Half-hourly Time Scale Research Articles

Plant Propagation Fronts and Wind Dispersal: An Analytical Model to Upscale from Seconds to Decades Using Superstatistics

Scale separation crossing many orders of magnitude is a consistent challenge in the ecological sciences. Wind dispersal of seed that generates plant propagation fronts is a typical case where timescales range from less than a second for fast turbulent processes to interannual timescales governing plant growth and climatic forcing. We show that the scale separation can be overcome by developing mechanistic and statistical links between processes at the different timescales. A mechanistic model is used to scale up from the turbulent regime to hourly timescales, while a superstatistical approach is used to relate the half-hourly timescales to annual vegetation migration speeds. We derive a semianalytical model to predict vegetation front movement as a function of wind-forcing statistics and characteristics of the species being dispersed. This model achieves better than order-of-magnitude agreement in a case study of tree dispersal from the early Holocene, a marked improvement over diffusion models. Plant migration is shown to depend nonlinearly on the wind environment forcing the movement but linearly on most physiological parameters. Applications of these analytical results to parameterizing models of plant dispersion and the implications of the superstatistical approach for addressing other ecological problems plagued by similar "dimensionality curses" are outlined.

Estimating net ecosystem exchange in a patterned ecosystem: Example from blanket bog

Net ecosystem exchange (NEE) was measured in a patterned peatland with eddy covariance (EC) and chamber methods during a 12-month period. The peatland surface was composed of microforms characterized by a difference in water level and vegetation composition. The distribution of microforms varied spatially within the peatland. To achieve correspondent half-hourly EC and chamber NEE estimates, we modelled microform level chamber fluxes, estimated them at each instance of weather recordings and integrated them over the year. We then scaled the fluxes up to the EC footprint. On a half-hourly time scale the correlation coefficient ( R) of the NEE between the methods was 0.80 and the slope of regression 0.91. Measurements made in summer and during daylight were more highly correlated than measurements made in winter and during darkness. When integrated on a monthly time scale the methods agreed better, with R of 0.98 and slope of regression 1.00. The annual NEE for the EC and chamber methods were 206 and 242 g(CO 2) m −2, respectively. The study confirmed that the surface pattern of the EC footprint in the blanket bog was sufficiently homogeneous, that the changing wind direction did not influence the half-hourly NEE. However, the chamber estimates found that the annual NEE of the driest area within the footprint was 130% larger than that in the wettest area, indicating that large spatial variation can be found in NEE.

Modelling plant carbon and nitrogen dynamics of a boreal aspen forest in CLASS — the Canadian Land Surface Scheme

The formulation of the physiological processes of plant carbon and nitrogen that have recently been implemented in CLASS — the Canadian Land Surface Scheme is presented, including plant photosynthesis of sunlit and shaded leaves, root nitrogen uptake, tissue respiration, growth, and senescence. Results from this formulation provide the vegetation parameters (e.g. stomatal resistance, leaf area index, and root length and distribution) for simulating energy and water exchange, and contribute to the carbon and nitrogen biogeochemical calculations and ecosystem CO 2 flux estimations in CLASS. The model was parameterized for deciduous trees and run at a time step of 30 min. Modelled results were tested with measurements of leaf CO 2 exchange, half-hourly and daily plant CO 2 exchange, and annual plant carbon budgets from the Old Aspen ( Populus tremuloides) site in the Southern Study Area (SSA-OA) of the Boreal Ecosystem-Atmosphere Study (BOREAS). Model tests at leaf level and half-hourly timescale improve confidence in the predictive capabilities of the model at canopy level and longer timescale. At half-hourly timescale, simulated plant CO 2 exchange explained 81% of the variance in the measured CO 2 flux derived from eddy correlation measurements and soil chamber measurements in the three comparing weeks in 1994. At a coarser timescale, simulated daily plant CO 2 exchange explained 86% of the variance in the measured values during 1994 and 1996. Annual gross primary production (GPP) accumulated from half-hourly values in the model in 1994 and 1996 averaged about 1053 g C m −2 for this aspen site, close to the estimations from field eddy correlation measurements. Of this carbon fixation, 59% was consumed by autotrophic respiration ( R a) for plant growth and maintenance in the model, similar to estimations from chamber measurements. Sensitivity analyses show that the modelled plant GPP, R a and net primary production NPP (=GPP− R a) were very sensitive to the changes in temperature, implying that this boreal aspen ecosystem is strongly constrained by temperature conditions.