The Tropical Rain Forest

Comparative measurements of radiation flux components and turbulent fluxes of energy and CO

Comparative measurements and seasonal variations in energy and carbon exchange over forest and pasture in South West Amazonia

Measurements of direct and diffuse solar radiation were made at fifteen randomly distributed locations beneath a pine canopy and a hardwood canopy. A model that identifies four components of the solar radiation appearing beneath tree canopies is used to analyse the data. Direct beam radiation appearing below the canopy was found to follow an exponential-extinction law in the pine forest and a constant-ratio law in the hardwood forest. The large number of randomly distributed radiometers permitted precise estimates of the number of instruments needed to sample the radiation for different averaging periods. Under the pine canopy, 412 radiometers would be needed to estimate the instantaneous radiation with a standard error of the mean of 10 mly/min. For a full-day average, the number is 10. Beneath the hardwood canopy, the number needed for an instantaneous average is 18, while only one is needed for a full-day average.

Spatial and temporal distribution of solar radiation beneath forest canopies.

Measurements of shortwave and longwave components of the radiation budget were made above and below tropical forest in the Amazon Basin for a total duration of 203 hours in September 1983. Albedo was (12.25±0.2)%, and showed a small variation with solar altitude. The net outward flux of longwave radiation was around 30 W m−2, and fairly constant over the day. The relationship between net radiation, R, and solar radiation, S, was adequately described by the expression R = (0.858±0.006)S - (35.0±1.9) (W m−2). The fraction of solar radiation reaching the bottom of the canopy was low, about one per cent: net radiation in this position was consistent with low soil heat flux and low soil evaporation.

Observations of radiation exchange above and below Amazonian forest.

Summarizing observed dry-canopy evaporation (hereafter, evaporation) data from earlier papers, we developed a scheme for forest categorization according to evaporation rates in the growing season. Evaporation rates were represented by the Priestley–Taylor coefficient α calculated for daytime. We examined relationships between forest properties (e.g. climatic regions, leaf types) and α values. We obtained α data for 67 forest sites from earlier papers. Based on these data, we found (i) a clear difference in α values between broad-leaved and coniferous forests, (ii) a greater variation in α values between individual coniferous forests than between individual broad-leaved forests, and (iii) a clear relationship between canopy height and α values for coniferous forests. These three results were supported by surface conductance data summarized from earlier papers. We concluded that forests should be primarily classified into broad-leaved and coniferous forests, and that coniferous forests should be further classified according to canopy height. This classification scheme is applicable only to forests with projected leaf area index (LAI) ≥3·0. Regardless of this LAI limitation, this classification will be useful because many forests satisfy this LAI limitation.

This paper shows valuable results in the following two respects. First, this paper explicitly shows the difference in evaporation rates between broad-leaved and coniferous forests. Although this difference would have been implicitly recognized, this difference has not been shown based on adequate amounts of observed data. Second, it is shown that classifying coniferous forests according to canopy height is as important as classifying forests according to leaf type (broad-leaved or coniferous). Although studies have recognized the effect of canopy height on evaporation rates, the significant effect of canopy height on evaporation rates, compared with the effects of other factors on evaporation rates, has not previously been shown. Copyright © 2005 John Wiley & Sons, Ltd.

Forest categorization according to dry‐canopy evaporation rates in the growing season: comparison of the Priestley–Taylor coefficient values from various observation sites

The present study involved determination of the experimental energy receipt partitioning over the tropical Amazon forest. Diurnal variation of net radiation (Q*), sensible heat flux (QH) and latent heat flux (QE) is presented. The daytimeQE is in phase withQ* and it is always an important term in the energy balance. The daily averagedQE is 59 to 100% of the dailyQ* whereasQH is 5 to 28% at the Amazon forest site (2° 57′ S; 59° 57′ W) for the sample periods. The results present evidence thatQE over the Amazon forest is greater thanQ* in the afternoon hours. The role of sensible heat advection in maintaining largeQE over the forest surface is discussed. Hourly Bowen ratio (β) values for two campaigns of the Amazon forest micrometeorological experiment are presented. During daylight hours, the differences inβ are not significant, and exhibit a systematic pattern. The only time that the variation in Bowen ratio increased significantly was at sunrise and sunset when the thermal structure of the air was changing from a strong inversion to lapse and vice versa. The diurnalβ values changed from −3.50 to 0.85. The mean hourlyβ calculated from values from 07.00 to 16.00 h, varied from 0.05 to 0.85.

L. D. de Abreu Sá

Papers

Energy flux partitioning over the Amazon forest