123
124

Contents

3.4.1 Introduction 125

3.4.1.1 Carbon dioxide in the environment 125

3.4.1.2 Definitions and units 125

3.4.2 Role of Carbon Dioxide in Tree

Seedling Growth and

Development 126

3.4.2.1 Carbon dioxide and photosynthesis 126

3.4.2.2 Growth response to carbon dioxide

enrichment 127

3.4.2.3 Interaction of carbon dioxide with other

factors 127

Light 127

Temperature 128

Water and mineral nutrients 128

3.4.3 Optimum Carbon Dioxide Levels

130

3.4.3.1 Establishment phase 130

3.4.3.2 Rapid growth phase 130

3.4.3.3 Hardening phase 130

3.4.4 Modifying Carbon Dioxide in

Container TreeNurseries 132

3.4.4.1 Increasing carbon dioxide availability with

ventilation 132

3.4.4.2 Supplementing carbon dioxide levels

132

Sources of carbon dioxide 132

Designing a carbon dioxide enrichment

system 134

Potential problems with combustion

systems 135

3.4.5 Carbon Dioxide Monitoring and

Control Systems 137

3.4.5.1 Measuring carbon dioxide levels 137

3.4.5.2 Carbon dioxide control systems 138

3.4.6 Conclusions and Recommendations

139

3.4.7 Literature Cited 140
125

on tree growth in the past few years, and much of this work has 

been done on forest tree seedlings (see section 3.4.2.2).

The ambient COZ2 level around a nursery can vary from 200 to 

400 ppm, depending on location; higher values can be found in 

industrial areas, due to combustion of fossil fuels, and in low wet 

areas, such as swamps and river bottoms, where plant materials 

are decomposing (Nelson 1985). Carbon dioxide concentration 

measured in weight per unit volume also decreases with elevation, 

decreasing about 40% from sea level to 4,500 m (14,800 feet) 

(Kramer and Kozlowski 1979).

3.4.1.2 Definitions and units

Carbon dioxide levels can be described and measured in several 

different ways. Because it is a gas, CO2 can be described in 

pressure units but these are not widely used for horticultural 

purposes. Plant physiologists measure photosynthesis by the 

amount of CO2 consumed per unit volume, expressed either by 

weight in milligrams per liter (mg/I) or by volume in microliters per 

liter (μl/I). Pallas (1986) provides a thorough discussion of units 

that are used in scientific research. However, for operational 

nursery work, concentration units—percentage (%) or parts per 

million (ppm)—are the simplest and most appropriate way to 

measure CO2.

3.4.1 Introduction

Carbon dioxide (CO2) is a colorless, odorless gas. Because it 

exists in relatively small amounts in the ambient atmosphere, CO2

is considered to be the principal limiting factor of photosynthesis 

under some growing conditions. In horticulture, carbon dioxide 

was the last of the limiting growth factors to be culturally 

managed. Initial experiments with CO2 enrichment were 

discouraging because elevated levels of CO 2produced a 

phytotoxic response, which was probably due to the presence of 

toxic gaseous contaminants. In 1918, the beneficial effects of CO2

enrichment for plant growth were firmly established, but 

commercial application did not become widespread until the 

mid-1960's (Bauerle and others 1986, Hicklenton 1988).

Carbon dioxide enrichment is not widely practiced in container tree 

nurseries. Only 17% of the nurseries in the United States and 

Canada reported CO2 management programs (Container Nursery 

Survey). The lack of interest should not be interpreted to mean that 

CO2 is not important to tree seedling culture, as many research 

studies have shown that increased COZ levels definitely accelerate 

photo synthetic rates (Kramer and Kozlowski 1979).

3.4.1.1 Carbon dioxide in the environment

Two familiar gases make up 99% of the earth's atmosphere by 

volume-nitrogen (78%) and oxygen (21 %)-whereas carbon dioxide 

concentrations currently average only 0.035% (350 ppm). This was 

not always the case, however, as CO2 was more common than 

oxygen in the primitive atmosphere. The rapid development of plant 

life quickly utilized this CO2 and released oxygen, which made 

possible the evolution of larger and more advanced organisms. The 

CO2 level of the earth's atmo sphere appears to have stabilized at 

its lowest level (280 ppm) in the mid-1800's. Since that time, the 

industrial revolution has resulted in a gradual increase in ambient 

CO2 concentrations. Massive deforestation and combustion of 

fossil fuels has caused the atmospheric CO2 level to increase 1 to 

2 ppm per year. This trend is unlikely to change in the near future 

(Hicklenton 1988). The concern over "global warming" has resulted 

in many new research studies of the effects of higher CO2 levels
3.4.2 Role of Carbon Dioxide in Tree

Seedling Growth and Development

Carbon is one of the 16 nutrients that are essential for plant 

growth. About 40% of the dry weight of a typical plant is composed 

of carbon. Plants obtain this carbon from the CO2 in the air through 

the stomata in the leaves (fig. 3.1.2). The normal ambient CO2 level 

of approximately 350 ppm is considered to be adequate for 

"normal" growth, although plants have the capacity to utilize much 

greater amounts. This capacity apparently stems back to primitive 

times when CO2 levels were 10 to 100 times higher than the 

present time (Nelson 1985).

