Summary
We examine the notion that scalars are transported along their mean concentration gradients in the air space of the canopy. Recent observations and theory indicate that this concept is both inappropriate and misleading. Independent measurements of the fluxes and gradients of heat, water vapour and CO2 in a forest canopy show that counter-gradient fluxes are common. The intermittency of the transport processes and their large scale are seen as important reasons for this.
Eddy-correlation and/or ecological techniques seem to be the only viable alternatives for measuring flux densities and source-sink strengths at present, but the logistical problems are formidable.
For modelling exchange processes at leaf surfaces, hence source-sink distributions, analyses based on the gradient-diffusion concept may not be too much in error in as much as they employ essentially correct descriptions of transfer across leaf boundary-layers, if not in the canopy air space. An empirical description of transport in the latter may suffice.
The utility of alternative models of scalar transport based on the nature of canopy turbulence is examined. Second-order closure models appear to have great pedagogic value in identifying the existence and relative importance of mechanisms for the production, transport and dissipation of scalar fluxes, but they are of limited use for prediction. Lagrangian models, though, appear to predict dispersion and profile development very well, provided the source distribution is known. However, the inverse problem of inferring source distributions from the concentration profiles remains a challenge.
Similar content being viewed by others
References
Brown KW, Covey W (1966) The energy-budget evaluation of the micrometeorological transfer processes within a cornfield. Agric Meteorol 3:73
Coppin PA, Raupach MR, Legg BJ (1986) Experiments on scalar dispersion within a model plant canopy Part II: An elevated plane source. Boundary-Layer Meteorol 35:167
Corrsin S (1974) Limitations of gradient transport models in random walks and in turbulence. Adv Geophys 18A:25
Cowan IR (1968) Mass, heat and momentum exchange between stands of plants and their atmospheric environment. Q J R Meteorol Soc 94:523
Denmead OT (1964) Evaporation sources and apparent diffusivities in a forest canopy. J Appl Meteorol 3:383
Denmead OT (1973) Relative significance of soil and plant evaporation in estimating evapotranspiration. In: Plant response to climatic factors. Proceedings Uppsala Symposium 1970, UNESCO, Paris, p 505
Denmead OT (1984) Plant physiological methods for studying evaporation: Problems of telling the forest from the trees. Agric Water Manag 8:167
Denmead OT, Bradley EF (1985) Flux-gradient relationships in a forest canopy. In: Hutchison BA, Hicks BB (eds) The forest-atmosphere interaction. Reidel, Dordrecht, p 421
Denmead OT, McIlroy IC (1971) Measurement of carbon dioxide exchange in the field. In: Sestak Z, Catsky J, Jarvis PG (eds) Plant photosynthetic production manual of methods. Junk, The Hague, p 467
Denmead OT, Freney JR, Simpson JR (1976) A closed ammonia cycle within a plant canopy. Soil Biol Biochem 8:161
Droppo JG, Hamilton HL (1973) Experimental variability in the determination of the energy balance in a deciduous forest. J Appl Meteorol 12:781
Druilhet A, Perrier A, Fontan J, Laurent JL (1971) Analysis of turbulent transfer in vegetation: Use of thoron for measuring the diffusivity profiles. Boundary-Layer Meteorol 2:173
Farquhar GD, Ball MC, von Caemmerer S, Roksandic Z (1982) Effect of salinity and humidity on δ13C value of halophytes-evidence for diffusional isotope fractionation determined by the ratio of intercellular/atmospheric partial pressure of CO2 under different environmental conditions. Oecologia (Berlin) 52:121
Finnigan JJ (1979) Turbulence in waving wheat II. Structure of momentum. Boundary-Layer Meteorol 16:213
Finnigan JJ (1985) Turbulent transfer in flexible plant canopies. In: Hutchison BA, Hicks BB (eds) The forest-atmosphere interaction. Reidel, Dordrecht, p 443
Finnigan JJ, Raupach MR (1987) Transfer processes in plant canopies in relation to stomatal characteristics. In: Zeiger E, Farquhar G, Cowan I (eds) Stomatal function. Stanford University Press, Stanford, CA (in press)
Goudriaan J, Waggoner PE (1972) Simulating both aerial microclimate and soil temperature from observations above the foliar canopy. Neth J Agric Sci 20:104
Impens II (1973) Daytime distribution of energy sinks and sources and transfer processes within a sunflower canopy. In: Plant response to climatic factors. Proceedings Uppsala Symposium 1970, UNESCO, Paris, p 357
