Tuesday, August 18, 2015

Life support - Trees

This will be a background post, with deeper data by species coming later.

 Fruit and nut trees are not often considered for hydroponics, and for good reasons. They take years to mature and several meters of vertical space. Less commonly known is that many trees control flowering by the gravitational flow of hormones, so zero-G orchards may not behave in predictable ways. Also, behavior of many species is seasonal and dependent on both temperature and daylight hour changes. Trees are often less efficient at converting light and CO2 than other plants, and significant amounts of their production is in the form of wood instead of food. As a result, the edible yield of trees is not competitive with vegetables. Experience with hydroponic orchard production is very limited. Almost all trees require pollinators or hand pollination and many require different varieties for fertility.

 These are not insurmountable problems for a permanent colony, particularly one on a body with gravity. Let's look at the benefits so we can make some comparisons. Nut trees produce nutritious food that can be stored for months at ambient conditions. Fruit trees produce a variety of fruits, good for improving the diversity of the diet; fruits also often have higher sugar content than hydroponic produce. The physical appearance of trees can have psychological benefits for colonists, as can the dietary contributions. Pollen from tree flowers can likewise improve the variety of diet for pollinators. Trees themselves are quite adaptable and almost certainly have enormous untapped potential for the development of space-adapted varieties. Trees produce wood, a useful material for handles, furniture, etc. Wood waste fibers can be made into paper. The cellulose in wood fibers can be extracted and used as a film, converted to polymers or converted with nitric acid into nitrocellulose as blasting charges. A variety of spices and medicinals are only available as trees or inconveniently large shrubs. Allspice, clove, cinnamon, nutmeg, bay, mace, ginko and witch hazel are all examples. Most of these are used in trace quantities, so low productivity is not a problem. More common species like willow can be used for aspirin (for example), though it might be more efficient to synthesize it; on the other hand willow is a good source of material for making activated carbon filters. Trees can soak up the area lighting in public spaces, turning energy that would otherwise have become heat into food and air. They can help reduce CO2 concentrations when a lot of people are in one spot and provide some fallback capacity if the air system is temporarily offline.

 My early data for fruit trees is based on semi-intensive orchard growing on Earth using semidwarf trees. Apples seem to be the best producers at around 5 kg/m², followed by pears and plums at 3 kg/m² and cherries and peaches at 2 kg/m². Noncommercial species like pawpaw or quince yield around 1.2 kg/m². In daily terms that's only 5.5 to 13.7 grams per square meter per day. It does mean that apples can be more productive than barley (for example, at about 12g/m²*day) on an energy basis, but not even close on a volume basis. Nuts, unfortunately, are much less productive than fruits; almonds weigh in around 0.2 kg/m² or about half a gram a day per square meter. Almond milk is not likely to find itself on the menu.

 For fruits, the difference in yield by species has little to do with inherent productivity. Apples have been the focus of commercial orchard breeding programs for longer periods and larger budgets than the other species. It's true that peaches and cherries are also undergoing heavy investment and there seems to be no reason why their yields could not exceed that of today's apples. Pears and plums have not been so intensively developed, though they started with a bit better output and have still seen improvement by many dedicated researchers and breeders. Crossing over into intensive hydroponic production has generally meant incredible productivity gains for most crops. Average yields can be expected to double at the very least simply by eliminating disease, pests and competition and providing optimal levels of light, water, nutrients, temperature and air composition. A more accurate projection might be to use Earth record yields instead of average yields. Space-specific pruning and training plus a good breeding program and the potential for shortened production cycles should at least double that number again. If everbearing varieties could be developed then yields could be pushed even higher. I think given enough time and resources that apple yields could be brought as high as 120 g/m²*day, comparable to many vegetable species. We won't know until it is attempted, and given the time it takes for each generation to mature this is a process that will take 2-3 decades of sustained effort.

 The drawbacks are pretty significant and need to be addressed. Not all species have suitable dwarfing rootstocks for grafting. (Most fruit trees are a franken-tree made from one variety or species {the rootstock} that resists pests and has good growth plus another variety {the graft} that has good flavor and yield.) At this point we must assume any orchard crop will require gravity, so they are restricted to some unknown threshold probably above 0.5 g. Even mini-dwarf apples require 2 meters of aboveground clearance while the smaller semidwarf trees need nearly 4 meters if they are pruned according to normal methods. The solution I think is to emphasize the psychological aspect and place trees in public spaces. They do not need the high light output, long light hours and high CO2 concentrations of super-productive vegetables, so their growing conditions are compatible with habitation areas. Trees can be trained to grow into space-saving shapes, a practice called espalier which can produce fencelike barriers; this produces a visually pleasing structure that can screen out noise and unwanted sights while producing edible fruits. These trees will provide a sense of steady change combined with a sense of permanence and will contribute to making public areas feel more organic and living. If you think that sounds like a bunch of hippie BS, consider that just having a couple of bean plants on the ISS made a significant measurable improvement in astronaut mood and outlook. Depressed people do not get along well; mental health is critical in an environment where carelessness can kill.

 I think a large enough colony will benefit from the investment of trees. I also think a successful program of adaptation will take much longer than similar programs for vegetables and other single-year crops, and much of the work will require access to reduced-gravity growing areas. In other words, this kind of research will be enabled by spin-gravity colonies or research stations; early stations will not be able to bank on specific productivity numbers if they include trees in their design.

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