Solar Radiation and Productivity in Tropical Ecosystems

J. L. MonteithThe Journal of Applied Ecology, Vol. 9, No. 3 (Dec

1972), 747-766

Presented by Dahl WintersGeog 595, March 20, 2007

Introduction

Ecosystems can be treated as thermodynamic machines. To assess their efficiency, we can divide the useful, absorbed energy in an ecosystem by the total energy input into the ecosystem.

Solar constant = solar energy received at the outer surface of the Earth’s atmosphere = 1360 W/m2 = 1.36 kJ/m2/s. Energy stored by plants = 17 kJ/g dry matter. Solar constant = production of dry matter at a rate of about 1 g/m2 every 12 s, 7 kg/m2/day, or 2.6 tons/m2/yr.

Annual agricultural crop yield = between 1 and 30-60 tons/ha = an efficiency of 0.2-0.004% of the integrated solar constant. (This is the amount of solar energy that gets converted by photosynthesis into dry matter available for living things.)

To understand how plants respond to their environment, we need to relate efficiencies of dry matter production to growth rate-determining factors, e.g. fraction of radiation intercepted by a leaf canopy irradiance of individual leaves diffusion resistance of stomata behavior of the photochemical system

Components of Efficiency

Efficiency of dry matter production in natural communities = the net amount of solar energy stored by photosynthesis in any period / the solar constant integrated over the same period.

is a product of 7 factors: g from Earth’s geometry with respect to the sun

a from the transparency of the atmosphere

s from the spectral composition of PAR and the optical properties of foliage

q from the number of photons needed in the photosynthetic process

i from the fraction of radiation intercepted by a canopy

d from the finite rate at which CO2 molecules diffuse from the air to the surfaces of photosynthetic cells

r from the fraction of assimilate not used for respiration

Components of Efficiency

The geometrical factor g

radiant energy flux = rate at which radiant energy is received outside the atmosphere on a plane parallel to Earth’s surface.

g = ratio of radiant energy flux to the solar constant integrated over the same period. It depends only on latitude and season.

g is relatively constant through the year in the tropics, but has a much larger seasonal change in temperate regions that is a major factor determining global climate and species distribution patterns.

Components of Efficiency

Atmospheric Transmission a

a = monthly mean insolation / corresponding extraterrestrial radiation

With known amounts of atmospheric water vapor and ozone, a computer program can calculate how much radiant energy would be received at the surface in the absence of clouds and aerosols, expressed as a fraction of the solar constant. This was done for 3 tropical stations.

The mean transmissivity of a clean, cloudless atmosphere has less annual variation in the tropics than in temperate latitudes because there is little seasonal change in the mean atmospheric depth.

a follows a different pattern at each site due to different seasonal distributions of clouds and aerosols. Maximum radiation losses occur during the dry months; during the wet months, the atmosphere is the cleanest.

Differences between the cloudless atmosphere line and the real atmosphere line represents the energy absorbed and scattered back to space by clouds and aerosols.

Components of Efficiency

Components of Efficiency

Spectral factor s

PAR = 400-700 nm (visible light).

The fraction of PAR received depends on atmospheric absorption and scattering; can be calculated for a given solar elevation as a function of atmospheric water vapor and dust content.

More PAR is diffuse rather than direct. On clear days, the diffuse component is visible as blue sky light.

Combining the direct and diffuse components in appropriate proportions, the ratio of PAR to total radiation is close to 0.50.

This ratio varies slightly with season (spring < winter) and with cloud cover (clear days < cloudy days because cloud droplets absorb a small fraction of IR energy).

chlorophyll absorbs blue and red light strongly, leaving green to be reflected and transmitted.

the fraction of PAR absorbed by leaves = between 80-90%.

the fraction of the whole spectrum absorbed by leaves = 0.5 x 0.85 = 0.425 = s.

Components of Efficiency

Components of Efficiency

Photochemical efficiency q

q = energy stored in carbohydrate formation / absorbed radiant energy

In a light-limited system, ~20% of the absorbed energy is stored in final photosynthetic products, and ~80% is lost as heat or used to form higher compounds such as proteins and fats.

1 carbohydrate molecule (CH2O) = 1 molecule CO2 and energy of 10 light quanta.

The average energy content of one quantum of PAR = 3.6x10-19 J.

The amount of heat stored in the simplest carbohydrate molecule = 7.7x10-19 J.

The maximum q =

When estimating q using dry matter weight, the high energy/weight ratios of proteins and fats must be accounted for. Since chemical compositions of plants are rarely reported, a figure of 16.7-20 kJ/g can be used depending on plant type. Using 16.7 kJ/g, 1 kJ of solar radiation = 54 mg dry matter. If P is the rate of dry matter production in g/m2/h and I is solar irradiance for the whole spectrum in kW/m2, then P = 20 I.

Components of Efficiency

Diffusion efficiency d for single leaves

Decrease in the availability of intercellular CO2 is responsible for the photosynthesis-light curve in Fig. 6. When irradiance increases, this CO2 concentration decreases because of their diffusion from the air to the chloroplasts. At light saturation, the photosynthetic rate is proportional to the CO2 concentration in the external atmosphere.

P = 1/b in very weak light; P = 1/a in very strong light (2-8 g CH2O/m2/h for less and more efficient species, respectively).

Stomatal control of photosynthesis is more important in more efficient species (maize and sugar cane); resistances are about 2 s/cm compared to 7 s/cm for the least efficient species (cotton, tobacco).