3.4.2.1 Carbon dioxide and photosynthesis

Plants consume CO2 during photosynthesis and give it off through 

respiration; during daylight hours these processes occur 

simultaneously (fig. 3.3.5). When the ambient CO2 level reaches 

the compensation point, the point at which photo synthesis and 

respiration are equal, the seedling will not grow but only maintain 

itself. In greenhouses, this situation only occurs on cool, bright 

days when vents remain closed. At CO2 levels above the 

compensation point, which is 40 to 60 ppm for most commonly 

grown seedlings, net photosynthesis is positive, as long as other 

critical factors are not limiting (Ludlow and Jarvis 1971, Higginbotham and others 1985).

Carbon dioxide levels follow a typical diurnal pattern in a closed 

greenhouse environment (fig. 3.4.1). At night, green plants 

release CO2 through respiration and so concentration in a closed 

greenhouse rises to over 400 ppm; however, at dawn, 

photosynthesis begins and the concentration drops rapidly. The 

CO2 concentration becomes critically low in a greenhouse on a 

cool, cloudy day when ventilation is not required; in a greenhouse 

with only two air exchanges per hour or less, the CO2

concentration often drops below 200 ppm and limits 

photosynthesis (Holley 1965). Environmental measurements 

inside a greenhouse showed that the average daily CO2

concentration varied significantly during the month of February 

and reached its lowest level when the outside temperature was 

cold and the vents remained closed (fig. 3.4.2). Under calm 

conditions, CO2 levels can even be limiting in dense

126
3.4.2.3 Interaction of carbon dioxide with other

factors

The physiological effects of CO2 must not be considered alone, for 

they are highly interrelated to the effects of other factors limiting to 

plant growth, especially light, temperature, water, and mineral 

nutrients (Blackman 1905). As the CO2 concentration changes 

and the photosynthetic rate varies, the optimum levels of the other 

environmental factors also change.

Light. Photosynthesis, as measured by CO2 utilization, increases 

linearly with light until the light saturation point is reached. As long as 

the other factors are not limiting, the light saturation point becomes 

progressively greater as light intensities and CO2 concentration 

increase (fig. 3.4.4). In fact, high CO2 levels can, to a degree, 

compensate for low light intensity (Tolley and Strain 1984), which

often occurs on cloudy winter days. When such conditions occur, 

supplemental photosynthetic lighting is sometimes used to promote 

crop growth; CO2 enrichment is essential in obtaining the full benefits 

from the additional light (Hicklenton 1988). (See chapter 3 in this 

volume for more information on light effects on photosynthesis.)

127

crops grown in open fields, resulting in a 10 to 20% decrease 

in photosynthetic rate (Chang 1968).

As CO2 concentrations are enhanced above ambient levels, the 

photosynthetic rate increases because high concentrations 

increase the diffusion gradient from the ambient air, through the 

stoma ta, to the mesosphyll cells where the chloroplasts use CO2 

in photosynthesis. Photorespiration, which occurs at high light 

levels and causes loss of CO2, is also suppressed by high 

concentrations of CO2, which further increase net photosynthesis 

(Tinus 1975, Pearcy and others 1987).

3.4.2.2 Growth response to carbon dioxide

enrichment

Seedling growth rate increases when the net photosynthate is 

exported from the leaves, where photosynthesis takes place, to the 

meristems, where growth occurs. There are numerous studies on 

the beneficial effects of enhanced CO2 on tree seedling growth. 

The height, stem diameter, and total volume of ponderosa pine 

seedlings increased with increasing CO2 concentration, although 

two seed sources had different patterns of growth response 

(Surano and others 1986). Tolley and Strain (1984) found 

increased shoot height, leaf area, and biomass for loblolly pine and 

sweetgum seedlings and reported that the growth response of 

sweetgum was greater than loblolly pine. Lodgepole pine 

seedlings grown in enhanced CO2 levels produced more leaf 

growth and shoot height than did controls. Analysis of the biomass 

partitioning showed that root growth was particularly stimulated 

(fig. 3.4.3). The authors concluded that the combination of 

increased leaf area and root production could shorten the nursery 

production period.

There is an upper limit to the stimulatory effect of increased CO2 

concentrations on many plants. High concentrations of CO2 can 

cause stomatal closure, which automatically reduces the amount 

of CO2 entering the leaf. In addition, faster growth caused by a 

higher concentration of CO2 usually results in a shortage of 

another factor that limits photosynthesis and therefore growth.
Solar radiation also has a direct effect on the temperature regime 

in a closed greenhouse environment. Because sunlight is 

converted to thermal radiation in a greenhouse, the amount of 

sunlight directly influences the need for cooling by ventilation. 

Thus, the average daily light level has a direct relationship with the 

CO2 concentration in a greenhouse during winter (fig. 3.4.2).

Temperature. For many plant species, the optimum temperature 

for photosynthesis increases when the CO2 concentration is 

enhanced. Therefore, the greenhouse vents can remain closed 

longer-keeping the temperature about 3 to 6 °C (5 to 10 ° F) 

warmer than otherwise-which in turn prolongs the seedlings' 

exposure to high CO2 (Nelson 1985). Likewise, tolerance for higher 

temperature means that shading is less likely to be needed. High 

CO2, more light, and higher temperature therefore act 

synergistically up to the point where other problems limit growth.

When stomata close at high CO2 levels, evapotranspiration 

ceases, and plants may become damaged by the high foliar 

temperatures. For example, Surano and others (1986) found that 

transpirational cooling was much reduced when ponderosa pine 

seedlings were grown in enhanced CO2 levels in special growth 

chambers and needle temperature approached 45 °C (114 °F). 

