My citrus trees are in a small basement room for the winter. I leave the door to the room open (the door opens to the rest of the basement) and I have a fan on which circulates the air. I have also been leaving a window in the room open a little purely because I have assumed in the same way fresh air is good for humans, it will be good for the plants. However, with the window open, the average humidty in the room, even with a small humidifier and trays of water, is only about 20%. I suspect that if I close the window, the humidity would get in to the 30-40% range.

Should I close the window? In other words, what would be better for my citrus- fresh air or increased humidity?

I think the air movement is more important than fresh air and you are supplying that with your fan. Most houses have a pretty fast air turnover--especially older homes.

Well,

most times citrus requires Carbondioxide for proper growth.
So leaving the window open little, will not be benefical in changing a large air mass, because the little air movement through the slit of a little open window isn't that high, as many people think. So it's much better to open the window three times a day wide, and exchange a large air mass, within 10 up to 15 Minutes, then close the window again.
That's benefical.

Your fan can provide the carbondioxide from you and all other in the house living mamals to the plant, but than the air flow should from the house to the plants, as from the outside over the plants to the house.... that's not benefical, in the view of carbondioxide flow, but our produced carbondioxide is very low, and thus the air flow is not realy to consider.

So open a window only a little even less benefical, as humidity.. if all other factors are benefical for a good culture....
so the air fan isn't nneded, but if you think, your plants enjoy, well, let it go!
The open window isn't realy benefical, but if you think, the plants enjoy that, let it open....

If my choices are limited ONLY to humidity and fresh air and no other choices, I would choose fresh air. However, as you live in Brooklyn, I don't think you have to worry about humidity anyway. - Millet

Millet, how much air exchange do you have in your greenhouse in winter?
Do you have fans to circulate air inside?

Millet - I agree.... fresh air is allways good.

Skeet, I have four ceiling fans that continually run 24 hours a day, recycling the upper air back down to the floor. I also four large fans at plant height that circulate the air around and around the greenhouse. Added to this I have two very large greenhouse exhaust fans, that are capable of totally exchanging 100 percent of the internal greenhouse air every one minute. I also have a large converted dye vat that hold 250 gallons of water and is capable to heating the water up to boiling (212F). The vat is great for supplying greenhouse humidity, plus I cover it with a 3/4-inch plywood cover and use it as a large heat mat for my younger trees. - Millet

My question was aimed at understanding the relative importance of air movement vs fresh air. My understanding is that the air movement is more important for health of the plants. When it is closed, my guess is that a greenhouse actually has less air infiltration than a home--my first home advertised an air exchange every hour in the builder's brochure. During the winter I would not think that you are running your exhaust fans at an exchange rate of 1 per minute--your heating bill would be astronomical.

Skeet, my greenhouse is NEVER 100 percent closed, even on the very coldest days of the winter, as the plants need fresh CO2 coming in at all times. If the vents were completely closed, the greenhouse crop would soon use up all the available internal CO2, then plant growth would stop no matter how much I circulated the air, as I would be circulating air that contained no CO2.. I have a vent open, or partially open, every single day. Another benefit that circling air has that many people do not think about is the constant distribution of CO2 up against the surface of the leaf blade. When the air is still, and the leaf stomata are open taking in CO2, soon a CO2 "vacuum", or "partial vacuum" develops adjacent the very leaf surface, thus restricting potential energy production by the tree. Circulating air that is moving constantly past the leaf surface, by use of a fan, keeps this from happening, by constantly supplying CO2 saturated air to the stomata. Lastly, fans also greatly help keep the temperature even throughout the greenhouse. - Millet

Millet,
your greenhouse isn't airtight! So even with all vents closed, you will never get the plants to need up all carbondioxide. That's a myth!

And the CO2 vakuum you talk abut, Millet, that evenly won't happen, because gas exchange generates a little, very little gas flow on und around the leaf, thus air will move, gently.. so do not mind!

Even a fully closed glasshouse (as one can find in may botanical gardens) has allways little air, by the door, vents opening and closing due temperature sensors in the glashouse and through all slits of doors, root and other points, so there will be not any CO2 deficiency happening.
Only some gardners rise artificially the Carbondioxide in the air of the glasshouse, to ensure a much better and faster growth of their plants, but the benefit of this methode is highly discussed and very controversial.
And even with 'no wind' there will be allways enogh little air movment, so simply provide enough gas exchange capacity for the leaf....

But, Millet, if you think it's good, do not mind: Don't do a change.

And don't forget that the bacterias of the soil release CO2. Even if your plant is completely closed it will grow using the CO2 coming from the destruction of the organic mater of the soil.
Indeed, if your soil is 100% mineral or hydroponic there could be some issues.

Steve said........"And the CO2 vacuum you talk abut, Millet, that evenly won't happen, because gas exchange generates a little, very little gas flow on und around the leaf, thus air will move, gently.. so do not mind! ".......

