Jonathan Millett | Loughborough University (original) (raw)

Papers by Jonathan Millett

Biology Letters, 2014

The traps of many carnivorous plants are red in colour. This has been widely hypothesised to serv... more The traps of many carnivorous plants are red in colour. This has been widely hypothesised to serve a prey attraction function; colour has also been hypothesised to function as camouflage, preventing prey avoidance. We tested these two hypotheses in-situ for the carnivorous plant Drosera rotundifolia. We conducted three separate studies: (i) prey attraction to artificial traps to isolate the influence of colour; (ii) prey attraction to artificial traps on artificial backgrounds to control the degree of contrast; and (iii) observation of prey capture by D. rotundifolia to determine colour impacts on prey capture. Prey were not attracted to green traps and were deterred from red traps. There was no evidence that camouflaged traps caught more prey. For D. rotundifolia there was a relationship between trap colour and prey capture. However, trap colour may be confounded with other leaf traits. Thus we conclude that for D. rotundifolia red trap colour does not serve a prey attraction or camouflage function.

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"1. Grazing mammals are often used to maintain and restore high conservation value plant communit... more "1. Grazing mammals are often used to maintain and restore high conservation value plant communities, but the evidence-base for management is lacking long-term studies.

2. We erected grazing exclosures in dune slacks to determine the impact of three different grazing regimes on the plant community: (1) rabbits and sheep excluded for 36 years, (2) continued rabbit grazing for 36 years and (3) rabbit grazing for 17 years followed by rabbit and sheep grazing for 19 years. We monitored plant community composition inside and outside the exclosures.

3. All of the plant communities changed over time, moving away from the original high value system and losing some characteristic species. Grazing slowed succession, reduced woody perennial cover and increased graminoid and forb cover and species diversity. The impact of adding sheep grazing to the existing rabbit grazing was additive at the functional group scale, but both complementary and additive (depending on the species) at the plant species scale.

4. Synthesis and applications. At the levels of grazing present in this study (2.5 sheep ha-1 year-1) sheep had similar impacts on dune slack plant communities to rabbits, making them suitable for replacing or augmenting rabbit grazing for conservation management. At the intensity present in this study, long-term grazing can help to maintain a species-rich dune slack community but is not sufficient for successful restoration.
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Embryo dunes are often ephemeral, but can develop to become established coastal foredunes. In 200... more Embryo dunes are often ephemeral, but can develop to become established coastal foredunes. In 2001 a patch of embryo dunes 13.11 m2 appeared on a beach in north Lincolnshire, UK and had expanded to over 3600 m2 by 2011. The rate of expansion is linked to storm occurrence, where expansion is slowed during years with a higher incidence of storm surges. From July 2009–October 2010 seasonal changes in dune field topography were determined using terrestrial laser scanning (TLS) data. Vegetation is important in the development of embryo dunes, but can cause errors in TLS data. Tests evaluating the impact of vegetation on the TLS data suggest the minimum elevation value from the TLS point cloud within a 0.05 m grid cell gives a good approximation of the ground surface. Digital elevation models (DEMs) of the dunes constructed using filtered data showed the embryo dunes underwent a classic seasonal cycle of erosion during the winter and accretion during the summer. For example from October 2009 to April 2010 over 375 m3 of sediment was eroded from the dunes whereas during spring and summer 2010 the dune field gained over 600 m3 of sand. The overall magnitude of change in dune height and volume from season to season exceeded the errors associated with the construction of the DEM from the TLS data and the vegetation filtering process, which suggests TLS can be useful for documenting topographic change in vegetated dunes. After 10 years, the patch of embryo dunes is still expanding but has not yet merged with more established foredunes to landward. Aeolian process measurements indicate that, at present, the embryo dunes do not prevent sand from reaching the foredunes, however the rate of foredune progradation has slowed concurrently with the expansion of the embryo dune field.

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"• Carnivory in plants is presumed to be an adaptation to a low nutrient environment. Nitrogen (N... more "• Carnivory in plants is presumed to be an adaptation to a low nutrient environment. Nitrogen (N) from carnivory is expected to become a less important component of their N budget as root N availability increases.
• We investigated the uptake of N via roots versus prey of the carnivorous plant Drosera rotundifolia growing in ombrotrophic bogs along a latitudinal nitrogen deposition gradient through Sweden, using a natural abundance stable isotope mass balance technique.
• D. rotundifolia plants receiving the lowest level of N deposition obtained a greater proportion of N from prey (57%) than plants on bogs with higher N deposition (22% at intermediate and 33% at the highest deposition). When adjusted for differences in plant mass this pattern was also present when considering total prey N uptake (66, 26 and 26 µg prey N plant-1 at the low, intermediate and high N deposition sites respectively). The pattern of mass adjusted root N uptake was opposite to this (47, 75 and 86 µg N plant-1).
• D. rotundifolia plants in this study switched from reliance on prey N to reliance on root derived N as a result of increasing N availability due to atmospheric N deposition."