Reduced transpiration

128

was accompanied by an increase in ethylene production, which 

may be indicative of heat stress. Stomatal response to high CO2 

varies between species: for example, stomatal opening of lodgepole pine seedlings was unaffected by CO2 concentrations up to 

2,000 ppm (Higginbotham and others 1985). Because small 

seedlings are so sensitive to heat, stomata[ closure may lead to 

heat injury in container nurseries with enriched CO2 levels.

Water and mineral nutrients. The rapid growth rates of plants 

grown in enhanced CO2 levels result in an increase in water and 

mineral nutrient uptake (Hicklenton 1988). Seedling water use will 

increase because the larger foliage has a greater transpirational 

surface. However, because stomata begin to close at higher CO2 

levels, the efficiency of water use by the seedlings should also 

increase. This was apparently the case in an experiment with 

radiata pine where water-stressed seedlings had a greater growth 

response than those supplied with adequate water (Conroy and 

others 1986).

The increased growth resulting from the higher photosynthetic rate 

will also create a demand for more mineral nutrients, but the 

uptake is not consistent for all nutrients. In Virginia pine, uptake of 

nitrogen and calcium increased during CO2 enrichment, but uptake 

of phosphorus and potassium was unaffected (Luxmore and 

others 1986). Aspen and white spruce seedlings responded 

differently to enhanced CO2 and nitrogen fertilization. Root, stem, 

and leaf biomass of aspen were greater at high and low nitrogen 

levels, whereas only leaf mass of white spruce was significantly 

increased and only at high nitrogen levels. However, the increased 

growth rate of the aspen seedlings did not persist, because 

mineral nutrient deficiencies were caused by rapid growth (Brown 

and Higginbotham 1986). A similar response was noted with sweet 

chestnut seedlings: leaves grown under enhanced CO2 became 

prematurely chlorotic, which was attributed to nutrient dilution 

resulting from the rapid growth rate (Mousseau and Enoch 1989).
Growers who supply extra CO2 to their crops will have to be aware 

of these potential problems and make appropriate adjustments to 

their cultural regimes. In general, growers who fertilize with every 

watering should have no problems, but those who fertilize 

intermittently may need to fertilize more often or with higher 

concentrations of nutrients.

129
130

lings became established. However, Yeatman (1970) found a 

definite positive response when germinants of four conifer 

species were grown under increased CO2 at two different light 

levels (fig. 3.4.5). In all cases, supplementing the CO2 levels to 

900 ppm was found to produce larger seedlings at 3 weeks 

after germination, but response to further CO2 enrichment was 

variable.

3.4.3.2 Rapid growth phase

Seedlings show the most response to CO2 enrichment in the rapid 

growth phase and the treatment usually begins about 1 month 

after sowing. Campagna and Margolis (1989) supplied enhanced 

CO2 levels for black spruce seedlings at different stages of 

development and found a significant growth response during the 

first 3 months of the growing season, when the seedlings had the 

greatest amount of active photosynthetic surface. Lodgepole pine 

seedlings showed an increasing response in both shoot height 

and leaf surface area (fig. 3.4.6) throughout a 5-month growing 

season (Higginbotham and others 1985). A 1,000 ppm CO2 level 

was more effective than a higher concentration. Ponderosa pine 

and blue spruce seedlings grown for 1 year in 1,200 PPM CO2 

were 50 to 70% heavier than seedlings grown in 325 ppm (Tinus 

1972).

3.4.3.3 Hardening phase

The benefits of CO2 enrichment decline during the hardening 

phase. Black spruce seedlings grown in enhanced CO2 during late 

summer showed no increase in height growth or biomass, 

although seedlings treated earlier in the growing season had significant growth responses (Campagna and Margolis 1989). An 

enriched CO2 environment may actually be detrimental to 

development of seedling cold hardiness. Black spruce seedlings 

grown under elevated CO2 levels hardened more slowly than control seedlings and subsequently suffered greater frost injury 

(Margolis and Vezina 1990). Enhanced CO2 levels prolong the 

succulence of seedlings, which makes them more susceptible to 

cold injury

3.4.3 Optimum Carbon Dioxide Levels

The optimum CO2 level will depend on the crop species and the 

stage of seedling development. Most ornamental greenhouses try 

to maintain CO2 concentrations between 600 to 1,500 ppm (Freeman 1985). Research trials with tree seedlings have shown 

positive growth responses at 1,000 ppm or below. Sionit and 

Kramer (1986) provide a comprehensive listing of the responses of 

woody plants to CO2 enrichment. Carbon dioxide levels in excess 

of 1,500 ppm have not resulted in any additional growth, although 

most research has been done at light intensities equivalent to 20 to 

30% of full sunlight and therefore may not accurately predict what 

will happen in a greenhouse. Phyto toxicity may occur above 2,500 

ppm in some species (table 3.4.1).

3.4.3.1 Establishment phase

There is some question whether enhancing CO2 levels during 

seed germination and early establishment is beneficial. Younger 

plants have slightly higher optimal requirements for CO2 than 

older plants (Chang 1968), but the small surface area of the 

cotyledons and primary needles means less photosynthetic 

capacity. Most research trials begin supplementing CO2 several 

weeks after the seed-
and other stresses. Enhanced CO2 retards leaf abscission in 

deciduous hardwoods and may stimulate budbreak and renewed 

growth. Therefore, container nursery managers should terminate 

CO2 treatment before beginning the stress treatments used to 

initiate dormancy.

