Coarser Time Scales Research Articles

Simulating forest productivity and surface-atmosphere carbon exchange in the BOREAS study region.

A process-based, general ecosystem model (BIOME-BGC) was used to simulate daily gross primary production, maintenance and heterotrophic respiration, net primary production and net ecosystem carbon exchange of boreal aspen, jack pine and black spruce stands. Model simulations of daily net carbon exchange of the ecosystem (NEE) explained 51.7% (SE = 1.32 g C m(-2) day(-1)) of the variance in daily NEE derived from stand eddy flux measurements of CO(2) during 1994. Differences between measured and simulated results were attributed to several factors including difficulties associated with measuring nighttime CO(2) fluxes and model assumptions of site homogeneity. However, comparisons between simulations and field data improved markedly at coarser time-scales. Model simulations explained 66.1% (SE = 0.97 g C m(-2) day(-1)) of the variance in measured NEE when 5-day means of daily results were compared. Annual simulations of aboveground net primary production ranged from 0.6-2.4 Mg C ha(-1) year(-1) and were concurrent with results derived from tree increment core measurements and allometric equations. Model simulations showed that all of the sites were net sinks (0.1-4.1 Mg C ha(-1) year(-1)) of atmospheric carbon for 1994. Older conifer stands showed narrow margins between uptake of carbon by net photosynthesis and carbon release through respiration. Younger stands were more productive than older stands, primarily because of lower maintenance respiration costs. However, all sites appeared to be less productive than temperate forests. Productivity simulations were strongly linked to stand morphology and site conditions. Old jack pine and aspen stands showed decreased productivity in response to simulated low soil water contents near the end of the 1994 growing season. Compared with the aspen stand, the jack pine stand appeared better adapted to conserve soil water through lower daily evapotranspiration losses but also exhibited a narrower margin between daily net photosynthesis and respiration. Stands subjected to water stress during the growing season may exist on the edge between being annual sources or sinks for atmospheric carbon.

The measurement of “sinuosity” in correlated random walks

Many animals appear to move according to a correlated random walk, that is to say, on a sufficiently fine time scale the direction of successive steps will be in approximately the same but on a sufficiently coarse time scale the direction of successive steps will be randomly distributed around a circle. Bovet & Benhamou (1988, J. theor. Biol. 131, 419–433) have examined the technical problems associated with analysing correlated random walks and present a useful technique for measuring the sinuosity of the path by sampling the path at successive points each the same Cartesian distance from the next. Bovet and Benhamou use simulations to obtain an empirical expression relating the true sinuosity to the value obtained using their sampling technique. In this paper we derive an alternative technique based on sampling the path at successive points equally spaced in time. We are able to derive rigorous expressions for the sinuosity and the variance in the estimate of the sinuosity.

A point rainfall model based on a three-state continuous Markov occurrence process

A four-parameter three-state Markov process model for rainfall occurrence at a point is described. The model gives rise to rainfall events which consist of clusters of showers in a similar fashion to existing Poisson cluster process models, but has a more direct physical interpretation since showers do not overlap in time. Parameters may be estimated by maximum likelihood from observed runs of wet and dry hours. Runs of dry hours follow a mixed geometric distribution which is verified using hourly occurrence data for Spokane in mid-winter. The sequential occurrence structure permits a two-parameter autocorrelated exponential (shot noise) intensity process to be associated with the occurrence process which in turn permits the model to match rainfall intensity and occurrence statistics for Denver in mid-summer better than an existing five-parameter rectangular pulse Poisson cluster process model. The occurrence model can be directly related to a simpler two-state model for applications at coarser time scales. Daily statistics for the corresponding two-state model are shown to be consistent with daily statistics for the three-state model. The occurrence model can also be extended to include additional weather states of hydrological significance, such as periods of nonprecipitating cloud.