MY RESPONSE: The amount of carbon dioxide supplied to the plant from the atmosphere depends upon the level of CO2 at the leaf surface, or even more precisely, the CO2 level at the cell surface on the leaf. In still air, a leaf can draw out most of the CO2 at the cell surface, thus creating an envelope of CO2 deficiency around the leaf. When there is no turbulence, CO2 replenishment is slow. A lack of turbulence can be a common problem in the winter greenhouse, triggering carbon dioxide deficiencies even though there is plenty of the gas in the air. Turbulent air around a leaf disperses this envelope of low carbon dioxide concentration, replenishing the air adjacent to the leaf with an ample supply of CO2. Nature accomplishes this with wind.. Research in the Netherlands by P. Gaastra in 1963 showed that the rate of photosynthesis can be increased by as much as 40 percent with no change in atmospheric CO2 when the wind velocity increases from 10-100 centimeters (4 to 40 inches) per second. So, for better greenhouse plant growth, keep the air constantly moving around during the day, especially in winter.

Steve & Sylvain said........."Winter greenhouse CO2 is supplied by slits in the door and other points.....vent opening.......by bacteria"........

MY RESPONSE: In Colorado during the winter, the greenhouse vents NEVER open due to high thermostate temperatures. Carbon dioxide in the air is essential to photosynthesis. The normal level of CO2 in the air is about 300 parts per million (ppm), or .03 of 1 percent of the air we breathe. Because about 50 percent of a plant is made of carbon (and all that must come from the air), you can see that plants have quite an appetite for CO2. When CO2 is scarce, plant growth slows. But when the supply of CO2 increases beyond the normal 300 ppm, up to a certain point, plant growth increases. When plants are in a sealed environment such as a greenhouse, they can actually deplete the supply of CO2 in the air. This is exactly what happens in a greenhouse in winter when the greenhouse is closed tight. The level of CO2 has been seen to drop from 300 ppm to 100 ppm by noon on an average winter day. This can slow plant growth by 68 percent--not a pleasant thought especially if your a greenhouse plant. This phenomenon occurs only in winter greenhouses when there is no outside ventilation and the structure is closed to the outside. What does a CO2 deficient plant look like? Well, that is the big problem. It's almost impossible to see a CO2 deficiency, because the only symptom is slower growth.

NOTE: = It was not until the late fifties and early sixties that researchers worked with levels higher that the normal ambient (300 ppm) amounts. Enhancing CO2 levels from a depleted greenhouse atmosphere of 100 ppm all the way to 1,500 ppm resulted in significant yield increases. Since the sixties many commercial greenhouse growers have been enhancing the CO2 levels in winter greenhouses to 1,200 - 1,500 ppm, with a crop yield increase of 30 percent. It's like fertilizing the plants through the air. It is thought that anything above 2,000 ppm is a wasted effort, and continuous exposure to levels of 5,000 ppm may be a problem to the health of the greenhouse workers. - Millet

CO2 in the air is now closer to 400 ppm, I do not know if that is volume or weight based, but I think it is volume based--therefore for every liter of air there is 400 microliters of CO2 or about 16 micromoles (a micromole of gas is 22.4 L at 0C and 1 atmosphere pressure-- I rounded to 25 L -- at CO altitude it would be even more volume). Converted to carbohydrate (CH2O) that is about 450 ug of carbon--it would take all the CO2 in 2000 L of air to make a little less than 1 g of CH2O (plant matter)--I can see it being possible to deplete CO2 in a closed greenhouse.

Skeeter,
it's diffucult!
Because we allways have some nitrobacter and nitrosomas bacteria in the ground, als many other fungal and bacterial collonies will use O2 and breath out CO2, so complete depletion might only be possible in a sealed glas house, and only due a longer periode of time.

As Cooper and Gruenberg 1998 published, for submer grown plants and transported their studies to greenhouse banana cultures, their will be a little stream and flow of gas around the leaf blade, allways. Due light reflection, by evaporation and other factors there will be allways gas exchange around a leaf, and thus in water a measureable streaming and in air a little, but important 'breeze', even if is completely wind still.

So wind has an other factor, like the study of cooper and gruenberg found: Wind increases evaporation and limits photosynthesis and thus CO2 usage. So high CO2 dosages will only do well, if evaporation levels do not increase to high. So a light breeze may help, but a to high 'wind' will reduce photosynthesis activity...

But we can discuse it here, to excess, and won't get to one point.. because there is no good or false, no black or white... it depends again how you cultivate the plants and how you do, and how your plants respond to it....
And that's more important: To get the qualification and the competence, to look at the plants and see what the need or do not like.... and do the necessary things... that's the point, we have to reach!

I understand Steve--I had never really thought about the point Millet made and just wanted to put some real numbers into perspective. My rough calculations were a surprise to me--ignoring the potential of decomposition sources (which I had mentioned earlier), I just wanted to get an idea of the volume of atmospheric CO2 it takes to create a gram of plant matter. The 1 g in this case would be dry matter and in a live plant matter would probably be about 5 g. I still think it is pretty surprising that it takes about 2000 L of air to make 1 g of CH2O (over 4000 L to make a g of reduced C).

It really shows the power of things like tropical forest to remove atmospheric CO2 if we were not also destroying them.