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132

feasible, has been found equivalent to a 50% enrichment in CO2 

levels. The potential for increasing ventilation is limited, however, 

as the wind velocity at maximum ventilation is around 42 cm/s (83 

feet per minute) in a fully enclosed greenhouse (Hanan and others 

1978).

Because new energy-efficient greenhouses are much more 

air-tight than older ones, growers can no longer depend on air 

leakage to provide CO2 replacement. Growers with shelterhouses 

should stimulate photosynthesis by raising the sides whenever air 

temperatures permit, and those with fully enclosed structures 

should use fan-jet ventilation systems that encourage good air 

mixing within the greenhouse. Even on days when outside 

temperatures are low, it may prove beneficial to bring in outside 

air and heat it, especially early in the morning when 

photosynthetic rates are highest. (Ventilation is discussed in more

detail in section 3.1.4.3 in this manual.)

3.4.4.2 Supplementing carbon dioxide levels

The potential of CO2 enrichment has been demonstrated in growth 

chamber experiments, where growth of tree seedlings has been 

increased 50 to 100% (Tinus 1972, 1976). In a fully enclosed 

greenhouse the response will be somewhat less because it is 

necessary to ventilate when air temperatures exceed optimum 

levels and it becomes impossible to maintain CO2 levels much 

above that of the outside air. The type of growing structure is also 

important; drafty houses have eight times the air exchange rate of 

well-sealed greenhouses (Hicklenton 1988). The feasibility of CO2 

enrichment will depend, therefore, on the type and the condition of 

growing structure and the proportion of the time the vents remain 

closed.

Sources of carbon dioxide. The use of decomposing organic 

matter, especially manure, to heat greenhouses and cold-frames 

incidentally provides supplemental CO2 (Bauerle and others 

1986). Organic matter will eventually release 1.4 times its weight 

of CO2 (Hanan and others 1978). Although these materials are 

cheap and readily available, they are rarely used in modern 

container nursery facilities because of problems with sanitation 

and disposal. Another serious drawback is the impossi3.4.4 Modifying Carbon Dioxide in

Container Tree Nurseries

The first thing to evaluate when considering supplementing CO2 is 

the type of nursery. There is obviously no potential for 

supplementation in open compounds, whereas in enclosed 

growing structures, the rationale for increasing CO2 levels will 

depend on the type of structure. Enrichment is less economical in 

structures that are not well-sealed, such as shelterhouses, than in 

fully enclosed greenhouses. Double-layer polyethylene covered 

houses are ideal because they have fewer air leaks than those 

glazed with glass, fiberglass, or plastic panels (Nelson 1985).

3.4.4.1 Increasing carbon dioxide availability with

ventilation

Good circulation of air is necessary to make CO2 available at the 

leaf surface. Therefore, growers can stimulate seedling growth by 

merely encouraging good ventilation. In the light when the air is 

stagnant, the CO2 concentration in the boundary air layer around 

the seedlings can be considerably lower than the ambient 

conditions outside the greenhouse, and so increasing the air flow 

over the leaves makes the gas more available for photo synthesis 

(Kramer and Kozlowski 1979, Hicklenton 1988). In studies with a 

field crop, Gaastra (1963) demonstrated that increasing the wind 

velocity over the leaf surface can significantly increase 

photosynthesis (table 3.4.2). In a greenhouse, an air flow rate of 50 

cm/s (99 feet per minute), which is fast but
1988). Complete combustion of natural gas and propane requires 

adequate oxygen and produces CO2 and water; propane 

produces proportionally less water than natural gas (Sheldrake 

(1964). Considerable heat is added by the burner: from 4,000 to 

5,000 kcal/kg (8,000 to 10,000 Btu per pound) of fuel, depending 

upon the type of fuel and the efficiency of the burner. If excessive 

heat is a problem, burners can be located outside the greenhouse 

and the flue gas piped in.

The principal disadvantage of these CO2 generators is that leaks or 

incomplete combustion can release phyto toxic gases into the 

greenhouse. Propane, which can contain a significant amount of 

propylene, is more of a concern because propylene can produce 

damage that is similar to ethylene (Abeles 1973). Natural gas is 

mainly a mixture of methane, ethane, and propane, but it too can 

contain potentially harmful contaminants, such as hy drogen 

sulfide, propylene, and ethylene, but in levels that are generally too 

low to cause problems. The burner flame should always be 

adjusted to a uniform blue color; traces of yellow indicate 

incomplete combustion, which can release toxic gases such as 

carbon monoxide and ethylene into the greenhouse environment 

(Hicklenton 1988). Carbon monoxide is dangerous to humans as 

well as to plants, and ethylene is a potent plant hormone that 

promotes dormancy and leaf abscission (Abeles 1973).

Exhaust gases from greenhouse heating systems that use 

propane or natural gas are another potential source of CO2, but 

they are available only when heating is required. Scrubbed stack 

gas from industrial or electric generating plants also produces 

waste CO2, as do fertilizer factories and some food processing 

plants. The practicality of these sources depends on the location 

of the greenhouse. The major benefit is that they are essentially 

free, and there is also the potential for utilizing waste heat from the 

source. Exhaust gases have the same potential problems as 

propane or natural gas, and incomplete combustion is more 

common and more difficult to detect than with the readily visible 

flames of CO2 generators (Hicklenton 1988).

