One of the things plants do is pump out oxygen. This comes, as I understand it, out of the leaves. I’m guessing that trees with leaves will pump out more oxygen than trees with needles because of their greater surface area. Have they measured the OQ’s (oxygen quotients) of various kinds of plants? And would the greater amount put out with deciduous trees with leaves be sufficiently greater to offset th the fact that they’re putting out this oxygen for only part of the year?
OK, let’s go back to basics.
The first point ot recognise is that trees “put out oxygen” only under very specific circumstances, most importantly they can only put out oxygen if there is a high light level and if they have sufficient water. We won’t go inot the details of why this is the case, but just accept that trees need a lot of light and a lot of water to produce oxygen. Followinig on from that, trees lose a lot of water from their leaves. The more oxygen they are producing the more water they are losing. The second point is that trees only produce oxygen as a byproduct of produicng tehir food. They don’t produce oxygen for the sake of producing oxygen, oxygen is just a waste product of food production.
So trees have to compromise between water loss and food/oxygen prooduction. If they want to produce a lot of food they need to have large, soft broad leaves that absorb a lot of light and lose a lot of water. If they want to use less water they have to reduce the size of the leaves.
When temperatures are below freezing plants have no water available at all. If the ground itself is frozen then of couorse there is no wate rin the soil, but even if only air temperatures are below freezing any water that falls can’t melt into the soil and the soil around the roots rapidly dries out. What that means is that to a plant cold weather is physiologically the same as a drought.
To overcome seasonal drought conditions plants have evolved three basic types of mechanisms. The first is to retain large soft leaves for the wet season, and simply shed the leaves when the soil dries out. This deciduous option is very expensive. Every time the leaves are shed the plant loses a lot of mineral nutrients. This simply isn’t an option in environments like deserts or swamps which are nutrient poor and where the shed leaves don’t rot very rapidly a deciduous. In those environment a plant trying to be deciduous will rapidily deplete the soil and die.
The second option is to produce small needle-like leaves that are retained year round. This reduces the amount of water the leaves can lose as well as reducing the maount of light absorbed, both of which also inhibit oxygen production, but the advantage is that the plant loses no nutrients. Needle-like leaves are mostly restricted to dry climates and nutrient deficient soils, but within those areas they usually predominate, not just amongst the conifers but amongst many of the flowering plants as well. The most extreme reduction of leaves to needles is found amongst the cacti.
The third option is to produce broad leaves and coat them in a thick layer of wax with just a few pores for gas exchange. The wax prevents water loss while the leaf remains broad enough to maximise light capture. This option is a compromise nutrient deficient environments.
OK , it took a while, but hopefully you shoud be able to work out the answer to your own questions now.
Yes, oxygen output of various plants has been well measured and it varies more with environmental conditions than between species. IOW leaf type is less important than where and when the plants are growing.
During periods with sufficient water and light plants with broad soft leaves will produce more oxygen than plants with needles or broad hard leaves. That’s why almost all leaves found in tropical rainforests are braod and soft. During periods of water stress such as drought or during freezing periods no plant will be producing much oxygen, if any. The deciduous plants will drop thier leaves and the other plants will shut down to avoid water loss.
Which answers your last question: evergreen plants in cold climates aren’t producing much oxygen in winter anyway. IOW whether a plant is decidous or evergreen in such climates is irrelvant, they all only produce oxygen during the warm part of the year.
One should not think needles are not dropped. They normally run a couple year cycle. It varies by species, but a typical cycle can be like this.
Year 1: New needles grow in late spring and produce food.
Year 2: Needles produce food.
Year 3: Needles produce food and die during the late summer.
This cycle will leave a tree with about a third of the needles in each years stage.
There are also evergreens with leaves that are waxy. Look to the euonymus for an example, and the holly.
Ilex aquifolium ‘Aureomarginata’
You should not get the idea that evergreens don’t have large leaf areas. Until you measure the oxygen output of different trees, you can’t say if an evergreen or deciduous plant will produce more oxygen. Leaving it to a contest between a single needle or single leaf, A large leaf will beat out the needle. I believe that what is being asked for though is the output of oxygen over a year for equal sized trees in areas that experience cold weather. The limitations of evergreens in winter is already well explained. I think a more grey area is present, about evergreens in climates where the ground doesn’t freeze, but trees drop their leaves.
This is the case for almost all non-decidupsu leaves. The point is that the leaves are retained for the maximum possible period.
Or eucalypts, or olives or almost any other arid zone plant.
Indeed, many evergreens of rainforests have extremely high leaf areas and high leaf area indices, that is the amount of leaf area per unit ground area. Many conifers also have large leaf areas.
However the amount of leaf area for leaf biomass is small simply because a cylinder isn’t an efficient way too produce surface area. IOW for a given weight of leaf a needle will produce less surface area and oxygen production is primarily dependent on surface area.
In which case we have to ask why the trees drop their leaves. In all the cases I can think of the effect is still caused because the soil becomes too dry to suport the leaves. The effect then remains the same: the conifers will still shut tend to down to avoid drought stress.
BTW be careful in your use of “evergreen”, I suspect you are misusing it. Most tropical rainforest trees for example are evergreen, but they are not needle leafed plants, nor are they gymnosperms.
Tangental question: Wiki says that arborvitae leaves are rich in vitamin C, and that American Indians and early European explorers used them as a cure for scurvy. How? Did they just munch on them? Or did they make tea?
I can’t help you on that specific item.
I have a small book that has old medicinal uses listed, but I don’t consider it to be trustworthy as to it’s accuracy. Many items were chewed after removal from the live plant, others made into teas, some into poultice, and others boiled with mineral spirits. The juniper berry was used as a diuretic. The inner bark of slippery elm was used for stomach problems and sore throats. I won’t go on about uses of plants further in this thread as it would totally derail it .
Blake – thank you very much for your extremely clear and extremely helpful answer. My ignorance has not just been fought, it’s been beaten into submission. 
Serious – this all makes an enormous amount of sense. Thanks.
Harmonious Discord – I’m well aware that there are evergreens with leaves – this is why I specified leaves vs. needles, not deciduous vs. evergreens, in the thread title.
I found a ton of references for this in a Google search for +“pine needle tea” +indians, but did not get anything I considered a worthy reference to post here. Lots and lots of mention of it, though, for what it’s worth, just nothing definitive before I lost patience. FWIW, a lot of the sites claimed that they did both, munched on them and made a tea out of them.
I know that.
I stated that for the people that are not into plants like you are.
Ah. Sorry for the snark, then.
Why do you use plus signs in a Google search?
The plus sign is a Google operator that means I want that word to be in the search.
The minus sign means do not include any results with that word.
Putting only words without operators Google won’t return only results with all the words.
Quotes mean give results for that exact phrase.
We’re in danger of derailing the thread, but since the OP’s question has been answered, I should point out that the above is incorrect:
The only time you need to use the “+” operator is when you specifically want to include a word in your search that Google would otherwise ignore as too common. See the page to which you link. The advanced search page to which you link only includes an “with all of the terms” search box because the other search boxes are on some other basis. On an ordinary search, it is the default anyway.