Modelling Grassland Ecosystems

In this contribution a view of the promise and difficulties of modelling grassland is given. This is largely centred around work with a grassland ecosystem simulator known as the Hurley Pasture Model. A brief introduction sets forth possible reasons for building a large ecosystem model, and stresses the importance of modelling objectives. It is suggested that a model is de rigeur for any research programme which aims to take a firm grasp of the complex responses of grassland. Mechanistic models are required to provide the understanding needed for intelligent and flexible management of grassland, whatever the prevailing environmental or economic objectives. The models are necessarily large, reflecting the complexity of the ‘real’ system, and, in a sense, are ‘big’ science. The challenge is to develop models of ‘engineering strength’. This requires an appropriate research environment, which should be reasonably stable, multidisciplinary, well-connected to experimental programmes, and permit adequate support for the three essential legs of an ecosystem model: development, documentation, and application. Some modelling researchers are dismayed by the wasteful fragmentation of many plant ecosystem modelling research programmes. Next an outline account of the Hurley Pasture Model (HPM) is given. Most plant ecosystem models are now quite similar at the qualitative level, and few would dispute the statement that a reasonable level of consensus is emerging. The HPM is a standard model of the genre. It comprises plant, animal, soil and water submodels. To-date there is no phenology submodel. There are environmental and management drivers, the former accepting monthly, daily or diurnal data, the latter permitting the simulation of fertilizer, grazing, and cutting scenarios, more or less ad libitum. Recent developments of the HPM include a submodel to take account of acclimation of photosynthesis to light, nitrogen, carbon dioxide (‘down-regulation’) and temperature; and a simple method of using the HPM to simulate legume dynamics in a grass-legume pasture. Finally, some applications of the model are presented, relating to fertilizer application, grazing, harvesting, and climate change. These are to illustrate the scope of the model, for both application and understanding. The last application shows that in grassland ecosystems climate change responses can be greatly affected by (i) a variable legume content; (ii) management, e.g. how the crop is grazed or cut; and (iii) water stress, as occurs in southern Britain. The impact of climate change on grassland ecosystems is of particular interest. It is known that, in a constant climate, a grassland ecosystem can take hundreds of years to come to equilibrium. Experiments cannot address the problem directly. Short-term experiments can give very variable responses, depending on conditions, which are often misleading, even opposite in sign from long-term responses. Mechanistic models provide a clear framework for unifying these variable results, understanding why they arise, and making predictions about the future time course of plant ecosystems. There seems to be no other way of doing this work

How to Measure Subdiffusion Parameters

We propose a method to measure the subdiffusion parameter $\alpha$ and subdiffusion coefficient $D_{\alpha}$ which are defined by means of the relation $=\frac{2D_\alpha} {\Gamma(1+\alpha)} t^\alpha$ where $$ denotes a mean square displacement of a random walker starting from $x=0$ at the initial time $t=0$. The method exploits a membrane system where a substance of interest is transported in a solvent from one vessel to another across a thin membrane which plays here only an auxiliary role. We experimentally study a diffusion of glucose and sucrose in a gel solvent, and we precisely determine the parameters $\alpha$ and $D_{\alpha}$, using a fully analytic solution of the fractional subdiffusion equation.Comment: short version of cond-mat/0309072, to appear in Phys. Rev. Let

The influence of emotional valence on word recognition in people with aphasia

Although several studies have demonstrated that emotional valence facilitates lexical processing in neurotypical adults, there has been limited work involving people with aphasia. This study explored the effects of valence (valenced/neutral) and polarity (positive/negative) on single word processing. Twenty people with chronic aphasia and 20 neurotypical controls completed a written lexical decision task in which valence was manipulated. An effect of valence (i.e. better performance for valenced vs. neutral words) was found in both accuracy and response time in the aphasic group and in response time for controls. Both groups showed an effect of polarity, with aphasic participants generating fewer errors for positive (vs. negative and neutral) stimuli, and controls responding more quickly to positive (vs. neutral) stimuli. Additionally, performance with positive words was impacted less by aphasia severity than negative and neutral words. The results highlight the importance of valence as a psycholinguistic factor in aphasia assessment and intervention