133

bility of controlling the decomposition rate and therefore the CO2 

concentration in the greenhouse. The potential has been 

demonstrated for forest nurseries, however, as white fir seedlings 

grown above an organic mulch were significantly taller than 

controls (Montano and others 1977). Hicklenton (1988) provides a 

good discussion of the possible applications for supplying CO2 

through controlled decomposition.

In modern container nurseries, there are two realistic options for 

supplying CO2: injection of pure CO2 and combustion of carbon 

fuels.

Pure carbon dioxide. The safest method of supplying CO2 in 

greenhouses is to inject pure CO2 from pressurized liquid tanks 

(Hicklenton 1988). The pressurized gas is distributed along the 

length of the greenhouse through perforated pipes. Hanan and 

others (1989) estimated an average CO2 usage of 10 to 26 kg/m2

(2 to 5 pounds per square foot) of greenhouse space for flower 

crops. Although effective, the major drawback of this method is the 

expense. In recent years, however, the higher costs and uncertain 

future of fossil fuels have made pure CO2 more economically 

attractive (Nelson 1985).

Combustion of carbon fuels. Complete combustion of any 

carbon compound will produce CO2 Kerosene was the first fuel to 

be used in greenhouses for CO2 generation, although its cost and 

concerns about sulfur dioxide phytotoxicity have made it 

unpopular in many parts of the world. Propane and natural gas 

are more commonly used in commercial greenhouses, and the 

difference is primarily one of cost and availability. Both sources 

are pure enough, and thus are generally considered the most 

economical source of generating CO2.

Carbon dioxide generators that burn propane or natural gas are 

commercially available (fig. 3.4.7). The CO2 production rate is 

controlled by modulation of the burner, and approximately 15 

burners per hectare (6 per acre) should generate 1,000 PPM CO2 

in a well-sealed greenhouse. The gas consumption of these 

generators is quite low—2 m3/hr (70 cubic feet per hour) 

(Hicklenton
Designing a carbon dioxide enrichment system. It is only 

practical to supply CO2 in well-sealed greenhouses. Even though 

polyethylene is permeable to CO2 greenhouses covered with 

polyethylene film are very air tight compared to glass or fiberglass 

structures.

Calculating carbon dioxide requirements. The quantity of CO2 

to supply depends on the amount used by the crop plus the 

leakage from the greenhouse. Specific calculations to determine 

the proper size of generator for a given size and type of 

greenhouse can be found in Aldrich and Bartok (1989), but table 

3.4.3 provides a typical example. Calculations for fuel use and 

amount of heat generated are also described in the same 

reference.

Distribution within the greenhouse. Once a CO2 generator has 

been installed, it is necessary to mix the gas with the greenhouse 

air to provide a uniform concentration at the crop level. This should 

not be a problem during cold days when the heating system 

creates normal convection currents, but Hicklenton (1988) reports 

that CO2 levels are typically higher in the immediate vicinity of the 

gener134
ator. Carbon dioxide generators should be mounted with air 

circulation and fire safety in mind (fig. 3.4.7C). Some brands 

contain fans to ensure that the CO2 is distributed horizontally 

across the greenhouse (fig. 3.4.713). Special vertical fan tube 

systems can be used to move CO2 down to the crop level.

If pure CO2 is injected from a point source, it must be distributed 

throughout the greenhouse. Some growers use fans to induce 

horizontal flow; others distribute the pressurized gas through 

finely perforated tubing installed along the length of the 

greenhouse at the crop level. Hicklenton (1988) discusses some 

common applications of injecting pure CO2 in horticultural 

greenhouses.

Potential problems with combustion systems.

Several potentially dangerous gases can result from improperly 

designed or maintained CO2 burners (table 3.4.4). Certain compounds, such as carbon monoxide and ethylene, are 

produced by incom135
136

plete combustion (Scarratt 1985). Sulfur dioxides result from 

burning high-sulfur fuel. The following fuels were suggested for 

CO2 generation because of their low sulfur content (Kretchman and 

Howlett 1970): propane, 0.01 % sulfur; natural gas, 0.02% sulfur; 

and kerosene, 0.02% sulfur.

A good way to ensure complete combustion is to provide a direct 

supply of outside air to the burner. An air intake area of 1 cm2 for 

every 80 kcal (1 square inch for every 2,000 Btu) of burner capacity should provide sufficient air for complete burning. The 

burner should be properly adjusted. If the flame is yellow, and 

especially if it is smo key, it may produce harmful products of 

partial combustion; if the flame is short and very turbulent, there is 

likely to be some gas that escaped unburned into the greenhouse. 

For a further discussion of combustion-generated pollutants that 

can be produced by CO2 generators, see Hicklenton (1988) and 

Hand (1986).
A more sophisticated and expensive ($1,400) monitor, the infrared 

gas analyzer, is popular because of its accuracy and precision (fig. 

