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Search Results: 1 - 10 of 622136 matches for " G. M. F. van der Heijden "
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Liana infestation impacts tree growth in a lowland tropical moist forest
G. M. F. van der Heijden ,O. L. Phillips
Biogeosciences (BG) & Discussions (BGD) , 2009,
Abstract: Ecosystem-level estimates of the effect of lianas on tree growth in mature tropical forests are needed to evaluate the functional impact of lianas and their potential to affect the ability of tropical forests to sequester carbon, but these are currently lacking. Using data collected on tree growth rates, local growing conditions and liana competition in five permanent sampling plots in Amazonian Peru, we present the first ecosystem-level estimates of the effect of lianas on above-ground productivity of trees. By first constructing a multi-level linear mixed effect model to predict individual-tree diameter growth model using individual-tree growth conditions, we were able to then estimate stand-level above-ground biomass (AGB) increment in the absence of lianas. We show that lianas, mainly by competing above-ground with trees, reduce tree annual above-ground stand-level biomass increment by ~10%, equivalent to 0.51 Mg dry weight ha 1 yr 1 or 0.25 Mg C ha 1 yr 1. AGB increment of lianas themselves was estimated to be 0.15 Mg dry weight ha 1 yr 1 or 0.07 Mg C ha 1 yr 1, thus only compensating ~29% of the liana-induced reduction in ecosystem AGB increment. Increasing liana pressure on tropical forests will therefore not only tend to reduce their carbon storage capacity, by indirectly promoting tree species with low-density wood, but also their rate of carbon uptake, with potential consequences for the rate of increase in atmospheric carbon dioxide.
Liana infestation impacts tree growth in a lowland tropical moist forest
G. M. F. van der Heijden,O. L. Phillips
Biogeosciences Discussions , 2009,
Abstract: Stand-level estimates of the effect of lianas on tree growth in mature tropical forests are needed to evaluate the functional impact of lianas and their potential to affect the ability of tropical forests to sequester carbon, but these are currently lacking. Using data collected on tree growth rates, local growing conditions and liana competition in five permanent sampling plots in Amazonian Peru, we present the first such estimates of the effect of lianas on above-ground productivity of trees. By constructing a multi-level linear mixed effect model to predict individual tree diameter growth model using individual tree growth conditions, we were able to estimate stand-level above-ground biomass (AGB) increment in the absence of lianas. We show that lianas, mainly by competing above-ground with trees, reduce tree annual above-ground stand-level biomass by ~10%, equivalent to 0.51 Mg dry weight ha 1 yr 1 or 0.25 Mg C ha 1 yr 1. AGB increment of lianas themselves was estimated to be 0.15 Mg dry weight ha 1 yr 1 or 0.07 Mg C ha 1 yr 1, thus only compensating ~29% of the liana-induced reduction in stand-level AGB increment. Increasing liana pressure on tropical forests may therefore not only reduce their carbon storage capacity, by indirectly promoting tree species with low-density wood, but also their rate of carbon uptake, with potential consequences for the rate of increase in atmospheric carbon dioxide.
Accounting for self-protective responses in randomized response data from a social security survey using the zero-inflated Poisson model
Maarten J. L. F. Cruyff,Ulf B?ckenholt,Ardo van den Hout,Peter G. M. van der Heijden
Statistics , 2008, DOI: 10.1214/07-AOAS135
Abstract: In 2004 the Dutch Department of Social Affairs conducted a survey to assess the extent of noncompliance with social security regulations. The survey was conducted among 870 recipients of social security benefits and included a series of sensitive questions about regulatory noncompliance. Due to the sensitive nature of the questions the randomized response design was used. Although randomized response protects the privacy of the respondent, it is unlikely that all respondents followed the design. In this paper we introduce a model that allows for respondents displaying self-protective response behavior by consistently giving the nonincriminating response, irrespective of the outcome of the randomizing device. The dependent variable denoting the total number of incriminating responses is assumed to be generated by the application of randomized response to a latent Poisson variable denoting the true number of rule violations. Since self-protective responses result in an excess of observed zeros in relation to the Poisson randomized response distribution, these are modeled as observed zero-inflation. The model includes predictors of the Poisson parameters, as well as predictors of the probability of self-protective response behavior.