Paleoenvironmental patterns in the evolution of post-Paleozoic benthic marine invertebrates

The ecological context of large-scale evolutionary patterns has been neglected. Several workers have recently reported a bathymetric bias in the evolution of benthic marine communities, such that nearshore assemblages tend to contain advanced taxa and community structures and offshore assemblages contain more archaic features. A clade-by-clade analysis is the most powerful approach for assessing the generality of the pattern and testing hypotheses on underlying mechanisms. We develop a paleoenvironmental framework for constructing time-environment histories for higher taxa that should be of general comparative utility, based on five environmental categories recognized by physical sedimentary and biostratinomic criteria. The robustness of negative data (absence of a given taxon from a particular time and environment) is evaluated using taphonomic control groupshigher taxa with ecological, morphological, and mineralogical properties similar to those of the group under study. Within this framework we have constructed three detailed time-environment histories from the primary literature, for the crinoid Order Isocrinida (based on 99 early Triassic-Recent occurrences), the bryozoan Order Cheilostomata (67 late Jurassic-late Eocene occurrences), and the bivalve Superfamily Tellinacea (70 late Triassic-Miocene occurrences). These three time-environment diagrams as well as numerous anecdotal reports suggest that the onshore-offshore trends previously reported for communities are actually underlain by individualistic clade histories that only appear to act in concert when viewed on a coarse time scale. For these three particular higher taxa differences in rates and timing of movement through environments also falsify causal hypotheses that invoke sea-level fluctuations or mass extinctions. We outline remaining viable hypotheses for driving mechanisms and suggest further tests; at present the data are consistent with a broad array of intrinsic biological mechanisms. These pervasive shifts through time of environmental preferences severely undermine approaches to paleoenvironmental reconstruction based on interpreting fossil content in the light of present-day faunal distributions. Production of detailed timeenvironment histories for additional higher taxa will permit paleoenvironmental analyses based on overlapping environmental range zones, in which particular co-occurrences are diagnostic of different habitats at different times.

A simulation model of population dynamics of the codling moth, Cydia pomonella

A simulation model has been developed that predicts numbers and phenology of a population of codling moth, Cydia pomonella (L.), in an apple orchard. The model is a general insect population model based on the interative-cohort technique. It operates at two time scales: a fine time scale (1 h) for temperature-dependent physiological processes, and a coarse time scale (1 day) for population processes. The population is divided into a specifiable number of stages, and each stage is described by four process functions, which may be of any convenient mathematical form, and may differ among stages. Each stage is divided into cohorts of individuals born or emerged on the same day, and individuals within a cohort are considered probabilistically identical. The model simulates the processes of development, transition among stages, and mortality by using probability distributions representing these processes, and incorporates the effects of pesticides on mortality of the insect. Model output was evaluated by comparison with records of pheromone trap catches of codling moths in commercial apple orchards in North Carolina. The model predicts timing of the first spring flight well, depending on the initial age distribution used. Time between peaks of numbers of adults in the model is about 15 days longer than the observed period between flight peaks in orchards. Sensitivity analysis indicates that this discrepancy may be related to differences between measured ambient temperature and tree canopy temperature. The sensitivities of numbers of insects produced by the model, and timing of peaks in numbers present were determined for each of the parameters in the model. The parameters with greatest effect on the model output were those which control the locations of developmental rate functions and survival functions on the temperature scale. In the model, pesticides had a much larger effect on numbers of adults present than records of moths caught in pheromone traps indicate actually occurred, suggesting that moths caught in traps in commercial orchards where effective pesticides are applied may be largely immigrants.

Exact fixed points in discrete hydrodynamics

Recently a discrete formulation of hydrodynamics was introduced, which was shown to be exactly renormalizable in a certain sense: a procedure was given for computing the equations of motion on a coarse space and time scale from those on a finer scale. In this paper we carry out this coarsening procedure explicitly, giving exact numerical results for a one-dimensional diffusive system. The coarsening transformation is found to have a one-parameter family of nontrivial fixed points, parameterized by a diffusion parameterD. This result gives a new way of understanding why so many systems obey Fick's lawj = − D'dn/dx. Any of an extremely broad class of microscopic equations of motion, when viewed on a coarse enough scale, obey the fixed-point equations (which are equivalent to Fick's law). The methods used here are equally applicable to higher-dimensionality systems such as fluids.