3.4.813). These analyzers can measure CO2 continuously and can 

interface directly with environmental computer systems (Pallas 

1986). Nurseries that do not have infrared gas analyzers may be 

able to have samples tested at a nearby university or agricultural 

research station. Samples can be taken by filling a clean inner 

tube with greenhouse air using a bicy cle pump. Butyl rubber is 

highly impermeable to

3.4.5 Carbon Dioxide Monitoring and

Control Systems

3.4.5.1 Measuring carbon dioxide levels

Once the generators have been adjusted to produce the desired 

levels, most nurseries do not monitor CO2 levels continuously, but 

check them occasionally. Small hand-pump CO2 testers with 

reasonable accuracy are available for around $200; the air is 

pumped through a glass tube that contains a CO2-sensitive 

chemical that changes color as an air sample is drawn through it 

(Pallas 1986) (fig. 3.4.8A).

137
138

The best way to control CO2 concentration is to use an automatic 

feedback system that continuously measures CO2 concentration 

and adds CO2 as needed. Many microcomputer control systems 

use infrared gas analyzers and a system of solenoid valves to 

regulate the CO2 concentration in the greenhouse to within 100 

ppm. These systems are expensive ($3,000 to 10,000), but new 

models are durable and have a proven record in operational 

greenhouses. Pallas (1986) and Hicklenton (1988) provide an 

excellent review of CO2 measurement and control systems and 

their suppliers.

CO2, and such samples should be good for days. Do not use 

plastic bags, unless the samples can be analyzed in a matter of 

minutes. Be sure no human breath is mixed with the sample.

3.4.5.2 Carbon dioxide control systems

A simple and inexpensive CO2 control system uses a photocell or 

time clock that is connected to the thermostat that controls the first 

stage of cooling. The generator comes on at dawn or about 1 hour 

before, boosting the CO2 concentration into the optimum range by 

the time the stomata open and photosynthesis begins. The 

generator remains on until cooling is required. When the vents 

open and fans come on, high C O2 levels can no longer be 

maintained and the thermostat shuts off the generator. The 

system may be activated again in the late afternoon when 

ventilation for cooling ceases. The time clock would normally shut 

off the generator about one-half hour before sunset.

With these systems, the generator may be on all day in winter; but 

during the warmer seasons, it is usually possible to raise the CO2 

level for only a few hours in the early morning and again in the 

evening. This relatively short period of increased CO2 is still 

effective because the stomata typically close during midday 

because of normal water stress, even if the plants are well 

watered. Under these conditions, photosynthesis is most rapid in 

the early morning and has a secondary peak in the late afternoon. 

In addition, the enhanced CO2 will keep the stomata partly closed, 

which will delay the midday moisture stress and perhaps prevent 

the dip in photosynthesis (Pettibone and others 1970).
3.4.6 Conclusions and Recommendations

Carbon dioxide is commonly the most limiting factor to 

photosynthesis in the natural environment, and this situation is 

exacerbated in greenhouses that restrict air exchange, especially 

in cool weath er when vents are closed. Although CO2 levels are 

not currently managed in most container tree nurseries, numerous 

research studies have demonstrated the benefits from doing so.

Growers can insure that their crops get enough CO2 in two ways: 

first, by encouraging adequate ventilation during periods when 

photosynthesis is greatest, and second, by direct 

supplementation. Fully controlled greenhouses should 

supplement CO2 levels to 1,000 ppm, and even growers with 

semicontrolled growing structures can increase photosynthesis 

and growth by promoting good air exchange during cool weather, 

especially during early morning hours. Nurseries that grow crops 

during winter months are particularly well-suited for CO2 

enrichment, especially at high latitudes where supplemental 

photosynthetic lighting is also used.

Carbon dioxide enrichment has been shown to increase the growth 

rate of many forest tree seedlings, but species variation should be 

expected. Enhanced CO2 levels will give the best results early in 

the growing season and can even help during the establishment 

phase. Because it may inhibit dormancy and cold hardiness, CO2 

supplementation should be stopped at the beginning of the harden-

ing phase. The accelerated seedling growth rates that are 

experienced will cause an increased demand for mineral nutrients 

and water, and so growers must adjust their fertilization and 

irrigation schedules when CO2 enrichment is used. Likewise, crops 

grown under high CO2 can tolerate higher temperatures and benefit 

from higher light intensities. Shorter crop rotations can be 

expected.

The practicality and economics of CO2 enrichment must be 

calculated for each individual container nursery but, considering 

that annual operating costs are only $1 to $2 per m2 ($0.10 to 

$0.15 per square foot), more growers should consider this cultural 

practice (Freeman 1985).

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140

3.4.7 Literature Cited

Abeles, F.B. 1973. Ethylene in plant biology. New York: Academic 

Press. 302 p.

Aldrich, R.A.; Bartok, J.W. Jr. 1989. Greenhouse engineering. 

Publ. NRAES-33. Ithaca, NY: Cornell University, Northeast 

Regional Agricultural Engineering Service. 203 p.

Bauerle, W.L.; Kretchman, D.; Tucker-Kelly, L. 1986. COZ 

enrichment in the U.S. In: Enoch, H.Z.; Kimball, B.A., eds. 

Carbon dioxide enrichment of greenhouse crops, vol. 1, status 

and COZ sources. Boca Raton, FL: CRC Press: 49-57.

Blackman, F.F. 1905. Optima and limiting factors. Annals of 

Botany 19:281.

Brown, K.; Higginbotham, K.O. 1986. Effects of carbon dioxide 

enrichment and nitrogen supply on growth of boreal tree 

seedlings. Tree Physiology 2(1-3):223-232.