Infrared dust emission in the outer disk of M51

We examine faint infrared emission features detected in Spitzer Space Telescope images of M51, which are associated with atomic hydrogen in the outer disk and tidal tail at R greater than R_25 (4.9', ~14 kpc at d=9.6 Mpc). The infrared colors of these features are consistent with the colors of dust associated with star formation in the bright disk. However, the star formation efficiency (as a ratio of star formation rate to neutral gas mass) implied in the outer disk is lower than that in the bright disk of M51 by an order of magnitude, assuming a similar relationship between infrared emission and star formation rate in the inner and outer disks.Comment: 13 pages in manuscript form, 2 figures; download PDF of manuscript with original-resolution Figure 1 at http://www.eg.bucknell.edu/physics/thornley/thornleym51.pd

Modelling allocation with transport/conversion processes.

A shoot:root carbon:nitrogen allocation model, based on the two processes of transport and chemical conversion, is described and explored. The view is proposed that all allocation models, whether built for the purposes of theoretical investigation or practical application, should start with this irreducible framework. In the present implementation, the processes operate according to: for substrate sources, dependence on shoot and root sizes, with possible product inhibition; for transport, movement down a substrate concentration gradient; for substrate sinks or utilization, linear bisubstrate kinetics. The dynamic and equilibrium properties of the model are explored. Failure of this approach to allocation will indicate to the modeller that additional mechanisms to control the processes are needed, and the mode of failure will indicate the type of mechanisms required. Additional mechanisms are discussed which may involve hormones or teleonomic (goal-seeking) controls, and may be added to the irreducible framework. However, these additions should not replace the irreducible framework of transport and chemical conversion, because they do not in reality. Modifications to the basic model to reflect some possibilities such as ontogenesis with the transition from exponential growth towards a steady state or with the scaling of within-plant transport resistances, the influence of hormones, and active transport, are described

White cell count in the normal range and short-term and long-term mortality: international comparisons of electronic health record cohorts in England and New Zealand

OBJECTIVES: Electronic health records offer the opportunity to discover new clinical implications for established blood tests, but international comparisons have been lacking. We tested the association of total white cell count (WBC) with all-cause mortality in England and New Zealand. SETTING: Primary care practices in England (ClinicAl research using LInked Bespoke studies and Electronic health Records (CALIBER)) and New Zealand (PREDICT). DESIGN: Analysis of linked electronic health record data sets: CALIBER (primary care, hospitalisation, mortality and acute coronary syndrome registry) and PREDICT (cardiovascular risk assessments in primary care, hospitalisations, mortality, dispensed medication and laboratory results). PARTICIPANTS: People aged 30-75 years with no prior cardiovascular disease (CALIBER: N=686 475, 92.0% white; PREDICT: N=194 513, 53.5% European, 14.7% Pacific, 13.4% Maori), followed until death, transfer out of practice (in CALIBER) or study end. PRIMARY OUTCOME MEASURE: HRs for mortality were estimated using Cox models adjusted for age, sex, smoking, diabetes, systolic blood pressure, ethnicity and total:high-density lipoprotein (HDL) cholesterol ratio. RESULTS: We found 'J'-shaped associations between WBC and mortality; the second quintile was associated with lowest risk in both cohorts. High WBC within the reference range (8.65-10.05×10(9)/L) was associated with significantly increased mortality compared to the middle quintile (6.25-7.25×10(9)/L); adjusted HR 1.51 (95% CI 1.43 to 1.59) in CALIBER and 1.33 (95% CI 1.06 to 1.65) in PREDICT. WBC outside the reference range was associated with even greater mortality. The association was stronger over the first 6 months of follow-up, but similar across ethnic groups. CONCLUSIONS: Clinically recorded WBC within the range considered 'normal' is associated with mortality in ethnically different populations from two countries, particularly within the first 6 months. Large-scale international comparisons of electronic health record cohorts might yield new insights from widely performed clinical tests. TRIAL REGISTRATION NUMBER: NCT02014610