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We measured the effect of elevated atmospheric CO2 on atmospheric nitrogen (N2) fixation for the ... more We measured the effect of elevated atmospheric CO2 on atmospheric nitrogen (N2) fixation for the tree species Alnus glutinosa growing in monoculture or in mixture with the non-N2-fixing tree species Betula pendula and Fagus sylvatica. We addressed the hypotheses that 1: N2 fixation in A. glutinosa will increase in response to increased atmospheric CO2 concentrations, when growing in monoculture, 2: the impact of elevated CO2 on N2 fixation in A. glutinosa is the same in mixture and in monoculture and 3: the impacts of elevated CO2 on N cycling will be evident in a decrease in leaf δ15N and in the soil-leaf enrichment factor (EF), and that these impacts will not differ between mixed and single species stands. Trees were grown in a forest plantation on former agricultural fields for 4 growing seasons, after which the trees were on average 3.8 m tall and canopy closure had occurred. Atmospheric CO2 concentrations were maintained at either ambient or elevated (by 200 ppm) concentrations using a free-air CO2 enrichment (FACE) system. Leaf δ15N was measured and used to estimate the amount (Ndfa) and proportion (%Ndfa) of N derived from atmospheric fixation. On average 62% of the N in A. glutinosa leaves was from fixation. %Ndfa and Ndfa for A. glutinosa trees in monoculture did not increase under elevated CO2, despite higher growth rates. However, N2 fixation did increase for trees growing in mixture, despite the absence of significant growth stimulation. There was evidence that fixed N2 was transferred from A. glutinosa to F. sylvatica and B. pendula, but no evidence that this affected their CO2 response. This study shows that N2 fixation in A. glutinosa may be higher in a future elevated CO2 world, but that this effect will only occur where the trees are growing in mixed species stands.

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We transplanted Sphagnum ‘turfs’ containing abundant Drosera rotundifolia into an existing nitrog... more We transplanted Sphagnum ‘turfs’ containing abundant Drosera rotundifolia into an existing nitrogen deposition experiment at Whim Moss near to Edinburgh. These mesocosms received simulated N deposition in the form of either NH4+ or NO3-, to give total N deposition rates of approximately 8, 16 or 32, or 64 kg N ha-1 year-1. Simulated N deposition was added in a realistic way (i.e. with rainfall throughout the year). The δ15N of this added N was elevated relative to background N. We measured the tissue chemistry and δ15N of S. papillosum and D. rotundifolia over two years after transplant. Our aim was to determine uptake of the deposited N and the impact on S. papillosum tissue chemistry and D. rotundifolia tissue chemistry and ecology.

We found clear, significant impacts of N deposition on S. papillosum, with increased capitula N content and reduced C:N ratio. Increased δ15N indicated uptake of deposited N. The response of D. rotundifolia was less clear with impacts only at the highest rate of N deposition. There was no evidence of differential uptake of reduced or oxidised wet N deposition by either S. papillosum or D. rotundifolia.

Using the natural abundance stale isotope method we estimated the minimum contribution of prey N to the total N in D. rotundifolia to be 35%.

The results suggest that differences in the uptake of reduced or oxidised wet N deposition might not be ecologically significant when wet N deposition is added realistically. They also support the suggestion that a model of N dynamics in Sphagnum dominated ecosystems that includes the role of Sphagnum as a small scale ecosystem engineer, is required to predict accurately vascular plant responses to N deposition.

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Woodland restoration on hard-rock waste tips is often unsuccessful because of inappropriate speci... more Woodland restoration on hard-rock waste tips is often unsuccessful because of inappropriate species selection for planting and poor growth of planted trees after initial establishment. In this study we monitored the growth of three native (Betula pendula, Sorbus aucuparia and Quercus petrea) and one exotic (Alnus cordata) tree species planted in a restoration scheme on slate-waste tips in North Wales. The objectives of this study were to: (i) investigate the relationship between the relative performance of planted species, and the species’ relative success in natural regeneration at the same site; and (ii) investigate how well early performance is correlated with the subsequent success of the species. The growth of the native species did reflect the extent of their natural regeneration at the site (B. pendula > S. aucuparia = Q. petraea). However, while not present as a natural coloniser, A. cordata was the best performing species. Species’ relative growth performance after eight years was predicted by that during the first few months after planting, suggesting that short-term monitoring has value in predicting longer-term establishment success. Of the species studied A. cordata is recommended for planting on slate waste. However, B. pendula is the best of the native species studied.

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We examined experimentally the effect of competition from Calluna vulgaris or Molinia caerulea on... 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.

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When attempting to expand existing woodland through natural regeneration, herbivory and competiti... 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.

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• Deciduous trees recycle nitrogen within their tissues. The aim of this study was to test the hy... 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.

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• The contribution of insect prey to total N in the carnivorous plants, Drosera rotundifolia and ... 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.

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Talks by Jonathan Millett

Prey derived N is predicted to become less important for carnivorous plants as N availability inc... more Prey derived N is predicted to become less important for carnivorous plants as N availability increases. We tested this prediction along a latitudinal N deposition gradient in Sweden. Using natural abundance stable isotope techniques we measured in-situ the contribution of prey N to the N budget of Drosera rotundifolia.