Campagna, M.A.; Margolis, H.A. 1989. Influence of short-term 

atmospheric COZ enrichment on growth, allocation patterns, 

and biochemistry of black spruce seedlings at different stages 

of development. Canadian Journal of Forest Research 

19(6):773-782.

Chang, J. 1968. Climate and agriculture: an ecological survey. 

Chicago: Aldine Publishing Co. 304 p.

Conroy, J.; Barlow, E.W.R.; Bevege, D.I. 1986. Response of 

Pinus radiata seedlings to carbon dioxide enrichment at 

different levels of water and phosphorus: growth, morphology 

and anatomy. Annals of Botany 57:165-177.

Freeman, R. 1985. The importance of carbon dioxide. In: Ball, V., 

ed. Ball RedBook, 14th ed. Reston, VA: Reston Publishing 

Company: 181-193.

Gaastra, P. 1963. Climatic control of photosynthesis and 

respiration. In: Evans, L.T., ed. Environmental control of plant 

growth. New York: Academic Press: 113-140.

Hanan, J.J.; Holley, W.D.; Goldsberry, K.L. 1978. Greenhouse 

management. Berlin: Springer-Verlag. 530 p.

Hand, D.W. 1986. COz sources and problems in burning 

hydrocarbon fuels for COZ enrichment. In: Enoch, H.Z.; Kimball, 

B.A., eds. 1986. Carbon dioxide enrichment of greenhouse 

crops, vol 1, status and COz sources. Boca Raton, FL: CRC 

Press: 99-121.

Hartmann, H.T.; Flocker, W.J.; Kofranek, A.M. 1981. Plant 

science: growth, development, and utilization of cultivated 

plants. Englewood Cliffs, NJ: Prentice-Hall. 676 p.

Hicklenton, P.R. 1988. COZ enrichment in the greenhouse: 

principles and practice. Portland, OR: Timber Press. 58 p.

Higginbotham, K.O.; Mayo, J.M.; L'Hirondelle, S.; Krystofiak, D.K. 

1985. Physiological ecology of lodgepole pine (Pinus contorta) 

in an enriched COZ environment. Canadian Journal of Forest 

Research 15:417-421.

Holley, W.D. 1965. The COZ story. In: Ball RedBook, 11th ed. 

West Chicago, IL: George J. Ball, Inc.: 94-119.

Kramer, P.J.; Kozlowski, T.T. 1979. Physiology of woody plants. 

New York: Academic Press. 811 p.

Kretchman, D.W., Howlett, R.S., 1970. COZ enrichment for 

vegetables. Transactions of the America Society of 

Agricultural Engineers 13(2):252-256.

Ludlow, M.M.; Jarvis, P.G. 1971. Photosynthesis in Sitka spruce 

(Picea sitchensis (Bong.) Carr.). I. General characteristics. 

Journal of Applied Ecology 8:925-953.

Luxmoore, R.J.; O'Neill, E.G.; Ells, J.M.; Rogers, H.H. 1986. 

Nutrient uptake and growth responses of Virginia pine to 

elevated atmospheric carbon dioxide. Journal of 

Environmental Quality 15(3): 244-251.

Margolis, H.A.; Vezina, L. 1990. Atmospheric COz enrichment and 

the development of frost hardiness in containerized black 

spruce seedlings. Canadian Journal of Forest Research 

20(9):1392-1398.

Montano, J.M.; Fisher, J.T.; Cotter, D.J. 1977. Sawdust for 

growing containerized forest tree seedlings. Tree Planters' 

Notes 28(2):6-9.

Mousseau, M.; Enoch, H.Z. 1989. Carbon dioxide enrichment 

reduces shoot growth in sweet chestnut seedlings (Castanea 

sativa Mill.). Plant, Cell and Environment 12(9):927-934.

Nelson, P.V. 1985. Greenhouse operation and management, 

3rd. ed. Englewood Cliffs, NJ: Prentice-Hall. 598 p.
141

Tolley, L.C.; Strain, B.R., 1984. Effects of COz enrichment on 

growth of Liquidambar styraciflua and Pinus taeda seedlings 

under different irradiance levels. Canadian Journal of Forestry 

Research 14:343-350.

Yeatman, C.W. 1970. COZ enriched air increased growth of 

conifer seedlings. Forestry Chronicle 46:229-230.

Pallas, J.E., Jr. 1986. COZ measurement and control. In: Enoch, 

H.Z.; Kimball, B.A., eds. Carbon dioxide enrighment of 

greenhouse crops, vol. 1, status and COz sources. Boca 

Raton, FL: CRC Press: 77-98.

Pearcy, R.W.; Bjorkman, O.; Caldwell, M.M.; Keeley, J.E.; 

Monson, R.K.; Strain, B.R., 1987. Carbon gain by plants in 

natural environments: carbon assimilation analysis provides 

an understanding of how plants function in diverse environments. BioScience 37(1):21-29.

Pettibone, C.A.; Matson, W.R.; Pheiffer, C.L.; et al. 1970. The 

control and effects of supplemental COZ in air-supported 

plastic greenhouses. Transactions of American Society of 

Agricultural Engineers 13(2):259-62, 268.

Scarratt, J.B. 1985. Greenhouse managers: beware combustion 

fumes in container greenhouses. Forestry Chronicle 61 

(4):308-311.

Sheldrake, R. 1964. COZ and ventiliation. American Vegetable 

Grower 12:30.