Below 400 W/m2 PAR and 300 ppm CO2, photosynthetic rate can be expressed as a function of irradiance:

Components of Efficiency

Diffusion efficiency d for stands

a simple model can be used to derive the diffusion efficiency of a stand.

1. calculate leaf area exposed to direct sunlight.

2. integrate equations 2 and 3 (area index of sunlit (A0) and shaded leaves (A1), respectively) over the daylight hours, assuming that irradiance changes sinusoidally and reaches a max at noon.

a and b describe the slope of the photosynthesis light curve

I = irradiance on a horizontal leaf

= fraction of radiation intercepted by a leaf that is transmitted and available for photosynthesis by lower leaves

Can now estimate the maximum gross photosynthesis for a crop from measured values of solar radiation or from an appropriate fraction of the solar constant if coefficients a and b are known.

Components of Efficiency

Line 1: Maize and sugar cane (efficient species)

Line 2: Rice, wheat, barley

Line 3: Dicots such as cotton, tobacco, and ground nuts

Components of Efficiency

Interception efficiency i

= actual rate of photosynthesis / maximum rate estimated for a stand of identical plants with enough leaves for full light interception.

i is proportional to LAI; can be easily measured or inferred this way.

The photosynthesis rate cannot be strictly proportional to i unless all the leaves are working at the same photochemical efficiency. This happens either in 1) weak light or if 2) photosynthesis in shaded leaves is negligible compared to sunlit leaves.

Proportionality is also preserved whenever sunny and cloudy conditions are relatively constant over a period of time.

To a good approximation for most crops and climates,

Components of Efficiency

i as a function of s and L when = 0.07

Components of Efficiency

Respiration factor r

r = 1-R/P where R = weight of carbohydrate used for respiration per day (mean dark respiration rate x 24) and P = weight of carbohydrate produced by photosynthesis per day (dark respiration rate + net update of photosynthesis).

Accurate estimates of r are rare because respiration’s importance in determining net rates of dry matter production have been underestimated. The traditional figure is 0.20-0.25, though higher values have been reported (0.25-0.50 for various crops, 0.75 for tropical rain forest, and 0.44 for white clover). The mean value has been arbitrarily considered to be 0.5.

Respiration is coupled to photosynthesis such that CO2 is produced at a rate that increases with the photosynthetic rate (and thus the irradiance).

True gross photosynthesis = AD, but the classical method of measuring this yields the smaller AB+BD=AC.

Components of Efficiency

Tests of the Analysis

Uncertainty in (g,i) is calculated to be 17% - a combination of random and systematic errors in the individual efficiency components.

The error in estimating dry matter production from constant values of r and a during vegetative growth and with i calculated from LAI is comparable with the error in the conventional measurement of production (about +/- 10-15% over a period of 1-2 weeks).

Tests of the Analysis

Tests of the Analysis

Generalization and Extensions

Primary Production

A method of predicting crop growth rates has been established and tested for the tropics and sub-tropics. Differences in the efficiency of different stands can now be assigned to specific physical or biological factors.

Latitude and cloudiness differences are accounted for by g and a. The interception factor i is largest for temperate zones because leaf area is zero in winter. Nutrient and water shortages restrict leaf development and affect both i and d.

Maximum photosynthetic efficiencies can be calculated assuming that i = 0.95 when the LAI is very large.

Class I crops have the highest efficiencies (g, r), at almost twice that of Class III crops.

i is a major discriminant of dry matter production. It accounts for productivity differences under different climate and management conditions, as well as differences between the mean and maximum production rates in a stand.

Generalization and Extensions

Secondary Production

Large mammal metabolism is nearly proportional to surface area. Using 60 W/m2 for the metabolic rate, we can calculate the area of herbivore that can be supported by a unit area of ground (skin area index).

If an energy supply of 4 x 10-2 W/m2 is available, 1500 m2 of ground area is needed to produce enough energy for 1 m2 of animal (herbivores only, not counting carnivores).

A person has a skin area of about 1.7 m2. The solar constant (1360 W/m2) = energy produced by 13.6 people/m2 or 13600 people/ha. In reality, # people per area cultivated land = f u (g,r), where

u = fraction of useful energy removed from a crop, and

f = fraction of total dry matter production retained after harvesting, storage, and preparation losses.

In a region where i = 0.05, the population would be 1.8 people/ha. The actual population densities in subsistence agriculture countries consistent with a range of values of (g,r) from 3-12x10-4, or of i from 0.03-0.12.

Generalization and Extensions

Conclusions

The efficiency with which plants store energy is the product of 7 factors: latitude and season cloudiness and aerosol content of the atmosphere spectral composition of the radiation photochemical process dependence on LAI photochemical process dependence and leaf arrangement CO2 concentration and diffusion resistance of individual leaves fraction of assimilates used in respiration

Estimates of dry matter production in the tropics agree well with vegetative growth measurements, but overestimate production during reproductive growth. This is either because photosynthesis rates are overestimated, respiration is underestimated, or both.

Main limitations and unknowns: Why exactly does d decreases in aging leaves? What is the way in which respiration rates are related to photosynthesis rates and the accumulated dry stand weight? How is the partitioning of assimilates controlled by environmental and endogenous factors? Still need to use measurements instead of predictions of LAI. How do leaf expansion rates depend on environmental factors such as temperature, water, and nutrient availability?