Long-distance seed dispersal is an important topic in ecology, but notoriously difficult to quantify. Previous modeling approaches have failed to simulate long-distance dispersal, and it has remained unclear which mechanisms determine long-distance dispersal and what their relative importance is. We simulated wind dispersal of grassland plant seeds with four mechanistic models of increasing complexity and realism to assess which processes and which attributes of plants and their environment determine dispersal distances. We compared simulation results of the models to each other and to data from field dispersal experiments. The more complex and realistic models predicted short-distance dispersal more accurately and were the only models able to simulate long-distance dispersal. The model comparisons showed that autocorrelated turbulent fluctuations in vertical wind velocity are the key mechanism for long-distance dispersal. Seed dispersal distances are longest under high wind velocity conditions, when mechanically produced turbulent air movements are large. Under very low wind velocity conditions seeds are dispersed farther when there is more surface heating, but never as far as during strong wind events. Model sensitivity analyses showed that mean horizontal wind velocity, seed release height, and vegetation height are crucial determinants of dispersal potential and dispersal distances. Between plant species (but not within a species), seed terminal velocity is an additional important determinant of long-distance dispersal. These results imply that seed release height is the most important plant-controlled dispersal parameter for grassland plants, and that the structure of the local vegetation greatly affects dispersal distances. Thus, management plans for grasslands should take into account that changes in vegetation structure, e.g., due to eutrophication, can reduce the seed dispersal ability of wind-dispersed plant species.

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Determinants of long-distance seed dispersal by wind in grasslands

Multi-site evaluation of terrestrial evaporation models using FLUXNET data

Thefluxcontributionofcoherentstructurestothetotalexchangeofenergy and matter is investigated in a spruce canopy of moderate density in heterogeneous, complex terrain. The study deploys two methods of analysis to estimate the coherent exchange: conditional averages in combination with wavelet analysis, and quadrant analysis. The data were obtained by high-frequency single-point measurements using sonic anemometers and gas analysers at five observation heights above and within the canopy and subcanopy, and represent a period of up to 2.5months. The study mainly addresses the momentum transfer and exchange of sensible heat throughout the roughness sublayer, while results are provided for the exchange of carbon dioxide and water vapour above the canopy. The magnitude of the flux contribution of coherent structures largely depends on the method of analysis, and it is demonstrated that these differences are attributed to differences in the sampling strategy between the two methods. Despite the differ- ences, relational properties such as sweep and ejection ratios and the variation of the flux contribution with height were in agreement for both methods. The sweep phase of coherentstructures is the dominantprocess close to and within the canopy, whereas the ejections gain importance with increasing distance to the canopy. The efficiency of the coherent exchange in transporting scalars exceeds that for momentum by a factor of two. The occurrence of coherent structures results in a flux error less than 4% for the eddy-covariance method. Based on the physical processes identified from the analysis of the ejection and sweep phases along the vertical profile in the rough- ness sublayer, a classification scheme for the identification of exchange regimes is developed. This scheme allows one to estimate the region of the canopy participat- ing in the exchange of energy and matter with the above-canopy air under varying environmental conditions.

Flux contribution of coherent structures and its implications for the exchange of energy and matter in a tall spruce canopy

In the atmospheric surface layer, it is widely accepted that ejection andsweep eddy motions, typically associated with coherent structures, areresponsible for much of the land-surface evaporation, sensible heat, andmomentum fluxes. The present study analyzes the ejection-sweep propertiesusing velocity and scalar fluctuation measurements over tall natural grassand bare soil surfaces. It is shown that momentum ejections and sweeps occurat equal frequencies (D eject ≈ D sweep ≈ 0.29) irrespective of surfaceroughness length or atmospheric stability conditions. Also, their magnitudesare comparable to values reported from open channel velocity measurements (Dsweep ≈ 0.33; D eject ≈: 0.30). The scalar D eject is constant andsimilar in magnitude to the momentum D eject( ≈ 0.29) over both surfacesand for a wide range of atmospheric stability conditions, in contrast to thescalar D sweep. The scalar sweep frequency is shown to depend on the scalarskewness for the dynamic convective and free convective sublayers, but isidentical to D eject for the dynamic sublayer. The threshold scalar skewnessat which the D sweep dependence occurs is 0.25, in agreement with theaccepted temperature skewness value at near-neutral conditions. In contrastto a previous surface-layer experiment, this investigation demonstrates thatthe third-order cumulant expansion method (CEM) reproduces the measuredrelative flux contribution of ejections and sweeps (ΔS0) for momentumand scalars at both sites. Furthermore, a linkage between ΔS0 and thescalar variance budget is derived via the third-order CEM in analogy tomomentum. It is shown that ΔS0 can be related to the flux divergenceterm and that such a relationship can be estimated from surface-layersimilarity theory, and the three sublayer model of Kader and Yaglom andproposed similarity functions.