Legg BJ (1975) Turbulent diffusion within a wheat canopy: I Measurement using nitrous oxide. Q J R Meteorol Soc 101:597
Legg BJ, Long IF (1975) Turbulent diffusion within a wheat canopy: II. Results and interpretation. Q J R Meteorol Soc 101:611
Legg BJ, Monteith J (1975) Heat and mass transfer within plant canopies. In: Vries DA de, Afgan NH (eds) Heat and mass transfer in the biosphere I. Transfer processes in the plant environment. Scripta, Washington DC, p 167
Legg BJ, Raupach MR (1982) Markov-chain simulation of particle dispersion in inhomogeneous flows: The mean drift velocity induced by a gradient in Eulerian velocity variance. Boundary-Layer Meteorol 24:3
Legg BJ, Raupach MR, Coppin PA (1986) Experiments on scalar dispersion within a plant canopy Part III: An elevated line source. Boundary-Layer Meteorol 35:277
Lemon ER, Wright JL (1969) Photosynthesis under field conditions. XA. Assessing sources and sinks of carbon dioxide in a corn (Zea mays L) crop using a momentum balance approach. Agron J 61:405
Leuning R, Ohtaki E, Denmead OT, Lang ARG (1982) Effects of heat and water vapor transport on eddy covariance measurements of CO2 fluxes. Boundary-Layer Meteorol 23:255
McBean GA (1968) An investigation of turbulence within the forest. J Appl Meteorol 7:410
Medina E, Minchin P (1980) Stratification of δ13C values of leaves in Amazonian rain forests. Oecologia (Berlin) 45:377
Penman HL, Long IF (1960) Weather in wheat: an essay in micro-meteorology. Q J R Meteorol Soc 86:16
Raupach MR (1986) Turbulent transfer in plant canopies. In: Russell G, Marshall B, Jarvis PG (eds) Plant canopies — their growth, form and function. Cambridge University Press, Cambridge (in press)
Raupach MR (1987) A Lagrangian analysis of scalar transfer in vegetation canopies. Q J R Meteorol Soc (in press)
Raupach MR, Coppin PA, Legg BJ (1986) Experiments on scalar dispersion within a model canopy Part I: The turbulence structure. Boundary-Layer Meteorol 35:21
Seginer I, Mulhearn PJ, Bradley EF, Finnigan JJ (1976) Turbulent flow in a model plant canopy. Boundary-Layer Meteorol 10:423
Shaw RH (1985) On diffusive and dispersive fluxes in forest canopies. In: Hutchison BA, Hicks BB (eds) The forest-atmosphere interaction. Reidel, Dordrecht, p 407
Shaw RH, Tavangar J, Ward DP (1983) Structure of the Reynolds stress in a canopy layer. J Clim Appl Meteorol 22:1922
Sinclair TR, Murphy Jr CE, Knoerr KR (1976) Development and evaluation of simplified models for simulating canopy photosynthesis and transpiration. J Appl Ecol 13:813
Stewart DW, Lemon ER (1969) The energy budget at the earth's surface: a simulation of net photosynthesis of field corn. US Army ECOM Technical Report 2-68I-6 US Army Electronics Command, Fort Huachuca, Arizona
Taylor GI (1921) Diffusion by continuous movements. Proc London Math Soc A20:196
Thom AS (1971) Momentum absorbed by vegetation. Q J R Meteorol Soc 97:414
Uchijima Z (1962) Studies on the microclimate within the plant communities. (1) On the turbulent transfer coefficient within plant layer. J Agric Metecrol (Tokyo) 18:1
Uchijima Z (1970) Carbon dioxide environment and flux within a corn crop canopy. In: Prediction and measurement of photosynthetic activity. Proceedings of IBP/PP Technical Meeting, Trebon, Czechoslovakia 1969. Centre for Agricultural Publishing and Documentation, Wageningen, Netherlands, p 179
Vogel JC (1978) Recycling of carbon in a forest environment. Oecol Plant 13:89
Waggoner PE, Furnival GM, Reifsnyder WE (1969) Simulation of the microclimate in a forest. Forest Sci 15:37
Webb EK, Pearman GI, Leuning R (1980) Correction of flux measurements for density effects due to heat and water vapour transfer. Q J R Meteorol Soc 106:85
Wilson JD, Legg BJ, Thomson DJ (1983) Calculation of particle trajectories in the presence of a gradient in turbulent-velocity variance. Boundary-Layer Meteorol 27:163
Wilson JD, Thurtell GW, Kidd GE (1981) Numerical simulation of particle trajectories in inhomogeneous turbulence. II: Systems with variable turbulent velocity scale. BoundaryLayer Meteorol 21:423
Wilson JD, Ward DP, Thurtell GW, Kidd GE (1982) Statistics of atmospheric turbulence within and above a corn canopy. Boundary-Layer Meteorol 24:495
Wilson NR, Shaw RH (1977) A higher order closure model for canopy flow. J. Appl Meteorol 16:1197
Wright JL, Brown KW (1967) Comparison of momentum and energy balance methods of computing vertical transfer within a crop. Agron J 59:427
Author information
Authors and Affiliations
Rights and permissions
About this article
Cite this article
Denmead, O.T., Bradley, E.F. On Scalar Transport in Plant Canopies. Irrig Sci 8, 131–149 (1987). https://doi.org/10.1007/BF00259477
Received:
Issue Date:
DOI: https://doi.org/10.1007/BF00259477