Tracking Fungal Community Responses to Maize Plants by DNA- and RNA-Based Pyrosequencing
Eiko E. Kuramae, Erik Verbruggen, Remy Hillekens, Mattias de Hollander, Wilfred F. M. R?ling, Marcel G. A. van der Heijden, George A. Kowalchuk
PLOS ONE , 2013, DOI: 10.1371/journal.pone.0069973
Abstract: We assessed soil fungal diversity and community structure at two sampling times (t1 = 47 days and t2 = 104 days of plant age) in pots associated with four maize cultivars, including two genetically modified (GM) cultivars by high-throughput pyrosequencing of the 18S rRNA gene using DNA and RNA templates. We detected no significant differences in soil fungal diversity and community structure associated with different plant cultivars. However, DNA-based analyses yielded lower fungal OTU richness as compared to RNA-based analyses. Clear differences in fungal community structure were also observed in relation to sampling time and the nucleic acid pool targeted (DNA versus RNA). The most abundant soil fungi, as recovered by DNA-based methods, did not necessary represent the most “active” fungi (as recovered via RNA). Interestingly, RNA-derived community compositions at t1 were highly similar to DNA-derived communities at t2, based on presence/absence measures of OTUs. We recovered large proportions of fungal sequences belonging to arbuscular mycorrhizal fungi and Basidiomycota, especially at the RNA level, suggesting that these important and potentially beneficial fungi are not affected by the plant cultivars nor by GM traits (Bt toxin production). Our results suggest that even though DNA- and RNA-derived soil fungal communities can be very different at a given time, RNA composition may have a predictive power of fungal community development through time.
What Do Patients Consider to Be the Most Important Outcomes for Effectiveness Studies on Migraine Treatment? Results of a Delphi Study
Antonia F. H. Smelt, Mark A. Louter, Dennis A. Kies, Jeanet W. Blom, Gisela M. Terwindt, Geert J. M. G. van der Heijden, Véronique De Gucht, Michel D. Ferrari, Willem J. J. Assendelft
PLOS ONE , 2014, DOI: 10.1371/journal.pone.0098933
Abstract: Background The outcome measures most frequently used in studies on the effectiveness of migraine treatment are whether the patient is free of pain, nausea, and free of photophobia/phonophobia within two hours. However, no patient-centred outcome measures are available. Therefore, we performed an online Delphi procedure to compile a list of outcome measures deemed most important to migraine patients. Methods From a large database of migraine patients, we randomly selected 150 males and 150 females patients. We asked the open-ended question: ‘If a new medicine was developed for migraine attacks, what would you wish the effect of this medication to be?’ In the second and third rounds, we presented the answers of the first round and asked the patients to rate the importance of each item. Results The initial response rate was 56% (n = 169). In the subsequent rounds the response rates were 90% (n = 152), and 97% (n = 147), respectively. Patients wanted their attack medication to treat the headache within 30 min, to prevent the attack from getting worse, to ensure they could function properly within 1 h, and prevent the recurrence of symptoms during the same day. Conclusions The currently used outcome measures in migraine research do not sufficiently reflect the wishes of patients. Patients want the medication to work faster, to take away pain at an earlier stage, to make them able to function properly quickly, and to prevent recurrence. These aspects should be considered in future evaluation of new attack medication for migraine.