/pdf/the-ejection-sweep-character-of-scalar-fluxes-in-the-7wjixea73g.pdf

The ejection-sweep character of scalar fluxes in the unstable surface layer

Ejection and sweep eddy motions in the atmospheric surface layer (ASL) are widely accepted as being responsible for much of land surface evaporation, sensible heat flux, and momentum flux; however, less is known about this type of eddy motion within the canopy sublayer (CSL) of forested systems. The present study analyzed the ejection-sweep properties at the canopy-atmosphere interface of a 13 m tall, uniformly aged southern loblolly pine stand and a 33 m tall, unevenly aged hardwood stand using velocity and scalar (temperature, water vapor, and carbon dioxide) fluctuation measurements at the canopy-atmosphere interface. It was found that the measured sweeps and ejections time fractions for scalars and momentum are comparable and are in good agreement with other laboratory and field experiments. This investigation demonstrates that the third-order cumulant expansion method (CEM) reproduces the measured relative flux contribution of ejections and sweeps (ΔS 0 ) and the difference between sweep and ejection time fractions for both momentum and scalars at the canopy-atmosphere interface in contrast to findings from a previous ASL experiment. A linkage between ΔS 0 and the scalar flux budget is derived and tested via the third-order CEM at the canopy-atmosphere interface for the pine and the hardwood stands. It is shown that ΔS 0 can be related to the dimensionless scalar flux transport term whose gradient is central to the scalar variance budget. Also, the derived relationship is independent of canopy roughness or scalar sources and sinks. Hence this investigation establishes an analytical linkage between second-order closure models, the ejection-sweep cycle, and third-order CEM at the canopy-atmosphere interface. Dissimilarity between the ejection-sweep cycle for scalar and momentum transport is considered via conditional probability distributions at both forest stands. In contrast to a laboratory heat dispersion experiment, it is shown that while the ejection-sweep cycles for scalar and momentum transport are intimately linked, they are not identical. Therefore the results from momentum ejection-sweeps investigations cannot be extrapolated to scalar transport. Comparisons with other laboratory experiments are also discussed, especially in relation to the scalar ejection and sweep time fractions.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/97JD00777

Turbulent eddy motion at the forest‐atmosphere interface

Eddy-correlation measurements above an uneven-aged forest, a uniform-irrigated bare soil field, and within a grass-covered forest clearing were used to investigate the usefulness of the fluxvariance method above uniform and non-uniform terrain. For this purpose, the Monin and Obukhov (1954) variance similarity functions were compared with direct measurements. Such comparisons were in close agreement for momentum and heat but not for water vapor. Deviations between measured and predicted similarity functions for water vapor were attributed to three factors: 1) the active role of temperature in surface-layer turbulence, 2) dissimilarity between sources and sinks of heat and water vapor at the ground surface, and 3) the non-uniformity in water vapor sources and sinks. It was demonstrated that the latter non-uniformity contributed to horizontal gradients that do not scale with the vertical flux. These three factors resulted in a turbulence regime that appeared more efficient in transporting heat than water vapor for the dynamic convective sublayer but not for the dynamic sublayer. The agreement between eddy-correlation measured and flux-variance predicted sensible heat flux was better than that for latent heat flux at all three sites. The flux-variance method systematically overestimated the latent heat flux when compared to eddy-correlation measurements. It was demonstrated that the non-uniformity in water vapor sources reduced the surface flux when compared to an “equivalent” uniform terrain subjected to identical shear stress, sensible heat flux, and atmospheric water vapor variance. Finally, the correlation between the temperature and water vapor fluctuations was related to the relative efficiency of surface-layer turbulence in removing heat and water vapor. These relations were used to assess critical assumptions in the derivation of the flux-variance formulation.

Yung Cheng

Papers

Estimation of surface heat and momentum fluxes using the flux-variance method above uniform and non-uniform terrain