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Carnivorous plants capture, digest and assimilate the nutrients contianed in insects. However, th... more Carnivorous plants capture, digest and assimilate the nutrients contianed in insects. However, the costs associated with capturing insects limits their growth to high moisture, high light, low nutrient environments. Thus they are particularly sensitive to changes in nutrient supply. Here, we measured the impact of increasing N deposition on the proportion of N derived from prey for D. rotundifolia using stable isotope analysis

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Conference Posters by Jonathan Millett

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Leaves of the carnivorous Drosera rotundifolia (sundew) exhibit variation in leaf pigment composi... more Leaves of the carnivorous Drosera rotundifolia (sundew) exhibit variation in leaf pigment composition. Data obtained from UK ombotrophic bogs with a wide range of N deposition, along with the manipulation of the light environment, is used to address the significance of light radiation and N availability in driving differences in D. rotundifolia leaf pigmentation.

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Elevated atmospheric CO2 is predicted to have a ‘fertilisation’ effect on forest growth – at leas... more Elevated atmospheric CO2 is predicted to have a ‘fertilisation’ effect on forest growth – at least in the short to medium term. However, this increased growth will have to be supported by an increase in nitrogen uptake or in the efficiency of N use. As a result it is predicted that forest will become progressively more N limited, eventually restricting their response to increasing atmospheric CO2. One possible exception is trees that are able to directly access N that is fixed from the atmosphere (e.g. Alnus spp.). In this study we measured the amount of N fixed by Alnus glutinosa trees growing in monoculture or in mixture with Betula pendula and Fagus sylvatica by comparing their N stable isotope composition.

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Biology Letters, 2014

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Using the natural abundance stale isotope method we estimated the minimum contribution of prey N to the total N in D. rotundifolia to be 35%.

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Understanding the drivers of vegetation change, and interactions between them, is fundamental to ... more Understanding the drivers of vegetation change, and interactions between them, is fundamental to understanding ecological systems. In this thesis I have measured the effects of competition and herbivory in driving change from heather (Calluna vulgaris) and grass (Molinia caerulea) dominated heath to birch (Betula pubescens) woodland. Invasion by birch marks the boundary of change between these two ecosystem states (heath and woodland), thus making this an interesting system for studying vegetation change. Furthermore, these two semi-natural vegetation types are both of local, national and global importance. Consequently, management often concentrates on encouraging change (for the purpose of woodland creation) or preventing change (for the purpose of conserving areas of heath).
In this thesis I report on the results of three studies in which I investigated the role of competition and browsing by large herbivores in determining the fitness of birch saplings that are established in heath vegetation. Factors influencing sapling growth and/or morphology have the potential for impacting on both the probability of invasion and the temporal and spatial pattern of invasion. Browsing by deer has previously been identified as a major driver of vegetation change in this system. However, interactions with other plants also have the potential to impact on sapling fitness, and alter the response to browsing damage.
In an observational field study I measured the morphological characteristics of B. pubescens saplings growing in C. vulgaris or M. caerulea under three different levels of herbivore densities. There was a strong effect of deer density on the growth of the saplings, indicating the importance of this driver. However, when deer densities (and therefore the amount of browsing damage) were low, sapling growth was influenced by the species of competing vegetation. Specifically, those growing with C. vulgaris were smaller than those growing with M. caerulea. This difference was absent when browsing pressure was higher.
•In an ex-situ experiment I 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 pre-senescence or at bud-burst. I measured the morphology and dry mass allocation response of the saplings over a period of two years. Additionally, by using 15N labeled fertilizer I was able to quantify N uptake, leaf-N withdrawal and remobilisation.
Competition reduced sapling dry mass by approximately 50%, but C. vulgaris reduced dry mass to a greater extent than did M. caerulea. This concurs with the patterns in-situ. The total competition intensity of C. vulgaris was greater than that of M. caerulea, due to an apparent facilitative effect of M. caerulea 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. Competition reduced B. pubescens leaf-N withdrawal and remobilisation, 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 remobilisation closely followed sapling dry mass. This indicates 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 remobilised N. The ability of saplings to compensate for browsing damage removed any potential effect of browsing on N dynamics.
Finally, I examined the role of above-ground foraging in determining the ability of B. pubescens to utilise canopy gaps and therefore, effectively compete above-ground. I manipulated the light environment of B. pubescens seedlings, creating a spatially heterogeneous environment. Additionally, I induced nutrient limitation and simulated herbivory by removing seedling meristems. Seedlings foraged for light by placing leaf area in the light side of the pot. The degree of foraging was influenced by meristem removal and nutrient limitation, due to their effect on growth. This could, in part, explain the lack of above-ground competitive effect observed in the ex-situ study.
This study highlights the importance of complexity in dynamic systems. Influences on the growth, and therefore, invasion success of birch were multifactorial. Additionally, the potential for interaction between factors further increases this complexity and contributes to the spatial and temporal heterogeneity that is characteristic of natural and semi-natural systems.

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