Quantified 'shock sensitivity' above the Maxwell load
J. M. T. Thompson,G. H. M. van der Heijden
Mathematics , 2013, DOI: 10.1142/S0218127414300092
Abstract: Using the static-dynamic analogy, work at Bath and Bristol has uncovered the vital organizing role of the Maxwell 'energy criterion' load in the advanced post-buckling of long-thin structures which exhibit severe shell-like imperfection sensitivity. It has become clear that above the Maxwell load, $P_M$, there are localized solutions offering an order-of-magnitude increase in sensitivity to lateral side-loads, whether static or dynamic. We propose to call this 'shock-sensitivity', and notice that so far only the seminal paper by Horak, Lord and Peletier in 2006 has quantified this in terms of an $E(P)$ energy-barrier versus load graph. In this paper we present three graphs of this nature for archetypal problems: the free twisted rod, the cylindrically constrained rod, and the strut on a softening elastic foundation. We find in all cases that the energy barrier of the localizing solution above $P_M$ is quite close to the energy of a single periodic wave. Now a single such wave is not kinematically admissible, and the corresponding periodic barrier must be for all the waves in the long structure, $N$, say. So in practice $N$ will be large, and does indeed tend to infinity with the length of the structure. Thus the sensitivity increases by a factor of a large $N$ as the Maxwell load is exceeded. This is important in its own right, and we do not seek to explain or fit curves to the scattered experimental buckling loads of shell structures.
Tension-induced multistability in inextensible helical ribbons
E. L. Starostin,G. H. M. van der Heijden
Physics , 2008, DOI: 10.1103/PhysRevLett.101.084301
Abstract: We study the non-monotonic force-extension behaviour of helical ribbons using a new model for inextensible elastic strips. Unlike previous rod models our model predicts hysteresis behaviour for low-pitch ribbons of arbitrary material properties. Associated with it is a first-order transition between two different helical states as observed in experiments with cholesterol ribbons. Numerical solutions show non-uniform uncoiling with hysteresis also occurring under controlled tension. They furthermore reveal a new uncoiling scenario in which a ribbon of very low pitch shears under tension and successively releases a sequence of almost planar loops. Our results may be relevant for nanoscale devices such as force probes.
Force and moment balance equations for geometric variational problems on curves
E. L. Starostin,G. H. M. van der Heijden
Physics , 2009, DOI: 10.1103/PhysRevE.79.066602
Abstract: We consider geometric variational problems for a functional defined on a curve in three-dimensional space. The functional is assumed to be written in a form invariant under the group of Euclidean motions. We present the Euler-Lagrange equations as equilibrium equations for the internal force and moment. Classical as well as new examples are discussed to illustrate our approach. This new form of the equations particularly serves to promote the study of bio- and nanofilaments.
Spatial chaos of an extensible conducting rod in a uniform magnetic field
D. Sinden,G. H. M. van der Heijden
Physics , 2009, DOI: 10.1088/1751-8113/42/37/375207
Abstract: The equilibrium equations for the isotropic Kirchhoff rod are known to form an integrable system. It is also known that the effects of extensibility and shearability of the rod do not break the integrable structure. Nor, as we have shown in a previous paper does the effect of a magnetic field on a conducting rod. Here we show, by means of Mel'nikov analysis, that, remarkably, the combined effects do destroy integrability; that is, the governing equations for an extensible current-carrying rod in a uniform magnetic field are nonintegrable. This result has implications for possible configurations of electrodynamic space tethers and may be relevant for electromechanical devices.
Cascade unlooping of a low-pitch helical spring under tension
E. L. Starostin,G. H. M. van der Heijden
Physics , 2008, DOI: 10.1016/j.jmps.2009.02.004
Abstract: We study the force vs extension behaviour of a helical spring made of a thin torsionally-stiff anisotropic elastic rod. Our focus is on springs of very low helical pitch. For certain parameters of the problem such a spring is found not to unwind when pulled but rather to form hockles that pop-out one by one and lead to a highly non-monotonic force-extension curve. Between abrupt loop pop-outs this curve is well described by the planar elastica whose relevant solutions are classified. Our results may be relevant for tightly coiled nanosprings in future micro- and nano(electro)mechanical devices.
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