentally the effect of competition from Calluna vulgaris or Molinia caerulea on Betula pubescens saplings subjected to simulated mammalian browsing damage. We tested three hypotheses: 1. that responses to competition will follow the balanced growth hypothesis with the predominant competition effect being below-ground, resulting in increased allocation to roots; 2. that above-ground competition effects from C. vulgaris are greater than M. caerulea, resulting in differences in growth allocation; 3. that, when only B. pubescens saplings are damaged, herbivory reduces its ability to compete with both species. We grew B. pubescens saplings in the presence of below- or above- and below-ground interactions from C. vulgaris or M. caerulea. Saplings were also subjected to simulated browsing by clipping (50% of current year’s growth), either presenescence or at bud-burst. We measured the morphology and dry mass allocation response of the saplings one year after the browsing was applied. Competition reduced sapling dry mass by approximately 50%, but C. vulgaris reduced dry mass to a greater extent than M. caerulea. This difference was due to a smaller impact of M. caerulea shoots than C. vulgaris shoots on birch growth. Saplings compensated for browsing damage, resulting in no difference in dry mass one year after damage. However, sapling morphological responses to browsing damage were dependent on the competing species. Despite the large competitive effect of below-ground interactions, saplings did not increase allocation to root growth as predicted. Additionally, in response to above-ground interactions from M. caeruela, saplings increased allocation to root growth relative to those growing with C. vulgaris. As such, growth responses to competition were not explained by the balanced growth hypothesis. This study highlights the importance of patterns, as well as intensity, of competition in determining plant responses to inter-specific interactions.

How do different competing species influence the response of Betula pubescens Ehrh. to browsing?

by Jonathan Millett

Basic and Applied Ecology, 7(2006), 123—132.

When attempting to expand existing woodland through natural regeneration, herbivory and competition from the existing... more When attempting to expand existing woodland through natural regeneration, herbivory and competition from the existing vegetation may impede the regenerating saplings. This work addresses how browsing and competition with other vegetation interact to drive sapling growth and morphology of the widespread tree species Betula pubescens. We took above-ground morphological measurements of B. pubescens saplings within an intimate mosaic of Calluna vulgaris and Molinia caerulea, comparing saplings growing with each of the two plant species under three different red deer (Cervus elaphus) densities, allowing comparison of different levels of both past and present levels of browsing damage. Saplings growing in M. caerulea-dominated vegetation responded to reduced browsing with faster growth than those growing in C. vulgaris-dominated vegetation. However, we found that when natural browsing levels were high, browsing masked any differences in inter-specific interactions between plant species. We propose that, in regeneration schemes where deer densities are reduced, these differences should confer a competitive advantage to saplings growing with M. caerulea over those growing with C. vulgaris. Additionally, our results highlight the importance of browsing history, rather than just current browsing levels, in determining sapling growth responses under different herbivore management regimes. This study highlights the importance of multi-factor interactions in determining plant growth and morphology under different conditions. In particular we identify the prevalence of interactions between competition, herbivory and time, as determining the potential growth and morphology of B. pubescens saplings in regeneration areas. This has important implications for the management of sites where browsing impedes the natural regeneration of trees and shrubs, or where herbivore densities have been reduced to encourage woodland regeneration.

Do competition and herbivory alter the internal nitrogen dynamics of birch saplings?

by Jonathan Millett

New Phytologist, 168(2005), 413-422.

• Deciduous trees recycle nitrogen within their tissues. The aim of this study was to test the hypothesis that... more • Deciduous trees recycle nitrogen within their tissues. The aim of this study was to test the hypothesis that reductions in plant growth, caused by competition and herbivory, reduce the sink strength for N during autumn nutrient withdrawal, and reduce the storage capacity and hence the amount of N remobilized in the following spring.
• We used 15N-labelled fertilizer to quantify N uptake, leaf N withdrawal and remobilization. Betula pubescens saplings were grown with either Molinia caerulea or Calluna vulgaris, and subjected to simulated browsing damage.
• Competition reduced B. pubescens leaf N withdrawal and remobilization, with C. vulgaris having a greater effect than M. caerulea. However, simulated browsing had no significant effect on sapling N dynamics. The patterns of leaf N withdrawal and remobilization closely followed sapling dry mass.
• We conclude that the effect of competition on sapling mass reduces their N storage capacity. This reduces sink strength for leaf N withdrawal and the source strength for remobilized N. The ability of saplings to compensate for browsing damage removed any potential effect of browsing on N dynamics.

The contribution of insect prey to the total nitrogen content of sundews (Drosera spp.) determined in situ by stable isotope analysis

by Jonathan Millett

New Phytologist, 158(2003), 527-534.

• The contribution of insect prey to total N in the carnivorous plants, Drosera rotundifolia and D. intermedia , was... more • The contribution of insect prey to total N in the carnivorous plants, Drosera rotundifolia and D. intermedia , was quantified in situ and without any experimental manipulation using natural abundance stable isotope analysis.
• Samples of D. rotundifolia and D. intermedia, insects and non carnivorous reference plants were collected from three contrasting locations across Britain. The proportion of Drosera nitrogen obtained from insect prey was calculated by a mixing model using δ15N values from the different plant groups.
• The mean proportion of Drosera N derived from prey was 50%. There were significant differences in this proportion between sites, and significant differences within sites. There were significant differences between plant tissues and a significant negative relationship between the proportion of N derived from prey and the C : N ratio of Drosera tissues.
• There was little evidence of differences in prey capture/utilisation in response to N availability, possibly due to a limited range in available N between the sites. However, evidence of a positive benefit of prey capture was apparent through the decrease in C : N ratio with increasing prey N concentrations in the plants.