Sionit, N.; Kramer, P.J. 1986. Woody plant reactions to COZ 

enrichment. In: Enoch, H.Z.; Kimball, B.A., eds. Carbon 

dioxide enrichment of greenhouse crops, vol. 2., physiology, 

yield, and economics. Boca Raton, FL: CRC Press: 69-85.

Surano, K.A.; Daley, P.F.; Houpis, J.L.J.; Shinn, J.H.; Helms, J.A.; 

Palassou, R.J.; Costella, M.P. 1986. Growth and physiological 

responses of Pinus ponderosa Dougl. ex P. Laws. to long-term 

elevated COZ concentrations. Tree Physiology 2(1-3):243-259.

Tinus, R.W. 1972. Carbon dioxide enriched atmosphere speeds 

growth of ponderosa pine and blue spruce seedlings. Tree 

Planters' Notes 23(1):12-15.

Tinus, R.W. 1975. Impact of the COZ requirement on plant 

water use. Stone, J.F., ed. Plant modification for more 

efficient water use. Amsterdam: Elsevier Scientific 

Publishing. 320 p.

Tinus, R.W. 1976. Photoperiod and atmosphere COZ interact to 

control black walnut (Juglans nigra L.) seedling growth. Plant 

Physiology 57(5) (Suppl.):Abstr. 554.

Tolley, L.C. 1982. The effects of atmospheric carbon dioxide 

enrichment, irradiance, and water stress on seedling growth 

and physiology of Liquidambar styraciflua and Pinus taeda. 

PhD dissertation. Durham, NC: Duke University.
142
143
Index of Common and

Scientific Names

Trees

alder

red alder Alnus rubra Bong.83

ash

green ash Fraxinus pennsylvanica Marsh. 108

aspen Populus spp. 128

birch Betula spp. 16, 59

chestnut, sweet Castan'ea sativa Mill. 128

cypress Cupressus spp. 16, 59

dogwood Cornus spp. 83

Douglas-fir Pseudotsuga menziesii (Mirb.)

Franco 9, 12, 16, 17, 57, 59, 83, 86, 88, 93

eucalyptus Eucalyptus spp. 59

fir Abies spp. 16, 59

California red fir A. magnifica A.Murr 17

noble fir A. procera Rehd. 9

Pacific silver fir A. amabilis Dougl. ex Forbes 9, 89

white fir A. concolor (Cold & G lend.) Lindl. ex

Hildbr. 133

hackberry Celtis spp. 17, 89

hemlock

eastern hemlock Tsuga canadensis (L.) Carr. 12

mountain hemlock T. mertensiana (Bong.) Carr 86,

89, 107

western hemlock T. heterophylla (Raf.)

Sarg. 17,83, 88, 89

juniper Juniperus spp. 16, 59

eastern redcedar J. virginiana L. 17, 89

Rocky Mountain juniper J. scopulorum Sarg. 17,

89

larch Larix spp. 16, 59

Siberian larch L. siberica Ledeb. 17, 83

maple Acer spp. 16, 59

144

oak Quercus spp. 15, 59

bur oak Q. macrocarpa Michaux. 17, 18, 89

northern red oak Q. rubra L. 83

white oak Q. alba L. 83

pine Pinus spp. 15, 59 

jack pine P. banksiana Lamb. 107, 130 

loblolly pine P. taeda L. 11, 14, 15, 17, 58, 83, 93, 127 

lodgepole pine P. contorta Dougl. ex Loud. 9, 12, 17, 18, 83, 

89, 127, 128, 130 

longleaf pine P. palustris Mill. 13, 14, 17, 93

Monterey ["radiata"] pine P. radiata D. Don 11, 17,

128

ponderosa pine P. ponderosa Doug]. ex Laws.

9, 12, 17, 83, 89, 127, 128, 130

red pine P. resinosa Ait. 93

Scotch pine P. sylvestris L. 17, 18, 86, 130

shortleaf pine P. echinata Mill. 13, 14

slash pine P. elliottii Engelm. 14

Virginia pine P. virginiana Mill. 128

poplar Populus spp. 55, 107

redwood Sequoia sempervirens (D. Don) Endl. 10,

16, 17, 59

spruce Picea spp. 16, 59, 1 14 

black spruce P. mariana (Mill.) B.S.P. 12, 14, 15, 

18, 90, 130 

["Colorado"] blue spruce P. engelmannii (Mill.) 

B. S. P. 11, 12, 15, 17, 82, 83, 86, 89, 107

interior spruce P. glanca x P. englemannii 89 

Norway spruce P. abies (L.) Karst. 130

Sitka spruce P. sitchensis (Bong.) Carr. 83, 88

white spruce P. glauca (Moench) Voss 12, 17, 18,

86, 88, 89, 107, 108, 128, 131

sweetgum Liquidambar styraciflua L.107

thuja Thuja spp. 59

western cedar Thuja plicata Donn. ex D. Don 88

walnut Juglans spp. 16, 59 black 

walnut J. nigra L. 17, 88

willow Salix spp. 55
145

Other Plants

ageratum 58

marigold 58

petunia 58

cucumber 58

Pathogens

algae 55, 70

dark-winged fungus gnats Bradysia spp. 55

fungi 55

grey mold Botrytis cinerea 28, 35, 55, 57, 60,

64, 93

liverworts 49, 55, 70

mosses 49, 55, 70